statespace model

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Chapter 18 The STATESPACE Procedure Chapter Table of Contents OVERVIEW  . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 999 GETTING ST ARTED  . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1002 Au to ma ti c St ate Sp ace Mod el Se le ct io n . . . . . . . . . . . . . . . . . . . . 10 03 Specifying the State Space Model . . . . . . . . . . . . . . . . . . . . . . . 1010 SYNTAX  . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1013 Functional Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1013 PROC ST A T ES PACE Statement . . . . . . . . . . . . . . . . . . . . . . . . 1014 BY Statement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1018 FORM Statement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1018 ID Statement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1019 INITIAL Statement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1019 RESTRICT Statement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1019 V AR Statement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1020 DETAILS  . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1021 Missing Values . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1021 Stationarity and Differencing . . . . . . . . . . . . . . . . . . . . . . . . . . 1021 Pr e li min ary Au to re gr es s ive Mod els . . . . . . . . . . . . . . . . . . . . . . 1 02 2 Canonical Correlation Analysis . . . . . . . . . . . . . . . . . . . . . . . . . 1026 Parameter Estimation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1029 Forecasting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1031 Relation of ARMA and State Space Forms . . . . . . . . . . . . . . . . . . . 1033 OUT= Data Set . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1034 OUT AR= Data Set . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1035 OUTMODEL= Data Set . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1036 Printed Output . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1037 ODS Table Names . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1039 EXAMPLE  . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1040 Ex ampl e 18 .1 Se ri es J fr om Bo x and Je nk in s . . . . . . . . . . . . . . . . . 10 40 REFERENCES  . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1047 997

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Page 1: Statespace Model

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Chapter 18

The STATESPACE Procedure

Chapter Table of Contents

OVERVIEW 999

GETTING STARTED 1002

Automatic State Space Model Selection 1003

Specifying the State Space Model 1010

SYNTAX 1013

Functional Summary 1013

PROC STATESPACE Statement 1014

BY Statement 1018

FORM Statement 1018

ID Statement 1019

INITIAL Statement 1019

RESTRICT Statement 1019

VAR Statement 1020

DETAILS 1021Missing Values 1021

Stationarity and Differencing 1021

Preliminary Autoregressive Models 1022

Canonical Correlation Analysis 1026

Parameter Estimation 1029

Forecasting 1031

Relation of ARMA and State Space Forms 1033

OUT= Data Set 1034

OUTAR= Data Set 1035

OUTMODEL= Data Set 1036

Printed Output 1037

ODS Table Names 1039

EXAMPLE 1040

Example 181 Series J from Box and Jenkins 1040

REFERENCES 1047

997

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Chapter 18

The STATESPACE Procedure

Overview

The STATESPACE procedure analyzes and forecasts multivariate time series using

the state space model The STATESPACE procedure is appropriate for jointly fore-

casting several related time series that have dynamic interactions By taking into

account the autocorrelations among the whole set of variables the STATESPACE

procedure may give better forecasts than methods that model each series separately

By default the STATESPACE procedure automatically selects a state space model

appropriate for the time series making the procedure a good tool for automatic fore-

casting of multivariate time series Alternatively you can specify the state spacemodel by giving the form of the state vector and the state transition and innovation

matrices

The methods used by the STATESPACE procedure assume that the time series are

jointly stationary Nonstationary series must be made stationary by some prelimi-

nary transformation usually by differencing The STATESPACE procedure allows

you to specify differencing of the input data When differencing is specified the

STATESPACE procedure automatically integrates forecasts of the differenced series

to produce forecasts of the original series

The State Space Model

The state space model represents a multivariate time series through auxiliary vari-ables some of which may not be directly observable These auxiliary variables are

called the state vector The state vector summarizes all the information from the

present and past values of the time series relevant to the prediction of future values

of the series The observed time series are expressed as linear combinations of the

state variables The state space model is also called a Markovian representation or

a canonical representation of a multivariate time series process The state space ap-

proach to modeling a multivariate stationary time series is summarized in Akaike

(1976)

The state space form encompasses a very rich class of models Any Gaussian multi-

variate stationary time series can be written in a state space form provided that the

dimension of the predictor space is finite In particular any autoregressive movingaverage (ARMA) process has a state space representation and conversely any state

space process can be expressed in an ARMA form (Akaike 1974) More details on

the relation of the state space and ARMA forms are given in Relation of ARMA and

State Space Forms later in this chapter

Letx

t

be ther 1

vector of observed variables after differencing (if differencing is

specified) and subtracting the sample mean Letz

t

be the state vector of dimension

s s

r where the first r components of z

t

consist of x

t

Let the notationx

t + k j t

999

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Part 2 General Information

represent the conditional expectation (or prediction) of x

t + k

based on the information

available at time t Then the lasts r

elements of z

t

consist of elements of xt + k j t

where k gt0 is specified or determined automatically by the procedure

There are various forms of the state space model in use The form of the state space

model used by the STATESPACE procedure is based on Akaike (1976) The model

is defined by the following state transition equation

z

t + 1

= F z

t

+ G e

t + 1

In the state transition equation thes s

coefficient matrix F is called the transition

matrix it determines the dynamic properties of the model

Thes r

coefficient matrix G is called the input matrix it determines the variance

structure of the transition equation For model identification the first r rows and

columns of G are set to anr r

identity matrix

The input vector et

is a sequence of independent normally distributed random vectors

of dimension r with mean 0 and covariance matrix

e e

The random error et

is

sometimes called the innovation vector or shock vector

In addition to the state transition equation state space models usually include a mea-

surement equation or observation equation that gives the observed valuesx

t

as a

function of the state vectorz

t

However since PROC STATESPACE always includes

the observed valuesx

t

in the state vectorz

t

the measurement equation in this case

merely represents the extraction of the first r components of the state vector

The measurement equation used by the STATESPACE procedure is

x

t

= I

r

0 z

t

whereI

r

is anr r

identity matrix In practice PROC STATESPACE performs the

extraction of x

t

fromz

t

without reference to an explicit measurement equation

In summary

xt

is an observation vector of dimension r

zt

is a state vector of dimension s whose first r elements are xt

and

whose lasts r

elements are conditional prediction of future xt

F is ans s

transition matrix

G is ans r

input matrix with the identity matrix Ir

forming the first

r rows and columns

et

is a sequence of independent normally distributed random vectors

of dimension r with mean 0 and covariance matrix

e e

How PROC STATESPACE Works

The design of the STATESPACE procedure closely follows the modeling strategy

proposed by Akaike (1976) This strategy employs canonical correlation analysis for

the automatic identification of the state space model

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Chapter 18 Overview

Following Akaike (1976) the procedure first fits a sequence of unrestricted vector

autoregressive (VAR) models and computes Akaikersquos information criterion (AIC) for

each model The vector autoregressive models are estimated using the sample au-

tocovariance matrices and the Yule-Walker equations The order of the VAR model

producing the smallest Akaike information criterion is chosen as the order (number

of lags into the past) to use in the canonical correlation analysis

The elements of the state vector are then determined via a sequence of canonical cor-

relation analyses of the sample autocovariance matrices through the selected order

This analysis computes the sample canonical correlations of the past with an increas-

ing number of steps into the future Variables that yield significant correlations are

added to the state vector those that yield insignificant correlations are excluded from

further consideration The importance of the correlation is judged on the basis of

another information criterion proposed by Akaike See the section Canonical Cor-

relation Analysis for details If you specify the state vector explicitly these model

identification steps are omitted

Once the state vector is determined the state space model is fit to the data The free

parameters in the F G and

e e

matrices are estimated by approximate maximumlikelihood By default the

F

andG

matrices are unrestricted except for identifia-

bility requirements Optionally conditional least-squares estimates can be computed

You can impose restrictions on elements of theF

andG

matrices

After the parameters are estimated forecasts are produced from the fitted state space

model using the Kalman filtering technique If differencing was specified the fore-

casts are integrated to produce forecasts of the original input variables

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Part 2 General Information

Getting Started

The following introductory example uses simulated data for two variables X and Y

The following statements generate the X and Y series

data inx=10 y=40

x1=0 y1=0

a1=0 b1=0

iseed=123

do t=-100 to 200

a=rannor(iseed)

b=rannor(iseed)

dx = 05x1 + 03y1 + a - 02a1 - 01b1

dy = 03x1 + 05y1 + b

x = x + d x + 2 5

y = y + d y + 2 5

if t gt= 0 then output

x1 = dx y1 = dy

a 1 = a b 1 = b

end

keep t x y

run

The simulated series X and Y are shown in Figure 181

Figure 181 Example Series

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Chapter 18 Getting Started

Automatic State Space Model Selection

The STATESPACE procedure is designed to automatically select the best state space

model for forecasting the series You can specify your own model if you wish and

you can use the output from PROC STATESPACE to help you identify a state space

model However the easiest way to use PROC STATESPACE is to let it choose the

model

Stationarity and Differencing

Although PROC STATESPACE selects the state space model automatically it does

assume that the input series are stationary If the series are nonstationary then the

process may fail Therefore the first step is to examine your data and test to see if

differencing is required (See the section Stationarity and Differencing later in this

chapter for further discussion of this issue)

The series shown in Figure 181 are nonstationary In order to forecast X and Y

with a state space model you must difference them (or use some other de-trending

method) If you fail to difference when needed and try to use PROC STATESPACE

with nonstationary data an inappropriate state space model may be selected and themodel estimation may fail to converge

The following statements identify and fit a state space model for the first differences

of X and Y and forecast X and Y 10 periods ahead

proc statespace data=in out=out lead=10

var x(1) y(1)

id t

run

The DATA= option specifies the input data set and the OUT= option specifies theoutput data set for the forecasts The LEAD= option specifies forecasting 10 obser-

vations past the end of the input data The VAR statement specifies the variables to

forecast and specifies differencing The notation X(1) Y(1) specifies that the state

space model analyzes the first differences of X and Y

Descriptive Statistics and Preliminary Autoregressions

The first page of the printed output produced by the preceding statements is shown in

Figure 182

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The STATESPACE Procedure

Number of Observations 200

Standard

Variable Mean Error

x 0144316 1233457 Has been differenced With period(s) = 1

y 0164871 1304358 Has been differenced

With period(s) = 1

The STATESPACE Procedure

Information Criterion for Autoregressive Models

Lag=0 Lag=1 Lag=2 Lag=3 Lag=4 Lag=5 Lag=6 Lag=7 Lag=8

149697 8387786 5517099 1205986 1536952 2179538 2400638 2988874 3355708

Information

Criterion for Autoregressive

Models

Lag=9 Lag=10

4117606 4770222

Schematic Representation of Correlations

NameLag 0 1 2 3 4 5 6 7 8 9 10

x ++ ++ ++ ++ ++ ++ + + +

y ++ ++ ++ ++ ++ + + + +

+ is gt 2std error - is lt -2std error is between

Figure 182 Descriptive Statistics and VAR Order Selection

Descriptive statistics are printed first giving the number of nonmissing observations

after differencing and the sample means and standard deviations of the differenced

series The sample means are subtracted before the series are modeled (unless the

NOCENTER option is specified) and the sample means are added back when the

forecasts are produced

LetX

t

andY

t

be the observed values of X and Y and letx

t

andy

t

be the values of X

and Y after differencing and subtracting the mean difference The seriesx

t

modeled

by the STATEPSPACE procedure is

x

t

=

x

t

y

t

=

1 B X

t

0 1 4 4 3 1 6

1 B Y

t

0 1 6 4 8 7 1

where B represents the backshift operator

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Chapter 18 Getting Started

After the descriptive statistics PROC STATESPACE prints the Akaike information

criterion (AIC) values for the autoregressive models fit to the series The smallest AIC

value in this case 5517 at lag 2 determines the number of autocovariance matrices

analyzed in the canonical correlation phase

A schematic representation of the autocorrelations is printed next This indicates

which elements of the autocorrelation matrices at different lags are significantlygreater or less than 0

The second page of the STATESPACE printed output is shown in Figure 183

The STATESPACE Procedure

Schematic Representation of Partial Autocorrelations

NameLag 1 2 3 4 5 6 7 8 9 10

x ++ +

y ++

+ is gt 2std error - is lt -2std error is between

Yule-Walker Estimates for Minimum AIC

--------Lag=1------- --------Lag=2-------

x y x y

x 0257438 0202237 0170812 0133554

y 0292177 0469297 -000537 -000048

Figure 183 Partial Autocorrelations and VAR Model

Figure 183 shows a schematic representation of the partial autocorrelations similar

to the autocorrelations shown in Figure 182 The selection of a second order autore-

gressive model by the AIC statistic looks reasonable in this case because the partialautocorrelations for lags greater than 2 are not significant

Next the Yule-Walker estimates for the selected autoregressive model are printed

This output shows the coefficient matrices of the vector autoregressive model at each

lag

Selected State Space Model Form and Preliminary Estimates

After the autoregressive order selection process has determined the number of lags to

consider the canonical correlation analysis phase selects the state vector By default

output for this process is not printed You can use the CANCORR option to print

details of the canonical correlation analysis See the section Canonical Correlation

Analysis later in this chapter for an explanation of this process

Once the state vector is selected the state space model is estimated by approximate

maximum likelihood Information from the canonical correlation analysis and from

the preliminary autoregression is used to form preliminary estimates of the state space

model parameters These preliminary estimates are used as starting values for the

iterative estimation process

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The form of the state vector and the preliminary estimates are printed next as shown

in Figure 184

The STATESPACE Procedure

Selected Statespace Form and Preliminary Estimates

State Vector

x(TT) y(TT) x(T+1T)

Estimate of Transition Matrix

0 0 1

0291536 0468762 -000411

024869 024484 0204257

Input Matrix for Innovation

1 0

0 1

0257438 0202237

Variance Matrix for Innovation

0945196 0100786

0100786 1014703

Figure 184 Preliminary Estimates of State Space Model

Figure 184 first prints the state vector as X[TT] Y[TT] X[T+1T] This notation

indicates that the state vector is

z

t

=

2

4

x

t j t

y

t j t

x

t + 1 j t

3

5

The notationx

t + 1 j t

indicates the conditional expectation or prediction of x

t + 1

based

on the information available at time t andx

t j t

andy

t j t

arex

t

andy

t

respectively

The remainder of Figure 184 shows the preliminary estimates of the transition matrix

F

the input matrixG

and the covariance matrix

e e

Estimated State Space Model

The next page of the STATESPACE output prints the final estimates of the fittedmodel as shown in Figure 185 This output has the same form as in Figure 184 but

shows the maximum likelihood estimates instead of the preliminary estimates

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Chapter 18 Getting Started

The STATESPACE Procedure

Selected Statespace Form and Fitted Model

State Vector

x(TT) y(TT) x(T+1T)

Estimate of Transition Matrix

0 0 1

0297273 047376 -001998

02301 0228425 0256031

Input Matrix for Innovation

1 0

0 1

0257284 0202273

Variance Matrix for Innovation

0945188 01007520100752 1014712

Figure 185 Fitted State Space Model

The estimated state space model shown in Figure 185 is

2

4

x

t + 1 j t + 1

y

t + 1 j t + 1

x

t + 2 j t + 1

3

5

=

2

4

0 0 1

0 2 9 7 0 4 7 4 0 0 2 0

0 2 3 0 0 2 2 8 0 2 5 6

3

5

2

4

x

t

y

t

x

t + 1 j t

3

5

+

2

4

1 0

0 1

0 2 5 7 0 2 0 2

3

5

e

t + 1

n

t + 1

v a r

e

t + 1

n

t + 1

=

0 9 4 5 0 1 0 1

0 1 0 1 1 0 1 5

The next page of the STATESPACE output lists the estimates of the free parameters

in theF

andG

matrices with standard errors and t statistics as shown in Figure 186

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Part 2 General Information

The STATESPACE Procedure

Parameter Estimates

Standard

Parameter Estimate Error t Value

F(21) 0297273 0129995 229

F(22) 0473760 0115688 410F(23) -001998 0313025 -006

F(31) 0230100 0126226 182

F(32) 0228425 0112978 202

F(33) 0256031 0305256 084

G(31) 0257284 0071060 362

G(32) 0202273 0068593 295

Figure 186 Final Parameter Estimates

Convergence Failures

The maximum likelihood estimates are computed by an iterative nonlinear maximiza-

tion algorithm which may not converge If the estimates fail to converge warning

messages are printed in the output

If you encounter convergence problems you should recheck the stationarity of the

data and ensure that the specified differencing orders are correct Attempting to fit

state space models to nonstationary data is a common cause of convergence failure

You can also use the MAXIT= option to increase the number of iterations allowed

or experiment with the convergence tolerance options DETTOL= and PARMTOL=

Forecast Data Set

The following statements print the output data set The WHERE statement excludes

the first 190 observations from the output so that only the forecasts and the last 10

actual observations are printed

proc print data=outid t

where t gt 190

run

The PROC PRINT output is shown in Figure 187

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Figure 188 Plot of Forecasts

Controlling Printed Output

By default the STATESPACE procedure produces a large amount of printed output

The NOPRINT option suppresses all printed output You can suppress the printed

output for the autoregressive model selection process with the PRINTOUT=NONE

option The descriptive statistics and state space model estimation output are still

printed when PRINTOUT=NONE is specified You can produce more detailed output

with the PRINTOUT=LONG option and by specifying the printing control optionsCANCORR COVB and PRINT

Specifying the State Space Model

Instead of allowing the STATESPACE procedure to select the model automatically

you can use FORM and RESTRICT statements to specify a state space model

Specifying the State Vector

Use the FORM statement to control the form of the state vector You can use this

feature to force PROC STATESPACE to estimate and forecast a model different from

the model it would select automatically You can also use this feature to reestimate

the automatically selected model (possibly with restrictions) without repeating thecanonical correlation analysis

The FORM statement specifies the number of lags of each variable to include in

the state vector For example the statement FORM X 3 forces the state vector to

includex

t j t

x

t + 1 j t

andx

t + 2 j t

The following statement specifies the state vector

x

t j t

y

t j t

x

t + 1 j t

which is the same state vector selected in the preceding example

f o r m x 2 y 1

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Chapter 18 Getting Started

You can specify the form for only some of the variables and allow PROC STATES-

PACE to select the form for the other variables If only some of the variables are

specified in the FORM statement canonical correlation analysis is used to determine

the number of lags included in the state vector for the remaining variables not spec-

ified by the FORM statement If the FORM statement includes specifications for all

the variables listed in the VAR statement the state vector is completely defined and

the canonical correlation analysis is not performed

Restricting the F and G matrices

After you know the form of the state vector you can use the RESTRICT statement

to fix some parameters in theF

andG

matrices to specified values One use of this

feature is to remove insignificant parameters by restricting them to 0

In the introductory example shown in the preceding section the F[23] parameter is

not significant (The parameters estimation output shown in Figure 186 gives the t

statistic for F[23] as -006 F[33] and F[31] also have low significance witht 2

)

The following statements reestimate this model with F[23] restricted to 0 The

FORM statement is used to specify the state vector and thus bypass the canonicalcorrelation analysis

proc statespace data=in out=out lead=10

var x(1) y(1)

id t

form x 2 y 1

restrict f(23)=0

run

The final estimates produced by these statements are shown in Figure 189

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The STATESPACE Procedure

Selected Statespace Form and Fitted Model

State Vector

x(TT) y(TT) x(T+1T)

Estimate of Transition Matrix

0 0 1

0290051 0467468 0

0227051 0226139 026436

Input Matrix for Innovation

1 0

0 1

0256826 0202022

Variance Matrix for Innovation

0945175 01006960100696 1014733

The STATESPACE Procedure

Parameter Estimates

Standard

Parameter Estimate Error t Value

F(21) 0290051 0063904 454

F(22) 0467468 0060430 774

F(31) 0227051 0125221 181

F(32) 0226139 0111711 202

F(33) 0264360 0299537 088G(31) 0256826 0070994 362

G(32) 0202022 0068507 295

Figure 189 Results using RESTRICT Statement

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Chapter 18 Syntax

Syntax

The STATESPACE procedure uses the following statements

PROC STATESPACE options

BY variable FORM variable value

ID variable

INITIAL F(rowcolumn)=value

G(rowcolumn)=value

RESTRICT F(rowcolumn)=value

G(rowcolumn)=value

VAR variable (difference difference

)

Functional Summary

The statements and options used by PROC STATESPACE are summarized in the

following table

Description Statement Option

Input Data Set Options

specify the input data set PROC STATESPACE DATA=

prevent subtraction of sample mean PROC STATESPACE NOCENTER

specify the ID variable ID

specify the observed series and differencing VAR

Options for Autoregressive Estimates

specify the maximum order PROC STATESPACE ARMAX=

specify maximum lag for autocovariances PROC STATESPACE LAGMAX=

output only minimum AIC model PROC STATESPACE MINIC

specify the amount of detail printed PROC STATESPACE PRINTOUT=

write preliminary AR models to a data set PROC STATESPACE OUTAR=

Options for Canonical Correlation Analysis

print the sequence of canonical correlations PROC STATESPACE CANCORR

specify upper limit of dimension of state

vector

PROC STATESPACE DIMMAX=

specify the minimum number of lags PROC STATESPACE PASTMIN=

specify the multiplier of the degrees of

freedom

PROC STATESPACE SIGCORR=

Options for State Space Model Estimation

specify starting values INITIAL

print covariance matrix of parameter estimates PROC STATESPACE COVB

specify the convergence criterion PROC STATESPACE DETTOL=

specify the convergence criterion PROC STATESPACE PARMTOL=

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Description Statement Option

print the details of the iterations PROC STATESPACE ITPRINT

specify an upper limit of the number of lags PROC STATESPACE KLAG=

specify maximum number of iterations

allowed

PROC STATESPACE MAXIT=

suppress the final estimation PROC STATESPACE NOEST

write the state space model parameter esti-

mates to an output data set

PROC STATESPACE OUTMODEL=

use conditional least squares for final estimates PROC STATESPACE RESIDEST

specify criterion for testing for singularity PROC STATESPACE SINGULAR=

Options for Forecasting

start forecasting before end of the input data PROC STATESPACE BACK=

specify the time interval between observations PROC STATESPACE INTERVAL=

specify multiple periods in the time series PROC STATESPACE INTPER=specify how many periods to forecast PROC STATESPACE LEAD=

specify the output data set for forecasts PROC STATESPACE OUT=

print forecasts PROC STATESPACE PRINT

Options to Specify the State Space Model

specify the state vector FORM

specify the parameter values RESTRICT

BY Groups

specify BY-group processing BY

Printing

suppresses all printed output NOPRINT

PROC STATESPACE Statement

PROC STATESPACE options

The following options can be specified in the PROC STATESPACE statement

Printing Options NOPRINT

suppresses all printed output

Input Data Options

DATA= SAS-data-set

specifies the name of the SAS data set to be used by the procedure If the DATA=

option is omitted the most recently created SAS data set is used

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Chapter 18 Syntax

LAGMAX= k

specifies the number of lags for which the sample autocovariance matrix is computed

The LAGMAX= option controls the number of lags printed in the schematic repre-

sentation of the autocorrelations

The sample autocovariance matrix of lag i denoted asC

i

is computed as

C

i

=

1

N 1

N

X

t = 1 + i

x

t

x

0

t i

wherex

t

is the differenced and centered data andN

is the number of observations

(If the NOCENTER option is specified 1 is not subtracted fromN

) LAGMAX= k

specifies thatC

0

throughC

k

are computed The default is LAGMAX=10

NOCENTER

prevents subtraction of the sample mean from the input series (after any specified

differencing) before the analysis

Options for Preliminary Autoregressive Models ARMAX= n

specifies the maximum order of the preliminary autoregressive models The AR-

MAX= option controls the autoregressive orders for which information criteria are

printed and controls the number of lags printed in the schematic representation of

partial autocorrelations The default is ARMAX=10 See Preliminary Autoregres-

sive Models later in this chapter for details

MINIC

writes to the OUTAR= data set only the preliminary Yule-Walker estimates for the

VAR model producing the minimum AIC See OUTAR= Data Set later in this chap-

ter for details

OUTAR= SAS-data-set

writes the Yule-Walker estimates of the preliminary autoregressive models to a SAS

data set See OUTAR= Data Set later in this chapter for details

PRINTOUT= SHORT | LONG | NONE

determines the amount of detail printed PRINTOUT=LONG prints the lagged co-

variance matrices the partial autoregressive matrices and estimates of the resid-

ual covariance matrices from the sequence of autoregressive models PRINT-

OUT=NONE suppresses the output for the preliminary autoregressive models The

descriptive statistics and state space model estimation output are still printed when

PRINTOUT=NONE is specified PRINTOUT=SHORT is the default

Canonical Correlation Analysis Options

CANCORR

prints the canonical correlations and information criterion for each candidate state

vector considered See Canonical Correlation Analysis later in this chapter for

details

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Part 2 General Information

DIMMAX= n

specifies the upper limit to the dimension of the state vector The DIMMAX= option

can be used to limit the size of the model selected The default is DIMMAX=10

PASTMIN= n

specifies the minimum number of lags to include in the canonical correlation analy-

sis The default is PASTMIN=0 See Canonical Correlation Analysis later in thischapter for details

SIGCORR= value

specifies the multiplier of the degrees of freedom for the penalty term in the informa-

tion criterion used to select the state space form The default is SIGCORR=2 The

larger the value of the SIGCORR= option the smaller the state vector tends to be

Hence a large value causes a simpler model to be fit See Canonical Correlations

Analysis later in this chapter for details

State Space Model Estimation Options

COVB

prints the inverse of the observed information matrix for the parameter estimatesThis matrix is an estimate of the covariance matrix for the parameter estimates

DETTOL= value

specifies the convergence criterion The DETTOL= and PARMTOL= option values

are used together to test for convergence of the estimation process If during an

iteration the relative change of the parameter estimates is less than the PARMTOL=

value and the relative change of the determinant of the innovation variance matrix

is less than the DETTOL= value then iteration ceases and the current estimates are

accepted The default is DETTOL=1E-5

ITPRINT

prints the iterations during the estimation process

KLAG= n

sets an upper limit for the number of lags of the sample autocovariance matrix used

in computing the approximate likelihood function If the data have a strong moving

average character a larger KLAG= value may be necessary to obtain good estimates

The default is KLAG=15 See Parameter Estimation later in this chapter for details

MAXIT= n

sets an upper limit to the number of iterations in the maximum likelihood or condi-

tional least-squares estimation The default is MAXIT=50

NOEST

suppresses the final maximum likelihood estimation of the selected model

OUTMODEL= SAS-data-set

writes the parameter estimates and their standard errors to a SAS data set See OUT-

MODEL= Data Set later in this chapter for details

PARMTOL= value

specifies the convergence criterion The DETTOL= and PARMTOL= option values

are used together to test for convergence of the estimation process If during an

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Chapter 18 Syntax

iteration the relative change of the parameter estimates is less than the PARMTOL=

value and the relative change of the determinant of the innovation variance matrix

is less than the DETTOL= value then iteration ceases and the current estimates are

accepted The default is PARMTOL=001

RESIDEST

computes the final estimates using conditional least squares on the raw data Thistype of estimation may be more stable than the default maximum likelihood method

but is usually more computationally expensive See Parameter Estimation later in

this chapter for details of the conditional least squares method

SINGULAR= value

specifies the criterion for testing for singularity of a matrix A matrix is declared

singular if a scaled pivot is less than the SINGULAR= value when sweeping the

matrix The default is SINGULAR=1E-7

Forecasting Options

BACK= n

starts forecasting n periods before the end of the input data The BACK= option valuemust not be greater than the number of observations The default is BACK=0

INTERVAL= interval

specifies the time interval between observations The INTERVAL= value is used

in conjunction with the ID variable to check that the input data are in order and

have no missing periods The INTERVAL= option is also used to extrapolate the ID

values past the end of the input data See Chapter 3 ldquoDate Intervals Formats and

Functionsrdquo for details on the INTERVAL= values allowed

INTPER= n

specifies that each input observation corresponds to n time periods For example

the options INTERVAL=MONTH and INTPER=2 specify bimonthly data and are

equivalent to specifying INTERVAL=MONTH2 If the INTERVAL= option is not

specified the INTPER= option controls the increment used to generate ID values for

the forecast observations The default is INTPER=1

LEAD= n

specifies how many forecast observations are produced The forecasts start at the

point set by the BACK= option The default is LEAD=0 which produces no fore-

casts

OUT= SAS-data-set

writes the residuals actual values forecasts and forecast standard errors to a SAS

data set See OUT= Data Set later in this chapter for details

PRINT

prints the forecasts

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Part 2 General Information

BY Statement

BY variable

A BY statement can be used with the STATESPACE procedure to obtain separateanalyses on observations in groups defined by the BY variables

FORM Statement

FORM variable value

The FORM statement specifies the number of times a variable is included in the state

vector Values can be specified for any variable listed in the VAR statement If a

value is specified for each variable in the VAR statement the state vector for the state

space model is entirely specified and automatic selection of the state space model is

not performed

The FORM statement forces the state vectorz

t

to contain a specific variable a given

number of times For example if Y is one of the variables inx

t

then the statement

form y 3

forces the state vector to containY

t

Y

t + 1 j t

andY

t + 2 j t

possibly along with other

variables

The following statements illustrate the use of the FORM statement

proc statespace data=in

var x y

f o r m x 3 y 2

run

These statements fit a state space model with the following state vector

z

t

=

2

6

6

6

6

4

x

t j t

y

t j t

x

t + 1 j t

y

t + 1 j t

x

t + 2 j t

3

7

7

7

7

5

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Chapter 18 Syntax

ID Statement

ID variable

The ID statement specifies a variable that identifies observations in the input data setThe variable specified in the ID statement is included in the OUT= data set The

values of the ID variable are extrapolated for the forecast observations based on the

values of the INTERVAL= and INTPER= options

INITIAL Statement

INITIAL F (rowcolumn)= value G(row column)= value

The INITIAL statement gives initial values to the specified elements of theF

andG

matrices These initial values are used as starting values for the iterative estimation

Parts of theF

andG

matrices represent fixed structural identities If an element

specified is a fixed structural element instead of a free parameter the corresponding

initialization is ignored

The following is an example of an INITIAL statement

initial f(32)=0 g(41)=0 g(51)=0

RESTRICT Statement

RESTRICT F(rowcolumn)= value G(rowcolumn)= value

The RESTRICT statement restricts the specified elements of theF

andG

matrices

to the specified values

To use the restrict statement you need to know the form of the model Either specify

the form of the model with the FORM statement or do a preliminary run perhaps

with the NOEST option to find the form of the model that PROC STATESPACE

selects for the data

The following is an example of a RESTRICT statement

restrict f(32)=0 g(41)=0 g(51)=0

Parts of theF

andG

matrices represent fixed structural identities If a restriction is

specified for an element that is a fixed structural element instead of a free parameter

the restriction is ignored

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Part 2 General Information

VAR Statement

VAR variable (difference difference )

The VAR statement specifies the variables in the input data set to model and fore-cast The VAR statement also specifies differencing of the input variables The VAR

statement is required

Differencing is specified by following the variable name with a list of difference

periods separated by commas See the section Stationarity and Differencing for

more information on differencing of input variables

The order in which variables are listed in the VAR statement controls the order in

which variables are included in the state vector Usually potential inputs should be

listed before potential outputs

For example assuming the input data are monthly the following VAR statement

specifies modeling and forecasting of the one period and seasonal second differenceof X and Y

var x(112) y(112)

In this example the vector time series analyzed is

x

t

=

1 B 1 B

1 2

X

t

x

1 B 1 B

1 2

Y

t

y

where B represents the back shift operator and x and y represent the means of thedifferenced series If the NOCENTER option is specified the mean differences are

not subtracted

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Chapter 18 Details

Details

Missing Values

The STATESPACE procedure does not support missing values The procedure uses

the first contiguous group of observations with no missing values for any of the VARstatement variables Observations at the beginning of the data set with missing values

for any VAR statement variable are not used or included in the output data set

Stationarity and Differencing

The state space model used by the STATESPACE procedure assumes that the time

series are stationary Hence the data should be checked for stationarity One way

to check for stationarity is to plot the series A graph of series over time can show a

time trend or variability changes

You can also check stationarity by using the sample autocorrelation functions dis-

played by the ARIMA procedure The autocorrelation functions of nonstationaryseries tend to decay slowly See Chapter 7 ldquoThe ARIMA Procedurerdquo for more in-

formation

Another alternative is to use the STATIONARITY= option on the IDENTIFY state-

ment in PROC ARIMA to apply Dickey-Fuller tests for unit roots in the time series

See Chapter 7 ldquoThe ARIMA Procedurerdquo for more information on Dickey-Fuller unit

root tests

The most popular way to transform a nonstationary series to stationarity is by dif-

ferencing Differencing of the time series is specified in the VAR statement For

example to take a simple first difference of the series X use this statement

var x(1)

In this example the change in X from one period to the next is analyzed When the se-

ries has a seasonal pattern differencing at a period equal to the length of the seasonal

cycle may be desirable For example suppose the variable X is measured quarterly

and shows a seasonal cycle over the year You can use the following statement to

analyze the series of changes from the same quarter in the previous year

var x(4)

To difference twice add another differencing period to the list For ex-

ample the following statement analyzes the series of second differences

X

t

X

t 1

X

t 1

X

t 2

= X

t

2 X

t 1

+ X

t 2

var x(11)

The following statement analyzes the seasonal second difference series

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Part 2 General Information

var x(14)

The series modeled is the 1-period difference of the 4-period difference

X

t

X

t 4

X

t 1

X

t 5

= X

t

X

t 1

X

t 4

+ X

t 5

Another way to obtain stationary series is to use a regression on time to de-trend the

data If the time series has a deterministic linear trend regressing the series on time

produces residuals that should be stationary The following statements write residuals

of X and Y to the variable RX and RY in the output data set DETREND

data a

set a

t=_n_

run

proc reg data=a

model x y = t

output out=detrend r=rx ry

run

You then use PROC STATESPACE to forecast the de-trended series RX and RY A

disadvantage of this method is that you need to add the trend back to the forecast

series in an additional step A more serious disadvantage of the de-trending method

is that it assumes a deterministic trend In practice most time series appear to have a

stochastic rather than a deterministic trend Differencing is a more flexible and often

more appropriate method

There are several other methods to handle nonstationary time series For more infor-

mation and examples refer to Brockwell and Davis (1991)

Preliminary Autoregressive Models

After computing the sample autocovariance matrices PROC STATESPACE fits a

sequence of vector autoregressive models These preliminary autoregressive models

are used to estimate the autoregressive order of the process and limit the order of the

autocovariances considered in the state vector selection process

Yule-Walker Equations for Forward and Backward Models

Unlike a univariate autoregressive model a multivariate autoregressive model has

different forms depending on whether the present observation is being predicted from

the past observations or from the future observations

Letx

t

be the r -component stationary time series given by the VAR statement after

differencing and subtracting the vector of sample means (If the NOCENTER option

is specified the mean is not subtracted) Let n be the number of observations of x

t

from the input data set

Lete

t

be a vector white noise sequence with mean vector 0 and variance matrix

p

and letn

t

be a vector white noise sequence with mean vector 0 and variance matrix

p

Let p be the order of the vector autoregressive model forx

t

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Chapter 18 Details

The forward autoregressive form based on the past observations is written as follows

x

t

=

p

X

i = 1

p

i

x

t i

+ e

t

The backward autoregressive form based on the future observations is written as fol-

lows

x

t

=

p

X

i = 1

p

i

x

t + i

+ n

t

LettingE

denote the expected value operator the autocovariance sequence for thex

t

series

i

is

i

= E x

t

x

0

t i

The Yule-Walker equations for the autoregressive model that matches the first p ele-

ments of the autocovariance sequence are

2

6

6

4

0

1

p 1

0

1

0

p 2

0

p 1

0

p 2

0

3

7

7

5

2

6

6

4

p

1

p

2

p

p

3

7

7

5

=

2

6

6

4

1

2

p

3

7

7

5

and

2

6

6

6

4

0

0

1

0

p 1

1

0

0

p 2

p 1

p 2

0

3

7

7

7

5

2

6

6

4

p

1

p

2

p

p

3

7

7

5

=

2

6

6

4

0

1

0

2

0

p

3

7

7

5

Here

p

i

are the coefficient matrices for the past observation form of the vector au-toregressive model and

p

i

are the coefficient matrices for the future observation

form More information on the Yule-Walker equations in the multivariate setting can

be found in Whittle (1963) and Ansley and Newbold (1979)

The innovation variance matrices for the two forms can be written as follows

p

=

0

p

X

i = 1

p

i

0

i

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Part 2 General Information

p

=

0

p

X

i = 1

p

i

i

The autoregressive models are fit to the data using the preceding Yule-Walker equa-

tions with

i

replaced by the sample covariance sequenceC

i

The covariance matri-

ces are calculated as

C

i

=

1

N 1

N

X

t = i + 1

x

t

x

0

t i

Let b

p

b

p

b

p

and b

p

represent the Yule-Walker estimates of

p

p

p

and

p

respectively These matrices are written to an output data set when the OUTAR=

option is specified

When the PRINTOUT=LONG option is specified the sequence of matrices b

p

and

the corresponding correlation matrices are printed The sequence of matricesb

p isused to compute Akaike information criteria for selection of the autoregressive order

of the process

Akaike Information Criterion

The Akaike information criterion or AIC is defined as -2( maximum of log likeli-

hood )+2(number of parameters) Since the vector autoregressive models are esti-

mates from the Yule-Walker equations not by maximum likelihood the exact likeli-

hood values are not available for computing the AIC However for the vector autore-

gressive model the maximum of the log likelihood can be approximated as

l n L

n

2

l n j

b

p

j

Thus the AIC for the order p model is computed as

A I C

p

= n l n j

b

p

j + 2 p r

2

You can use the printed AIC array to compute a likelihood ratio test of the autoregres-

sive order The log-likelihood ratio test statistic for testing the order p model against

the order p 1

model is

n l n j

b

p

j + n l n j

b

p 1

j

This quantity is asymptotically distributed as a

2 withr

2 degrees of freedom if the

series is autoregressive of order p 1

It can be computed from the AIC array as

A I C

p 1

A I C

p

+ 2 r

2

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Chapter 18 Details

You can evaluate the significance of these test statistics with the PROBCHI function

in a SAS DATA step or with a

2 table

Determining the Autoregressive Order

Although the autoregressive models can be used for prediction their primary value is

to aid in the selection of a suitable portion of the sample covariance matrix for use in

computing canonical correlations If the multivariate time series x

t is of autoregres-sive order p then the vector of past values to lag p is considered to contain essentially

all the information relevant for prediction of future values of the time series

By default PROC STATESPACE selects the order p producing the autoregressive

model with the smallestA I C

p

If the value p for the minimumA I C

p

is less than the

value of the PASTMIN= option then p is set to the PASTMIN= value Alternatively

you can use the ARMAX= and PASTMIN= options to force PROC STATESPACE to

use an order you select

Significance Limits for Partial Autocorrelations

The STATESPACE procedure prints a schematic representation of the partial autocor-

relation matrices indicating which partial autocorrelations are significantly greater orsignificantly less than 0 Figure 1810 shows an example of this table

The STATESPACE Procedure

Schematic Representation of Partial Autocorrelations

NameLag 1 2 3 4 5 6 7 8 9 10

x ++ +

y ++

+ is gt 2std error - is lt -2std error is between

Figure 1810 Significant Partial Autocorrelations

The partial autocorrelations are from the sample partial autoregressive matrices b

p

p

The standard errors used for the significance limits of the partial autocorrelations are

computed from the sequence of matrices

p

and

p

Under the assumption that the observed series arises from an autoregressive process

of order p 1

the pth sample partial autoregressive matrix b

p

p

has an asymptotic

variance matrix 1

n

1

p

p

The significance limits for b

p

p

used in the schematic plot of the sample partial autore-

gressive sequence are derived by replacing

p

and

p

with their sample estimators

to produce the variance estimate as follows

d

V a r

b

p

p

=

1

n r p

b

1

p

b

p

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Part 2 General Information

Canonical Correlation Analysis

Given the order p letp

t

be the vector of current and past values relevant to prediction

of x

t + 1

p

t

= x

0

t

x

0

t 1

x

0

t p

0

Letf

t

be the vector of current and future values

f

t

= x

0

t

x

0

t + 1

x

0

t + p

0

In the canonical correlation analysis consider submatrices of the sample covariance

matrix of p

t

andf

t

This covariance matrixV

has a block Hankel form

V =

2

6

6

6

4

C

0

C

0

1

C

0

2

C

0

p

C

0

1

C

0

2

C

0

3

C

0

p + 1

C

0

p

C

0

p + 1

C

0

p + 2

C

0

2 p

3

7

7

7

5

State Vector Selection Process

The canonical correlation analysis forms a sequence of potential state vectorsz

j

t

Examine a sequencef

j

t

of subvectors of f

t

and form the submatrixV

j consisting

of the rows and columns of V

corresponding to the components of f

j

t

and compute

its canonical correlations

The smallest canonical correlation of V

j is then used in the selection of the com-

ponents of the state vector The selection process is described in the following For

more details about this process refer to Akaike (1976)

In the following discussion the notationx

t + k j t

denotes the wide sense conditional

expectation (best linear predictor) of x

t + k

given allx

s

with s less than or equal to t

In the notationx

i t + 1

the first subscript denotes the ith component of x

t + 1

The initial state vectorz

1

t

is set tox

t

The sequencef

j

t

is initialized by setting

f

1

t

= z

1

0

t

x

1 t + 1 j t

0

= x

0

t

x

1 t + 1 j t

0

That is start by considering whether to addx

1 t + 1 j t

to the initial state vectorz

1

t

The procedure forms the submatrixV

1 corresponding tof

1

t

and computes its canon-

ical correlations Denote the smallest canonical correlation of V

1 as

m i n

If

m i n

is

significantly greater than 0x

1 t + 1 j t

is added to the state vector

If the smallest canonical correlation of V

1 is not significantly greater than 0 then a

linear combination of f

1

t

is uncorrelated with the pastp

t

Assuming that the determi-

nant of C

0

is not 0 (that is no input series is a constant) you can take the coefficient

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Chapter 18 Details

of x

1 t + 1 j t

in this linear combination to be 1 Denote the coefficients of z

1

t

in this

linear combination as

This gives the relationship

x

1 t + 1 j t

=

0

x

t

Therefore the current state vector already contains all the past information useful forpredicting

x

1 t + 1

and any greater leads of x

1 t

The variablex

1 t + 1 j t

is not added to

the state vector nor are any termsx

1 t + k j t

considered as possible components of the

state vector The variablex

1

is no longer active for state vector selection

The process described forx

1 t + 1 j t

is repeated for the remaining elements of f

t

The

next candidate for inclusion in the state vector is the next component of f

t

corre-

sponding to an active variable Components of f

t

corresponding to inactive variables

that produced a zero

m i n

in a previous step are skipped

Denote the next candidate asx

l t + k j t

The vectorf

j

t

is formed from the current state

vector andx

l t + k j t

as follows

f

j

t

= z

j

0

t

x

l t + k j t

0

The matrixV

j is formed fromf

j

t

and its canonical correlations are computed The

smallest canonical correlation of V

j is judged to be either greater than or equal to 0

If it is judged to be greater than 0x

l t + k j t

is added to the state vector If it is judged

to be 0 then a linear combination of f

j

t

is uncorrelated with thep

t

and the variable

x

l

is now inactive

The state vector selection process continues until no active variables remain

Testing Significance of Canonical Correlations For each step in the canonical correlation sequence the significance of the small-

est canonical correlation

m i n

is judged by an information criterion from Akaike

(1976) This information criterion is

n l n 1

2

m i n

r p + 1 q + 1

where q is the dimension of f

j

t

at the current step r is the order of the state vector p

is the order of the vector autoregressive process and

is the value of the SIGCORR=

option The default is SIGCORR=2 If this information criterion is less than or equal

to 0

m i n

is taken to be 0 otherwise it is taken to be significantly greater than 0

(Do not confuse this information criterion with the AIC)

Variables inx

t + p j t

are not added in the model even with positive information crite-

rion because of the singularity of V

You can force the consideration of more candi-

date state variables by increasing the size of theV

matrix by specifying a PASTMIN=

option value larger than p

Printing the Canonical Correlations

To print the details of the canonical correlation analysis process specify the CAN-

CORR option in the PROC STATESPACE statement The CANCORR option prints

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Part 2 General Information

the candidate state vectors the canonical correlations and the information criteria for

testing the significance of the smallest canonical correlation

Bartlettrsquos

2 and its degrees of freedom are also printed when the CANCORR option

is specified The formula used for Bartlettrsquos

2 is

2

= n 5 r p + 1 q + 1 l n 1

2

m i n

withr p + 1 q + 1

degrees of freedom

Figure 1811 shows the output of the CANCORR option for the introductory example

shown in the Getting Started section of this chapter

The STATESPACE Procedure

Canonical Correlations Analysis

Information Chi

x(TT) y(TT) x(T+1T) Criterion Square DF

1 1 0237045 3566167 114505 4

Information Chi

x(TT) y(TT) x(T+1T) y(T+1T) Criterion Square DF

1 1 0238244 0056565 -535906 0636134 3

Information Chi

x(TT) y(TT) x(T+1T) x(T+2T) Criterion Square DF

1 1 0237602 0087493 -446312 1525353 3

Figure 1811 Canonical Correlations Analysis

New variables are added to the state vector if the information criteria are positive Inthis example

y

t + 1 j t

andx

t + 2 j t

are not added to the state space vector because the

information criteria for these models are negative

If the information criterion is nearly 0 then you may want to investigate models

that arise if the opposite decision is made regarding

m i n

This investigation can be

accomplished by using a FORM statement to specify part or all of the state vector

Preliminary Estimates of F

When a candidate variablex

l t + k j t

yields a zero

m i n

and is not added to the state

vector a linear combination of f

j

t

is uncorrelated with thep

t

Because of the method

used to construct thef

j

t

sequence the coefficient of x

l t + k j t

inl

can be taken as 1

Denote the coefficients of z

j

t

in this linear combination as l

This gives the relationship

x

l t + k j t

= l

0

z

j

t

The vectorl

is used as a preliminary estimate of the first r columns of the row of the

transition matrixF

corresponding tox

l t + k 1 j t

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Chapter 18 Details

Parameter Estimation

The model isz

t + 1

= F z

t

+ G e

t + 1

wheree

t

is a sequence of independent multivari-

ate normal innovations with mean vector 0 and variance

e e

The observed sequence

x

t

composes the first r components of z

t

and thusx

t

= H z

t

where H is ther s

matrix I

r

0

LetE

be ther n

matrix of innovations

E = e

1

e

n

If the number of observations n is reasonably large the log likelihood L can be

approximated up to an additive constant as follows

L =

n

2

l n j

e e

j

1

2

t r a c e

1

e e

E E

0

The elements of

e e

are taken as free parameters and are estimated as follows

S

0

=

1

n

E E

0

Replacing

e e

byS

0

in the likelihood equation the log likelihood up to an additive

constant is

L =

n

2

l n j S

0

j

Letting B be the backshift operator the formal relation betweenx

t

ande

t

is

x

t

= H I B F

1

G e

t

e

t

= H I B F

1

G

1

x

t

=

1

X

i = 0

i

x

t i

LettingC

i

be the ith lagged sample covariance of x

t

and neglecting end effects the

matrixS

0

is

S

0

=

1

X

i j = 0

i

C

i + j

0

j

For the computation of S

0

the infinite sum is truncated at the value of the KLAG=

option The value of the KLAG= option should be large enough that the sequence

i

is approximately 0 beyond that point

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Part 2 General Information

Let

be the vector of free parameters in theF

andG

matrices The derivative of the

log likelihood with respect to the parameter

is

L

=

n

2

t r a c e

S

1

0

S

0

The second derivative is

2

L

0

=

n

2

t r a c e

S

1

0

S

0

0

S

1

0

S

0

t r a c e

S

1

0

2

S

0

0

Near the maximum the first term is unimportant and the second term can be approx-

imated to give the following second derivative approximation

2

L

0

=

n t r a c e

S

1

0

E

E

0

0

The first derivative matrix and this second derivative matrix approximation are com-

puted from the sample covariance matrixC

0

and the truncated sequence

i

The

approximate likelihood function is maximized by a modified Newton-Raphson algo-

rithm employing these derivative matrices

The matrixS

0

is used as the estimate of the innovation covariance matrix

e e

The

negative of the inverse of the second derivative matrix at the maximum is used as an

approximate covariance matrix for the parameter estimates The standard errors of

the parameter estimates printed in the parameter estimates tables are taken from the

diagonal of this covariance matrix The parameter covariance matrix is printed when

the COVB option is specified

If the data are nearly nonstationary a better estimate of

e e

and the other parameters

can sometimes be obtained by specifying the RESIDEST option The RESIDEST

option estimates the parameters using conditional least squares instead of maximum

likelihood

The residuals are computed using the state space equation and the sample mean val-

ues of the variables in the model as start-up values The estimate of S

0

is then com-

puted using the residuals from the ith observation on where i is the maximum number

of times any variable occurs in the state vector A multivariate Gauss-Marquardt al-

gorithm is used to minimizej S

0

j

Refer to Harvey (1981a) for a further description

of this method

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Chapter 18 Details

Forecasting

Given estimates of F

G

and

e e

forecasts of x

t

are computed from the conditional

expectation of z

t

In forecasting the parameters F G and

e e

are replaced with the estimates or by val-

ues specified in the RESTRICT statement One-step-ahead forecasting is performedfor the observation

x

t

wheret n b

Heren

is the number of observations and

b is the value of the BACK= option For the observationx

t

wheret n b

m-

step-ahead forecasting is performed form = t n + b

The forecasts are generated

recursively with the initial conditionz

0

= 0

The m-step-ahead forecast of z

t + m

isz

t + m j t

wherez

t + m j t

denotes the conditional

expectation of z

t + m

given the information available at time t The m-step-ahead

forecast of x

t + m

isx

t + m j t

= H z

t + m j t

where the matrixH = I

r

0

Let

i

= F

i

G

Note that the lasts r

elements of z

t

consist of the elements of x

u j t

foru t

The state vector z

t + m

can be represented as

z

t + m

= F

m

z

t

+

m 1

X

i = 0

i

e

t + m i

Sincee

t + i j t

= 0

fori 0

the m-step-ahead forecastz

t + m j t

is

z

t + m j t

= F

m

z

t

= F z

t + m 1 j t

Therefore the m-step-ahead forecast of x

t + m

is

x

t + m j t

= H z

t + m j t

The m-step-ahead forecast error is

z

t + m

z

t + m j t

=

m 1

X

i = 0

i

e

t + m i

The variance of the m-step-ahead forecast error is

V

z m

=

m 1

X

i = 0

i

e e

0

i

LettingV

z 0

= 0

the variance of the m-step-ahead forecast error of z

t + m

V

z m

can

be computed recursively as follows

V

z m

= V

z m 1

+

m 1

e e

0

m 1

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Part 2 General Information

The variance of the m-step-ahead forecast error of x

t + m

is ther r

left upper sub-

matrix of V

z m

that is

V

x m

= H V

z m

H

0

Unless the NOCENTER option is specified the sample mean vector is added to the

forecast When differencing is specified the forecasts xt + m j t

plus the sample mean

vector are integrated back to produce forecasts for the original series

Lety

t

be the original series specified by the VAR statement with some 0 values

appended corresponding to the unobserved past observations Let B be the backshift

operator and let B

be thes s

matrix polynomial in the backshift operator

corresponding to the differencing specified by the VAR statement The off-diagonal

elements of

i

are 0 Note that

0

= I

s

whereI

s

is thes s

identity matrix Then

z

t

= B y

t

This gives the relationship

y

t

=

1

B z

t

=

1

X

i = 0

i

z

t i

where

1

B =

P

1

i = 0

i

B

i and

0

= I

s

The m-step-ahead forecast of y

t + m

is

y

t + m j t

=

m 1

X

i = 0

i

z

t + m i j t

+

1

X

i = m

i

z

t + m i

The m-step-ahead forecast error of y

t + m

is

m 1

X

i = 0

i

z

t + m i

z

t + m i j t

=

m 1

X

i = 0

i

X

u = 0

u

i u

e

t + m i

LettingV

y 0

= 0

the variance of the m-step-ahead forecast error of y

t + m

V

y m

is

V

y m

=

m 1

X

i = 0

i

X

u = 0

u

i u

e e

i

X

u = 0

u

i u

0

= V

y m 1

+

m 1

X

u = 0

u

m 1 u

e e

m 1

X

u = 0

u

m 1 u

0

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Chapter 18 Details

Relation of ARMA and State Space Forms

Every state space model has an ARMA representation and conversely every ARMA

model has a state space representation This section discusses this equivalence The

following material is adapted from Akaike (1974) where there is a more complete

discussion Pham-Dinh-Tuan (1978) also contains a discussion of this material

Suppose you are given the following ARMA model

B x

t

= B e

t

or in more detail

x

t

1

x

t 1

p

x

t p

= e

t

+

1

e

t 1

+ +

q

e

t q

(1)

wheree

t

is a sequence of independent multivariate normal random vectors with mean

0 and variance matrix

e e

B is the backshift operator (B x

t

= x

t 1

) B

and

B

are matrix polynomials in B andx

t

is the observed process

If the roots of the determinantial equationj B j = 0

are outside the unit circle in

the complex plane the model can also be written as

x

t

=

1

B B e

t

=

1

X

i = 0

i

e

t i

The

i

matrices are known as the impulse response matrices and can be computed

as

1

B B

You can assume p q

since if this is not initially true you can add more terms

i

that are identically 0 without changing the model

To write this set of equations in a state space form proceed as follows Letx

t + i j t

be

the conditional expectation of x

t + i

givenx

w

forw t

The following relations hold

x

t + i j t

=

1

X

j = i

j

e

t + i j

x

t + i j t + 1

= x

t + i j t

+

i 1

e

t + 1

However from equation (1) you can derive the following relationship

x

t + p j t

=

1

x

t + p 1 j t

+ +

p

x

t

(2)

Hence wheni = p

you can substitute forx

t + p j t

in the right-hand side of equation

(2) and close the system of equations

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Part 2 General Information

This substitution results in the following model in the state space form

z

t + 1

= F z

t

+ G e

t + 1

2

6

6

6

4

x

t + 1

x

t + 2 j t + 1

x

t + p j t + 1

3

7

7

7

5

=

2

6

6

4

0 I 0 0

0 0 I 0

p

p 1

1

3

7

7

5

2

6

6

6

4

x

t

x

t + 1 j t

x

t + p 1 j t

3

7

7

7

5

+

2

6

6

4

I

1

p 1

3

7

7

5

e

t + 1

Note that the state vectorz

t

is composed of conditional expectations of x

t

and the

first r components of z

t

are equal tox

t

The state space form can be cast into an ARMA form by solving the system of dif-

ference equations for the first r components

When converting from an ARMA form to a state space form you can generate a

state vector larger than needed that is the state space model may not be a minimal

representation When going from a state space form to an ARMA form you can have

nontrivial common factors in the autoregressive and moving average operators that

yield an ARMA model larger than necessary

If the state space form used is not a minimal representation some but not all compo-

nents of x

t + i j t

may be linearly dependent This situation corresponds to

p

p 1

being of less than full rank when B

and B

have no common nontrivial

left factors In this casez

t

consists of a subset of the possible components of

x

t + i j t

i = 1 2 p 1

However once a component of x

t + i j t

(for example the

jth one) is linearly dependent on the previous conditional expectations then all sub-

sequent jth components of x

t + k j t

fork i

must also be linearly dependent Note

that in this case equivalent but seemingly different structures can arise if the order of

the components withinx

t

is changed

OUT= Data Set

The forecasts are contained in the output data set specified by the OUT= option on the

PROC STATESPACE statement The OUT= data set contains the following variables

the BY variables

the ID variable

the VAR statement variables These variables contain the actual values from

the input data set

FORi numeric variables containing the forecasts The variable FORi contains

the forecasts for the ith variable in the VAR statement list Forecasts are one-

step-ahead predictions until the end of the data or until the observation specified

by the BACK= option

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Chapter 18 Details

RESi numeric variables containing the residual for the forecast of the ith vari-

able in the VAR statement list For forecast observations the actual values are

missing and the RESi variables contain missing values

STDi numeric variables containing the standard deviation for the forecast of

the ith variable in the VAR statement list The values of the STDi variables can

be used to construct univariate confidence limits for the corresponding fore-casts However such confidence limits do not take into account the covariance

of the forecasts

OUTAR= Data Set

The OUTAR= data set contains the estimates of the preliminary autoregressive mod-

els The OUTAR= data set contains the following variables

ORDER a numeric variable containing the order p of the autoregressive model

that the observation represents

AIC a numeric variable containing the value of the information criterionA I C

p

SIGFl numeric variables containing the estimate of the innovation covariance

matrices for the forward autoregressive models The variable SIGFl contains

the lth column of b

p

in the observations with ORDER= p

SIGBl numeric variables containing the estimate of the innovation covariance

matrices for the backward autoregressive models The variable SIGBl contains

the lth column of b

p

in the observations with ORDER= p

FORk ndashl numeric variables containing the estimates of the autoregressive pa-

rameter matrices for the forward models The variable FORk ndashl contains the

lth column of the lag k autoregressive parameter matrix b

p

k

in the observations

with ORDER= p

BACk ndashl numeric variables containing the estimates of the autoregressive pa-

rameter matrices for the backward models The variable BACk ndashl contains the

lth column of the lag k autoregressive parameter matrix b

p

k

in the observations

with ORDER= p

The estimates for the order p autoregressive model can be selected as those observa-

tions with ORDER= p Within these observations the k lth element of

p

i

is given by

the value of the FORindashl variable in the k th observation The k lth element of

p

i

is

given by the value of BACindashl variable in the k th observation The k lth element of

p

is given by SIGFl in the k th observation The k lth element of

p

is given by SIGBlin the k th observation

Table 181 shows an example of the OUTAR= data set with ARMAX=3 andx

t

of

dimension 2 In Table 181 i j

indicate the ijth element of the matrix

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Part 2 General Information

Table 181 Values in the OUTAR= Data Set

Obs ORDER AIC SIGF1 SIGF2 SIGB1 SIGB2 FOR1ndash 1 FOR1ndash 2 FOR2ndash 1 FOR2ndash 2 FOR3ndash 1

1 0 AIC0

0 1 1

0 1 2

0 1 1

0 1 2

2 0 AIC0

0 2 1

0 2 2

0 2 1

0 2 2

3 1 AIC1

1 1 1

1 1 2

1 1 1

1 1 2

1

1

1 1

1

1

1 2

4 1 AIC1

1 2 1

1 1 2

1 2 1

1 1 2

1

1

2 1

1

1

2 2

5 2 AIC2

2 1 1

2 1 2

2 1 1

2 1 2

2

1

1 1

2

1

1 2

2

2

1 1

2

2

1 2

6 2 AIC2

2 2 1

2 1 2

2 2 1

2 1 2

2

1

2 1

2

1

2 2

2

2

2 1

2

2

2 2

7 3 AIC3

3 1 1

3 1 2

3 1 1

3 1 2

3

1

1 1

3

1

1 2

3

2

1 1

3

2

1 2

3

3

1 1

8 3 AIC3

3 2 1

3 1 2

3 2 1

3 1 2

3

1

2 1

3

1

2 2

3

2

2 1

3

2

2 2

3

3

2 1

Obs FOR3ndash 2 BACK1ndash 1 BACK1ndash 2 BACK2ndash 1 BACK2ndash 2 BACK3ndash 1 BACK3ndash 2

1

2

3

1

1

1 1

1

1

1 2

4

1

1

2 1

1

1

2 2

5

2

1

1 1

2

1

1 2

2

2

1 1

2

2

1 2

6

2

1

2 1

2

1

2 2

2

2

2 1

2

2

2 2

7

3

3

1 2

3

1

1 1

3

1

1 2

3

2

1 1

3

2

1 2

3

3

1 1

3

3

1 2

8

3

3

2 2

3

1

2 1

3

1

2 2

3

2

2 1

3

2

2 2

3

3

2 1

3

3

2 2

The estimated autoregressive parameters can be used in the IML procedure to obtain

autoregressive estimates of the spectral density function or forecasts based on the

autoregressive models

OUTMODEL= Data Set

The OUTMODEL= data set contains the estimates of theF

andG

matrices and their

standard errors the names of the components of the state vector and the estimates of

the innovation covariance matrix The variables contained in the OUTMODEL= data

set are as follows

the BY variables

STATEVEC a character variable containing the name of the component of the

state vector corresponding to the observation The STATEVEC variable has

the value STD for standard deviations observations which contain the standarderrors for the estimates given in the preceding observation

Fndash j numeric variables containing the columns of theF

matrix The variable

Fndash j contains the jth column of F

The number of Fndash j variables is equal to

the value of the DIMMAX= option If the model is of smaller dimension the

extraneous variables are set to missing

Gndash j numeric variables containing the columns of theG

matrix The variable

Gndash j contains the jth column of G

The number of Gndash j variables is equal to r

the dimension of x

t

given by the number of variables in the VAR statement

SIGndash j numeric variables containing the columns of the innovation covariance

matrix The variable SIGndash j contains the jth column of

e e

There are r vari-ables SIGndash j

Table 182 shows an example of the OUTMODEL= data set withx

t

= x

t

y

t

0

z

t

= x

t

y

t

x

t + 1 j t

0 and DIMMAX=4 In Table 182F

i j

andG

i j

are the ijth

elements of F

andG

respectively Note that all elements for Fndash4 are missing because

F

is a3 3

matrix

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Chapter 18 Details

Table 182 Value in the OUTMODEL= Data Set

Obs STATEVEC Fndash 1 Fndash 2 Fndash 3 Fndash 4 Gndash 1 Gndash 2 SIGndash 1 SIGndash 2

1 X(TT) 0 0 1 1 0

1 1

1 2

2 STD

3 Y(TT) F2 1

F2 2

F2 3

0 1

2 1

2 2

4 STD std F2 1

std F2 2

std F2 3

5 X(T+1T) F3 1

F3 2

F3 3

G3 1

G3 2

6 STD std F3 1

std F3 2

std F3 3

std G3 1

std G3 2

Printed Output

The printed output produced by the STATESPACE procedure is described in the fol-

lowing

1 descriptive statistics which include the number of observations used the

names of the variables their means and standard deviations (Std) and the dif-

ferencing operations used

2 the Akaike information criteria for the sequence of preliminary autoregressive

models

3 if the PRINTOUT=LONG option is specified the sample autocovariance ma-

trices of the input series at various lags

4 if the PRINTOUT=LONG option is specified the sample autocorrelation ma-

trices of the input series

5 a schematic representation of the autocorrelation matrices showing the signif-

icant autocorrelations

6 if the PRINTOUT=LONG option is specified the partial autoregressive ma-

trices (These are

p

p

as described in Preliminary Autoregressive Modelsearlier in this chapter)

7 a schematic representation of the partial autocorrelation matrices showing the

significant partial autocorrelations

8 the Yule-Walker estimates of the autoregressive parameters for the autoregres-

sive model with the minimum AIC

9 if the PRINTOUT=LONG option is specified the autocovariance matrices of

the residuals of the minimum AIC model This is the sequence of estimated

innovation variance matrices for the solutions of the Yule-Walker equations

10 if the PRINTOUT=LONG option is specified the autocorrelation matrices of the residuals of the minimum AIC model

11 If the CANCORR option is specified the canonical correlations analysis for

each potential state vector considered in the state vector selection process

This includes the potential state vector the canonical correlations the informa-

tion criterion for the smallest canonical correlation Bartlettrsquos

2 statistic (ldquoChi

Squarerdquo) for the smallest canonical correlation and the degrees of freedom of

Bartlettrsquos

2

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Part 2 General Information

12 the components of the chosen state vector

13 the preliminary estimate of the transition matrixF

the input matrixG

and

the variance matrix for the innovations

e e

14 if the ITPRINT option is specified the iteration history of the likelihood max-

imization For each iteration this shows the iteration number the number of step halvings the determinant of the innovation variance matrix the damping

factor Lambda and the values of the parameters

15 the state vector printed again to aid interpretation of the following listing of F

andG

16 the final estimate of the transition matrixF

17 the final estimate of the input matrixG

18 the final estimate of the variance matrix for the innovations

e e

19 a table listing the estimates of the free parameters inF

andG

and their standarderrors and t statistics

20 if the COVB option is specified the covariance matrix of the parameter esti-

mates

21 if the COVB option is specified the correlation matrix of the parameter esti-

mates

22 if the PRINT option is specified the forecasts and their standard errors

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Chapter 18 Details

ODS Table Names

PROC STATESPACE assigns a name to each table it creates You can use these

names to reference the table when using the Output Delivery System (ODS) to select

tables and create output data sets These names are listed in the following table For

more information on ODS see Chapter 6 ldquoUsing the Output Delivery Systemrdquo

Table 183 ODS Tables Produced in PROC STATESPACE

ODS Table Name Description Option

NObs Number of observations default

Summary Simple summary statistics table default

InfoCriterion Information criterion table default

CovLags Covariance Matrices of Input Series PRINTOUT=LONG

CorrLags Correlation Matrices of Input Series PRINTOUT=LONG

PartialAR Partial Autoregressive Matrices PRINTOUT=LONG

YWEstimates Yule-Walker Estimates for Minimum AIC default

CovResiduals Covariance of Residuals PRINTOUT=LONGCorrResiduals Residual Correlations from AR Models PRINTOUT=LONG

StateVector State vector table default

CorrGraph Schematic Representation of Correlations default

TransitionMatrix Transition Matrix default

InputMatrix Input Matrix default

VarInnov Variance Matrix for the Innovation default

CovB Covariance of Parameter Estimates COVB

CorrB Correlation of Parameter Estimates COVB

CanCorr Canonical Correlation Analysis CANCORR

IterHistory Iterative Fitting table ITPRINT

ParameterEstimates Parameter Estimates Table default

Forecasts Forecasts Table PRINT

ConvergenceStatus Convergence Status Table default

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Part 2 General Information

Example

Example 181 Series J from Box and Jenkins

This example analyzes the gas furnace data (series J) from Box and Jenkins (The

data are not shown Refer to Box and Jenkins (1976) for the data)

First a model is selected and fit automatically using the following statements

title1 rsquoGas Furnace Datarsquo

title2 rsquoBox amp Jenkins Series Jrsquo

title3 rsquoAutomatically Selected Modelrsquo

proc statespace data=seriesj cancorr

var x y

run

The results for the automatically selected model are shown in Output 1811

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Chapter 18 Example

Output 1811 Results for Automatically Selected Model

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Number of Observations 296

Standard

Variable Mean Error

x -005683 1072766

y 5350912 3202121

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Information Criterion for Autoregressive Models

Lag=0 Lag=1 Lag=2 Lag=3 Lag=4 Lag=5 Lag=6 Lag=7 Lag=8

6513862 -103357 -163296 -164512 -165152 -164891 -164934 -164315 -163856

Information

Criterion for

Autoregressive

Models

Lag=9 Lag=10

-16348 -163359

Schematic Representation of Correlations

NameLag 0 1 2 3 4 5 6 7 8 9 10

x +- +- +- +- +- +- +- +- +- +- +-

y -+ -+ -+ -+ -+ -+ -+ -+ -+ -+ -+

+ is gt 2std error - is lt -2std error is between

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Part 2 General Information

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Schematic Representation of Partial Autocorrelations

NameLag 1 2 3 4 5 6 7 8 9 10

x + - + -

y -+ -- - + +

+ is gt 2std error - is lt -2std error is between

Yule-Walker Estimates for Minimum AIC

------Lag=1------ ------Lag=2------ ------Lag=3------ ------Lag=4------

x y x y x y x y

x 1925887 -000124 -120166 0004224 0116918 -000867 0104236 0003268

y 0050496 1299793 -002046 -03277 -071182 -025701 0195411 0133417

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Canonical Correlations Analysis

Information Chi

x(TT) y(TT) x(T+1T) Criterion Square DF

1 1 0804883 2929228 3047481 8

Information Chix(TT) y(TT) x(T+1T) y(T+1T) Criterion Square DF

1 1 0906681 0607529 1223358 1347237 7

Information Chi

x(TT) y(TT) x (T+1T) y (T+1T) x (T+2T) Criterion Square DF

1 1 0909434 0610278 0186274 -154701 1034705 6

Information Chi

x(TT) y(TT) x (T+1T) y (T+1T) y (T+2T) Criterion Square DF

1 1 091014 0618937 0206823 0940392 1280924 6

Information Chi

x(TT) y(TT) x(T+1T) y(T+1T) y(T+2T) y(T+3T) Criterion Square DF

1 1 0912963 0628785 0226598 0083258 -794103 2041584 5

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Chapter 18 Example

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Selected Statespace Form and Preliminary Estimates

State Vector

x(TT) y(TT) x(T+1T) y(T+1T) y(T+2T)

Estimate of Transition Matrix

0 0 1 0 0

0 0 0 1 0

-084718 0026794 1711715 -005019 0

0 0 0 0 1

-019785 0334274 -018174 -123557 1787475

Input Matrix for Innovation

1 0

0 11925887 -000124

0050496 1299793

0142421 1361696

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Selected Statespace Form and Preliminary Estimates

Variance Matrix for Innovation

0035274 -000734

-000734 0097569

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Part 2 General Information

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Selected Statespace Form and Fitted Model

State Vector

x(TT) y(TT) x(T+1T) y(T+1T) y(T+2T)

Estimate of Transition Matrix

0 0 1 0 0

0 0 0 1 0

-086192 0030609 1724235 -005483 0

0 0 0 0 1

-034839 0292124 -009435 -109823 1671418

Input Matrix for Innovation

1 0

0 1192442 -000416

0015621 1258495

008058 1353204

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Selected Statespace Form and Fitted Model

Variance Matrix for Innovation

0035579 -000728

-000728 0095577

Parameter Estimates

Standard

Parameter Estimate Error t Value

F(31) -086192 0072961 -1181

F(32) 0030609 0026167 117

F(33) 1724235 0061599 2799

F(34) -005483 0030169 -182

F(51) -034839 0135253 -258

F(52) 0292124 0046299 631

F(53) -009435 0096527 -098F(54) -109823 0109525 -1003

F(55) 1671418 0083737 1996

G(31) 1924420 0058162 3309

G(32) -000416 0035255 -012

G(41) 0015621 0095771 016

G(42) 1258495 0055742 2258

G(51) 0080580 0151622 053

G(52) 1353204 0091388 1481

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Chapter 18 Example

The two series are believed to have a transfer function relation with the gas rate

(variable X) as the input and the CO2

concentration (variable Y) as the output Since

the parameter estimates shown in Output 1811 support this kind of model the model

is reestimated with the feedback parameters restricted to 0 The following statements

fit the transfer function (no feedback) model

title3 rsquoTransfer Function Modelrsquo

proc statespace data=seriesj printout=none

var x y

restrict f(32)=0 f(34)=0

g(32)=0 g(41)=0 g(51)=0

run

The last 2 pages of the output are shown in Output 1812

Output 1812 STATESPACE Output for Transfer Function Model

Gas Furnace Data

Box amp Jenkins Series J

Transfer Function Model

The STATESPACE Procedure

Selected Statespace Form and Fitted Model

State Vector

x(TT) y(TT) x(T+1T) y(T+1T) y(T+2T)

Estimate of Transition Matrix

0 0 1 0 0

0 0 0 1 0

-068882 0 1598717 0 0

0 0 0 0 1

-035944 0284179 -00963 -107313 1650047

Input Matrix for Innovation

1 0

0 1

1923446 0

0 1260856

0 1346332

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Part 2 General Information

Gas Furnace Data

Box amp Jenkins Series J

Transfer Function Model

The STATESPACE Procedure

Selected Statespace Form and Fitted Model

Variance Matrix for Innovation

0036995 -00072

-00072 0095712

Parameter Estimates

Standard

Parameter Estimate Error t Value

F(31) -068882 0050549 -1363

F(33) 1598717 0050924 3139

F(51) -035944 0229044 -157

F(52) 0284179 0096944 293

F(53) -009630 0140876 -068

F(54) -107313 0250385 -429

F(55) 1650047 0188533 875G(31) 1923446 0056328 3415

G(42) 1260856 0056464 2233

G(52) 1346332 0091086 1478

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Chapter 18 References

References

Akaike H (1974) Markovian Representation of Stochastic Processes and Its Ap-

plication to the Analysis of Autoregressive Moving Average Processes Annals

of the Institute of Statistical Mathematics 26 363-387

Akaike H (1976) Canonical Correlations Analysis of Time Series and the Use of

an Information Criterion in Advances and Case Studies in System Identification

eds R Mehra and DG Lainiotis New York Academic Press

Anderson TW (1971) The Statistical Analysis of Time Series New York John

Wiley amp Sons

Ansley CF and Newbold P (1979) Multivariate Partial Autocorrelations Pro-

ceedings of the Business and Economic Statistics Section American Statistical

Association 349-353

Box GEP and Jenkins G (1976) Time Series Analysis Forecasting and Control

San Francisco Holden-Day

Brockwell PJ and Davis RA (1991) Time Series Theory and Methods 2nd Edi-

tion Springer-Verlag

Hannan EJ (1970) Multiple Time Series New York John Wiley amp Sons

Hannan EJ (1976) The Identification and Parameterization of ARMAX and State

Space Forms Econometrica 44 713-722

Harvey AC (1981a) The Econometric Analysis of Time Series New York John

Wiley amp Sons

Harvey AC (1981b) Time Series Models New York John Wiley amp Sons

Jones RH (1974) Identification and Autoregressive Spectrum Estimation IEEE Transactions on Automatic Control AC-19 894-897

Pham-Dinh-Tuan (1978) On the Fitting of Multivariate Processes of the

Autoregressive-Moving Average Type Biometrika 65 99-107

Priestley MB (1980) System Identification Kalman Filtering and Stochastic Con-

trol in Directions in Time Series eds DR Brillinger and GC Tiao Institute

of Mathematical Statistics

Whittle P (1963) On the Fitting of Multivariate Autoregressions and the Approxi-

mate Canonical Factorization of a Spectral Density Matrix Biometrika 50 129-

134

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The correct bibliographic citation for this manual is as follows SAS Institute Inc SAS

ET S U serrsquos Gui de Version 8 Cary NC SAS Institute Inc 1999 1546 pp

SASET S Userrsquos Guide Version 8

Copyright copy 1999 by S AS I ns t it u t e I nc Ca ry N C U S A

ISBN 1ndash58025ndash489ndash6

All r ight s res erved P r int ed in t he U n it ed St a t es of A merica N o pa rt of t his p u blica t ionma y b e reprodu ced s t ored in a ret r ieva l s ys t em or t ra ns mit t ed in a ny form or b y a nymeans electronic mechanical photocopying or otherwise without the prior writtenpermission of the publisher SAS Institute Inc

US Government Restricted Rights Notice Use du plicat ion or disclosure of the

software by the government is subject to restrictions as set forth in FAR 52227ndash19Commercial Computer Softw ar e-Restricted Rights (J une 1987)

SAS Institute Inc SAS Campus Drive Cary North Carolina 27513

1st print ing October 1999

S AS reg a nd a ll ot her SA S I ns t it u t e I nc p rodu ct or s ervice na m es a re regis t ered t ra dema rkst d k f SA S I t it t I i t h U S A d t h t i reg i di t U SA

Page 2: Statespace Model

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Chapter 18

The STATESPACE Procedure

Overview

The STATESPACE procedure analyzes and forecasts multivariate time series using

the state space model The STATESPACE procedure is appropriate for jointly fore-

casting several related time series that have dynamic interactions By taking into

account the autocorrelations among the whole set of variables the STATESPACE

procedure may give better forecasts than methods that model each series separately

By default the STATESPACE procedure automatically selects a state space model

appropriate for the time series making the procedure a good tool for automatic fore-

casting of multivariate time series Alternatively you can specify the state spacemodel by giving the form of the state vector and the state transition and innovation

matrices

The methods used by the STATESPACE procedure assume that the time series are

jointly stationary Nonstationary series must be made stationary by some prelimi-

nary transformation usually by differencing The STATESPACE procedure allows

you to specify differencing of the input data When differencing is specified the

STATESPACE procedure automatically integrates forecasts of the differenced series

to produce forecasts of the original series

The State Space Model

The state space model represents a multivariate time series through auxiliary vari-ables some of which may not be directly observable These auxiliary variables are

called the state vector The state vector summarizes all the information from the

present and past values of the time series relevant to the prediction of future values

of the series The observed time series are expressed as linear combinations of the

state variables The state space model is also called a Markovian representation or

a canonical representation of a multivariate time series process The state space ap-

proach to modeling a multivariate stationary time series is summarized in Akaike

(1976)

The state space form encompasses a very rich class of models Any Gaussian multi-

variate stationary time series can be written in a state space form provided that the

dimension of the predictor space is finite In particular any autoregressive movingaverage (ARMA) process has a state space representation and conversely any state

space process can be expressed in an ARMA form (Akaike 1974) More details on

the relation of the state space and ARMA forms are given in Relation of ARMA and

State Space Forms later in this chapter

Letx

t

be ther 1

vector of observed variables after differencing (if differencing is

specified) and subtracting the sample mean Letz

t

be the state vector of dimension

s s

r where the first r components of z

t

consist of x

t

Let the notationx

t + k j t

999

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Part 2 General Information

represent the conditional expectation (or prediction) of x

t + k

based on the information

available at time t Then the lasts r

elements of z

t

consist of elements of xt + k j t

where k gt0 is specified or determined automatically by the procedure

There are various forms of the state space model in use The form of the state space

model used by the STATESPACE procedure is based on Akaike (1976) The model

is defined by the following state transition equation

z

t + 1

= F z

t

+ G e

t + 1

In the state transition equation thes s

coefficient matrix F is called the transition

matrix it determines the dynamic properties of the model

Thes r

coefficient matrix G is called the input matrix it determines the variance

structure of the transition equation For model identification the first r rows and

columns of G are set to anr r

identity matrix

The input vector et

is a sequence of independent normally distributed random vectors

of dimension r with mean 0 and covariance matrix

e e

The random error et

is

sometimes called the innovation vector or shock vector

In addition to the state transition equation state space models usually include a mea-

surement equation or observation equation that gives the observed valuesx

t

as a

function of the state vectorz

t

However since PROC STATESPACE always includes

the observed valuesx

t

in the state vectorz

t

the measurement equation in this case

merely represents the extraction of the first r components of the state vector

The measurement equation used by the STATESPACE procedure is

x

t

= I

r

0 z

t

whereI

r

is anr r

identity matrix In practice PROC STATESPACE performs the

extraction of x

t

fromz

t

without reference to an explicit measurement equation

In summary

xt

is an observation vector of dimension r

zt

is a state vector of dimension s whose first r elements are xt

and

whose lasts r

elements are conditional prediction of future xt

F is ans s

transition matrix

G is ans r

input matrix with the identity matrix Ir

forming the first

r rows and columns

et

is a sequence of independent normally distributed random vectors

of dimension r with mean 0 and covariance matrix

e e

How PROC STATESPACE Works

The design of the STATESPACE procedure closely follows the modeling strategy

proposed by Akaike (1976) This strategy employs canonical correlation analysis for

the automatic identification of the state space model

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Chapter 18 Overview

Following Akaike (1976) the procedure first fits a sequence of unrestricted vector

autoregressive (VAR) models and computes Akaikersquos information criterion (AIC) for

each model The vector autoregressive models are estimated using the sample au-

tocovariance matrices and the Yule-Walker equations The order of the VAR model

producing the smallest Akaike information criterion is chosen as the order (number

of lags into the past) to use in the canonical correlation analysis

The elements of the state vector are then determined via a sequence of canonical cor-

relation analyses of the sample autocovariance matrices through the selected order

This analysis computes the sample canonical correlations of the past with an increas-

ing number of steps into the future Variables that yield significant correlations are

added to the state vector those that yield insignificant correlations are excluded from

further consideration The importance of the correlation is judged on the basis of

another information criterion proposed by Akaike See the section Canonical Cor-

relation Analysis for details If you specify the state vector explicitly these model

identification steps are omitted

Once the state vector is determined the state space model is fit to the data The free

parameters in the F G and

e e

matrices are estimated by approximate maximumlikelihood By default the

F

andG

matrices are unrestricted except for identifia-

bility requirements Optionally conditional least-squares estimates can be computed

You can impose restrictions on elements of theF

andG

matrices

After the parameters are estimated forecasts are produced from the fitted state space

model using the Kalman filtering technique If differencing was specified the fore-

casts are integrated to produce forecasts of the original input variables

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Part 2 General Information

Getting Started

The following introductory example uses simulated data for two variables X and Y

The following statements generate the X and Y series

data inx=10 y=40

x1=0 y1=0

a1=0 b1=0

iseed=123

do t=-100 to 200

a=rannor(iseed)

b=rannor(iseed)

dx = 05x1 + 03y1 + a - 02a1 - 01b1

dy = 03x1 + 05y1 + b

x = x + d x + 2 5

y = y + d y + 2 5

if t gt= 0 then output

x1 = dx y1 = dy

a 1 = a b 1 = b

end

keep t x y

run

The simulated series X and Y are shown in Figure 181

Figure 181 Example Series

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Chapter 18 Getting Started

Automatic State Space Model Selection

The STATESPACE procedure is designed to automatically select the best state space

model for forecasting the series You can specify your own model if you wish and

you can use the output from PROC STATESPACE to help you identify a state space

model However the easiest way to use PROC STATESPACE is to let it choose the

model

Stationarity and Differencing

Although PROC STATESPACE selects the state space model automatically it does

assume that the input series are stationary If the series are nonstationary then the

process may fail Therefore the first step is to examine your data and test to see if

differencing is required (See the section Stationarity and Differencing later in this

chapter for further discussion of this issue)

The series shown in Figure 181 are nonstationary In order to forecast X and Y

with a state space model you must difference them (or use some other de-trending

method) If you fail to difference when needed and try to use PROC STATESPACE

with nonstationary data an inappropriate state space model may be selected and themodel estimation may fail to converge

The following statements identify and fit a state space model for the first differences

of X and Y and forecast X and Y 10 periods ahead

proc statespace data=in out=out lead=10

var x(1) y(1)

id t

run

The DATA= option specifies the input data set and the OUT= option specifies theoutput data set for the forecasts The LEAD= option specifies forecasting 10 obser-

vations past the end of the input data The VAR statement specifies the variables to

forecast and specifies differencing The notation X(1) Y(1) specifies that the state

space model analyzes the first differences of X and Y

Descriptive Statistics and Preliminary Autoregressions

The first page of the printed output produced by the preceding statements is shown in

Figure 182

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Part 2 General Information

The STATESPACE Procedure

Number of Observations 200

Standard

Variable Mean Error

x 0144316 1233457 Has been differenced With period(s) = 1

y 0164871 1304358 Has been differenced

With period(s) = 1

The STATESPACE Procedure

Information Criterion for Autoregressive Models

Lag=0 Lag=1 Lag=2 Lag=3 Lag=4 Lag=5 Lag=6 Lag=7 Lag=8

149697 8387786 5517099 1205986 1536952 2179538 2400638 2988874 3355708

Information

Criterion for Autoregressive

Models

Lag=9 Lag=10

4117606 4770222

Schematic Representation of Correlations

NameLag 0 1 2 3 4 5 6 7 8 9 10

x ++ ++ ++ ++ ++ ++ + + +

y ++ ++ ++ ++ ++ + + + +

+ is gt 2std error - is lt -2std error is between

Figure 182 Descriptive Statistics and VAR Order Selection

Descriptive statistics are printed first giving the number of nonmissing observations

after differencing and the sample means and standard deviations of the differenced

series The sample means are subtracted before the series are modeled (unless the

NOCENTER option is specified) and the sample means are added back when the

forecasts are produced

LetX

t

andY

t

be the observed values of X and Y and letx

t

andy

t

be the values of X

and Y after differencing and subtracting the mean difference The seriesx

t

modeled

by the STATEPSPACE procedure is

x

t

=

x

t

y

t

=

1 B X

t

0 1 4 4 3 1 6

1 B Y

t

0 1 6 4 8 7 1

where B represents the backshift operator

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Chapter 18 Getting Started

After the descriptive statistics PROC STATESPACE prints the Akaike information

criterion (AIC) values for the autoregressive models fit to the series The smallest AIC

value in this case 5517 at lag 2 determines the number of autocovariance matrices

analyzed in the canonical correlation phase

A schematic representation of the autocorrelations is printed next This indicates

which elements of the autocorrelation matrices at different lags are significantlygreater or less than 0

The second page of the STATESPACE printed output is shown in Figure 183

The STATESPACE Procedure

Schematic Representation of Partial Autocorrelations

NameLag 1 2 3 4 5 6 7 8 9 10

x ++ +

y ++

+ is gt 2std error - is lt -2std error is between

Yule-Walker Estimates for Minimum AIC

--------Lag=1------- --------Lag=2-------

x y x y

x 0257438 0202237 0170812 0133554

y 0292177 0469297 -000537 -000048

Figure 183 Partial Autocorrelations and VAR Model

Figure 183 shows a schematic representation of the partial autocorrelations similar

to the autocorrelations shown in Figure 182 The selection of a second order autore-

gressive model by the AIC statistic looks reasonable in this case because the partialautocorrelations for lags greater than 2 are not significant

Next the Yule-Walker estimates for the selected autoregressive model are printed

This output shows the coefficient matrices of the vector autoregressive model at each

lag

Selected State Space Model Form and Preliminary Estimates

After the autoregressive order selection process has determined the number of lags to

consider the canonical correlation analysis phase selects the state vector By default

output for this process is not printed You can use the CANCORR option to print

details of the canonical correlation analysis See the section Canonical Correlation

Analysis later in this chapter for an explanation of this process

Once the state vector is selected the state space model is estimated by approximate

maximum likelihood Information from the canonical correlation analysis and from

the preliminary autoregression is used to form preliminary estimates of the state space

model parameters These preliminary estimates are used as starting values for the

iterative estimation process

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Part 2 General Information

The form of the state vector and the preliminary estimates are printed next as shown

in Figure 184

The STATESPACE Procedure

Selected Statespace Form and Preliminary Estimates

State Vector

x(TT) y(TT) x(T+1T)

Estimate of Transition Matrix

0 0 1

0291536 0468762 -000411

024869 024484 0204257

Input Matrix for Innovation

1 0

0 1

0257438 0202237

Variance Matrix for Innovation

0945196 0100786

0100786 1014703

Figure 184 Preliminary Estimates of State Space Model

Figure 184 first prints the state vector as X[TT] Y[TT] X[T+1T] This notation

indicates that the state vector is

z

t

=

2

4

x

t j t

y

t j t

x

t + 1 j t

3

5

The notationx

t + 1 j t

indicates the conditional expectation or prediction of x

t + 1

based

on the information available at time t andx

t j t

andy

t j t

arex

t

andy

t

respectively

The remainder of Figure 184 shows the preliminary estimates of the transition matrix

F

the input matrixG

and the covariance matrix

e e

Estimated State Space Model

The next page of the STATESPACE output prints the final estimates of the fittedmodel as shown in Figure 185 This output has the same form as in Figure 184 but

shows the maximum likelihood estimates instead of the preliminary estimates

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Chapter 18 Getting Started

The STATESPACE Procedure

Selected Statespace Form and Fitted Model

State Vector

x(TT) y(TT) x(T+1T)

Estimate of Transition Matrix

0 0 1

0297273 047376 -001998

02301 0228425 0256031

Input Matrix for Innovation

1 0

0 1

0257284 0202273

Variance Matrix for Innovation

0945188 01007520100752 1014712

Figure 185 Fitted State Space Model

The estimated state space model shown in Figure 185 is

2

4

x

t + 1 j t + 1

y

t + 1 j t + 1

x

t + 2 j t + 1

3

5

=

2

4

0 0 1

0 2 9 7 0 4 7 4 0 0 2 0

0 2 3 0 0 2 2 8 0 2 5 6

3

5

2

4

x

t

y

t

x

t + 1 j t

3

5

+

2

4

1 0

0 1

0 2 5 7 0 2 0 2

3

5

e

t + 1

n

t + 1

v a r

e

t + 1

n

t + 1

=

0 9 4 5 0 1 0 1

0 1 0 1 1 0 1 5

The next page of the STATESPACE output lists the estimates of the free parameters

in theF

andG

matrices with standard errors and t statistics as shown in Figure 186

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Part 2 General Information

The STATESPACE Procedure

Parameter Estimates

Standard

Parameter Estimate Error t Value

F(21) 0297273 0129995 229

F(22) 0473760 0115688 410F(23) -001998 0313025 -006

F(31) 0230100 0126226 182

F(32) 0228425 0112978 202

F(33) 0256031 0305256 084

G(31) 0257284 0071060 362

G(32) 0202273 0068593 295

Figure 186 Final Parameter Estimates

Convergence Failures

The maximum likelihood estimates are computed by an iterative nonlinear maximiza-

tion algorithm which may not converge If the estimates fail to converge warning

messages are printed in the output

If you encounter convergence problems you should recheck the stationarity of the

data and ensure that the specified differencing orders are correct Attempting to fit

state space models to nonstationary data is a common cause of convergence failure

You can also use the MAXIT= option to increase the number of iterations allowed

or experiment with the convergence tolerance options DETTOL= and PARMTOL=

Forecast Data Set

The following statements print the output data set The WHERE statement excludes

the first 190 observations from the output so that only the forecasts and the last 10

actual observations are printed

proc print data=outid t

where t gt 190

run

The PROC PRINT output is shown in Figure 187

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Part 2 General Information

Figure 188 Plot of Forecasts

Controlling Printed Output

By default the STATESPACE procedure produces a large amount of printed output

The NOPRINT option suppresses all printed output You can suppress the printed

output for the autoregressive model selection process with the PRINTOUT=NONE

option The descriptive statistics and state space model estimation output are still

printed when PRINTOUT=NONE is specified You can produce more detailed output

with the PRINTOUT=LONG option and by specifying the printing control optionsCANCORR COVB and PRINT

Specifying the State Space Model

Instead of allowing the STATESPACE procedure to select the model automatically

you can use FORM and RESTRICT statements to specify a state space model

Specifying the State Vector

Use the FORM statement to control the form of the state vector You can use this

feature to force PROC STATESPACE to estimate and forecast a model different from

the model it would select automatically You can also use this feature to reestimate

the automatically selected model (possibly with restrictions) without repeating thecanonical correlation analysis

The FORM statement specifies the number of lags of each variable to include in

the state vector For example the statement FORM X 3 forces the state vector to

includex

t j t

x

t + 1 j t

andx

t + 2 j t

The following statement specifies the state vector

x

t j t

y

t j t

x

t + 1 j t

which is the same state vector selected in the preceding example

f o r m x 2 y 1

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Chapter 18 Getting Started

You can specify the form for only some of the variables and allow PROC STATES-

PACE to select the form for the other variables If only some of the variables are

specified in the FORM statement canonical correlation analysis is used to determine

the number of lags included in the state vector for the remaining variables not spec-

ified by the FORM statement If the FORM statement includes specifications for all

the variables listed in the VAR statement the state vector is completely defined and

the canonical correlation analysis is not performed

Restricting the F and G matrices

After you know the form of the state vector you can use the RESTRICT statement

to fix some parameters in theF

andG

matrices to specified values One use of this

feature is to remove insignificant parameters by restricting them to 0

In the introductory example shown in the preceding section the F[23] parameter is

not significant (The parameters estimation output shown in Figure 186 gives the t

statistic for F[23] as -006 F[33] and F[31] also have low significance witht 2

)

The following statements reestimate this model with F[23] restricted to 0 The

FORM statement is used to specify the state vector and thus bypass the canonicalcorrelation analysis

proc statespace data=in out=out lead=10

var x(1) y(1)

id t

form x 2 y 1

restrict f(23)=0

run

The final estimates produced by these statements are shown in Figure 189

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Part 2 General Information

The STATESPACE Procedure

Selected Statespace Form and Fitted Model

State Vector

x(TT) y(TT) x(T+1T)

Estimate of Transition Matrix

0 0 1

0290051 0467468 0

0227051 0226139 026436

Input Matrix for Innovation

1 0

0 1

0256826 0202022

Variance Matrix for Innovation

0945175 01006960100696 1014733

The STATESPACE Procedure

Parameter Estimates

Standard

Parameter Estimate Error t Value

F(21) 0290051 0063904 454

F(22) 0467468 0060430 774

F(31) 0227051 0125221 181

F(32) 0226139 0111711 202

F(33) 0264360 0299537 088G(31) 0256826 0070994 362

G(32) 0202022 0068507 295

Figure 189 Results using RESTRICT Statement

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Chapter 18 Syntax

Syntax

The STATESPACE procedure uses the following statements

PROC STATESPACE options

BY variable FORM variable value

ID variable

INITIAL F(rowcolumn)=value

G(rowcolumn)=value

RESTRICT F(rowcolumn)=value

G(rowcolumn)=value

VAR variable (difference difference

)

Functional Summary

The statements and options used by PROC STATESPACE are summarized in the

following table

Description Statement Option

Input Data Set Options

specify the input data set PROC STATESPACE DATA=

prevent subtraction of sample mean PROC STATESPACE NOCENTER

specify the ID variable ID

specify the observed series and differencing VAR

Options for Autoregressive Estimates

specify the maximum order PROC STATESPACE ARMAX=

specify maximum lag for autocovariances PROC STATESPACE LAGMAX=

output only minimum AIC model PROC STATESPACE MINIC

specify the amount of detail printed PROC STATESPACE PRINTOUT=

write preliminary AR models to a data set PROC STATESPACE OUTAR=

Options for Canonical Correlation Analysis

print the sequence of canonical correlations PROC STATESPACE CANCORR

specify upper limit of dimension of state

vector

PROC STATESPACE DIMMAX=

specify the minimum number of lags PROC STATESPACE PASTMIN=

specify the multiplier of the degrees of

freedom

PROC STATESPACE SIGCORR=

Options for State Space Model Estimation

specify starting values INITIAL

print covariance matrix of parameter estimates PROC STATESPACE COVB

specify the convergence criterion PROC STATESPACE DETTOL=

specify the convergence criterion PROC STATESPACE PARMTOL=

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Part 2 General Information

Description Statement Option

print the details of the iterations PROC STATESPACE ITPRINT

specify an upper limit of the number of lags PROC STATESPACE KLAG=

specify maximum number of iterations

allowed

PROC STATESPACE MAXIT=

suppress the final estimation PROC STATESPACE NOEST

write the state space model parameter esti-

mates to an output data set

PROC STATESPACE OUTMODEL=

use conditional least squares for final estimates PROC STATESPACE RESIDEST

specify criterion for testing for singularity PROC STATESPACE SINGULAR=

Options for Forecasting

start forecasting before end of the input data PROC STATESPACE BACK=

specify the time interval between observations PROC STATESPACE INTERVAL=

specify multiple periods in the time series PROC STATESPACE INTPER=specify how many periods to forecast PROC STATESPACE LEAD=

specify the output data set for forecasts PROC STATESPACE OUT=

print forecasts PROC STATESPACE PRINT

Options to Specify the State Space Model

specify the state vector FORM

specify the parameter values RESTRICT

BY Groups

specify BY-group processing BY

Printing

suppresses all printed output NOPRINT

PROC STATESPACE Statement

PROC STATESPACE options

The following options can be specified in the PROC STATESPACE statement

Printing Options NOPRINT

suppresses all printed output

Input Data Options

DATA= SAS-data-set

specifies the name of the SAS data set to be used by the procedure If the DATA=

option is omitted the most recently created SAS data set is used

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Chapter 18 Syntax

LAGMAX= k

specifies the number of lags for which the sample autocovariance matrix is computed

The LAGMAX= option controls the number of lags printed in the schematic repre-

sentation of the autocorrelations

The sample autocovariance matrix of lag i denoted asC

i

is computed as

C

i

=

1

N 1

N

X

t = 1 + i

x

t

x

0

t i

wherex

t

is the differenced and centered data andN

is the number of observations

(If the NOCENTER option is specified 1 is not subtracted fromN

) LAGMAX= k

specifies thatC

0

throughC

k

are computed The default is LAGMAX=10

NOCENTER

prevents subtraction of the sample mean from the input series (after any specified

differencing) before the analysis

Options for Preliminary Autoregressive Models ARMAX= n

specifies the maximum order of the preliminary autoregressive models The AR-

MAX= option controls the autoregressive orders for which information criteria are

printed and controls the number of lags printed in the schematic representation of

partial autocorrelations The default is ARMAX=10 See Preliminary Autoregres-

sive Models later in this chapter for details

MINIC

writes to the OUTAR= data set only the preliminary Yule-Walker estimates for the

VAR model producing the minimum AIC See OUTAR= Data Set later in this chap-

ter for details

OUTAR= SAS-data-set

writes the Yule-Walker estimates of the preliminary autoregressive models to a SAS

data set See OUTAR= Data Set later in this chapter for details

PRINTOUT= SHORT | LONG | NONE

determines the amount of detail printed PRINTOUT=LONG prints the lagged co-

variance matrices the partial autoregressive matrices and estimates of the resid-

ual covariance matrices from the sequence of autoregressive models PRINT-

OUT=NONE suppresses the output for the preliminary autoregressive models The

descriptive statistics and state space model estimation output are still printed when

PRINTOUT=NONE is specified PRINTOUT=SHORT is the default

Canonical Correlation Analysis Options

CANCORR

prints the canonical correlations and information criterion for each candidate state

vector considered See Canonical Correlation Analysis later in this chapter for

details

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Part 2 General Information

DIMMAX= n

specifies the upper limit to the dimension of the state vector The DIMMAX= option

can be used to limit the size of the model selected The default is DIMMAX=10

PASTMIN= n

specifies the minimum number of lags to include in the canonical correlation analy-

sis The default is PASTMIN=0 See Canonical Correlation Analysis later in thischapter for details

SIGCORR= value

specifies the multiplier of the degrees of freedom for the penalty term in the informa-

tion criterion used to select the state space form The default is SIGCORR=2 The

larger the value of the SIGCORR= option the smaller the state vector tends to be

Hence a large value causes a simpler model to be fit See Canonical Correlations

Analysis later in this chapter for details

State Space Model Estimation Options

COVB

prints the inverse of the observed information matrix for the parameter estimatesThis matrix is an estimate of the covariance matrix for the parameter estimates

DETTOL= value

specifies the convergence criterion The DETTOL= and PARMTOL= option values

are used together to test for convergence of the estimation process If during an

iteration the relative change of the parameter estimates is less than the PARMTOL=

value and the relative change of the determinant of the innovation variance matrix

is less than the DETTOL= value then iteration ceases and the current estimates are

accepted The default is DETTOL=1E-5

ITPRINT

prints the iterations during the estimation process

KLAG= n

sets an upper limit for the number of lags of the sample autocovariance matrix used

in computing the approximate likelihood function If the data have a strong moving

average character a larger KLAG= value may be necessary to obtain good estimates

The default is KLAG=15 See Parameter Estimation later in this chapter for details

MAXIT= n

sets an upper limit to the number of iterations in the maximum likelihood or condi-

tional least-squares estimation The default is MAXIT=50

NOEST

suppresses the final maximum likelihood estimation of the selected model

OUTMODEL= SAS-data-set

writes the parameter estimates and their standard errors to a SAS data set See OUT-

MODEL= Data Set later in this chapter for details

PARMTOL= value

specifies the convergence criterion The DETTOL= and PARMTOL= option values

are used together to test for convergence of the estimation process If during an

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Chapter 18 Syntax

iteration the relative change of the parameter estimates is less than the PARMTOL=

value and the relative change of the determinant of the innovation variance matrix

is less than the DETTOL= value then iteration ceases and the current estimates are

accepted The default is PARMTOL=001

RESIDEST

computes the final estimates using conditional least squares on the raw data Thistype of estimation may be more stable than the default maximum likelihood method

but is usually more computationally expensive See Parameter Estimation later in

this chapter for details of the conditional least squares method

SINGULAR= value

specifies the criterion for testing for singularity of a matrix A matrix is declared

singular if a scaled pivot is less than the SINGULAR= value when sweeping the

matrix The default is SINGULAR=1E-7

Forecasting Options

BACK= n

starts forecasting n periods before the end of the input data The BACK= option valuemust not be greater than the number of observations The default is BACK=0

INTERVAL= interval

specifies the time interval between observations The INTERVAL= value is used

in conjunction with the ID variable to check that the input data are in order and

have no missing periods The INTERVAL= option is also used to extrapolate the ID

values past the end of the input data See Chapter 3 ldquoDate Intervals Formats and

Functionsrdquo for details on the INTERVAL= values allowed

INTPER= n

specifies that each input observation corresponds to n time periods For example

the options INTERVAL=MONTH and INTPER=2 specify bimonthly data and are

equivalent to specifying INTERVAL=MONTH2 If the INTERVAL= option is not

specified the INTPER= option controls the increment used to generate ID values for

the forecast observations The default is INTPER=1

LEAD= n

specifies how many forecast observations are produced The forecasts start at the

point set by the BACK= option The default is LEAD=0 which produces no fore-

casts

OUT= SAS-data-set

writes the residuals actual values forecasts and forecast standard errors to a SAS

data set See OUT= Data Set later in this chapter for details

PRINT

prints the forecasts

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Part 2 General Information

BY Statement

BY variable

A BY statement can be used with the STATESPACE procedure to obtain separateanalyses on observations in groups defined by the BY variables

FORM Statement

FORM variable value

The FORM statement specifies the number of times a variable is included in the state

vector Values can be specified for any variable listed in the VAR statement If a

value is specified for each variable in the VAR statement the state vector for the state

space model is entirely specified and automatic selection of the state space model is

not performed

The FORM statement forces the state vectorz

t

to contain a specific variable a given

number of times For example if Y is one of the variables inx

t

then the statement

form y 3

forces the state vector to containY

t

Y

t + 1 j t

andY

t + 2 j t

possibly along with other

variables

The following statements illustrate the use of the FORM statement

proc statespace data=in

var x y

f o r m x 3 y 2

run

These statements fit a state space model with the following state vector

z

t

=

2

6

6

6

6

4

x

t j t

y

t j t

x

t + 1 j t

y

t + 1 j t

x

t + 2 j t

3

7

7

7

7

5

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Chapter 18 Syntax

ID Statement

ID variable

The ID statement specifies a variable that identifies observations in the input data setThe variable specified in the ID statement is included in the OUT= data set The

values of the ID variable are extrapolated for the forecast observations based on the

values of the INTERVAL= and INTPER= options

INITIAL Statement

INITIAL F (rowcolumn)= value G(row column)= value

The INITIAL statement gives initial values to the specified elements of theF

andG

matrices These initial values are used as starting values for the iterative estimation

Parts of theF

andG

matrices represent fixed structural identities If an element

specified is a fixed structural element instead of a free parameter the corresponding

initialization is ignored

The following is an example of an INITIAL statement

initial f(32)=0 g(41)=0 g(51)=0

RESTRICT Statement

RESTRICT F(rowcolumn)= value G(rowcolumn)= value

The RESTRICT statement restricts the specified elements of theF

andG

matrices

to the specified values

To use the restrict statement you need to know the form of the model Either specify

the form of the model with the FORM statement or do a preliminary run perhaps

with the NOEST option to find the form of the model that PROC STATESPACE

selects for the data

The following is an example of a RESTRICT statement

restrict f(32)=0 g(41)=0 g(51)=0

Parts of theF

andG

matrices represent fixed structural identities If a restriction is

specified for an element that is a fixed structural element instead of a free parameter

the restriction is ignored

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Part 2 General Information

VAR Statement

VAR variable (difference difference )

The VAR statement specifies the variables in the input data set to model and fore-cast The VAR statement also specifies differencing of the input variables The VAR

statement is required

Differencing is specified by following the variable name with a list of difference

periods separated by commas See the section Stationarity and Differencing for

more information on differencing of input variables

The order in which variables are listed in the VAR statement controls the order in

which variables are included in the state vector Usually potential inputs should be

listed before potential outputs

For example assuming the input data are monthly the following VAR statement

specifies modeling and forecasting of the one period and seasonal second differenceof X and Y

var x(112) y(112)

In this example the vector time series analyzed is

x

t

=

1 B 1 B

1 2

X

t

x

1 B 1 B

1 2

Y

t

y

where B represents the back shift operator and x and y represent the means of thedifferenced series If the NOCENTER option is specified the mean differences are

not subtracted

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Chapter 18 Details

Details

Missing Values

The STATESPACE procedure does not support missing values The procedure uses

the first contiguous group of observations with no missing values for any of the VARstatement variables Observations at the beginning of the data set with missing values

for any VAR statement variable are not used or included in the output data set

Stationarity and Differencing

The state space model used by the STATESPACE procedure assumes that the time

series are stationary Hence the data should be checked for stationarity One way

to check for stationarity is to plot the series A graph of series over time can show a

time trend or variability changes

You can also check stationarity by using the sample autocorrelation functions dis-

played by the ARIMA procedure The autocorrelation functions of nonstationaryseries tend to decay slowly See Chapter 7 ldquoThe ARIMA Procedurerdquo for more in-

formation

Another alternative is to use the STATIONARITY= option on the IDENTIFY state-

ment in PROC ARIMA to apply Dickey-Fuller tests for unit roots in the time series

See Chapter 7 ldquoThe ARIMA Procedurerdquo for more information on Dickey-Fuller unit

root tests

The most popular way to transform a nonstationary series to stationarity is by dif-

ferencing Differencing of the time series is specified in the VAR statement For

example to take a simple first difference of the series X use this statement

var x(1)

In this example the change in X from one period to the next is analyzed When the se-

ries has a seasonal pattern differencing at a period equal to the length of the seasonal

cycle may be desirable For example suppose the variable X is measured quarterly

and shows a seasonal cycle over the year You can use the following statement to

analyze the series of changes from the same quarter in the previous year

var x(4)

To difference twice add another differencing period to the list For ex-

ample the following statement analyzes the series of second differences

X

t

X

t 1

X

t 1

X

t 2

= X

t

2 X

t 1

+ X

t 2

var x(11)

The following statement analyzes the seasonal second difference series

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Part 2 General Information

var x(14)

The series modeled is the 1-period difference of the 4-period difference

X

t

X

t 4

X

t 1

X

t 5

= X

t

X

t 1

X

t 4

+ X

t 5

Another way to obtain stationary series is to use a regression on time to de-trend the

data If the time series has a deterministic linear trend regressing the series on time

produces residuals that should be stationary The following statements write residuals

of X and Y to the variable RX and RY in the output data set DETREND

data a

set a

t=_n_

run

proc reg data=a

model x y = t

output out=detrend r=rx ry

run

You then use PROC STATESPACE to forecast the de-trended series RX and RY A

disadvantage of this method is that you need to add the trend back to the forecast

series in an additional step A more serious disadvantage of the de-trending method

is that it assumes a deterministic trend In practice most time series appear to have a

stochastic rather than a deterministic trend Differencing is a more flexible and often

more appropriate method

There are several other methods to handle nonstationary time series For more infor-

mation and examples refer to Brockwell and Davis (1991)

Preliminary Autoregressive Models

After computing the sample autocovariance matrices PROC STATESPACE fits a

sequence of vector autoregressive models These preliminary autoregressive models

are used to estimate the autoregressive order of the process and limit the order of the

autocovariances considered in the state vector selection process

Yule-Walker Equations for Forward and Backward Models

Unlike a univariate autoregressive model a multivariate autoregressive model has

different forms depending on whether the present observation is being predicted from

the past observations or from the future observations

Letx

t

be the r -component stationary time series given by the VAR statement after

differencing and subtracting the vector of sample means (If the NOCENTER option

is specified the mean is not subtracted) Let n be the number of observations of x

t

from the input data set

Lete

t

be a vector white noise sequence with mean vector 0 and variance matrix

p

and letn

t

be a vector white noise sequence with mean vector 0 and variance matrix

p

Let p be the order of the vector autoregressive model forx

t

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Chapter 18 Details

The forward autoregressive form based on the past observations is written as follows

x

t

=

p

X

i = 1

p

i

x

t i

+ e

t

The backward autoregressive form based on the future observations is written as fol-

lows

x

t

=

p

X

i = 1

p

i

x

t + i

+ n

t

LettingE

denote the expected value operator the autocovariance sequence for thex

t

series

i

is

i

= E x

t

x

0

t i

The Yule-Walker equations for the autoregressive model that matches the first p ele-

ments of the autocovariance sequence are

2

6

6

4

0

1

p 1

0

1

0

p 2

0

p 1

0

p 2

0

3

7

7

5

2

6

6

4

p

1

p

2

p

p

3

7

7

5

=

2

6

6

4

1

2

p

3

7

7

5

and

2

6

6

6

4

0

0

1

0

p 1

1

0

0

p 2

p 1

p 2

0

3

7

7

7

5

2

6

6

4

p

1

p

2

p

p

3

7

7

5

=

2

6

6

4

0

1

0

2

0

p

3

7

7

5

Here

p

i

are the coefficient matrices for the past observation form of the vector au-toregressive model and

p

i

are the coefficient matrices for the future observation

form More information on the Yule-Walker equations in the multivariate setting can

be found in Whittle (1963) and Ansley and Newbold (1979)

The innovation variance matrices for the two forms can be written as follows

p

=

0

p

X

i = 1

p

i

0

i

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Part 2 General Information

p

=

0

p

X

i = 1

p

i

i

The autoregressive models are fit to the data using the preceding Yule-Walker equa-

tions with

i

replaced by the sample covariance sequenceC

i

The covariance matri-

ces are calculated as

C

i

=

1

N 1

N

X

t = i + 1

x

t

x

0

t i

Let b

p

b

p

b

p

and b

p

represent the Yule-Walker estimates of

p

p

p

and

p

respectively These matrices are written to an output data set when the OUTAR=

option is specified

When the PRINTOUT=LONG option is specified the sequence of matrices b

p

and

the corresponding correlation matrices are printed The sequence of matricesb

p isused to compute Akaike information criteria for selection of the autoregressive order

of the process

Akaike Information Criterion

The Akaike information criterion or AIC is defined as -2( maximum of log likeli-

hood )+2(number of parameters) Since the vector autoregressive models are esti-

mates from the Yule-Walker equations not by maximum likelihood the exact likeli-

hood values are not available for computing the AIC However for the vector autore-

gressive model the maximum of the log likelihood can be approximated as

l n L

n

2

l n j

b

p

j

Thus the AIC for the order p model is computed as

A I C

p

= n l n j

b

p

j + 2 p r

2

You can use the printed AIC array to compute a likelihood ratio test of the autoregres-

sive order The log-likelihood ratio test statistic for testing the order p model against

the order p 1

model is

n l n j

b

p

j + n l n j

b

p 1

j

This quantity is asymptotically distributed as a

2 withr

2 degrees of freedom if the

series is autoregressive of order p 1

It can be computed from the AIC array as

A I C

p 1

A I C

p

+ 2 r

2

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Chapter 18 Details

You can evaluate the significance of these test statistics with the PROBCHI function

in a SAS DATA step or with a

2 table

Determining the Autoregressive Order

Although the autoregressive models can be used for prediction their primary value is

to aid in the selection of a suitable portion of the sample covariance matrix for use in

computing canonical correlations If the multivariate time series x

t is of autoregres-sive order p then the vector of past values to lag p is considered to contain essentially

all the information relevant for prediction of future values of the time series

By default PROC STATESPACE selects the order p producing the autoregressive

model with the smallestA I C

p

If the value p for the minimumA I C

p

is less than the

value of the PASTMIN= option then p is set to the PASTMIN= value Alternatively

you can use the ARMAX= and PASTMIN= options to force PROC STATESPACE to

use an order you select

Significance Limits for Partial Autocorrelations

The STATESPACE procedure prints a schematic representation of the partial autocor-

relation matrices indicating which partial autocorrelations are significantly greater orsignificantly less than 0 Figure 1810 shows an example of this table

The STATESPACE Procedure

Schematic Representation of Partial Autocorrelations

NameLag 1 2 3 4 5 6 7 8 9 10

x ++ +

y ++

+ is gt 2std error - is lt -2std error is between

Figure 1810 Significant Partial Autocorrelations

The partial autocorrelations are from the sample partial autoregressive matrices b

p

p

The standard errors used for the significance limits of the partial autocorrelations are

computed from the sequence of matrices

p

and

p

Under the assumption that the observed series arises from an autoregressive process

of order p 1

the pth sample partial autoregressive matrix b

p

p

has an asymptotic

variance matrix 1

n

1

p

p

The significance limits for b

p

p

used in the schematic plot of the sample partial autore-

gressive sequence are derived by replacing

p

and

p

with their sample estimators

to produce the variance estimate as follows

d

V a r

b

p

p

=

1

n r p

b

1

p

b

p

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Part 2 General Information

Canonical Correlation Analysis

Given the order p letp

t

be the vector of current and past values relevant to prediction

of x

t + 1

p

t

= x

0

t

x

0

t 1

x

0

t p

0

Letf

t

be the vector of current and future values

f

t

= x

0

t

x

0

t + 1

x

0

t + p

0

In the canonical correlation analysis consider submatrices of the sample covariance

matrix of p

t

andf

t

This covariance matrixV

has a block Hankel form

V =

2

6

6

6

4

C

0

C

0

1

C

0

2

C

0

p

C

0

1

C

0

2

C

0

3

C

0

p + 1

C

0

p

C

0

p + 1

C

0

p + 2

C

0

2 p

3

7

7

7

5

State Vector Selection Process

The canonical correlation analysis forms a sequence of potential state vectorsz

j

t

Examine a sequencef

j

t

of subvectors of f

t

and form the submatrixV

j consisting

of the rows and columns of V

corresponding to the components of f

j

t

and compute

its canonical correlations

The smallest canonical correlation of V

j is then used in the selection of the com-

ponents of the state vector The selection process is described in the following For

more details about this process refer to Akaike (1976)

In the following discussion the notationx

t + k j t

denotes the wide sense conditional

expectation (best linear predictor) of x

t + k

given allx

s

with s less than or equal to t

In the notationx

i t + 1

the first subscript denotes the ith component of x

t + 1

The initial state vectorz

1

t

is set tox

t

The sequencef

j

t

is initialized by setting

f

1

t

= z

1

0

t

x

1 t + 1 j t

0

= x

0

t

x

1 t + 1 j t

0

That is start by considering whether to addx

1 t + 1 j t

to the initial state vectorz

1

t

The procedure forms the submatrixV

1 corresponding tof

1

t

and computes its canon-

ical correlations Denote the smallest canonical correlation of V

1 as

m i n

If

m i n

is

significantly greater than 0x

1 t + 1 j t

is added to the state vector

If the smallest canonical correlation of V

1 is not significantly greater than 0 then a

linear combination of f

1

t

is uncorrelated with the pastp

t

Assuming that the determi-

nant of C

0

is not 0 (that is no input series is a constant) you can take the coefficient

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Chapter 18 Details

of x

1 t + 1 j t

in this linear combination to be 1 Denote the coefficients of z

1

t

in this

linear combination as

This gives the relationship

x

1 t + 1 j t

=

0

x

t

Therefore the current state vector already contains all the past information useful forpredicting

x

1 t + 1

and any greater leads of x

1 t

The variablex

1 t + 1 j t

is not added to

the state vector nor are any termsx

1 t + k j t

considered as possible components of the

state vector The variablex

1

is no longer active for state vector selection

The process described forx

1 t + 1 j t

is repeated for the remaining elements of f

t

The

next candidate for inclusion in the state vector is the next component of f

t

corre-

sponding to an active variable Components of f

t

corresponding to inactive variables

that produced a zero

m i n

in a previous step are skipped

Denote the next candidate asx

l t + k j t

The vectorf

j

t

is formed from the current state

vector andx

l t + k j t

as follows

f

j

t

= z

j

0

t

x

l t + k j t

0

The matrixV

j is formed fromf

j

t

and its canonical correlations are computed The

smallest canonical correlation of V

j is judged to be either greater than or equal to 0

If it is judged to be greater than 0x

l t + k j t

is added to the state vector If it is judged

to be 0 then a linear combination of f

j

t

is uncorrelated with thep

t

and the variable

x

l

is now inactive

The state vector selection process continues until no active variables remain

Testing Significance of Canonical Correlations For each step in the canonical correlation sequence the significance of the small-

est canonical correlation

m i n

is judged by an information criterion from Akaike

(1976) This information criterion is

n l n 1

2

m i n

r p + 1 q + 1

where q is the dimension of f

j

t

at the current step r is the order of the state vector p

is the order of the vector autoregressive process and

is the value of the SIGCORR=

option The default is SIGCORR=2 If this information criterion is less than or equal

to 0

m i n

is taken to be 0 otherwise it is taken to be significantly greater than 0

(Do not confuse this information criterion with the AIC)

Variables inx

t + p j t

are not added in the model even with positive information crite-

rion because of the singularity of V

You can force the consideration of more candi-

date state variables by increasing the size of theV

matrix by specifying a PASTMIN=

option value larger than p

Printing the Canonical Correlations

To print the details of the canonical correlation analysis process specify the CAN-

CORR option in the PROC STATESPACE statement The CANCORR option prints

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the candidate state vectors the canonical correlations and the information criteria for

testing the significance of the smallest canonical correlation

Bartlettrsquos

2 and its degrees of freedom are also printed when the CANCORR option

is specified The formula used for Bartlettrsquos

2 is

2

= n 5 r p + 1 q + 1 l n 1

2

m i n

withr p + 1 q + 1

degrees of freedom

Figure 1811 shows the output of the CANCORR option for the introductory example

shown in the Getting Started section of this chapter

The STATESPACE Procedure

Canonical Correlations Analysis

Information Chi

x(TT) y(TT) x(T+1T) Criterion Square DF

1 1 0237045 3566167 114505 4

Information Chi

x(TT) y(TT) x(T+1T) y(T+1T) Criterion Square DF

1 1 0238244 0056565 -535906 0636134 3

Information Chi

x(TT) y(TT) x(T+1T) x(T+2T) Criterion Square DF

1 1 0237602 0087493 -446312 1525353 3

Figure 1811 Canonical Correlations Analysis

New variables are added to the state vector if the information criteria are positive Inthis example

y

t + 1 j t

andx

t + 2 j t

are not added to the state space vector because the

information criteria for these models are negative

If the information criterion is nearly 0 then you may want to investigate models

that arise if the opposite decision is made regarding

m i n

This investigation can be

accomplished by using a FORM statement to specify part or all of the state vector

Preliminary Estimates of F

When a candidate variablex

l t + k j t

yields a zero

m i n

and is not added to the state

vector a linear combination of f

j

t

is uncorrelated with thep

t

Because of the method

used to construct thef

j

t

sequence the coefficient of x

l t + k j t

inl

can be taken as 1

Denote the coefficients of z

j

t

in this linear combination as l

This gives the relationship

x

l t + k j t

= l

0

z

j

t

The vectorl

is used as a preliminary estimate of the first r columns of the row of the

transition matrixF

corresponding tox

l t + k 1 j t

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Chapter 18 Details

Parameter Estimation

The model isz

t + 1

= F z

t

+ G e

t + 1

wheree

t

is a sequence of independent multivari-

ate normal innovations with mean vector 0 and variance

e e

The observed sequence

x

t

composes the first r components of z

t

and thusx

t

= H z

t

where H is ther s

matrix I

r

0

LetE

be ther n

matrix of innovations

E = e

1

e

n

If the number of observations n is reasonably large the log likelihood L can be

approximated up to an additive constant as follows

L =

n

2

l n j

e e

j

1

2

t r a c e

1

e e

E E

0

The elements of

e e

are taken as free parameters and are estimated as follows

S

0

=

1

n

E E

0

Replacing

e e

byS

0

in the likelihood equation the log likelihood up to an additive

constant is

L =

n

2

l n j S

0

j

Letting B be the backshift operator the formal relation betweenx

t

ande

t

is

x

t

= H I B F

1

G e

t

e

t

= H I B F

1

G

1

x

t

=

1

X

i = 0

i

x

t i

LettingC

i

be the ith lagged sample covariance of x

t

and neglecting end effects the

matrixS

0

is

S

0

=

1

X

i j = 0

i

C

i + j

0

j

For the computation of S

0

the infinite sum is truncated at the value of the KLAG=

option The value of the KLAG= option should be large enough that the sequence

i

is approximately 0 beyond that point

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Let

be the vector of free parameters in theF

andG

matrices The derivative of the

log likelihood with respect to the parameter

is

L

=

n

2

t r a c e

S

1

0

S

0

The second derivative is

2

L

0

=

n

2

t r a c e

S

1

0

S

0

0

S

1

0

S

0

t r a c e

S

1

0

2

S

0

0

Near the maximum the first term is unimportant and the second term can be approx-

imated to give the following second derivative approximation

2

L

0

=

n t r a c e

S

1

0

E

E

0

0

The first derivative matrix and this second derivative matrix approximation are com-

puted from the sample covariance matrixC

0

and the truncated sequence

i

The

approximate likelihood function is maximized by a modified Newton-Raphson algo-

rithm employing these derivative matrices

The matrixS

0

is used as the estimate of the innovation covariance matrix

e e

The

negative of the inverse of the second derivative matrix at the maximum is used as an

approximate covariance matrix for the parameter estimates The standard errors of

the parameter estimates printed in the parameter estimates tables are taken from the

diagonal of this covariance matrix The parameter covariance matrix is printed when

the COVB option is specified

If the data are nearly nonstationary a better estimate of

e e

and the other parameters

can sometimes be obtained by specifying the RESIDEST option The RESIDEST

option estimates the parameters using conditional least squares instead of maximum

likelihood

The residuals are computed using the state space equation and the sample mean val-

ues of the variables in the model as start-up values The estimate of S

0

is then com-

puted using the residuals from the ith observation on where i is the maximum number

of times any variable occurs in the state vector A multivariate Gauss-Marquardt al-

gorithm is used to minimizej S

0

j

Refer to Harvey (1981a) for a further description

of this method

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Forecasting

Given estimates of F

G

and

e e

forecasts of x

t

are computed from the conditional

expectation of z

t

In forecasting the parameters F G and

e e

are replaced with the estimates or by val-

ues specified in the RESTRICT statement One-step-ahead forecasting is performedfor the observation

x

t

wheret n b

Heren

is the number of observations and

b is the value of the BACK= option For the observationx

t

wheret n b

m-

step-ahead forecasting is performed form = t n + b

The forecasts are generated

recursively with the initial conditionz

0

= 0

The m-step-ahead forecast of z

t + m

isz

t + m j t

wherez

t + m j t

denotes the conditional

expectation of z

t + m

given the information available at time t The m-step-ahead

forecast of x

t + m

isx

t + m j t

= H z

t + m j t

where the matrixH = I

r

0

Let

i

= F

i

G

Note that the lasts r

elements of z

t

consist of the elements of x

u j t

foru t

The state vector z

t + m

can be represented as

z

t + m

= F

m

z

t

+

m 1

X

i = 0

i

e

t + m i

Sincee

t + i j t

= 0

fori 0

the m-step-ahead forecastz

t + m j t

is

z

t + m j t

= F

m

z

t

= F z

t + m 1 j t

Therefore the m-step-ahead forecast of x

t + m

is

x

t + m j t

= H z

t + m j t

The m-step-ahead forecast error is

z

t + m

z

t + m j t

=

m 1

X

i = 0

i

e

t + m i

The variance of the m-step-ahead forecast error is

V

z m

=

m 1

X

i = 0

i

e e

0

i

LettingV

z 0

= 0

the variance of the m-step-ahead forecast error of z

t + m

V

z m

can

be computed recursively as follows

V

z m

= V

z m 1

+

m 1

e e

0

m 1

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The variance of the m-step-ahead forecast error of x

t + m

is ther r

left upper sub-

matrix of V

z m

that is

V

x m

= H V

z m

H

0

Unless the NOCENTER option is specified the sample mean vector is added to the

forecast When differencing is specified the forecasts xt + m j t

plus the sample mean

vector are integrated back to produce forecasts for the original series

Lety

t

be the original series specified by the VAR statement with some 0 values

appended corresponding to the unobserved past observations Let B be the backshift

operator and let B

be thes s

matrix polynomial in the backshift operator

corresponding to the differencing specified by the VAR statement The off-diagonal

elements of

i

are 0 Note that

0

= I

s

whereI

s

is thes s

identity matrix Then

z

t

= B y

t

This gives the relationship

y

t

=

1

B z

t

=

1

X

i = 0

i

z

t i

where

1

B =

P

1

i = 0

i

B

i and

0

= I

s

The m-step-ahead forecast of y

t + m

is

y

t + m j t

=

m 1

X

i = 0

i

z

t + m i j t

+

1

X

i = m

i

z

t + m i

The m-step-ahead forecast error of y

t + m

is

m 1

X

i = 0

i

z

t + m i

z

t + m i j t

=

m 1

X

i = 0

i

X

u = 0

u

i u

e

t + m i

LettingV

y 0

= 0

the variance of the m-step-ahead forecast error of y

t + m

V

y m

is

V

y m

=

m 1

X

i = 0

i

X

u = 0

u

i u

e e

i

X

u = 0

u

i u

0

= V

y m 1

+

m 1

X

u = 0

u

m 1 u

e e

m 1

X

u = 0

u

m 1 u

0

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Chapter 18 Details

Relation of ARMA and State Space Forms

Every state space model has an ARMA representation and conversely every ARMA

model has a state space representation This section discusses this equivalence The

following material is adapted from Akaike (1974) where there is a more complete

discussion Pham-Dinh-Tuan (1978) also contains a discussion of this material

Suppose you are given the following ARMA model

B x

t

= B e

t

or in more detail

x

t

1

x

t 1

p

x

t p

= e

t

+

1

e

t 1

+ +

q

e

t q

(1)

wheree

t

is a sequence of independent multivariate normal random vectors with mean

0 and variance matrix

e e

B is the backshift operator (B x

t

= x

t 1

) B

and

B

are matrix polynomials in B andx

t

is the observed process

If the roots of the determinantial equationj B j = 0

are outside the unit circle in

the complex plane the model can also be written as

x

t

=

1

B B e

t

=

1

X

i = 0

i

e

t i

The

i

matrices are known as the impulse response matrices and can be computed

as

1

B B

You can assume p q

since if this is not initially true you can add more terms

i

that are identically 0 without changing the model

To write this set of equations in a state space form proceed as follows Letx

t + i j t

be

the conditional expectation of x

t + i

givenx

w

forw t

The following relations hold

x

t + i j t

=

1

X

j = i

j

e

t + i j

x

t + i j t + 1

= x

t + i j t

+

i 1

e

t + 1

However from equation (1) you can derive the following relationship

x

t + p j t

=

1

x

t + p 1 j t

+ +

p

x

t

(2)

Hence wheni = p

you can substitute forx

t + p j t

in the right-hand side of equation

(2) and close the system of equations

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This substitution results in the following model in the state space form

z

t + 1

= F z

t

+ G e

t + 1

2

6

6

6

4

x

t + 1

x

t + 2 j t + 1

x

t + p j t + 1

3

7

7

7

5

=

2

6

6

4

0 I 0 0

0 0 I 0

p

p 1

1

3

7

7

5

2

6

6

6

4

x

t

x

t + 1 j t

x

t + p 1 j t

3

7

7

7

5

+

2

6

6

4

I

1

p 1

3

7

7

5

e

t + 1

Note that the state vectorz

t

is composed of conditional expectations of x

t

and the

first r components of z

t

are equal tox

t

The state space form can be cast into an ARMA form by solving the system of dif-

ference equations for the first r components

When converting from an ARMA form to a state space form you can generate a

state vector larger than needed that is the state space model may not be a minimal

representation When going from a state space form to an ARMA form you can have

nontrivial common factors in the autoregressive and moving average operators that

yield an ARMA model larger than necessary

If the state space form used is not a minimal representation some but not all compo-

nents of x

t + i j t

may be linearly dependent This situation corresponds to

p

p 1

being of less than full rank when B

and B

have no common nontrivial

left factors In this casez

t

consists of a subset of the possible components of

x

t + i j t

i = 1 2 p 1

However once a component of x

t + i j t

(for example the

jth one) is linearly dependent on the previous conditional expectations then all sub-

sequent jth components of x

t + k j t

fork i

must also be linearly dependent Note

that in this case equivalent but seemingly different structures can arise if the order of

the components withinx

t

is changed

OUT= Data Set

The forecasts are contained in the output data set specified by the OUT= option on the

PROC STATESPACE statement The OUT= data set contains the following variables

the BY variables

the ID variable

the VAR statement variables These variables contain the actual values from

the input data set

FORi numeric variables containing the forecasts The variable FORi contains

the forecasts for the ith variable in the VAR statement list Forecasts are one-

step-ahead predictions until the end of the data or until the observation specified

by the BACK= option

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RESi numeric variables containing the residual for the forecast of the ith vari-

able in the VAR statement list For forecast observations the actual values are

missing and the RESi variables contain missing values

STDi numeric variables containing the standard deviation for the forecast of

the ith variable in the VAR statement list The values of the STDi variables can

be used to construct univariate confidence limits for the corresponding fore-casts However such confidence limits do not take into account the covariance

of the forecasts

OUTAR= Data Set

The OUTAR= data set contains the estimates of the preliminary autoregressive mod-

els The OUTAR= data set contains the following variables

ORDER a numeric variable containing the order p of the autoregressive model

that the observation represents

AIC a numeric variable containing the value of the information criterionA I C

p

SIGFl numeric variables containing the estimate of the innovation covariance

matrices for the forward autoregressive models The variable SIGFl contains

the lth column of b

p

in the observations with ORDER= p

SIGBl numeric variables containing the estimate of the innovation covariance

matrices for the backward autoregressive models The variable SIGBl contains

the lth column of b

p

in the observations with ORDER= p

FORk ndashl numeric variables containing the estimates of the autoregressive pa-

rameter matrices for the forward models The variable FORk ndashl contains the

lth column of the lag k autoregressive parameter matrix b

p

k

in the observations

with ORDER= p

BACk ndashl numeric variables containing the estimates of the autoregressive pa-

rameter matrices for the backward models The variable BACk ndashl contains the

lth column of the lag k autoregressive parameter matrix b

p

k

in the observations

with ORDER= p

The estimates for the order p autoregressive model can be selected as those observa-

tions with ORDER= p Within these observations the k lth element of

p

i

is given by

the value of the FORindashl variable in the k th observation The k lth element of

p

i

is

given by the value of BACindashl variable in the k th observation The k lth element of

p

is given by SIGFl in the k th observation The k lth element of

p

is given by SIGBlin the k th observation

Table 181 shows an example of the OUTAR= data set with ARMAX=3 andx

t

of

dimension 2 In Table 181 i j

indicate the ijth element of the matrix

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Table 181 Values in the OUTAR= Data Set

Obs ORDER AIC SIGF1 SIGF2 SIGB1 SIGB2 FOR1ndash 1 FOR1ndash 2 FOR2ndash 1 FOR2ndash 2 FOR3ndash 1

1 0 AIC0

0 1 1

0 1 2

0 1 1

0 1 2

2 0 AIC0

0 2 1

0 2 2

0 2 1

0 2 2

3 1 AIC1

1 1 1

1 1 2

1 1 1

1 1 2

1

1

1 1

1

1

1 2

4 1 AIC1

1 2 1

1 1 2

1 2 1

1 1 2

1

1

2 1

1

1

2 2

5 2 AIC2

2 1 1

2 1 2

2 1 1

2 1 2

2

1

1 1

2

1

1 2

2

2

1 1

2

2

1 2

6 2 AIC2

2 2 1

2 1 2

2 2 1

2 1 2

2

1

2 1

2

1

2 2

2

2

2 1

2

2

2 2

7 3 AIC3

3 1 1

3 1 2

3 1 1

3 1 2

3

1

1 1

3

1

1 2

3

2

1 1

3

2

1 2

3

3

1 1

8 3 AIC3

3 2 1

3 1 2

3 2 1

3 1 2

3

1

2 1

3

1

2 2

3

2

2 1

3

2

2 2

3

3

2 1

Obs FOR3ndash 2 BACK1ndash 1 BACK1ndash 2 BACK2ndash 1 BACK2ndash 2 BACK3ndash 1 BACK3ndash 2

1

2

3

1

1

1 1

1

1

1 2

4

1

1

2 1

1

1

2 2

5

2

1

1 1

2

1

1 2

2

2

1 1

2

2

1 2

6

2

1

2 1

2

1

2 2

2

2

2 1

2

2

2 2

7

3

3

1 2

3

1

1 1

3

1

1 2

3

2

1 1

3

2

1 2

3

3

1 1

3

3

1 2

8

3

3

2 2

3

1

2 1

3

1

2 2

3

2

2 1

3

2

2 2

3

3

2 1

3

3

2 2

The estimated autoregressive parameters can be used in the IML procedure to obtain

autoregressive estimates of the spectral density function or forecasts based on the

autoregressive models

OUTMODEL= Data Set

The OUTMODEL= data set contains the estimates of theF

andG

matrices and their

standard errors the names of the components of the state vector and the estimates of

the innovation covariance matrix The variables contained in the OUTMODEL= data

set are as follows

the BY variables

STATEVEC a character variable containing the name of the component of the

state vector corresponding to the observation The STATEVEC variable has

the value STD for standard deviations observations which contain the standarderrors for the estimates given in the preceding observation

Fndash j numeric variables containing the columns of theF

matrix The variable

Fndash j contains the jth column of F

The number of Fndash j variables is equal to

the value of the DIMMAX= option If the model is of smaller dimension the

extraneous variables are set to missing

Gndash j numeric variables containing the columns of theG

matrix The variable

Gndash j contains the jth column of G

The number of Gndash j variables is equal to r

the dimension of x

t

given by the number of variables in the VAR statement

SIGndash j numeric variables containing the columns of the innovation covariance

matrix The variable SIGndash j contains the jth column of

e e

There are r vari-ables SIGndash j

Table 182 shows an example of the OUTMODEL= data set withx

t

= x

t

y

t

0

z

t

= x

t

y

t

x

t + 1 j t

0 and DIMMAX=4 In Table 182F

i j

andG

i j

are the ijth

elements of F

andG

respectively Note that all elements for Fndash4 are missing because

F

is a3 3

matrix

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Table 182 Value in the OUTMODEL= Data Set

Obs STATEVEC Fndash 1 Fndash 2 Fndash 3 Fndash 4 Gndash 1 Gndash 2 SIGndash 1 SIGndash 2

1 X(TT) 0 0 1 1 0

1 1

1 2

2 STD

3 Y(TT) F2 1

F2 2

F2 3

0 1

2 1

2 2

4 STD std F2 1

std F2 2

std F2 3

5 X(T+1T) F3 1

F3 2

F3 3

G3 1

G3 2

6 STD std F3 1

std F3 2

std F3 3

std G3 1

std G3 2

Printed Output

The printed output produced by the STATESPACE procedure is described in the fol-

lowing

1 descriptive statistics which include the number of observations used the

names of the variables their means and standard deviations (Std) and the dif-

ferencing operations used

2 the Akaike information criteria for the sequence of preliminary autoregressive

models

3 if the PRINTOUT=LONG option is specified the sample autocovariance ma-

trices of the input series at various lags

4 if the PRINTOUT=LONG option is specified the sample autocorrelation ma-

trices of the input series

5 a schematic representation of the autocorrelation matrices showing the signif-

icant autocorrelations

6 if the PRINTOUT=LONG option is specified the partial autoregressive ma-

trices (These are

p

p

as described in Preliminary Autoregressive Modelsearlier in this chapter)

7 a schematic representation of the partial autocorrelation matrices showing the

significant partial autocorrelations

8 the Yule-Walker estimates of the autoregressive parameters for the autoregres-

sive model with the minimum AIC

9 if the PRINTOUT=LONG option is specified the autocovariance matrices of

the residuals of the minimum AIC model This is the sequence of estimated

innovation variance matrices for the solutions of the Yule-Walker equations

10 if the PRINTOUT=LONG option is specified the autocorrelation matrices of the residuals of the minimum AIC model

11 If the CANCORR option is specified the canonical correlations analysis for

each potential state vector considered in the state vector selection process

This includes the potential state vector the canonical correlations the informa-

tion criterion for the smallest canonical correlation Bartlettrsquos

2 statistic (ldquoChi

Squarerdquo) for the smallest canonical correlation and the degrees of freedom of

Bartlettrsquos

2

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Part 2 General Information

12 the components of the chosen state vector

13 the preliminary estimate of the transition matrixF

the input matrixG

and

the variance matrix for the innovations

e e

14 if the ITPRINT option is specified the iteration history of the likelihood max-

imization For each iteration this shows the iteration number the number of step halvings the determinant of the innovation variance matrix the damping

factor Lambda and the values of the parameters

15 the state vector printed again to aid interpretation of the following listing of F

andG

16 the final estimate of the transition matrixF

17 the final estimate of the input matrixG

18 the final estimate of the variance matrix for the innovations

e e

19 a table listing the estimates of the free parameters inF

andG

and their standarderrors and t statistics

20 if the COVB option is specified the covariance matrix of the parameter esti-

mates

21 if the COVB option is specified the correlation matrix of the parameter esti-

mates

22 if the PRINT option is specified the forecasts and their standard errors

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Chapter 18 Details

ODS Table Names

PROC STATESPACE assigns a name to each table it creates You can use these

names to reference the table when using the Output Delivery System (ODS) to select

tables and create output data sets These names are listed in the following table For

more information on ODS see Chapter 6 ldquoUsing the Output Delivery Systemrdquo

Table 183 ODS Tables Produced in PROC STATESPACE

ODS Table Name Description Option

NObs Number of observations default

Summary Simple summary statistics table default

InfoCriterion Information criterion table default

CovLags Covariance Matrices of Input Series PRINTOUT=LONG

CorrLags Correlation Matrices of Input Series PRINTOUT=LONG

PartialAR Partial Autoregressive Matrices PRINTOUT=LONG

YWEstimates Yule-Walker Estimates for Minimum AIC default

CovResiduals Covariance of Residuals PRINTOUT=LONGCorrResiduals Residual Correlations from AR Models PRINTOUT=LONG

StateVector State vector table default

CorrGraph Schematic Representation of Correlations default

TransitionMatrix Transition Matrix default

InputMatrix Input Matrix default

VarInnov Variance Matrix for the Innovation default

CovB Covariance of Parameter Estimates COVB

CorrB Correlation of Parameter Estimates COVB

CanCorr Canonical Correlation Analysis CANCORR

IterHistory Iterative Fitting table ITPRINT

ParameterEstimates Parameter Estimates Table default

Forecasts Forecasts Table PRINT

ConvergenceStatus Convergence Status Table default

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Part 2 General Information

Example

Example 181 Series J from Box and Jenkins

This example analyzes the gas furnace data (series J) from Box and Jenkins (The

data are not shown Refer to Box and Jenkins (1976) for the data)

First a model is selected and fit automatically using the following statements

title1 rsquoGas Furnace Datarsquo

title2 rsquoBox amp Jenkins Series Jrsquo

title3 rsquoAutomatically Selected Modelrsquo

proc statespace data=seriesj cancorr

var x y

run

The results for the automatically selected model are shown in Output 1811

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Chapter 18 Example

Output 1811 Results for Automatically Selected Model

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Number of Observations 296

Standard

Variable Mean Error

x -005683 1072766

y 5350912 3202121

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Information Criterion for Autoregressive Models

Lag=0 Lag=1 Lag=2 Lag=3 Lag=4 Lag=5 Lag=6 Lag=7 Lag=8

6513862 -103357 -163296 -164512 -165152 -164891 -164934 -164315 -163856

Information

Criterion for

Autoregressive

Models

Lag=9 Lag=10

-16348 -163359

Schematic Representation of Correlations

NameLag 0 1 2 3 4 5 6 7 8 9 10

x +- +- +- +- +- +- +- +- +- +- +-

y -+ -+ -+ -+ -+ -+ -+ -+ -+ -+ -+

+ is gt 2std error - is lt -2std error is between

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Part 2 General Information

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Schematic Representation of Partial Autocorrelations

NameLag 1 2 3 4 5 6 7 8 9 10

x + - + -

y -+ -- - + +

+ is gt 2std error - is lt -2std error is between

Yule-Walker Estimates for Minimum AIC

------Lag=1------ ------Lag=2------ ------Lag=3------ ------Lag=4------

x y x y x y x y

x 1925887 -000124 -120166 0004224 0116918 -000867 0104236 0003268

y 0050496 1299793 -002046 -03277 -071182 -025701 0195411 0133417

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Canonical Correlations Analysis

Information Chi

x(TT) y(TT) x(T+1T) Criterion Square DF

1 1 0804883 2929228 3047481 8

Information Chix(TT) y(TT) x(T+1T) y(T+1T) Criterion Square DF

1 1 0906681 0607529 1223358 1347237 7

Information Chi

x(TT) y(TT) x (T+1T) y (T+1T) x (T+2T) Criterion Square DF

1 1 0909434 0610278 0186274 -154701 1034705 6

Information Chi

x(TT) y(TT) x (T+1T) y (T+1T) y (T+2T) Criterion Square DF

1 1 091014 0618937 0206823 0940392 1280924 6

Information Chi

x(TT) y(TT) x(T+1T) y(T+1T) y(T+2T) y(T+3T) Criterion Square DF

1 1 0912963 0628785 0226598 0083258 -794103 2041584 5

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Chapter 18 Example

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Selected Statespace Form and Preliminary Estimates

State Vector

x(TT) y(TT) x(T+1T) y(T+1T) y(T+2T)

Estimate of Transition Matrix

0 0 1 0 0

0 0 0 1 0

-084718 0026794 1711715 -005019 0

0 0 0 0 1

-019785 0334274 -018174 -123557 1787475

Input Matrix for Innovation

1 0

0 11925887 -000124

0050496 1299793

0142421 1361696

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Selected Statespace Form and Preliminary Estimates

Variance Matrix for Innovation

0035274 -000734

-000734 0097569

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Part 2 General Information

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Selected Statespace Form and Fitted Model

State Vector

x(TT) y(TT) x(T+1T) y(T+1T) y(T+2T)

Estimate of Transition Matrix

0 0 1 0 0

0 0 0 1 0

-086192 0030609 1724235 -005483 0

0 0 0 0 1

-034839 0292124 -009435 -109823 1671418

Input Matrix for Innovation

1 0

0 1192442 -000416

0015621 1258495

008058 1353204

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Selected Statespace Form and Fitted Model

Variance Matrix for Innovation

0035579 -000728

-000728 0095577

Parameter Estimates

Standard

Parameter Estimate Error t Value

F(31) -086192 0072961 -1181

F(32) 0030609 0026167 117

F(33) 1724235 0061599 2799

F(34) -005483 0030169 -182

F(51) -034839 0135253 -258

F(52) 0292124 0046299 631

F(53) -009435 0096527 -098F(54) -109823 0109525 -1003

F(55) 1671418 0083737 1996

G(31) 1924420 0058162 3309

G(32) -000416 0035255 -012

G(41) 0015621 0095771 016

G(42) 1258495 0055742 2258

G(51) 0080580 0151622 053

G(52) 1353204 0091388 1481

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Chapter 18 Example

The two series are believed to have a transfer function relation with the gas rate

(variable X) as the input and the CO2

concentration (variable Y) as the output Since

the parameter estimates shown in Output 1811 support this kind of model the model

is reestimated with the feedback parameters restricted to 0 The following statements

fit the transfer function (no feedback) model

title3 rsquoTransfer Function Modelrsquo

proc statespace data=seriesj printout=none

var x y

restrict f(32)=0 f(34)=0

g(32)=0 g(41)=0 g(51)=0

run

The last 2 pages of the output are shown in Output 1812

Output 1812 STATESPACE Output for Transfer Function Model

Gas Furnace Data

Box amp Jenkins Series J

Transfer Function Model

The STATESPACE Procedure

Selected Statespace Form and Fitted Model

State Vector

x(TT) y(TT) x(T+1T) y(T+1T) y(T+2T)

Estimate of Transition Matrix

0 0 1 0 0

0 0 0 1 0

-068882 0 1598717 0 0

0 0 0 0 1

-035944 0284179 -00963 -107313 1650047

Input Matrix for Innovation

1 0

0 1

1923446 0

0 1260856

0 1346332

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Part 2 General Information

Gas Furnace Data

Box amp Jenkins Series J

Transfer Function Model

The STATESPACE Procedure

Selected Statespace Form and Fitted Model

Variance Matrix for Innovation

0036995 -00072

-00072 0095712

Parameter Estimates

Standard

Parameter Estimate Error t Value

F(31) -068882 0050549 -1363

F(33) 1598717 0050924 3139

F(51) -035944 0229044 -157

F(52) 0284179 0096944 293

F(53) -009630 0140876 -068

F(54) -107313 0250385 -429

F(55) 1650047 0188533 875G(31) 1923446 0056328 3415

G(42) 1260856 0056464 2233

G(52) 1346332 0091086 1478

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Chapter 18 References

References

Akaike H (1974) Markovian Representation of Stochastic Processes and Its Ap-

plication to the Analysis of Autoregressive Moving Average Processes Annals

of the Institute of Statistical Mathematics 26 363-387

Akaike H (1976) Canonical Correlations Analysis of Time Series and the Use of

an Information Criterion in Advances and Case Studies in System Identification

eds R Mehra and DG Lainiotis New York Academic Press

Anderson TW (1971) The Statistical Analysis of Time Series New York John

Wiley amp Sons

Ansley CF and Newbold P (1979) Multivariate Partial Autocorrelations Pro-

ceedings of the Business and Economic Statistics Section American Statistical

Association 349-353

Box GEP and Jenkins G (1976) Time Series Analysis Forecasting and Control

San Francisco Holden-Day

Brockwell PJ and Davis RA (1991) Time Series Theory and Methods 2nd Edi-

tion Springer-Verlag

Hannan EJ (1970) Multiple Time Series New York John Wiley amp Sons

Hannan EJ (1976) The Identification and Parameterization of ARMAX and State

Space Forms Econometrica 44 713-722

Harvey AC (1981a) The Econometric Analysis of Time Series New York John

Wiley amp Sons

Harvey AC (1981b) Time Series Models New York John Wiley amp Sons

Jones RH (1974) Identification and Autoregressive Spectrum Estimation IEEE Transactions on Automatic Control AC-19 894-897

Pham-Dinh-Tuan (1978) On the Fitting of Multivariate Processes of the

Autoregressive-Moving Average Type Biometrika 65 99-107

Priestley MB (1980) System Identification Kalman Filtering and Stochastic Con-

trol in Directions in Time Series eds DR Brillinger and GC Tiao Institute

of Mathematical Statistics

Whittle P (1963) On the Fitting of Multivariate Autoregressions and the Approxi-

mate Canonical Factorization of a Spectral Density Matrix Biometrika 50 129-

134

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The correct bibliographic citation for this manual is as follows SAS Institute Inc SAS

ET S U serrsquos Gui de Version 8 Cary NC SAS Institute Inc 1999 1546 pp

SASET S Userrsquos Guide Version 8

Copyright copy 1999 by S AS I ns t it u t e I nc Ca ry N C U S A

ISBN 1ndash58025ndash489ndash6

All r ight s res erved P r int ed in t he U n it ed St a t es of A merica N o pa rt of t his p u blica t ionma y b e reprodu ced s t ored in a ret r ieva l s ys t em or t ra ns mit t ed in a ny form or b y a nymeans electronic mechanical photocopying or otherwise without the prior writtenpermission of the publisher SAS Institute Inc

US Government Restricted Rights Notice Use du plicat ion or disclosure of the

software by the government is subject to restrictions as set forth in FAR 52227ndash19Commercial Computer Softw ar e-Restricted Rights (J une 1987)

SAS Institute Inc SAS Campus Drive Cary North Carolina 27513

1st print ing October 1999

S AS reg a nd a ll ot her SA S I ns t it u t e I nc p rodu ct or s ervice na m es a re regis t ered t ra dema rkst d k f SA S I t it t I i t h U S A d t h t i reg i di t U SA

Page 3: Statespace Model

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Chapter 18

The STATESPACE Procedure

Overview

The STATESPACE procedure analyzes and forecasts multivariate time series using

the state space model The STATESPACE procedure is appropriate for jointly fore-

casting several related time series that have dynamic interactions By taking into

account the autocorrelations among the whole set of variables the STATESPACE

procedure may give better forecasts than methods that model each series separately

By default the STATESPACE procedure automatically selects a state space model

appropriate for the time series making the procedure a good tool for automatic fore-

casting of multivariate time series Alternatively you can specify the state spacemodel by giving the form of the state vector and the state transition and innovation

matrices

The methods used by the STATESPACE procedure assume that the time series are

jointly stationary Nonstationary series must be made stationary by some prelimi-

nary transformation usually by differencing The STATESPACE procedure allows

you to specify differencing of the input data When differencing is specified the

STATESPACE procedure automatically integrates forecasts of the differenced series

to produce forecasts of the original series

The State Space Model

The state space model represents a multivariate time series through auxiliary vari-ables some of which may not be directly observable These auxiliary variables are

called the state vector The state vector summarizes all the information from the

present and past values of the time series relevant to the prediction of future values

of the series The observed time series are expressed as linear combinations of the

state variables The state space model is also called a Markovian representation or

a canonical representation of a multivariate time series process The state space ap-

proach to modeling a multivariate stationary time series is summarized in Akaike

(1976)

The state space form encompasses a very rich class of models Any Gaussian multi-

variate stationary time series can be written in a state space form provided that the

dimension of the predictor space is finite In particular any autoregressive movingaverage (ARMA) process has a state space representation and conversely any state

space process can be expressed in an ARMA form (Akaike 1974) More details on

the relation of the state space and ARMA forms are given in Relation of ARMA and

State Space Forms later in this chapter

Letx

t

be ther 1

vector of observed variables after differencing (if differencing is

specified) and subtracting the sample mean Letz

t

be the state vector of dimension

s s

r where the first r components of z

t

consist of x

t

Let the notationx

t + k j t

999

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Part 2 General Information

represent the conditional expectation (or prediction) of x

t + k

based on the information

available at time t Then the lasts r

elements of z

t

consist of elements of xt + k j t

where k gt0 is specified or determined automatically by the procedure

There are various forms of the state space model in use The form of the state space

model used by the STATESPACE procedure is based on Akaike (1976) The model

is defined by the following state transition equation

z

t + 1

= F z

t

+ G e

t + 1

In the state transition equation thes s

coefficient matrix F is called the transition

matrix it determines the dynamic properties of the model

Thes r

coefficient matrix G is called the input matrix it determines the variance

structure of the transition equation For model identification the first r rows and

columns of G are set to anr r

identity matrix

The input vector et

is a sequence of independent normally distributed random vectors

of dimension r with mean 0 and covariance matrix

e e

The random error et

is

sometimes called the innovation vector or shock vector

In addition to the state transition equation state space models usually include a mea-

surement equation or observation equation that gives the observed valuesx

t

as a

function of the state vectorz

t

However since PROC STATESPACE always includes

the observed valuesx

t

in the state vectorz

t

the measurement equation in this case

merely represents the extraction of the first r components of the state vector

The measurement equation used by the STATESPACE procedure is

x

t

= I

r

0 z

t

whereI

r

is anr r

identity matrix In practice PROC STATESPACE performs the

extraction of x

t

fromz

t

without reference to an explicit measurement equation

In summary

xt

is an observation vector of dimension r

zt

is a state vector of dimension s whose first r elements are xt

and

whose lasts r

elements are conditional prediction of future xt

F is ans s

transition matrix

G is ans r

input matrix with the identity matrix Ir

forming the first

r rows and columns

et

is a sequence of independent normally distributed random vectors

of dimension r with mean 0 and covariance matrix

e e

How PROC STATESPACE Works

The design of the STATESPACE procedure closely follows the modeling strategy

proposed by Akaike (1976) This strategy employs canonical correlation analysis for

the automatic identification of the state space model

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Chapter 18 Overview

Following Akaike (1976) the procedure first fits a sequence of unrestricted vector

autoregressive (VAR) models and computes Akaikersquos information criterion (AIC) for

each model The vector autoregressive models are estimated using the sample au-

tocovariance matrices and the Yule-Walker equations The order of the VAR model

producing the smallest Akaike information criterion is chosen as the order (number

of lags into the past) to use in the canonical correlation analysis

The elements of the state vector are then determined via a sequence of canonical cor-

relation analyses of the sample autocovariance matrices through the selected order

This analysis computes the sample canonical correlations of the past with an increas-

ing number of steps into the future Variables that yield significant correlations are

added to the state vector those that yield insignificant correlations are excluded from

further consideration The importance of the correlation is judged on the basis of

another information criterion proposed by Akaike See the section Canonical Cor-

relation Analysis for details If you specify the state vector explicitly these model

identification steps are omitted

Once the state vector is determined the state space model is fit to the data The free

parameters in the F G and

e e

matrices are estimated by approximate maximumlikelihood By default the

F

andG

matrices are unrestricted except for identifia-

bility requirements Optionally conditional least-squares estimates can be computed

You can impose restrictions on elements of theF

andG

matrices

After the parameters are estimated forecasts are produced from the fitted state space

model using the Kalman filtering technique If differencing was specified the fore-

casts are integrated to produce forecasts of the original input variables

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Part 2 General Information

Getting Started

The following introductory example uses simulated data for two variables X and Y

The following statements generate the X and Y series

data inx=10 y=40

x1=0 y1=0

a1=0 b1=0

iseed=123

do t=-100 to 200

a=rannor(iseed)

b=rannor(iseed)

dx = 05x1 + 03y1 + a - 02a1 - 01b1

dy = 03x1 + 05y1 + b

x = x + d x + 2 5

y = y + d y + 2 5

if t gt= 0 then output

x1 = dx y1 = dy

a 1 = a b 1 = b

end

keep t x y

run

The simulated series X and Y are shown in Figure 181

Figure 181 Example Series

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Chapter 18 Getting Started

Automatic State Space Model Selection

The STATESPACE procedure is designed to automatically select the best state space

model for forecasting the series You can specify your own model if you wish and

you can use the output from PROC STATESPACE to help you identify a state space

model However the easiest way to use PROC STATESPACE is to let it choose the

model

Stationarity and Differencing

Although PROC STATESPACE selects the state space model automatically it does

assume that the input series are stationary If the series are nonstationary then the

process may fail Therefore the first step is to examine your data and test to see if

differencing is required (See the section Stationarity and Differencing later in this

chapter for further discussion of this issue)

The series shown in Figure 181 are nonstationary In order to forecast X and Y

with a state space model you must difference them (or use some other de-trending

method) If you fail to difference when needed and try to use PROC STATESPACE

with nonstationary data an inappropriate state space model may be selected and themodel estimation may fail to converge

The following statements identify and fit a state space model for the first differences

of X and Y and forecast X and Y 10 periods ahead

proc statespace data=in out=out lead=10

var x(1) y(1)

id t

run

The DATA= option specifies the input data set and the OUT= option specifies theoutput data set for the forecasts The LEAD= option specifies forecasting 10 obser-

vations past the end of the input data The VAR statement specifies the variables to

forecast and specifies differencing The notation X(1) Y(1) specifies that the state

space model analyzes the first differences of X and Y

Descriptive Statistics and Preliminary Autoregressions

The first page of the printed output produced by the preceding statements is shown in

Figure 182

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Part 2 General Information

The STATESPACE Procedure

Number of Observations 200

Standard

Variable Mean Error

x 0144316 1233457 Has been differenced With period(s) = 1

y 0164871 1304358 Has been differenced

With period(s) = 1

The STATESPACE Procedure

Information Criterion for Autoregressive Models

Lag=0 Lag=1 Lag=2 Lag=3 Lag=4 Lag=5 Lag=6 Lag=7 Lag=8

149697 8387786 5517099 1205986 1536952 2179538 2400638 2988874 3355708

Information

Criterion for Autoregressive

Models

Lag=9 Lag=10

4117606 4770222

Schematic Representation of Correlations

NameLag 0 1 2 3 4 5 6 7 8 9 10

x ++ ++ ++ ++ ++ ++ + + +

y ++ ++ ++ ++ ++ + + + +

+ is gt 2std error - is lt -2std error is between

Figure 182 Descriptive Statistics and VAR Order Selection

Descriptive statistics are printed first giving the number of nonmissing observations

after differencing and the sample means and standard deviations of the differenced

series The sample means are subtracted before the series are modeled (unless the

NOCENTER option is specified) and the sample means are added back when the

forecasts are produced

LetX

t

andY

t

be the observed values of X and Y and letx

t

andy

t

be the values of X

and Y after differencing and subtracting the mean difference The seriesx

t

modeled

by the STATEPSPACE procedure is

x

t

=

x

t

y

t

=

1 B X

t

0 1 4 4 3 1 6

1 B Y

t

0 1 6 4 8 7 1

where B represents the backshift operator

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Chapter 18 Getting Started

After the descriptive statistics PROC STATESPACE prints the Akaike information

criterion (AIC) values for the autoregressive models fit to the series The smallest AIC

value in this case 5517 at lag 2 determines the number of autocovariance matrices

analyzed in the canonical correlation phase

A schematic representation of the autocorrelations is printed next This indicates

which elements of the autocorrelation matrices at different lags are significantlygreater or less than 0

The second page of the STATESPACE printed output is shown in Figure 183

The STATESPACE Procedure

Schematic Representation of Partial Autocorrelations

NameLag 1 2 3 4 5 6 7 8 9 10

x ++ +

y ++

+ is gt 2std error - is lt -2std error is between

Yule-Walker Estimates for Minimum AIC

--------Lag=1------- --------Lag=2-------

x y x y

x 0257438 0202237 0170812 0133554

y 0292177 0469297 -000537 -000048

Figure 183 Partial Autocorrelations and VAR Model

Figure 183 shows a schematic representation of the partial autocorrelations similar

to the autocorrelations shown in Figure 182 The selection of a second order autore-

gressive model by the AIC statistic looks reasonable in this case because the partialautocorrelations for lags greater than 2 are not significant

Next the Yule-Walker estimates for the selected autoregressive model are printed

This output shows the coefficient matrices of the vector autoregressive model at each

lag

Selected State Space Model Form and Preliminary Estimates

After the autoregressive order selection process has determined the number of lags to

consider the canonical correlation analysis phase selects the state vector By default

output for this process is not printed You can use the CANCORR option to print

details of the canonical correlation analysis See the section Canonical Correlation

Analysis later in this chapter for an explanation of this process

Once the state vector is selected the state space model is estimated by approximate

maximum likelihood Information from the canonical correlation analysis and from

the preliminary autoregression is used to form preliminary estimates of the state space

model parameters These preliminary estimates are used as starting values for the

iterative estimation process

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Part 2 General Information

The form of the state vector and the preliminary estimates are printed next as shown

in Figure 184

The STATESPACE Procedure

Selected Statespace Form and Preliminary Estimates

State Vector

x(TT) y(TT) x(T+1T)

Estimate of Transition Matrix

0 0 1

0291536 0468762 -000411

024869 024484 0204257

Input Matrix for Innovation

1 0

0 1

0257438 0202237

Variance Matrix for Innovation

0945196 0100786

0100786 1014703

Figure 184 Preliminary Estimates of State Space Model

Figure 184 first prints the state vector as X[TT] Y[TT] X[T+1T] This notation

indicates that the state vector is

z

t

=

2

4

x

t j t

y

t j t

x

t + 1 j t

3

5

The notationx

t + 1 j t

indicates the conditional expectation or prediction of x

t + 1

based

on the information available at time t andx

t j t

andy

t j t

arex

t

andy

t

respectively

The remainder of Figure 184 shows the preliminary estimates of the transition matrix

F

the input matrixG

and the covariance matrix

e e

Estimated State Space Model

The next page of the STATESPACE output prints the final estimates of the fittedmodel as shown in Figure 185 This output has the same form as in Figure 184 but

shows the maximum likelihood estimates instead of the preliminary estimates

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Chapter 18 Getting Started

The STATESPACE Procedure

Selected Statespace Form and Fitted Model

State Vector

x(TT) y(TT) x(T+1T)

Estimate of Transition Matrix

0 0 1

0297273 047376 -001998

02301 0228425 0256031

Input Matrix for Innovation

1 0

0 1

0257284 0202273

Variance Matrix for Innovation

0945188 01007520100752 1014712

Figure 185 Fitted State Space Model

The estimated state space model shown in Figure 185 is

2

4

x

t + 1 j t + 1

y

t + 1 j t + 1

x

t + 2 j t + 1

3

5

=

2

4

0 0 1

0 2 9 7 0 4 7 4 0 0 2 0

0 2 3 0 0 2 2 8 0 2 5 6

3

5

2

4

x

t

y

t

x

t + 1 j t

3

5

+

2

4

1 0

0 1

0 2 5 7 0 2 0 2

3

5

e

t + 1

n

t + 1

v a r

e

t + 1

n

t + 1

=

0 9 4 5 0 1 0 1

0 1 0 1 1 0 1 5

The next page of the STATESPACE output lists the estimates of the free parameters

in theF

andG

matrices with standard errors and t statistics as shown in Figure 186

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Part 2 General Information

The STATESPACE Procedure

Parameter Estimates

Standard

Parameter Estimate Error t Value

F(21) 0297273 0129995 229

F(22) 0473760 0115688 410F(23) -001998 0313025 -006

F(31) 0230100 0126226 182

F(32) 0228425 0112978 202

F(33) 0256031 0305256 084

G(31) 0257284 0071060 362

G(32) 0202273 0068593 295

Figure 186 Final Parameter Estimates

Convergence Failures

The maximum likelihood estimates are computed by an iterative nonlinear maximiza-

tion algorithm which may not converge If the estimates fail to converge warning

messages are printed in the output

If you encounter convergence problems you should recheck the stationarity of the

data and ensure that the specified differencing orders are correct Attempting to fit

state space models to nonstationary data is a common cause of convergence failure

You can also use the MAXIT= option to increase the number of iterations allowed

or experiment with the convergence tolerance options DETTOL= and PARMTOL=

Forecast Data Set

The following statements print the output data set The WHERE statement excludes

the first 190 observations from the output so that only the forecasts and the last 10

actual observations are printed

proc print data=outid t

where t gt 190

run

The PROC PRINT output is shown in Figure 187

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Part 2 General Information

Figure 188 Plot of Forecasts

Controlling Printed Output

By default the STATESPACE procedure produces a large amount of printed output

The NOPRINT option suppresses all printed output You can suppress the printed

output for the autoregressive model selection process with the PRINTOUT=NONE

option The descriptive statistics and state space model estimation output are still

printed when PRINTOUT=NONE is specified You can produce more detailed output

with the PRINTOUT=LONG option and by specifying the printing control optionsCANCORR COVB and PRINT

Specifying the State Space Model

Instead of allowing the STATESPACE procedure to select the model automatically

you can use FORM and RESTRICT statements to specify a state space model

Specifying the State Vector

Use the FORM statement to control the form of the state vector You can use this

feature to force PROC STATESPACE to estimate and forecast a model different from

the model it would select automatically You can also use this feature to reestimate

the automatically selected model (possibly with restrictions) without repeating thecanonical correlation analysis

The FORM statement specifies the number of lags of each variable to include in

the state vector For example the statement FORM X 3 forces the state vector to

includex

t j t

x

t + 1 j t

andx

t + 2 j t

The following statement specifies the state vector

x

t j t

y

t j t

x

t + 1 j t

which is the same state vector selected in the preceding example

f o r m x 2 y 1

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Chapter 18 Getting Started

You can specify the form for only some of the variables and allow PROC STATES-

PACE to select the form for the other variables If only some of the variables are

specified in the FORM statement canonical correlation analysis is used to determine

the number of lags included in the state vector for the remaining variables not spec-

ified by the FORM statement If the FORM statement includes specifications for all

the variables listed in the VAR statement the state vector is completely defined and

the canonical correlation analysis is not performed

Restricting the F and G matrices

After you know the form of the state vector you can use the RESTRICT statement

to fix some parameters in theF

andG

matrices to specified values One use of this

feature is to remove insignificant parameters by restricting them to 0

In the introductory example shown in the preceding section the F[23] parameter is

not significant (The parameters estimation output shown in Figure 186 gives the t

statistic for F[23] as -006 F[33] and F[31] also have low significance witht 2

)

The following statements reestimate this model with F[23] restricted to 0 The

FORM statement is used to specify the state vector and thus bypass the canonicalcorrelation analysis

proc statespace data=in out=out lead=10

var x(1) y(1)

id t

form x 2 y 1

restrict f(23)=0

run

The final estimates produced by these statements are shown in Figure 189

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Part 2 General Information

The STATESPACE Procedure

Selected Statespace Form and Fitted Model

State Vector

x(TT) y(TT) x(T+1T)

Estimate of Transition Matrix

0 0 1

0290051 0467468 0

0227051 0226139 026436

Input Matrix for Innovation

1 0

0 1

0256826 0202022

Variance Matrix for Innovation

0945175 01006960100696 1014733

The STATESPACE Procedure

Parameter Estimates

Standard

Parameter Estimate Error t Value

F(21) 0290051 0063904 454

F(22) 0467468 0060430 774

F(31) 0227051 0125221 181

F(32) 0226139 0111711 202

F(33) 0264360 0299537 088G(31) 0256826 0070994 362

G(32) 0202022 0068507 295

Figure 189 Results using RESTRICT Statement

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Chapter 18 Syntax

Syntax

The STATESPACE procedure uses the following statements

PROC STATESPACE options

BY variable FORM variable value

ID variable

INITIAL F(rowcolumn)=value

G(rowcolumn)=value

RESTRICT F(rowcolumn)=value

G(rowcolumn)=value

VAR variable (difference difference

)

Functional Summary

The statements and options used by PROC STATESPACE are summarized in the

following table

Description Statement Option

Input Data Set Options

specify the input data set PROC STATESPACE DATA=

prevent subtraction of sample mean PROC STATESPACE NOCENTER

specify the ID variable ID

specify the observed series and differencing VAR

Options for Autoregressive Estimates

specify the maximum order PROC STATESPACE ARMAX=

specify maximum lag for autocovariances PROC STATESPACE LAGMAX=

output only minimum AIC model PROC STATESPACE MINIC

specify the amount of detail printed PROC STATESPACE PRINTOUT=

write preliminary AR models to a data set PROC STATESPACE OUTAR=

Options for Canonical Correlation Analysis

print the sequence of canonical correlations PROC STATESPACE CANCORR

specify upper limit of dimension of state

vector

PROC STATESPACE DIMMAX=

specify the minimum number of lags PROC STATESPACE PASTMIN=

specify the multiplier of the degrees of

freedom

PROC STATESPACE SIGCORR=

Options for State Space Model Estimation

specify starting values INITIAL

print covariance matrix of parameter estimates PROC STATESPACE COVB

specify the convergence criterion PROC STATESPACE DETTOL=

specify the convergence criterion PROC STATESPACE PARMTOL=

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Part 2 General Information

Description Statement Option

print the details of the iterations PROC STATESPACE ITPRINT

specify an upper limit of the number of lags PROC STATESPACE KLAG=

specify maximum number of iterations

allowed

PROC STATESPACE MAXIT=

suppress the final estimation PROC STATESPACE NOEST

write the state space model parameter esti-

mates to an output data set

PROC STATESPACE OUTMODEL=

use conditional least squares for final estimates PROC STATESPACE RESIDEST

specify criterion for testing for singularity PROC STATESPACE SINGULAR=

Options for Forecasting

start forecasting before end of the input data PROC STATESPACE BACK=

specify the time interval between observations PROC STATESPACE INTERVAL=

specify multiple periods in the time series PROC STATESPACE INTPER=specify how many periods to forecast PROC STATESPACE LEAD=

specify the output data set for forecasts PROC STATESPACE OUT=

print forecasts PROC STATESPACE PRINT

Options to Specify the State Space Model

specify the state vector FORM

specify the parameter values RESTRICT

BY Groups

specify BY-group processing BY

Printing

suppresses all printed output NOPRINT

PROC STATESPACE Statement

PROC STATESPACE options

The following options can be specified in the PROC STATESPACE statement

Printing Options NOPRINT

suppresses all printed output

Input Data Options

DATA= SAS-data-set

specifies the name of the SAS data set to be used by the procedure If the DATA=

option is omitted the most recently created SAS data set is used

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Chapter 18 Syntax

LAGMAX= k

specifies the number of lags for which the sample autocovariance matrix is computed

The LAGMAX= option controls the number of lags printed in the schematic repre-

sentation of the autocorrelations

The sample autocovariance matrix of lag i denoted asC

i

is computed as

C

i

=

1

N 1

N

X

t = 1 + i

x

t

x

0

t i

wherex

t

is the differenced and centered data andN

is the number of observations

(If the NOCENTER option is specified 1 is not subtracted fromN

) LAGMAX= k

specifies thatC

0

throughC

k

are computed The default is LAGMAX=10

NOCENTER

prevents subtraction of the sample mean from the input series (after any specified

differencing) before the analysis

Options for Preliminary Autoregressive Models ARMAX= n

specifies the maximum order of the preliminary autoregressive models The AR-

MAX= option controls the autoregressive orders for which information criteria are

printed and controls the number of lags printed in the schematic representation of

partial autocorrelations The default is ARMAX=10 See Preliminary Autoregres-

sive Models later in this chapter for details

MINIC

writes to the OUTAR= data set only the preliminary Yule-Walker estimates for the

VAR model producing the minimum AIC See OUTAR= Data Set later in this chap-

ter for details

OUTAR= SAS-data-set

writes the Yule-Walker estimates of the preliminary autoregressive models to a SAS

data set See OUTAR= Data Set later in this chapter for details

PRINTOUT= SHORT | LONG | NONE

determines the amount of detail printed PRINTOUT=LONG prints the lagged co-

variance matrices the partial autoregressive matrices and estimates of the resid-

ual covariance matrices from the sequence of autoregressive models PRINT-

OUT=NONE suppresses the output for the preliminary autoregressive models The

descriptive statistics and state space model estimation output are still printed when

PRINTOUT=NONE is specified PRINTOUT=SHORT is the default

Canonical Correlation Analysis Options

CANCORR

prints the canonical correlations and information criterion for each candidate state

vector considered See Canonical Correlation Analysis later in this chapter for

details

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Part 2 General Information

DIMMAX= n

specifies the upper limit to the dimension of the state vector The DIMMAX= option

can be used to limit the size of the model selected The default is DIMMAX=10

PASTMIN= n

specifies the minimum number of lags to include in the canonical correlation analy-

sis The default is PASTMIN=0 See Canonical Correlation Analysis later in thischapter for details

SIGCORR= value

specifies the multiplier of the degrees of freedom for the penalty term in the informa-

tion criterion used to select the state space form The default is SIGCORR=2 The

larger the value of the SIGCORR= option the smaller the state vector tends to be

Hence a large value causes a simpler model to be fit See Canonical Correlations

Analysis later in this chapter for details

State Space Model Estimation Options

COVB

prints the inverse of the observed information matrix for the parameter estimatesThis matrix is an estimate of the covariance matrix for the parameter estimates

DETTOL= value

specifies the convergence criterion The DETTOL= and PARMTOL= option values

are used together to test for convergence of the estimation process If during an

iteration the relative change of the parameter estimates is less than the PARMTOL=

value and the relative change of the determinant of the innovation variance matrix

is less than the DETTOL= value then iteration ceases and the current estimates are

accepted The default is DETTOL=1E-5

ITPRINT

prints the iterations during the estimation process

KLAG= n

sets an upper limit for the number of lags of the sample autocovariance matrix used

in computing the approximate likelihood function If the data have a strong moving

average character a larger KLAG= value may be necessary to obtain good estimates

The default is KLAG=15 See Parameter Estimation later in this chapter for details

MAXIT= n

sets an upper limit to the number of iterations in the maximum likelihood or condi-

tional least-squares estimation The default is MAXIT=50

NOEST

suppresses the final maximum likelihood estimation of the selected model

OUTMODEL= SAS-data-set

writes the parameter estimates and their standard errors to a SAS data set See OUT-

MODEL= Data Set later in this chapter for details

PARMTOL= value

specifies the convergence criterion The DETTOL= and PARMTOL= option values

are used together to test for convergence of the estimation process If during an

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Chapter 18 Syntax

iteration the relative change of the parameter estimates is less than the PARMTOL=

value and the relative change of the determinant of the innovation variance matrix

is less than the DETTOL= value then iteration ceases and the current estimates are

accepted The default is PARMTOL=001

RESIDEST

computes the final estimates using conditional least squares on the raw data Thistype of estimation may be more stable than the default maximum likelihood method

but is usually more computationally expensive See Parameter Estimation later in

this chapter for details of the conditional least squares method

SINGULAR= value

specifies the criterion for testing for singularity of a matrix A matrix is declared

singular if a scaled pivot is less than the SINGULAR= value when sweeping the

matrix The default is SINGULAR=1E-7

Forecasting Options

BACK= n

starts forecasting n periods before the end of the input data The BACK= option valuemust not be greater than the number of observations The default is BACK=0

INTERVAL= interval

specifies the time interval between observations The INTERVAL= value is used

in conjunction with the ID variable to check that the input data are in order and

have no missing periods The INTERVAL= option is also used to extrapolate the ID

values past the end of the input data See Chapter 3 ldquoDate Intervals Formats and

Functionsrdquo for details on the INTERVAL= values allowed

INTPER= n

specifies that each input observation corresponds to n time periods For example

the options INTERVAL=MONTH and INTPER=2 specify bimonthly data and are

equivalent to specifying INTERVAL=MONTH2 If the INTERVAL= option is not

specified the INTPER= option controls the increment used to generate ID values for

the forecast observations The default is INTPER=1

LEAD= n

specifies how many forecast observations are produced The forecasts start at the

point set by the BACK= option The default is LEAD=0 which produces no fore-

casts

OUT= SAS-data-set

writes the residuals actual values forecasts and forecast standard errors to a SAS

data set See OUT= Data Set later in this chapter for details

PRINT

prints the forecasts

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Part 2 General Information

BY Statement

BY variable

A BY statement can be used with the STATESPACE procedure to obtain separateanalyses on observations in groups defined by the BY variables

FORM Statement

FORM variable value

The FORM statement specifies the number of times a variable is included in the state

vector Values can be specified for any variable listed in the VAR statement If a

value is specified for each variable in the VAR statement the state vector for the state

space model is entirely specified and automatic selection of the state space model is

not performed

The FORM statement forces the state vectorz

t

to contain a specific variable a given

number of times For example if Y is one of the variables inx

t

then the statement

form y 3

forces the state vector to containY

t

Y

t + 1 j t

andY

t + 2 j t

possibly along with other

variables

The following statements illustrate the use of the FORM statement

proc statespace data=in

var x y

f o r m x 3 y 2

run

These statements fit a state space model with the following state vector

z

t

=

2

6

6

6

6

4

x

t j t

y

t j t

x

t + 1 j t

y

t + 1 j t

x

t + 2 j t

3

7

7

7

7

5

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Chapter 18 Syntax

ID Statement

ID variable

The ID statement specifies a variable that identifies observations in the input data setThe variable specified in the ID statement is included in the OUT= data set The

values of the ID variable are extrapolated for the forecast observations based on the

values of the INTERVAL= and INTPER= options

INITIAL Statement

INITIAL F (rowcolumn)= value G(row column)= value

The INITIAL statement gives initial values to the specified elements of theF

andG

matrices These initial values are used as starting values for the iterative estimation

Parts of theF

andG

matrices represent fixed structural identities If an element

specified is a fixed structural element instead of a free parameter the corresponding

initialization is ignored

The following is an example of an INITIAL statement

initial f(32)=0 g(41)=0 g(51)=0

RESTRICT Statement

RESTRICT F(rowcolumn)= value G(rowcolumn)= value

The RESTRICT statement restricts the specified elements of theF

andG

matrices

to the specified values

To use the restrict statement you need to know the form of the model Either specify

the form of the model with the FORM statement or do a preliminary run perhaps

with the NOEST option to find the form of the model that PROC STATESPACE

selects for the data

The following is an example of a RESTRICT statement

restrict f(32)=0 g(41)=0 g(51)=0

Parts of theF

andG

matrices represent fixed structural identities If a restriction is

specified for an element that is a fixed structural element instead of a free parameter

the restriction is ignored

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Part 2 General Information

VAR Statement

VAR variable (difference difference )

The VAR statement specifies the variables in the input data set to model and fore-cast The VAR statement also specifies differencing of the input variables The VAR

statement is required

Differencing is specified by following the variable name with a list of difference

periods separated by commas See the section Stationarity and Differencing for

more information on differencing of input variables

The order in which variables are listed in the VAR statement controls the order in

which variables are included in the state vector Usually potential inputs should be

listed before potential outputs

For example assuming the input data are monthly the following VAR statement

specifies modeling and forecasting of the one period and seasonal second differenceof X and Y

var x(112) y(112)

In this example the vector time series analyzed is

x

t

=

1 B 1 B

1 2

X

t

x

1 B 1 B

1 2

Y

t

y

where B represents the back shift operator and x and y represent the means of thedifferenced series If the NOCENTER option is specified the mean differences are

not subtracted

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Chapter 18 Details

Details

Missing Values

The STATESPACE procedure does not support missing values The procedure uses

the first contiguous group of observations with no missing values for any of the VARstatement variables Observations at the beginning of the data set with missing values

for any VAR statement variable are not used or included in the output data set

Stationarity and Differencing

The state space model used by the STATESPACE procedure assumes that the time

series are stationary Hence the data should be checked for stationarity One way

to check for stationarity is to plot the series A graph of series over time can show a

time trend or variability changes

You can also check stationarity by using the sample autocorrelation functions dis-

played by the ARIMA procedure The autocorrelation functions of nonstationaryseries tend to decay slowly See Chapter 7 ldquoThe ARIMA Procedurerdquo for more in-

formation

Another alternative is to use the STATIONARITY= option on the IDENTIFY state-

ment in PROC ARIMA to apply Dickey-Fuller tests for unit roots in the time series

See Chapter 7 ldquoThe ARIMA Procedurerdquo for more information on Dickey-Fuller unit

root tests

The most popular way to transform a nonstationary series to stationarity is by dif-

ferencing Differencing of the time series is specified in the VAR statement For

example to take a simple first difference of the series X use this statement

var x(1)

In this example the change in X from one period to the next is analyzed When the se-

ries has a seasonal pattern differencing at a period equal to the length of the seasonal

cycle may be desirable For example suppose the variable X is measured quarterly

and shows a seasonal cycle over the year You can use the following statement to

analyze the series of changes from the same quarter in the previous year

var x(4)

To difference twice add another differencing period to the list For ex-

ample the following statement analyzes the series of second differences

X

t

X

t 1

X

t 1

X

t 2

= X

t

2 X

t 1

+ X

t 2

var x(11)

The following statement analyzes the seasonal second difference series

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Part 2 General Information

var x(14)

The series modeled is the 1-period difference of the 4-period difference

X

t

X

t 4

X

t 1

X

t 5

= X

t

X

t 1

X

t 4

+ X

t 5

Another way to obtain stationary series is to use a regression on time to de-trend the

data If the time series has a deterministic linear trend regressing the series on time

produces residuals that should be stationary The following statements write residuals

of X and Y to the variable RX and RY in the output data set DETREND

data a

set a

t=_n_

run

proc reg data=a

model x y = t

output out=detrend r=rx ry

run

You then use PROC STATESPACE to forecast the de-trended series RX and RY A

disadvantage of this method is that you need to add the trend back to the forecast

series in an additional step A more serious disadvantage of the de-trending method

is that it assumes a deterministic trend In practice most time series appear to have a

stochastic rather than a deterministic trend Differencing is a more flexible and often

more appropriate method

There are several other methods to handle nonstationary time series For more infor-

mation and examples refer to Brockwell and Davis (1991)

Preliminary Autoregressive Models

After computing the sample autocovariance matrices PROC STATESPACE fits a

sequence of vector autoregressive models These preliminary autoregressive models

are used to estimate the autoregressive order of the process and limit the order of the

autocovariances considered in the state vector selection process

Yule-Walker Equations for Forward and Backward Models

Unlike a univariate autoregressive model a multivariate autoregressive model has

different forms depending on whether the present observation is being predicted from

the past observations or from the future observations

Letx

t

be the r -component stationary time series given by the VAR statement after

differencing and subtracting the vector of sample means (If the NOCENTER option

is specified the mean is not subtracted) Let n be the number of observations of x

t

from the input data set

Lete

t

be a vector white noise sequence with mean vector 0 and variance matrix

p

and letn

t

be a vector white noise sequence with mean vector 0 and variance matrix

p

Let p be the order of the vector autoregressive model forx

t

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Chapter 18 Details

The forward autoregressive form based on the past observations is written as follows

x

t

=

p

X

i = 1

p

i

x

t i

+ e

t

The backward autoregressive form based on the future observations is written as fol-

lows

x

t

=

p

X

i = 1

p

i

x

t + i

+ n

t

LettingE

denote the expected value operator the autocovariance sequence for thex

t

series

i

is

i

= E x

t

x

0

t i

The Yule-Walker equations for the autoregressive model that matches the first p ele-

ments of the autocovariance sequence are

2

6

6

4

0

1

p 1

0

1

0

p 2

0

p 1

0

p 2

0

3

7

7

5

2

6

6

4

p

1

p

2

p

p

3

7

7

5

=

2

6

6

4

1

2

p

3

7

7

5

and

2

6

6

6

4

0

0

1

0

p 1

1

0

0

p 2

p 1

p 2

0

3

7

7

7

5

2

6

6

4

p

1

p

2

p

p

3

7

7

5

=

2

6

6

4

0

1

0

2

0

p

3

7

7

5

Here

p

i

are the coefficient matrices for the past observation form of the vector au-toregressive model and

p

i

are the coefficient matrices for the future observation

form More information on the Yule-Walker equations in the multivariate setting can

be found in Whittle (1963) and Ansley and Newbold (1979)

The innovation variance matrices for the two forms can be written as follows

p

=

0

p

X

i = 1

p

i

0

i

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Part 2 General Information

p

=

0

p

X

i = 1

p

i

i

The autoregressive models are fit to the data using the preceding Yule-Walker equa-

tions with

i

replaced by the sample covariance sequenceC

i

The covariance matri-

ces are calculated as

C

i

=

1

N 1

N

X

t = i + 1

x

t

x

0

t i

Let b

p

b

p

b

p

and b

p

represent the Yule-Walker estimates of

p

p

p

and

p

respectively These matrices are written to an output data set when the OUTAR=

option is specified

When the PRINTOUT=LONG option is specified the sequence of matrices b

p

and

the corresponding correlation matrices are printed The sequence of matricesb

p isused to compute Akaike information criteria for selection of the autoregressive order

of the process

Akaike Information Criterion

The Akaike information criterion or AIC is defined as -2( maximum of log likeli-

hood )+2(number of parameters) Since the vector autoregressive models are esti-

mates from the Yule-Walker equations not by maximum likelihood the exact likeli-

hood values are not available for computing the AIC However for the vector autore-

gressive model the maximum of the log likelihood can be approximated as

l n L

n

2

l n j

b

p

j

Thus the AIC for the order p model is computed as

A I C

p

= n l n j

b

p

j + 2 p r

2

You can use the printed AIC array to compute a likelihood ratio test of the autoregres-

sive order The log-likelihood ratio test statistic for testing the order p model against

the order p 1

model is

n l n j

b

p

j + n l n j

b

p 1

j

This quantity is asymptotically distributed as a

2 withr

2 degrees of freedom if the

series is autoregressive of order p 1

It can be computed from the AIC array as

A I C

p 1

A I C

p

+ 2 r

2

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You can evaluate the significance of these test statistics with the PROBCHI function

in a SAS DATA step or with a

2 table

Determining the Autoregressive Order

Although the autoregressive models can be used for prediction their primary value is

to aid in the selection of a suitable portion of the sample covariance matrix for use in

computing canonical correlations If the multivariate time series x

t is of autoregres-sive order p then the vector of past values to lag p is considered to contain essentially

all the information relevant for prediction of future values of the time series

By default PROC STATESPACE selects the order p producing the autoregressive

model with the smallestA I C

p

If the value p for the minimumA I C

p

is less than the

value of the PASTMIN= option then p is set to the PASTMIN= value Alternatively

you can use the ARMAX= and PASTMIN= options to force PROC STATESPACE to

use an order you select

Significance Limits for Partial Autocorrelations

The STATESPACE procedure prints a schematic representation of the partial autocor-

relation matrices indicating which partial autocorrelations are significantly greater orsignificantly less than 0 Figure 1810 shows an example of this table

The STATESPACE Procedure

Schematic Representation of Partial Autocorrelations

NameLag 1 2 3 4 5 6 7 8 9 10

x ++ +

y ++

+ is gt 2std error - is lt -2std error is between

Figure 1810 Significant Partial Autocorrelations

The partial autocorrelations are from the sample partial autoregressive matrices b

p

p

The standard errors used for the significance limits of the partial autocorrelations are

computed from the sequence of matrices

p

and

p

Under the assumption that the observed series arises from an autoregressive process

of order p 1

the pth sample partial autoregressive matrix b

p

p

has an asymptotic

variance matrix 1

n

1

p

p

The significance limits for b

p

p

used in the schematic plot of the sample partial autore-

gressive sequence are derived by replacing

p

and

p

with their sample estimators

to produce the variance estimate as follows

d

V a r

b

p

p

=

1

n r p

b

1

p

b

p

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Canonical Correlation Analysis

Given the order p letp

t

be the vector of current and past values relevant to prediction

of x

t + 1

p

t

= x

0

t

x

0

t 1

x

0

t p

0

Letf

t

be the vector of current and future values

f

t

= x

0

t

x

0

t + 1

x

0

t + p

0

In the canonical correlation analysis consider submatrices of the sample covariance

matrix of p

t

andf

t

This covariance matrixV

has a block Hankel form

V =

2

6

6

6

4

C

0

C

0

1

C

0

2

C

0

p

C

0

1

C

0

2

C

0

3

C

0

p + 1

C

0

p

C

0

p + 1

C

0

p + 2

C

0

2 p

3

7

7

7

5

State Vector Selection Process

The canonical correlation analysis forms a sequence of potential state vectorsz

j

t

Examine a sequencef

j

t

of subvectors of f

t

and form the submatrixV

j consisting

of the rows and columns of V

corresponding to the components of f

j

t

and compute

its canonical correlations

The smallest canonical correlation of V

j is then used in the selection of the com-

ponents of the state vector The selection process is described in the following For

more details about this process refer to Akaike (1976)

In the following discussion the notationx

t + k j t

denotes the wide sense conditional

expectation (best linear predictor) of x

t + k

given allx

s

with s less than or equal to t

In the notationx

i t + 1

the first subscript denotes the ith component of x

t + 1

The initial state vectorz

1

t

is set tox

t

The sequencef

j

t

is initialized by setting

f

1

t

= z

1

0

t

x

1 t + 1 j t

0

= x

0

t

x

1 t + 1 j t

0

That is start by considering whether to addx

1 t + 1 j t

to the initial state vectorz

1

t

The procedure forms the submatrixV

1 corresponding tof

1

t

and computes its canon-

ical correlations Denote the smallest canonical correlation of V

1 as

m i n

If

m i n

is

significantly greater than 0x

1 t + 1 j t

is added to the state vector

If the smallest canonical correlation of V

1 is not significantly greater than 0 then a

linear combination of f

1

t

is uncorrelated with the pastp

t

Assuming that the determi-

nant of C

0

is not 0 (that is no input series is a constant) you can take the coefficient

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of x

1 t + 1 j t

in this linear combination to be 1 Denote the coefficients of z

1

t

in this

linear combination as

This gives the relationship

x

1 t + 1 j t

=

0

x

t

Therefore the current state vector already contains all the past information useful forpredicting

x

1 t + 1

and any greater leads of x

1 t

The variablex

1 t + 1 j t

is not added to

the state vector nor are any termsx

1 t + k j t

considered as possible components of the

state vector The variablex

1

is no longer active for state vector selection

The process described forx

1 t + 1 j t

is repeated for the remaining elements of f

t

The

next candidate for inclusion in the state vector is the next component of f

t

corre-

sponding to an active variable Components of f

t

corresponding to inactive variables

that produced a zero

m i n

in a previous step are skipped

Denote the next candidate asx

l t + k j t

The vectorf

j

t

is formed from the current state

vector andx

l t + k j t

as follows

f

j

t

= z

j

0

t

x

l t + k j t

0

The matrixV

j is formed fromf

j

t

and its canonical correlations are computed The

smallest canonical correlation of V

j is judged to be either greater than or equal to 0

If it is judged to be greater than 0x

l t + k j t

is added to the state vector If it is judged

to be 0 then a linear combination of f

j

t

is uncorrelated with thep

t

and the variable

x

l

is now inactive

The state vector selection process continues until no active variables remain

Testing Significance of Canonical Correlations For each step in the canonical correlation sequence the significance of the small-

est canonical correlation

m i n

is judged by an information criterion from Akaike

(1976) This information criterion is

n l n 1

2

m i n

r p + 1 q + 1

where q is the dimension of f

j

t

at the current step r is the order of the state vector p

is the order of the vector autoregressive process and

is the value of the SIGCORR=

option The default is SIGCORR=2 If this information criterion is less than or equal

to 0

m i n

is taken to be 0 otherwise it is taken to be significantly greater than 0

(Do not confuse this information criterion with the AIC)

Variables inx

t + p j t

are not added in the model even with positive information crite-

rion because of the singularity of V

You can force the consideration of more candi-

date state variables by increasing the size of theV

matrix by specifying a PASTMIN=

option value larger than p

Printing the Canonical Correlations

To print the details of the canonical correlation analysis process specify the CAN-

CORR option in the PROC STATESPACE statement The CANCORR option prints

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the candidate state vectors the canonical correlations and the information criteria for

testing the significance of the smallest canonical correlation

Bartlettrsquos

2 and its degrees of freedom are also printed when the CANCORR option

is specified The formula used for Bartlettrsquos

2 is

2

= n 5 r p + 1 q + 1 l n 1

2

m i n

withr p + 1 q + 1

degrees of freedom

Figure 1811 shows the output of the CANCORR option for the introductory example

shown in the Getting Started section of this chapter

The STATESPACE Procedure

Canonical Correlations Analysis

Information Chi

x(TT) y(TT) x(T+1T) Criterion Square DF

1 1 0237045 3566167 114505 4

Information Chi

x(TT) y(TT) x(T+1T) y(T+1T) Criterion Square DF

1 1 0238244 0056565 -535906 0636134 3

Information Chi

x(TT) y(TT) x(T+1T) x(T+2T) Criterion Square DF

1 1 0237602 0087493 -446312 1525353 3

Figure 1811 Canonical Correlations Analysis

New variables are added to the state vector if the information criteria are positive Inthis example

y

t + 1 j t

andx

t + 2 j t

are not added to the state space vector because the

information criteria for these models are negative

If the information criterion is nearly 0 then you may want to investigate models

that arise if the opposite decision is made regarding

m i n

This investigation can be

accomplished by using a FORM statement to specify part or all of the state vector

Preliminary Estimates of F

When a candidate variablex

l t + k j t

yields a zero

m i n

and is not added to the state

vector a linear combination of f

j

t

is uncorrelated with thep

t

Because of the method

used to construct thef

j

t

sequence the coefficient of x

l t + k j t

inl

can be taken as 1

Denote the coefficients of z

j

t

in this linear combination as l

This gives the relationship

x

l t + k j t

= l

0

z

j

t

The vectorl

is used as a preliminary estimate of the first r columns of the row of the

transition matrixF

corresponding tox

l t + k 1 j t

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Parameter Estimation

The model isz

t + 1

= F z

t

+ G e

t + 1

wheree

t

is a sequence of independent multivari-

ate normal innovations with mean vector 0 and variance

e e

The observed sequence

x

t

composes the first r components of z

t

and thusx

t

= H z

t

where H is ther s

matrix I

r

0

LetE

be ther n

matrix of innovations

E = e

1

e

n

If the number of observations n is reasonably large the log likelihood L can be

approximated up to an additive constant as follows

L =

n

2

l n j

e e

j

1

2

t r a c e

1

e e

E E

0

The elements of

e e

are taken as free parameters and are estimated as follows

S

0

=

1

n

E E

0

Replacing

e e

byS

0

in the likelihood equation the log likelihood up to an additive

constant is

L =

n

2

l n j S

0

j

Letting B be the backshift operator the formal relation betweenx

t

ande

t

is

x

t

= H I B F

1

G e

t

e

t

= H I B F

1

G

1

x

t

=

1

X

i = 0

i

x

t i

LettingC

i

be the ith lagged sample covariance of x

t

and neglecting end effects the

matrixS

0

is

S

0

=

1

X

i j = 0

i

C

i + j

0

j

For the computation of S

0

the infinite sum is truncated at the value of the KLAG=

option The value of the KLAG= option should be large enough that the sequence

i

is approximately 0 beyond that point

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Let

be the vector of free parameters in theF

andG

matrices The derivative of the

log likelihood with respect to the parameter

is

L

=

n

2

t r a c e

S

1

0

S

0

The second derivative is

2

L

0

=

n

2

t r a c e

S

1

0

S

0

0

S

1

0

S

0

t r a c e

S

1

0

2

S

0

0

Near the maximum the first term is unimportant and the second term can be approx-

imated to give the following second derivative approximation

2

L

0

=

n t r a c e

S

1

0

E

E

0

0

The first derivative matrix and this second derivative matrix approximation are com-

puted from the sample covariance matrixC

0

and the truncated sequence

i

The

approximate likelihood function is maximized by a modified Newton-Raphson algo-

rithm employing these derivative matrices

The matrixS

0

is used as the estimate of the innovation covariance matrix

e e

The

negative of the inverse of the second derivative matrix at the maximum is used as an

approximate covariance matrix for the parameter estimates The standard errors of

the parameter estimates printed in the parameter estimates tables are taken from the

diagonal of this covariance matrix The parameter covariance matrix is printed when

the COVB option is specified

If the data are nearly nonstationary a better estimate of

e e

and the other parameters

can sometimes be obtained by specifying the RESIDEST option The RESIDEST

option estimates the parameters using conditional least squares instead of maximum

likelihood

The residuals are computed using the state space equation and the sample mean val-

ues of the variables in the model as start-up values The estimate of S

0

is then com-

puted using the residuals from the ith observation on where i is the maximum number

of times any variable occurs in the state vector A multivariate Gauss-Marquardt al-

gorithm is used to minimizej S

0

j

Refer to Harvey (1981a) for a further description

of this method

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Forecasting

Given estimates of F

G

and

e e

forecasts of x

t

are computed from the conditional

expectation of z

t

In forecasting the parameters F G and

e e

are replaced with the estimates or by val-

ues specified in the RESTRICT statement One-step-ahead forecasting is performedfor the observation

x

t

wheret n b

Heren

is the number of observations and

b is the value of the BACK= option For the observationx

t

wheret n b

m-

step-ahead forecasting is performed form = t n + b

The forecasts are generated

recursively with the initial conditionz

0

= 0

The m-step-ahead forecast of z

t + m

isz

t + m j t

wherez

t + m j t

denotes the conditional

expectation of z

t + m

given the information available at time t The m-step-ahead

forecast of x

t + m

isx

t + m j t

= H z

t + m j t

where the matrixH = I

r

0

Let

i

= F

i

G

Note that the lasts r

elements of z

t

consist of the elements of x

u j t

foru t

The state vector z

t + m

can be represented as

z

t + m

= F

m

z

t

+

m 1

X

i = 0

i

e

t + m i

Sincee

t + i j t

= 0

fori 0

the m-step-ahead forecastz

t + m j t

is

z

t + m j t

= F

m

z

t

= F z

t + m 1 j t

Therefore the m-step-ahead forecast of x

t + m

is

x

t + m j t

= H z

t + m j t

The m-step-ahead forecast error is

z

t + m

z

t + m j t

=

m 1

X

i = 0

i

e

t + m i

The variance of the m-step-ahead forecast error is

V

z m

=

m 1

X

i = 0

i

e e

0

i

LettingV

z 0

= 0

the variance of the m-step-ahead forecast error of z

t + m

V

z m

can

be computed recursively as follows

V

z m

= V

z m 1

+

m 1

e e

0

m 1

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The variance of the m-step-ahead forecast error of x

t + m

is ther r

left upper sub-

matrix of V

z m

that is

V

x m

= H V

z m

H

0

Unless the NOCENTER option is specified the sample mean vector is added to the

forecast When differencing is specified the forecasts xt + m j t

plus the sample mean

vector are integrated back to produce forecasts for the original series

Lety

t

be the original series specified by the VAR statement with some 0 values

appended corresponding to the unobserved past observations Let B be the backshift

operator and let B

be thes s

matrix polynomial in the backshift operator

corresponding to the differencing specified by the VAR statement The off-diagonal

elements of

i

are 0 Note that

0

= I

s

whereI

s

is thes s

identity matrix Then

z

t

= B y

t

This gives the relationship

y

t

=

1

B z

t

=

1

X

i = 0

i

z

t i

where

1

B =

P

1

i = 0

i

B

i and

0

= I

s

The m-step-ahead forecast of y

t + m

is

y

t + m j t

=

m 1

X

i = 0

i

z

t + m i j t

+

1

X

i = m

i

z

t + m i

The m-step-ahead forecast error of y

t + m

is

m 1

X

i = 0

i

z

t + m i

z

t + m i j t

=

m 1

X

i = 0

i

X

u = 0

u

i u

e

t + m i

LettingV

y 0

= 0

the variance of the m-step-ahead forecast error of y

t + m

V

y m

is

V

y m

=

m 1

X

i = 0

i

X

u = 0

u

i u

e e

i

X

u = 0

u

i u

0

= V

y m 1

+

m 1

X

u = 0

u

m 1 u

e e

m 1

X

u = 0

u

m 1 u

0

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Relation of ARMA and State Space Forms

Every state space model has an ARMA representation and conversely every ARMA

model has a state space representation This section discusses this equivalence The

following material is adapted from Akaike (1974) where there is a more complete

discussion Pham-Dinh-Tuan (1978) also contains a discussion of this material

Suppose you are given the following ARMA model

B x

t

= B e

t

or in more detail

x

t

1

x

t 1

p

x

t p

= e

t

+

1

e

t 1

+ +

q

e

t q

(1)

wheree

t

is a sequence of independent multivariate normal random vectors with mean

0 and variance matrix

e e

B is the backshift operator (B x

t

= x

t 1

) B

and

B

are matrix polynomials in B andx

t

is the observed process

If the roots of the determinantial equationj B j = 0

are outside the unit circle in

the complex plane the model can also be written as

x

t

=

1

B B e

t

=

1

X

i = 0

i

e

t i

The

i

matrices are known as the impulse response matrices and can be computed

as

1

B B

You can assume p q

since if this is not initially true you can add more terms

i

that are identically 0 without changing the model

To write this set of equations in a state space form proceed as follows Letx

t + i j t

be

the conditional expectation of x

t + i

givenx

w

forw t

The following relations hold

x

t + i j t

=

1

X

j = i

j

e

t + i j

x

t + i j t + 1

= x

t + i j t

+

i 1

e

t + 1

However from equation (1) you can derive the following relationship

x

t + p j t

=

1

x

t + p 1 j t

+ +

p

x

t

(2)

Hence wheni = p

you can substitute forx

t + p j t

in the right-hand side of equation

(2) and close the system of equations

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This substitution results in the following model in the state space form

z

t + 1

= F z

t

+ G e

t + 1

2

6

6

6

4

x

t + 1

x

t + 2 j t + 1

x

t + p j t + 1

3

7

7

7

5

=

2

6

6

4

0 I 0 0

0 0 I 0

p

p 1

1

3

7

7

5

2

6

6

6

4

x

t

x

t + 1 j t

x

t + p 1 j t

3

7

7

7

5

+

2

6

6

4

I

1

p 1

3

7

7

5

e

t + 1

Note that the state vectorz

t

is composed of conditional expectations of x

t

and the

first r components of z

t

are equal tox

t

The state space form can be cast into an ARMA form by solving the system of dif-

ference equations for the first r components

When converting from an ARMA form to a state space form you can generate a

state vector larger than needed that is the state space model may not be a minimal

representation When going from a state space form to an ARMA form you can have

nontrivial common factors in the autoregressive and moving average operators that

yield an ARMA model larger than necessary

If the state space form used is not a minimal representation some but not all compo-

nents of x

t + i j t

may be linearly dependent This situation corresponds to

p

p 1

being of less than full rank when B

and B

have no common nontrivial

left factors In this casez

t

consists of a subset of the possible components of

x

t + i j t

i = 1 2 p 1

However once a component of x

t + i j t

(for example the

jth one) is linearly dependent on the previous conditional expectations then all sub-

sequent jth components of x

t + k j t

fork i

must also be linearly dependent Note

that in this case equivalent but seemingly different structures can arise if the order of

the components withinx

t

is changed

OUT= Data Set

The forecasts are contained in the output data set specified by the OUT= option on the

PROC STATESPACE statement The OUT= data set contains the following variables

the BY variables

the ID variable

the VAR statement variables These variables contain the actual values from

the input data set

FORi numeric variables containing the forecasts The variable FORi contains

the forecasts for the ith variable in the VAR statement list Forecasts are one-

step-ahead predictions until the end of the data or until the observation specified

by the BACK= option

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RESi numeric variables containing the residual for the forecast of the ith vari-

able in the VAR statement list For forecast observations the actual values are

missing and the RESi variables contain missing values

STDi numeric variables containing the standard deviation for the forecast of

the ith variable in the VAR statement list The values of the STDi variables can

be used to construct univariate confidence limits for the corresponding fore-casts However such confidence limits do not take into account the covariance

of the forecasts

OUTAR= Data Set

The OUTAR= data set contains the estimates of the preliminary autoregressive mod-

els The OUTAR= data set contains the following variables

ORDER a numeric variable containing the order p of the autoregressive model

that the observation represents

AIC a numeric variable containing the value of the information criterionA I C

p

SIGFl numeric variables containing the estimate of the innovation covariance

matrices for the forward autoregressive models The variable SIGFl contains

the lth column of b

p

in the observations with ORDER= p

SIGBl numeric variables containing the estimate of the innovation covariance

matrices for the backward autoregressive models The variable SIGBl contains

the lth column of b

p

in the observations with ORDER= p

FORk ndashl numeric variables containing the estimates of the autoregressive pa-

rameter matrices for the forward models The variable FORk ndashl contains the

lth column of the lag k autoregressive parameter matrix b

p

k

in the observations

with ORDER= p

BACk ndashl numeric variables containing the estimates of the autoregressive pa-

rameter matrices for the backward models The variable BACk ndashl contains the

lth column of the lag k autoregressive parameter matrix b

p

k

in the observations

with ORDER= p

The estimates for the order p autoregressive model can be selected as those observa-

tions with ORDER= p Within these observations the k lth element of

p

i

is given by

the value of the FORindashl variable in the k th observation The k lth element of

p

i

is

given by the value of BACindashl variable in the k th observation The k lth element of

p

is given by SIGFl in the k th observation The k lth element of

p

is given by SIGBlin the k th observation

Table 181 shows an example of the OUTAR= data set with ARMAX=3 andx

t

of

dimension 2 In Table 181 i j

indicate the ijth element of the matrix

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Part 2 General Information

Table 181 Values in the OUTAR= Data Set

Obs ORDER AIC SIGF1 SIGF2 SIGB1 SIGB2 FOR1ndash 1 FOR1ndash 2 FOR2ndash 1 FOR2ndash 2 FOR3ndash 1

1 0 AIC0

0 1 1

0 1 2

0 1 1

0 1 2

2 0 AIC0

0 2 1

0 2 2

0 2 1

0 2 2

3 1 AIC1

1 1 1

1 1 2

1 1 1

1 1 2

1

1

1 1

1

1

1 2

4 1 AIC1

1 2 1

1 1 2

1 2 1

1 1 2

1

1

2 1

1

1

2 2

5 2 AIC2

2 1 1

2 1 2

2 1 1

2 1 2

2

1

1 1

2

1

1 2

2

2

1 1

2

2

1 2

6 2 AIC2

2 2 1

2 1 2

2 2 1

2 1 2

2

1

2 1

2

1

2 2

2

2

2 1

2

2

2 2

7 3 AIC3

3 1 1

3 1 2

3 1 1

3 1 2

3

1

1 1

3

1

1 2

3

2

1 1

3

2

1 2

3

3

1 1

8 3 AIC3

3 2 1

3 1 2

3 2 1

3 1 2

3

1

2 1

3

1

2 2

3

2

2 1

3

2

2 2

3

3

2 1

Obs FOR3ndash 2 BACK1ndash 1 BACK1ndash 2 BACK2ndash 1 BACK2ndash 2 BACK3ndash 1 BACK3ndash 2

1

2

3

1

1

1 1

1

1

1 2

4

1

1

2 1

1

1

2 2

5

2

1

1 1

2

1

1 2

2

2

1 1

2

2

1 2

6

2

1

2 1

2

1

2 2

2

2

2 1

2

2

2 2

7

3

3

1 2

3

1

1 1

3

1

1 2

3

2

1 1

3

2

1 2

3

3

1 1

3

3

1 2

8

3

3

2 2

3

1

2 1

3

1

2 2

3

2

2 1

3

2

2 2

3

3

2 1

3

3

2 2

The estimated autoregressive parameters can be used in the IML procedure to obtain

autoregressive estimates of the spectral density function or forecasts based on the

autoregressive models

OUTMODEL= Data Set

The OUTMODEL= data set contains the estimates of theF

andG

matrices and their

standard errors the names of the components of the state vector and the estimates of

the innovation covariance matrix The variables contained in the OUTMODEL= data

set are as follows

the BY variables

STATEVEC a character variable containing the name of the component of the

state vector corresponding to the observation The STATEVEC variable has

the value STD for standard deviations observations which contain the standarderrors for the estimates given in the preceding observation

Fndash j numeric variables containing the columns of theF

matrix The variable

Fndash j contains the jth column of F

The number of Fndash j variables is equal to

the value of the DIMMAX= option If the model is of smaller dimension the

extraneous variables are set to missing

Gndash j numeric variables containing the columns of theG

matrix The variable

Gndash j contains the jth column of G

The number of Gndash j variables is equal to r

the dimension of x

t

given by the number of variables in the VAR statement

SIGndash j numeric variables containing the columns of the innovation covariance

matrix The variable SIGndash j contains the jth column of

e e

There are r vari-ables SIGndash j

Table 182 shows an example of the OUTMODEL= data set withx

t

= x

t

y

t

0

z

t

= x

t

y

t

x

t + 1 j t

0 and DIMMAX=4 In Table 182F

i j

andG

i j

are the ijth

elements of F

andG

respectively Note that all elements for Fndash4 are missing because

F

is a3 3

matrix

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Chapter 18 Details

Table 182 Value in the OUTMODEL= Data Set

Obs STATEVEC Fndash 1 Fndash 2 Fndash 3 Fndash 4 Gndash 1 Gndash 2 SIGndash 1 SIGndash 2

1 X(TT) 0 0 1 1 0

1 1

1 2

2 STD

3 Y(TT) F2 1

F2 2

F2 3

0 1

2 1

2 2

4 STD std F2 1

std F2 2

std F2 3

5 X(T+1T) F3 1

F3 2

F3 3

G3 1

G3 2

6 STD std F3 1

std F3 2

std F3 3

std G3 1

std G3 2

Printed Output

The printed output produced by the STATESPACE procedure is described in the fol-

lowing

1 descriptive statistics which include the number of observations used the

names of the variables their means and standard deviations (Std) and the dif-

ferencing operations used

2 the Akaike information criteria for the sequence of preliminary autoregressive

models

3 if the PRINTOUT=LONG option is specified the sample autocovariance ma-

trices of the input series at various lags

4 if the PRINTOUT=LONG option is specified the sample autocorrelation ma-

trices of the input series

5 a schematic representation of the autocorrelation matrices showing the signif-

icant autocorrelations

6 if the PRINTOUT=LONG option is specified the partial autoregressive ma-

trices (These are

p

p

as described in Preliminary Autoregressive Modelsearlier in this chapter)

7 a schematic representation of the partial autocorrelation matrices showing the

significant partial autocorrelations

8 the Yule-Walker estimates of the autoregressive parameters for the autoregres-

sive model with the minimum AIC

9 if the PRINTOUT=LONG option is specified the autocovariance matrices of

the residuals of the minimum AIC model This is the sequence of estimated

innovation variance matrices for the solutions of the Yule-Walker equations

10 if the PRINTOUT=LONG option is specified the autocorrelation matrices of the residuals of the minimum AIC model

11 If the CANCORR option is specified the canonical correlations analysis for

each potential state vector considered in the state vector selection process

This includes the potential state vector the canonical correlations the informa-

tion criterion for the smallest canonical correlation Bartlettrsquos

2 statistic (ldquoChi

Squarerdquo) for the smallest canonical correlation and the degrees of freedom of

Bartlettrsquos

2

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Part 2 General Information

12 the components of the chosen state vector

13 the preliminary estimate of the transition matrixF

the input matrixG

and

the variance matrix for the innovations

e e

14 if the ITPRINT option is specified the iteration history of the likelihood max-

imization For each iteration this shows the iteration number the number of step halvings the determinant of the innovation variance matrix the damping

factor Lambda and the values of the parameters

15 the state vector printed again to aid interpretation of the following listing of F

andG

16 the final estimate of the transition matrixF

17 the final estimate of the input matrixG

18 the final estimate of the variance matrix for the innovations

e e

19 a table listing the estimates of the free parameters inF

andG

and their standarderrors and t statistics

20 if the COVB option is specified the covariance matrix of the parameter esti-

mates

21 if the COVB option is specified the correlation matrix of the parameter esti-

mates

22 if the PRINT option is specified the forecasts and their standard errors

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Chapter 18 Details

ODS Table Names

PROC STATESPACE assigns a name to each table it creates You can use these

names to reference the table when using the Output Delivery System (ODS) to select

tables and create output data sets These names are listed in the following table For

more information on ODS see Chapter 6 ldquoUsing the Output Delivery Systemrdquo

Table 183 ODS Tables Produced in PROC STATESPACE

ODS Table Name Description Option

NObs Number of observations default

Summary Simple summary statistics table default

InfoCriterion Information criterion table default

CovLags Covariance Matrices of Input Series PRINTOUT=LONG

CorrLags Correlation Matrices of Input Series PRINTOUT=LONG

PartialAR Partial Autoregressive Matrices PRINTOUT=LONG

YWEstimates Yule-Walker Estimates for Minimum AIC default

CovResiduals Covariance of Residuals PRINTOUT=LONGCorrResiduals Residual Correlations from AR Models PRINTOUT=LONG

StateVector State vector table default

CorrGraph Schematic Representation of Correlations default

TransitionMatrix Transition Matrix default

InputMatrix Input Matrix default

VarInnov Variance Matrix for the Innovation default

CovB Covariance of Parameter Estimates COVB

CorrB Correlation of Parameter Estimates COVB

CanCorr Canonical Correlation Analysis CANCORR

IterHistory Iterative Fitting table ITPRINT

ParameterEstimates Parameter Estimates Table default

Forecasts Forecasts Table PRINT

ConvergenceStatus Convergence Status Table default

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Part 2 General Information

Example

Example 181 Series J from Box and Jenkins

This example analyzes the gas furnace data (series J) from Box and Jenkins (The

data are not shown Refer to Box and Jenkins (1976) for the data)

First a model is selected and fit automatically using the following statements

title1 rsquoGas Furnace Datarsquo

title2 rsquoBox amp Jenkins Series Jrsquo

title3 rsquoAutomatically Selected Modelrsquo

proc statespace data=seriesj cancorr

var x y

run

The results for the automatically selected model are shown in Output 1811

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Chapter 18 Example

Output 1811 Results for Automatically Selected Model

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Number of Observations 296

Standard

Variable Mean Error

x -005683 1072766

y 5350912 3202121

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Information Criterion for Autoregressive Models

Lag=0 Lag=1 Lag=2 Lag=3 Lag=4 Lag=5 Lag=6 Lag=7 Lag=8

6513862 -103357 -163296 -164512 -165152 -164891 -164934 -164315 -163856

Information

Criterion for

Autoregressive

Models

Lag=9 Lag=10

-16348 -163359

Schematic Representation of Correlations

NameLag 0 1 2 3 4 5 6 7 8 9 10

x +- +- +- +- +- +- +- +- +- +- +-

y -+ -+ -+ -+ -+ -+ -+ -+ -+ -+ -+

+ is gt 2std error - is lt -2std error is between

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Part 2 General Information

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Schematic Representation of Partial Autocorrelations

NameLag 1 2 3 4 5 6 7 8 9 10

x + - + -

y -+ -- - + +

+ is gt 2std error - is lt -2std error is between

Yule-Walker Estimates for Minimum AIC

------Lag=1------ ------Lag=2------ ------Lag=3------ ------Lag=4------

x y x y x y x y

x 1925887 -000124 -120166 0004224 0116918 -000867 0104236 0003268

y 0050496 1299793 -002046 -03277 -071182 -025701 0195411 0133417

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Canonical Correlations Analysis

Information Chi

x(TT) y(TT) x(T+1T) Criterion Square DF

1 1 0804883 2929228 3047481 8

Information Chix(TT) y(TT) x(T+1T) y(T+1T) Criterion Square DF

1 1 0906681 0607529 1223358 1347237 7

Information Chi

x(TT) y(TT) x (T+1T) y (T+1T) x (T+2T) Criterion Square DF

1 1 0909434 0610278 0186274 -154701 1034705 6

Information Chi

x(TT) y(TT) x (T+1T) y (T+1T) y (T+2T) Criterion Square DF

1 1 091014 0618937 0206823 0940392 1280924 6

Information Chi

x(TT) y(TT) x(T+1T) y(T+1T) y(T+2T) y(T+3T) Criterion Square DF

1 1 0912963 0628785 0226598 0083258 -794103 2041584 5

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Chapter 18 Example

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Selected Statespace Form and Preliminary Estimates

State Vector

x(TT) y(TT) x(T+1T) y(T+1T) y(T+2T)

Estimate of Transition Matrix

0 0 1 0 0

0 0 0 1 0

-084718 0026794 1711715 -005019 0

0 0 0 0 1

-019785 0334274 -018174 -123557 1787475

Input Matrix for Innovation

1 0

0 11925887 -000124

0050496 1299793

0142421 1361696

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Selected Statespace Form and Preliminary Estimates

Variance Matrix for Innovation

0035274 -000734

-000734 0097569

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Part 2 General Information

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Selected Statespace Form and Fitted Model

State Vector

x(TT) y(TT) x(T+1T) y(T+1T) y(T+2T)

Estimate of Transition Matrix

0 0 1 0 0

0 0 0 1 0

-086192 0030609 1724235 -005483 0

0 0 0 0 1

-034839 0292124 -009435 -109823 1671418

Input Matrix for Innovation

1 0

0 1192442 -000416

0015621 1258495

008058 1353204

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Selected Statespace Form and Fitted Model

Variance Matrix for Innovation

0035579 -000728

-000728 0095577

Parameter Estimates

Standard

Parameter Estimate Error t Value

F(31) -086192 0072961 -1181

F(32) 0030609 0026167 117

F(33) 1724235 0061599 2799

F(34) -005483 0030169 -182

F(51) -034839 0135253 -258

F(52) 0292124 0046299 631

F(53) -009435 0096527 -098F(54) -109823 0109525 -1003

F(55) 1671418 0083737 1996

G(31) 1924420 0058162 3309

G(32) -000416 0035255 -012

G(41) 0015621 0095771 016

G(42) 1258495 0055742 2258

G(51) 0080580 0151622 053

G(52) 1353204 0091388 1481

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Chapter 18 Example

The two series are believed to have a transfer function relation with the gas rate

(variable X) as the input and the CO2

concentration (variable Y) as the output Since

the parameter estimates shown in Output 1811 support this kind of model the model

is reestimated with the feedback parameters restricted to 0 The following statements

fit the transfer function (no feedback) model

title3 rsquoTransfer Function Modelrsquo

proc statespace data=seriesj printout=none

var x y

restrict f(32)=0 f(34)=0

g(32)=0 g(41)=0 g(51)=0

run

The last 2 pages of the output are shown in Output 1812

Output 1812 STATESPACE Output for Transfer Function Model

Gas Furnace Data

Box amp Jenkins Series J

Transfer Function Model

The STATESPACE Procedure

Selected Statespace Form and Fitted Model

State Vector

x(TT) y(TT) x(T+1T) y(T+1T) y(T+2T)

Estimate of Transition Matrix

0 0 1 0 0

0 0 0 1 0

-068882 0 1598717 0 0

0 0 0 0 1

-035944 0284179 -00963 -107313 1650047

Input Matrix for Innovation

1 0

0 1

1923446 0

0 1260856

0 1346332

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Part 2 General Information

Gas Furnace Data

Box amp Jenkins Series J

Transfer Function Model

The STATESPACE Procedure

Selected Statespace Form and Fitted Model

Variance Matrix for Innovation

0036995 -00072

-00072 0095712

Parameter Estimates

Standard

Parameter Estimate Error t Value

F(31) -068882 0050549 -1363

F(33) 1598717 0050924 3139

F(51) -035944 0229044 -157

F(52) 0284179 0096944 293

F(53) -009630 0140876 -068

F(54) -107313 0250385 -429

F(55) 1650047 0188533 875G(31) 1923446 0056328 3415

G(42) 1260856 0056464 2233

G(52) 1346332 0091086 1478

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Chapter 18 References

References

Akaike H (1974) Markovian Representation of Stochastic Processes and Its Ap-

plication to the Analysis of Autoregressive Moving Average Processes Annals

of the Institute of Statistical Mathematics 26 363-387

Akaike H (1976) Canonical Correlations Analysis of Time Series and the Use of

an Information Criterion in Advances and Case Studies in System Identification

eds R Mehra and DG Lainiotis New York Academic Press

Anderson TW (1971) The Statistical Analysis of Time Series New York John

Wiley amp Sons

Ansley CF and Newbold P (1979) Multivariate Partial Autocorrelations Pro-

ceedings of the Business and Economic Statistics Section American Statistical

Association 349-353

Box GEP and Jenkins G (1976) Time Series Analysis Forecasting and Control

San Francisco Holden-Day

Brockwell PJ and Davis RA (1991) Time Series Theory and Methods 2nd Edi-

tion Springer-Verlag

Hannan EJ (1970) Multiple Time Series New York John Wiley amp Sons

Hannan EJ (1976) The Identification and Parameterization of ARMAX and State

Space Forms Econometrica 44 713-722

Harvey AC (1981a) The Econometric Analysis of Time Series New York John

Wiley amp Sons

Harvey AC (1981b) Time Series Models New York John Wiley amp Sons

Jones RH (1974) Identification and Autoregressive Spectrum Estimation IEEE Transactions on Automatic Control AC-19 894-897

Pham-Dinh-Tuan (1978) On the Fitting of Multivariate Processes of the

Autoregressive-Moving Average Type Biometrika 65 99-107

Priestley MB (1980) System Identification Kalman Filtering and Stochastic Con-

trol in Directions in Time Series eds DR Brillinger and GC Tiao Institute

of Mathematical Statistics

Whittle P (1963) On the Fitting of Multivariate Autoregressions and the Approxi-

mate Canonical Factorization of a Spectral Density Matrix Biometrika 50 129-

134

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The correct bibliographic citation for this manual is as follows SAS Institute Inc SAS

ET S U serrsquos Gui de Version 8 Cary NC SAS Institute Inc 1999 1546 pp

SASET S Userrsquos Guide Version 8

Copyright copy 1999 by S AS I ns t it u t e I nc Ca ry N C U S A

ISBN 1ndash58025ndash489ndash6

All r ight s res erved P r int ed in t he U n it ed St a t es of A merica N o pa rt of t his p u blica t ionma y b e reprodu ced s t ored in a ret r ieva l s ys t em or t ra ns mit t ed in a ny form or b y a nymeans electronic mechanical photocopying or otherwise without the prior writtenpermission of the publisher SAS Institute Inc

US Government Restricted Rights Notice Use du plicat ion or disclosure of the

software by the government is subject to restrictions as set forth in FAR 52227ndash19Commercial Computer Softw ar e-Restricted Rights (J une 1987)

SAS Institute Inc SAS Campus Drive Cary North Carolina 27513

1st print ing October 1999

S AS reg a nd a ll ot her SA S I ns t it u t e I nc p rodu ct or s ervice na m es a re regis t ered t ra dema rkst d k f SA S I t it t I i t h U S A d t h t i reg i di t U SA

Page 4: Statespace Model

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Part 2 General Information

represent the conditional expectation (or prediction) of x

t + k

based on the information

available at time t Then the lasts r

elements of z

t

consist of elements of xt + k j t

where k gt0 is specified or determined automatically by the procedure

There are various forms of the state space model in use The form of the state space

model used by the STATESPACE procedure is based on Akaike (1976) The model

is defined by the following state transition equation

z

t + 1

= F z

t

+ G e

t + 1

In the state transition equation thes s

coefficient matrix F is called the transition

matrix it determines the dynamic properties of the model

Thes r

coefficient matrix G is called the input matrix it determines the variance

structure of the transition equation For model identification the first r rows and

columns of G are set to anr r

identity matrix

The input vector et

is a sequence of independent normally distributed random vectors

of dimension r with mean 0 and covariance matrix

e e

The random error et

is

sometimes called the innovation vector or shock vector

In addition to the state transition equation state space models usually include a mea-

surement equation or observation equation that gives the observed valuesx

t

as a

function of the state vectorz

t

However since PROC STATESPACE always includes

the observed valuesx

t

in the state vectorz

t

the measurement equation in this case

merely represents the extraction of the first r components of the state vector

The measurement equation used by the STATESPACE procedure is

x

t

= I

r

0 z

t

whereI

r

is anr r

identity matrix In practice PROC STATESPACE performs the

extraction of x

t

fromz

t

without reference to an explicit measurement equation

In summary

xt

is an observation vector of dimension r

zt

is a state vector of dimension s whose first r elements are xt

and

whose lasts r

elements are conditional prediction of future xt

F is ans s

transition matrix

G is ans r

input matrix with the identity matrix Ir

forming the first

r rows and columns

et

is a sequence of independent normally distributed random vectors

of dimension r with mean 0 and covariance matrix

e e

How PROC STATESPACE Works

The design of the STATESPACE procedure closely follows the modeling strategy

proposed by Akaike (1976) This strategy employs canonical correlation analysis for

the automatic identification of the state space model

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Chapter 18 Overview

Following Akaike (1976) the procedure first fits a sequence of unrestricted vector

autoregressive (VAR) models and computes Akaikersquos information criterion (AIC) for

each model The vector autoregressive models are estimated using the sample au-

tocovariance matrices and the Yule-Walker equations The order of the VAR model

producing the smallest Akaike information criterion is chosen as the order (number

of lags into the past) to use in the canonical correlation analysis

The elements of the state vector are then determined via a sequence of canonical cor-

relation analyses of the sample autocovariance matrices through the selected order

This analysis computes the sample canonical correlations of the past with an increas-

ing number of steps into the future Variables that yield significant correlations are

added to the state vector those that yield insignificant correlations are excluded from

further consideration The importance of the correlation is judged on the basis of

another information criterion proposed by Akaike See the section Canonical Cor-

relation Analysis for details If you specify the state vector explicitly these model

identification steps are omitted

Once the state vector is determined the state space model is fit to the data The free

parameters in the F G and

e e

matrices are estimated by approximate maximumlikelihood By default the

F

andG

matrices are unrestricted except for identifia-

bility requirements Optionally conditional least-squares estimates can be computed

You can impose restrictions on elements of theF

andG

matrices

After the parameters are estimated forecasts are produced from the fitted state space

model using the Kalman filtering technique If differencing was specified the fore-

casts are integrated to produce forecasts of the original input variables

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Part 2 General Information

Getting Started

The following introductory example uses simulated data for two variables X and Y

The following statements generate the X and Y series

data inx=10 y=40

x1=0 y1=0

a1=0 b1=0

iseed=123

do t=-100 to 200

a=rannor(iseed)

b=rannor(iseed)

dx = 05x1 + 03y1 + a - 02a1 - 01b1

dy = 03x1 + 05y1 + b

x = x + d x + 2 5

y = y + d y + 2 5

if t gt= 0 then output

x1 = dx y1 = dy

a 1 = a b 1 = b

end

keep t x y

run

The simulated series X and Y are shown in Figure 181

Figure 181 Example Series

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Chapter 18 Getting Started

Automatic State Space Model Selection

The STATESPACE procedure is designed to automatically select the best state space

model for forecasting the series You can specify your own model if you wish and

you can use the output from PROC STATESPACE to help you identify a state space

model However the easiest way to use PROC STATESPACE is to let it choose the

model

Stationarity and Differencing

Although PROC STATESPACE selects the state space model automatically it does

assume that the input series are stationary If the series are nonstationary then the

process may fail Therefore the first step is to examine your data and test to see if

differencing is required (See the section Stationarity and Differencing later in this

chapter for further discussion of this issue)

The series shown in Figure 181 are nonstationary In order to forecast X and Y

with a state space model you must difference them (or use some other de-trending

method) If you fail to difference when needed and try to use PROC STATESPACE

with nonstationary data an inappropriate state space model may be selected and themodel estimation may fail to converge

The following statements identify and fit a state space model for the first differences

of X and Y and forecast X and Y 10 periods ahead

proc statespace data=in out=out lead=10

var x(1) y(1)

id t

run

The DATA= option specifies the input data set and the OUT= option specifies theoutput data set for the forecasts The LEAD= option specifies forecasting 10 obser-

vations past the end of the input data The VAR statement specifies the variables to

forecast and specifies differencing The notation X(1) Y(1) specifies that the state

space model analyzes the first differences of X and Y

Descriptive Statistics and Preliminary Autoregressions

The first page of the printed output produced by the preceding statements is shown in

Figure 182

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Part 2 General Information

The STATESPACE Procedure

Number of Observations 200

Standard

Variable Mean Error

x 0144316 1233457 Has been differenced With period(s) = 1

y 0164871 1304358 Has been differenced

With period(s) = 1

The STATESPACE Procedure

Information Criterion for Autoregressive Models

Lag=0 Lag=1 Lag=2 Lag=3 Lag=4 Lag=5 Lag=6 Lag=7 Lag=8

149697 8387786 5517099 1205986 1536952 2179538 2400638 2988874 3355708

Information

Criterion for Autoregressive

Models

Lag=9 Lag=10

4117606 4770222

Schematic Representation of Correlations

NameLag 0 1 2 3 4 5 6 7 8 9 10

x ++ ++ ++ ++ ++ ++ + + +

y ++ ++ ++ ++ ++ + + + +

+ is gt 2std error - is lt -2std error is between

Figure 182 Descriptive Statistics and VAR Order Selection

Descriptive statistics are printed first giving the number of nonmissing observations

after differencing and the sample means and standard deviations of the differenced

series The sample means are subtracted before the series are modeled (unless the

NOCENTER option is specified) and the sample means are added back when the

forecasts are produced

LetX

t

andY

t

be the observed values of X and Y and letx

t

andy

t

be the values of X

and Y after differencing and subtracting the mean difference The seriesx

t

modeled

by the STATEPSPACE procedure is

x

t

=

x

t

y

t

=

1 B X

t

0 1 4 4 3 1 6

1 B Y

t

0 1 6 4 8 7 1

where B represents the backshift operator

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Chapter 18 Getting Started

After the descriptive statistics PROC STATESPACE prints the Akaike information

criterion (AIC) values for the autoregressive models fit to the series The smallest AIC

value in this case 5517 at lag 2 determines the number of autocovariance matrices

analyzed in the canonical correlation phase

A schematic representation of the autocorrelations is printed next This indicates

which elements of the autocorrelation matrices at different lags are significantlygreater or less than 0

The second page of the STATESPACE printed output is shown in Figure 183

The STATESPACE Procedure

Schematic Representation of Partial Autocorrelations

NameLag 1 2 3 4 5 6 7 8 9 10

x ++ +

y ++

+ is gt 2std error - is lt -2std error is between

Yule-Walker Estimates for Minimum AIC

--------Lag=1------- --------Lag=2-------

x y x y

x 0257438 0202237 0170812 0133554

y 0292177 0469297 -000537 -000048

Figure 183 Partial Autocorrelations and VAR Model

Figure 183 shows a schematic representation of the partial autocorrelations similar

to the autocorrelations shown in Figure 182 The selection of a second order autore-

gressive model by the AIC statistic looks reasonable in this case because the partialautocorrelations for lags greater than 2 are not significant

Next the Yule-Walker estimates for the selected autoregressive model are printed

This output shows the coefficient matrices of the vector autoregressive model at each

lag

Selected State Space Model Form and Preliminary Estimates

After the autoregressive order selection process has determined the number of lags to

consider the canonical correlation analysis phase selects the state vector By default

output for this process is not printed You can use the CANCORR option to print

details of the canonical correlation analysis See the section Canonical Correlation

Analysis later in this chapter for an explanation of this process

Once the state vector is selected the state space model is estimated by approximate

maximum likelihood Information from the canonical correlation analysis and from

the preliminary autoregression is used to form preliminary estimates of the state space

model parameters These preliminary estimates are used as starting values for the

iterative estimation process

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Part 2 General Information

The form of the state vector and the preliminary estimates are printed next as shown

in Figure 184

The STATESPACE Procedure

Selected Statespace Form and Preliminary Estimates

State Vector

x(TT) y(TT) x(T+1T)

Estimate of Transition Matrix

0 0 1

0291536 0468762 -000411

024869 024484 0204257

Input Matrix for Innovation

1 0

0 1

0257438 0202237

Variance Matrix for Innovation

0945196 0100786

0100786 1014703

Figure 184 Preliminary Estimates of State Space Model

Figure 184 first prints the state vector as X[TT] Y[TT] X[T+1T] This notation

indicates that the state vector is

z

t

=

2

4

x

t j t

y

t j t

x

t + 1 j t

3

5

The notationx

t + 1 j t

indicates the conditional expectation or prediction of x

t + 1

based

on the information available at time t andx

t j t

andy

t j t

arex

t

andy

t

respectively

The remainder of Figure 184 shows the preliminary estimates of the transition matrix

F

the input matrixG

and the covariance matrix

e e

Estimated State Space Model

The next page of the STATESPACE output prints the final estimates of the fittedmodel as shown in Figure 185 This output has the same form as in Figure 184 but

shows the maximum likelihood estimates instead of the preliminary estimates

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Chapter 18 Getting Started

The STATESPACE Procedure

Selected Statespace Form and Fitted Model

State Vector

x(TT) y(TT) x(T+1T)

Estimate of Transition Matrix

0 0 1

0297273 047376 -001998

02301 0228425 0256031

Input Matrix for Innovation

1 0

0 1

0257284 0202273

Variance Matrix for Innovation

0945188 01007520100752 1014712

Figure 185 Fitted State Space Model

The estimated state space model shown in Figure 185 is

2

4

x

t + 1 j t + 1

y

t + 1 j t + 1

x

t + 2 j t + 1

3

5

=

2

4

0 0 1

0 2 9 7 0 4 7 4 0 0 2 0

0 2 3 0 0 2 2 8 0 2 5 6

3

5

2

4

x

t

y

t

x

t + 1 j t

3

5

+

2

4

1 0

0 1

0 2 5 7 0 2 0 2

3

5

e

t + 1

n

t + 1

v a r

e

t + 1

n

t + 1

=

0 9 4 5 0 1 0 1

0 1 0 1 1 0 1 5

The next page of the STATESPACE output lists the estimates of the free parameters

in theF

andG

matrices with standard errors and t statistics as shown in Figure 186

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Part 2 General Information

The STATESPACE Procedure

Parameter Estimates

Standard

Parameter Estimate Error t Value

F(21) 0297273 0129995 229

F(22) 0473760 0115688 410F(23) -001998 0313025 -006

F(31) 0230100 0126226 182

F(32) 0228425 0112978 202

F(33) 0256031 0305256 084

G(31) 0257284 0071060 362

G(32) 0202273 0068593 295

Figure 186 Final Parameter Estimates

Convergence Failures

The maximum likelihood estimates are computed by an iterative nonlinear maximiza-

tion algorithm which may not converge If the estimates fail to converge warning

messages are printed in the output

If you encounter convergence problems you should recheck the stationarity of the

data and ensure that the specified differencing orders are correct Attempting to fit

state space models to nonstationary data is a common cause of convergence failure

You can also use the MAXIT= option to increase the number of iterations allowed

or experiment with the convergence tolerance options DETTOL= and PARMTOL=

Forecast Data Set

The following statements print the output data set The WHERE statement excludes

the first 190 observations from the output so that only the forecasts and the last 10

actual observations are printed

proc print data=outid t

where t gt 190

run

The PROC PRINT output is shown in Figure 187

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Part 2 General Information

Figure 188 Plot of Forecasts

Controlling Printed Output

By default the STATESPACE procedure produces a large amount of printed output

The NOPRINT option suppresses all printed output You can suppress the printed

output for the autoregressive model selection process with the PRINTOUT=NONE

option The descriptive statistics and state space model estimation output are still

printed when PRINTOUT=NONE is specified You can produce more detailed output

with the PRINTOUT=LONG option and by specifying the printing control optionsCANCORR COVB and PRINT

Specifying the State Space Model

Instead of allowing the STATESPACE procedure to select the model automatically

you can use FORM and RESTRICT statements to specify a state space model

Specifying the State Vector

Use the FORM statement to control the form of the state vector You can use this

feature to force PROC STATESPACE to estimate and forecast a model different from

the model it would select automatically You can also use this feature to reestimate

the automatically selected model (possibly with restrictions) without repeating thecanonical correlation analysis

The FORM statement specifies the number of lags of each variable to include in

the state vector For example the statement FORM X 3 forces the state vector to

includex

t j t

x

t + 1 j t

andx

t + 2 j t

The following statement specifies the state vector

x

t j t

y

t j t

x

t + 1 j t

which is the same state vector selected in the preceding example

f o r m x 2 y 1

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Chapter 18 Getting Started

You can specify the form for only some of the variables and allow PROC STATES-

PACE to select the form for the other variables If only some of the variables are

specified in the FORM statement canonical correlation analysis is used to determine

the number of lags included in the state vector for the remaining variables not spec-

ified by the FORM statement If the FORM statement includes specifications for all

the variables listed in the VAR statement the state vector is completely defined and

the canonical correlation analysis is not performed

Restricting the F and G matrices

After you know the form of the state vector you can use the RESTRICT statement

to fix some parameters in theF

andG

matrices to specified values One use of this

feature is to remove insignificant parameters by restricting them to 0

In the introductory example shown in the preceding section the F[23] parameter is

not significant (The parameters estimation output shown in Figure 186 gives the t

statistic for F[23] as -006 F[33] and F[31] also have low significance witht 2

)

The following statements reestimate this model with F[23] restricted to 0 The

FORM statement is used to specify the state vector and thus bypass the canonicalcorrelation analysis

proc statespace data=in out=out lead=10

var x(1) y(1)

id t

form x 2 y 1

restrict f(23)=0

run

The final estimates produced by these statements are shown in Figure 189

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Part 2 General Information

The STATESPACE Procedure

Selected Statespace Form and Fitted Model

State Vector

x(TT) y(TT) x(T+1T)

Estimate of Transition Matrix

0 0 1

0290051 0467468 0

0227051 0226139 026436

Input Matrix for Innovation

1 0

0 1

0256826 0202022

Variance Matrix for Innovation

0945175 01006960100696 1014733

The STATESPACE Procedure

Parameter Estimates

Standard

Parameter Estimate Error t Value

F(21) 0290051 0063904 454

F(22) 0467468 0060430 774

F(31) 0227051 0125221 181

F(32) 0226139 0111711 202

F(33) 0264360 0299537 088G(31) 0256826 0070994 362

G(32) 0202022 0068507 295

Figure 189 Results using RESTRICT Statement

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Chapter 18 Syntax

Syntax

The STATESPACE procedure uses the following statements

PROC STATESPACE options

BY variable FORM variable value

ID variable

INITIAL F(rowcolumn)=value

G(rowcolumn)=value

RESTRICT F(rowcolumn)=value

G(rowcolumn)=value

VAR variable (difference difference

)

Functional Summary

The statements and options used by PROC STATESPACE are summarized in the

following table

Description Statement Option

Input Data Set Options

specify the input data set PROC STATESPACE DATA=

prevent subtraction of sample mean PROC STATESPACE NOCENTER

specify the ID variable ID

specify the observed series and differencing VAR

Options for Autoregressive Estimates

specify the maximum order PROC STATESPACE ARMAX=

specify maximum lag for autocovariances PROC STATESPACE LAGMAX=

output only minimum AIC model PROC STATESPACE MINIC

specify the amount of detail printed PROC STATESPACE PRINTOUT=

write preliminary AR models to a data set PROC STATESPACE OUTAR=

Options for Canonical Correlation Analysis

print the sequence of canonical correlations PROC STATESPACE CANCORR

specify upper limit of dimension of state

vector

PROC STATESPACE DIMMAX=

specify the minimum number of lags PROC STATESPACE PASTMIN=

specify the multiplier of the degrees of

freedom

PROC STATESPACE SIGCORR=

Options for State Space Model Estimation

specify starting values INITIAL

print covariance matrix of parameter estimates PROC STATESPACE COVB

specify the convergence criterion PROC STATESPACE DETTOL=

specify the convergence criterion PROC STATESPACE PARMTOL=

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Part 2 General Information

Description Statement Option

print the details of the iterations PROC STATESPACE ITPRINT

specify an upper limit of the number of lags PROC STATESPACE KLAG=

specify maximum number of iterations

allowed

PROC STATESPACE MAXIT=

suppress the final estimation PROC STATESPACE NOEST

write the state space model parameter esti-

mates to an output data set

PROC STATESPACE OUTMODEL=

use conditional least squares for final estimates PROC STATESPACE RESIDEST

specify criterion for testing for singularity PROC STATESPACE SINGULAR=

Options for Forecasting

start forecasting before end of the input data PROC STATESPACE BACK=

specify the time interval between observations PROC STATESPACE INTERVAL=

specify multiple periods in the time series PROC STATESPACE INTPER=specify how many periods to forecast PROC STATESPACE LEAD=

specify the output data set for forecasts PROC STATESPACE OUT=

print forecasts PROC STATESPACE PRINT

Options to Specify the State Space Model

specify the state vector FORM

specify the parameter values RESTRICT

BY Groups

specify BY-group processing BY

Printing

suppresses all printed output NOPRINT

PROC STATESPACE Statement

PROC STATESPACE options

The following options can be specified in the PROC STATESPACE statement

Printing Options NOPRINT

suppresses all printed output

Input Data Options

DATA= SAS-data-set

specifies the name of the SAS data set to be used by the procedure If the DATA=

option is omitted the most recently created SAS data set is used

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Chapter 18 Syntax

LAGMAX= k

specifies the number of lags for which the sample autocovariance matrix is computed

The LAGMAX= option controls the number of lags printed in the schematic repre-

sentation of the autocorrelations

The sample autocovariance matrix of lag i denoted asC

i

is computed as

C

i

=

1

N 1

N

X

t = 1 + i

x

t

x

0

t i

wherex

t

is the differenced and centered data andN

is the number of observations

(If the NOCENTER option is specified 1 is not subtracted fromN

) LAGMAX= k

specifies thatC

0

throughC

k

are computed The default is LAGMAX=10

NOCENTER

prevents subtraction of the sample mean from the input series (after any specified

differencing) before the analysis

Options for Preliminary Autoregressive Models ARMAX= n

specifies the maximum order of the preliminary autoregressive models The AR-

MAX= option controls the autoregressive orders for which information criteria are

printed and controls the number of lags printed in the schematic representation of

partial autocorrelations The default is ARMAX=10 See Preliminary Autoregres-

sive Models later in this chapter for details

MINIC

writes to the OUTAR= data set only the preliminary Yule-Walker estimates for the

VAR model producing the minimum AIC See OUTAR= Data Set later in this chap-

ter for details

OUTAR= SAS-data-set

writes the Yule-Walker estimates of the preliminary autoregressive models to a SAS

data set See OUTAR= Data Set later in this chapter for details

PRINTOUT= SHORT | LONG | NONE

determines the amount of detail printed PRINTOUT=LONG prints the lagged co-

variance matrices the partial autoregressive matrices and estimates of the resid-

ual covariance matrices from the sequence of autoregressive models PRINT-

OUT=NONE suppresses the output for the preliminary autoregressive models The

descriptive statistics and state space model estimation output are still printed when

PRINTOUT=NONE is specified PRINTOUT=SHORT is the default

Canonical Correlation Analysis Options

CANCORR

prints the canonical correlations and information criterion for each candidate state

vector considered See Canonical Correlation Analysis later in this chapter for

details

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Part 2 General Information

DIMMAX= n

specifies the upper limit to the dimension of the state vector The DIMMAX= option

can be used to limit the size of the model selected The default is DIMMAX=10

PASTMIN= n

specifies the minimum number of lags to include in the canonical correlation analy-

sis The default is PASTMIN=0 See Canonical Correlation Analysis later in thischapter for details

SIGCORR= value

specifies the multiplier of the degrees of freedom for the penalty term in the informa-

tion criterion used to select the state space form The default is SIGCORR=2 The

larger the value of the SIGCORR= option the smaller the state vector tends to be

Hence a large value causes a simpler model to be fit See Canonical Correlations

Analysis later in this chapter for details

State Space Model Estimation Options

COVB

prints the inverse of the observed information matrix for the parameter estimatesThis matrix is an estimate of the covariance matrix for the parameter estimates

DETTOL= value

specifies the convergence criterion The DETTOL= and PARMTOL= option values

are used together to test for convergence of the estimation process If during an

iteration the relative change of the parameter estimates is less than the PARMTOL=

value and the relative change of the determinant of the innovation variance matrix

is less than the DETTOL= value then iteration ceases and the current estimates are

accepted The default is DETTOL=1E-5

ITPRINT

prints the iterations during the estimation process

KLAG= n

sets an upper limit for the number of lags of the sample autocovariance matrix used

in computing the approximate likelihood function If the data have a strong moving

average character a larger KLAG= value may be necessary to obtain good estimates

The default is KLAG=15 See Parameter Estimation later in this chapter for details

MAXIT= n

sets an upper limit to the number of iterations in the maximum likelihood or condi-

tional least-squares estimation The default is MAXIT=50

NOEST

suppresses the final maximum likelihood estimation of the selected model

OUTMODEL= SAS-data-set

writes the parameter estimates and their standard errors to a SAS data set See OUT-

MODEL= Data Set later in this chapter for details

PARMTOL= value

specifies the convergence criterion The DETTOL= and PARMTOL= option values

are used together to test for convergence of the estimation process If during an

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Chapter 18 Syntax

iteration the relative change of the parameter estimates is less than the PARMTOL=

value and the relative change of the determinant of the innovation variance matrix

is less than the DETTOL= value then iteration ceases and the current estimates are

accepted The default is PARMTOL=001

RESIDEST

computes the final estimates using conditional least squares on the raw data Thistype of estimation may be more stable than the default maximum likelihood method

but is usually more computationally expensive See Parameter Estimation later in

this chapter for details of the conditional least squares method

SINGULAR= value

specifies the criterion for testing for singularity of a matrix A matrix is declared

singular if a scaled pivot is less than the SINGULAR= value when sweeping the

matrix The default is SINGULAR=1E-7

Forecasting Options

BACK= n

starts forecasting n periods before the end of the input data The BACK= option valuemust not be greater than the number of observations The default is BACK=0

INTERVAL= interval

specifies the time interval between observations The INTERVAL= value is used

in conjunction with the ID variable to check that the input data are in order and

have no missing periods The INTERVAL= option is also used to extrapolate the ID

values past the end of the input data See Chapter 3 ldquoDate Intervals Formats and

Functionsrdquo for details on the INTERVAL= values allowed

INTPER= n

specifies that each input observation corresponds to n time periods For example

the options INTERVAL=MONTH and INTPER=2 specify bimonthly data and are

equivalent to specifying INTERVAL=MONTH2 If the INTERVAL= option is not

specified the INTPER= option controls the increment used to generate ID values for

the forecast observations The default is INTPER=1

LEAD= n

specifies how many forecast observations are produced The forecasts start at the

point set by the BACK= option The default is LEAD=0 which produces no fore-

casts

OUT= SAS-data-set

writes the residuals actual values forecasts and forecast standard errors to a SAS

data set See OUT= Data Set later in this chapter for details

PRINT

prints the forecasts

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Part 2 General Information

BY Statement

BY variable

A BY statement can be used with the STATESPACE procedure to obtain separateanalyses on observations in groups defined by the BY variables

FORM Statement

FORM variable value

The FORM statement specifies the number of times a variable is included in the state

vector Values can be specified for any variable listed in the VAR statement If a

value is specified for each variable in the VAR statement the state vector for the state

space model is entirely specified and automatic selection of the state space model is

not performed

The FORM statement forces the state vectorz

t

to contain a specific variable a given

number of times For example if Y is one of the variables inx

t

then the statement

form y 3

forces the state vector to containY

t

Y

t + 1 j t

andY

t + 2 j t

possibly along with other

variables

The following statements illustrate the use of the FORM statement

proc statespace data=in

var x y

f o r m x 3 y 2

run

These statements fit a state space model with the following state vector

z

t

=

2

6

6

6

6

4

x

t j t

y

t j t

x

t + 1 j t

y

t + 1 j t

x

t + 2 j t

3

7

7

7

7

5

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Chapter 18 Syntax

ID Statement

ID variable

The ID statement specifies a variable that identifies observations in the input data setThe variable specified in the ID statement is included in the OUT= data set The

values of the ID variable are extrapolated for the forecast observations based on the

values of the INTERVAL= and INTPER= options

INITIAL Statement

INITIAL F (rowcolumn)= value G(row column)= value

The INITIAL statement gives initial values to the specified elements of theF

andG

matrices These initial values are used as starting values for the iterative estimation

Parts of theF

andG

matrices represent fixed structural identities If an element

specified is a fixed structural element instead of a free parameter the corresponding

initialization is ignored

The following is an example of an INITIAL statement

initial f(32)=0 g(41)=0 g(51)=0

RESTRICT Statement

RESTRICT F(rowcolumn)= value G(rowcolumn)= value

The RESTRICT statement restricts the specified elements of theF

andG

matrices

to the specified values

To use the restrict statement you need to know the form of the model Either specify

the form of the model with the FORM statement or do a preliminary run perhaps

with the NOEST option to find the form of the model that PROC STATESPACE

selects for the data

The following is an example of a RESTRICT statement

restrict f(32)=0 g(41)=0 g(51)=0

Parts of theF

andG

matrices represent fixed structural identities If a restriction is

specified for an element that is a fixed structural element instead of a free parameter

the restriction is ignored

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Part 2 General Information

VAR Statement

VAR variable (difference difference )

The VAR statement specifies the variables in the input data set to model and fore-cast The VAR statement also specifies differencing of the input variables The VAR

statement is required

Differencing is specified by following the variable name with a list of difference

periods separated by commas See the section Stationarity and Differencing for

more information on differencing of input variables

The order in which variables are listed in the VAR statement controls the order in

which variables are included in the state vector Usually potential inputs should be

listed before potential outputs

For example assuming the input data are monthly the following VAR statement

specifies modeling and forecasting of the one period and seasonal second differenceof X and Y

var x(112) y(112)

In this example the vector time series analyzed is

x

t

=

1 B 1 B

1 2

X

t

x

1 B 1 B

1 2

Y

t

y

where B represents the back shift operator and x and y represent the means of thedifferenced series If the NOCENTER option is specified the mean differences are

not subtracted

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Chapter 18 Details

Details

Missing Values

The STATESPACE procedure does not support missing values The procedure uses

the first contiguous group of observations with no missing values for any of the VARstatement variables Observations at the beginning of the data set with missing values

for any VAR statement variable are not used or included in the output data set

Stationarity and Differencing

The state space model used by the STATESPACE procedure assumes that the time

series are stationary Hence the data should be checked for stationarity One way

to check for stationarity is to plot the series A graph of series over time can show a

time trend or variability changes

You can also check stationarity by using the sample autocorrelation functions dis-

played by the ARIMA procedure The autocorrelation functions of nonstationaryseries tend to decay slowly See Chapter 7 ldquoThe ARIMA Procedurerdquo for more in-

formation

Another alternative is to use the STATIONARITY= option on the IDENTIFY state-

ment in PROC ARIMA to apply Dickey-Fuller tests for unit roots in the time series

See Chapter 7 ldquoThe ARIMA Procedurerdquo for more information on Dickey-Fuller unit

root tests

The most popular way to transform a nonstationary series to stationarity is by dif-

ferencing Differencing of the time series is specified in the VAR statement For

example to take a simple first difference of the series X use this statement

var x(1)

In this example the change in X from one period to the next is analyzed When the se-

ries has a seasonal pattern differencing at a period equal to the length of the seasonal

cycle may be desirable For example suppose the variable X is measured quarterly

and shows a seasonal cycle over the year You can use the following statement to

analyze the series of changes from the same quarter in the previous year

var x(4)

To difference twice add another differencing period to the list For ex-

ample the following statement analyzes the series of second differences

X

t

X

t 1

X

t 1

X

t 2

= X

t

2 X

t 1

+ X

t 2

var x(11)

The following statement analyzes the seasonal second difference series

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Part 2 General Information

var x(14)

The series modeled is the 1-period difference of the 4-period difference

X

t

X

t 4

X

t 1

X

t 5

= X

t

X

t 1

X

t 4

+ X

t 5

Another way to obtain stationary series is to use a regression on time to de-trend the

data If the time series has a deterministic linear trend regressing the series on time

produces residuals that should be stationary The following statements write residuals

of X and Y to the variable RX and RY in the output data set DETREND

data a

set a

t=_n_

run

proc reg data=a

model x y = t

output out=detrend r=rx ry

run

You then use PROC STATESPACE to forecast the de-trended series RX and RY A

disadvantage of this method is that you need to add the trend back to the forecast

series in an additional step A more serious disadvantage of the de-trending method

is that it assumes a deterministic trend In practice most time series appear to have a

stochastic rather than a deterministic trend Differencing is a more flexible and often

more appropriate method

There are several other methods to handle nonstationary time series For more infor-

mation and examples refer to Brockwell and Davis (1991)

Preliminary Autoregressive Models

After computing the sample autocovariance matrices PROC STATESPACE fits a

sequence of vector autoregressive models These preliminary autoregressive models

are used to estimate the autoregressive order of the process and limit the order of the

autocovariances considered in the state vector selection process

Yule-Walker Equations for Forward and Backward Models

Unlike a univariate autoregressive model a multivariate autoregressive model has

different forms depending on whether the present observation is being predicted from

the past observations or from the future observations

Letx

t

be the r -component stationary time series given by the VAR statement after

differencing and subtracting the vector of sample means (If the NOCENTER option

is specified the mean is not subtracted) Let n be the number of observations of x

t

from the input data set

Lete

t

be a vector white noise sequence with mean vector 0 and variance matrix

p

and letn

t

be a vector white noise sequence with mean vector 0 and variance matrix

p

Let p be the order of the vector autoregressive model forx

t

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The forward autoregressive form based on the past observations is written as follows

x

t

=

p

X

i = 1

p

i

x

t i

+ e

t

The backward autoregressive form based on the future observations is written as fol-

lows

x

t

=

p

X

i = 1

p

i

x

t + i

+ n

t

LettingE

denote the expected value operator the autocovariance sequence for thex

t

series

i

is

i

= E x

t

x

0

t i

The Yule-Walker equations for the autoregressive model that matches the first p ele-

ments of the autocovariance sequence are

2

6

6

4

0

1

p 1

0

1

0

p 2

0

p 1

0

p 2

0

3

7

7

5

2

6

6

4

p

1

p

2

p

p

3

7

7

5

=

2

6

6

4

1

2

p

3

7

7

5

and

2

6

6

6

4

0

0

1

0

p 1

1

0

0

p 2

p 1

p 2

0

3

7

7

7

5

2

6

6

4

p

1

p

2

p

p

3

7

7

5

=

2

6

6

4

0

1

0

2

0

p

3

7

7

5

Here

p

i

are the coefficient matrices for the past observation form of the vector au-toregressive model and

p

i

are the coefficient matrices for the future observation

form More information on the Yule-Walker equations in the multivariate setting can

be found in Whittle (1963) and Ansley and Newbold (1979)

The innovation variance matrices for the two forms can be written as follows

p

=

0

p

X

i = 1

p

i

0

i

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p

=

0

p

X

i = 1

p

i

i

The autoregressive models are fit to the data using the preceding Yule-Walker equa-

tions with

i

replaced by the sample covariance sequenceC

i

The covariance matri-

ces are calculated as

C

i

=

1

N 1

N

X

t = i + 1

x

t

x

0

t i

Let b

p

b

p

b

p

and b

p

represent the Yule-Walker estimates of

p

p

p

and

p

respectively These matrices are written to an output data set when the OUTAR=

option is specified

When the PRINTOUT=LONG option is specified the sequence of matrices b

p

and

the corresponding correlation matrices are printed The sequence of matricesb

p isused to compute Akaike information criteria for selection of the autoregressive order

of the process

Akaike Information Criterion

The Akaike information criterion or AIC is defined as -2( maximum of log likeli-

hood )+2(number of parameters) Since the vector autoregressive models are esti-

mates from the Yule-Walker equations not by maximum likelihood the exact likeli-

hood values are not available for computing the AIC However for the vector autore-

gressive model the maximum of the log likelihood can be approximated as

l n L

n

2

l n j

b

p

j

Thus the AIC for the order p model is computed as

A I C

p

= n l n j

b

p

j + 2 p r

2

You can use the printed AIC array to compute a likelihood ratio test of the autoregres-

sive order The log-likelihood ratio test statistic for testing the order p model against

the order p 1

model is

n l n j

b

p

j + n l n j

b

p 1

j

This quantity is asymptotically distributed as a

2 withr

2 degrees of freedom if the

series is autoregressive of order p 1

It can be computed from the AIC array as

A I C

p 1

A I C

p

+ 2 r

2

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You can evaluate the significance of these test statistics with the PROBCHI function

in a SAS DATA step or with a

2 table

Determining the Autoregressive Order

Although the autoregressive models can be used for prediction their primary value is

to aid in the selection of a suitable portion of the sample covariance matrix for use in

computing canonical correlations If the multivariate time series x

t is of autoregres-sive order p then the vector of past values to lag p is considered to contain essentially

all the information relevant for prediction of future values of the time series

By default PROC STATESPACE selects the order p producing the autoregressive

model with the smallestA I C

p

If the value p for the minimumA I C

p

is less than the

value of the PASTMIN= option then p is set to the PASTMIN= value Alternatively

you can use the ARMAX= and PASTMIN= options to force PROC STATESPACE to

use an order you select

Significance Limits for Partial Autocorrelations

The STATESPACE procedure prints a schematic representation of the partial autocor-

relation matrices indicating which partial autocorrelations are significantly greater orsignificantly less than 0 Figure 1810 shows an example of this table

The STATESPACE Procedure

Schematic Representation of Partial Autocorrelations

NameLag 1 2 3 4 5 6 7 8 9 10

x ++ +

y ++

+ is gt 2std error - is lt -2std error is between

Figure 1810 Significant Partial Autocorrelations

The partial autocorrelations are from the sample partial autoregressive matrices b

p

p

The standard errors used for the significance limits of the partial autocorrelations are

computed from the sequence of matrices

p

and

p

Under the assumption that the observed series arises from an autoregressive process

of order p 1

the pth sample partial autoregressive matrix b

p

p

has an asymptotic

variance matrix 1

n

1

p

p

The significance limits for b

p

p

used in the schematic plot of the sample partial autore-

gressive sequence are derived by replacing

p

and

p

with their sample estimators

to produce the variance estimate as follows

d

V a r

b

p

p

=

1

n r p

b

1

p

b

p

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Canonical Correlation Analysis

Given the order p letp

t

be the vector of current and past values relevant to prediction

of x

t + 1

p

t

= x

0

t

x

0

t 1

x

0

t p

0

Letf

t

be the vector of current and future values

f

t

= x

0

t

x

0

t + 1

x

0

t + p

0

In the canonical correlation analysis consider submatrices of the sample covariance

matrix of p

t

andf

t

This covariance matrixV

has a block Hankel form

V =

2

6

6

6

4

C

0

C

0

1

C

0

2

C

0

p

C

0

1

C

0

2

C

0

3

C

0

p + 1

C

0

p

C

0

p + 1

C

0

p + 2

C

0

2 p

3

7

7

7

5

State Vector Selection Process

The canonical correlation analysis forms a sequence of potential state vectorsz

j

t

Examine a sequencef

j

t

of subvectors of f

t

and form the submatrixV

j consisting

of the rows and columns of V

corresponding to the components of f

j

t

and compute

its canonical correlations

The smallest canonical correlation of V

j is then used in the selection of the com-

ponents of the state vector The selection process is described in the following For

more details about this process refer to Akaike (1976)

In the following discussion the notationx

t + k j t

denotes the wide sense conditional

expectation (best linear predictor) of x

t + k

given allx

s

with s less than or equal to t

In the notationx

i t + 1

the first subscript denotes the ith component of x

t + 1

The initial state vectorz

1

t

is set tox

t

The sequencef

j

t

is initialized by setting

f

1

t

= z

1

0

t

x

1 t + 1 j t

0

= x

0

t

x

1 t + 1 j t

0

That is start by considering whether to addx

1 t + 1 j t

to the initial state vectorz

1

t

The procedure forms the submatrixV

1 corresponding tof

1

t

and computes its canon-

ical correlations Denote the smallest canonical correlation of V

1 as

m i n

If

m i n

is

significantly greater than 0x

1 t + 1 j t

is added to the state vector

If the smallest canonical correlation of V

1 is not significantly greater than 0 then a

linear combination of f

1

t

is uncorrelated with the pastp

t

Assuming that the determi-

nant of C

0

is not 0 (that is no input series is a constant) you can take the coefficient

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Chapter 18 Details

of x

1 t + 1 j t

in this linear combination to be 1 Denote the coefficients of z

1

t

in this

linear combination as

This gives the relationship

x

1 t + 1 j t

=

0

x

t

Therefore the current state vector already contains all the past information useful forpredicting

x

1 t + 1

and any greater leads of x

1 t

The variablex

1 t + 1 j t

is not added to

the state vector nor are any termsx

1 t + k j t

considered as possible components of the

state vector The variablex

1

is no longer active for state vector selection

The process described forx

1 t + 1 j t

is repeated for the remaining elements of f

t

The

next candidate for inclusion in the state vector is the next component of f

t

corre-

sponding to an active variable Components of f

t

corresponding to inactive variables

that produced a zero

m i n

in a previous step are skipped

Denote the next candidate asx

l t + k j t

The vectorf

j

t

is formed from the current state

vector andx

l t + k j t

as follows

f

j

t

= z

j

0

t

x

l t + k j t

0

The matrixV

j is formed fromf

j

t

and its canonical correlations are computed The

smallest canonical correlation of V

j is judged to be either greater than or equal to 0

If it is judged to be greater than 0x

l t + k j t

is added to the state vector If it is judged

to be 0 then a linear combination of f

j

t

is uncorrelated with thep

t

and the variable

x

l

is now inactive

The state vector selection process continues until no active variables remain

Testing Significance of Canonical Correlations For each step in the canonical correlation sequence the significance of the small-

est canonical correlation

m i n

is judged by an information criterion from Akaike

(1976) This information criterion is

n l n 1

2

m i n

r p + 1 q + 1

where q is the dimension of f

j

t

at the current step r is the order of the state vector p

is the order of the vector autoregressive process and

is the value of the SIGCORR=

option The default is SIGCORR=2 If this information criterion is less than or equal

to 0

m i n

is taken to be 0 otherwise it is taken to be significantly greater than 0

(Do not confuse this information criterion with the AIC)

Variables inx

t + p j t

are not added in the model even with positive information crite-

rion because of the singularity of V

You can force the consideration of more candi-

date state variables by increasing the size of theV

matrix by specifying a PASTMIN=

option value larger than p

Printing the Canonical Correlations

To print the details of the canonical correlation analysis process specify the CAN-

CORR option in the PROC STATESPACE statement The CANCORR option prints

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the candidate state vectors the canonical correlations and the information criteria for

testing the significance of the smallest canonical correlation

Bartlettrsquos

2 and its degrees of freedom are also printed when the CANCORR option

is specified The formula used for Bartlettrsquos

2 is

2

= n 5 r p + 1 q + 1 l n 1

2

m i n

withr p + 1 q + 1

degrees of freedom

Figure 1811 shows the output of the CANCORR option for the introductory example

shown in the Getting Started section of this chapter

The STATESPACE Procedure

Canonical Correlations Analysis

Information Chi

x(TT) y(TT) x(T+1T) Criterion Square DF

1 1 0237045 3566167 114505 4

Information Chi

x(TT) y(TT) x(T+1T) y(T+1T) Criterion Square DF

1 1 0238244 0056565 -535906 0636134 3

Information Chi

x(TT) y(TT) x(T+1T) x(T+2T) Criterion Square DF

1 1 0237602 0087493 -446312 1525353 3

Figure 1811 Canonical Correlations Analysis

New variables are added to the state vector if the information criteria are positive Inthis example

y

t + 1 j t

andx

t + 2 j t

are not added to the state space vector because the

information criteria for these models are negative

If the information criterion is nearly 0 then you may want to investigate models

that arise if the opposite decision is made regarding

m i n

This investigation can be

accomplished by using a FORM statement to specify part or all of the state vector

Preliminary Estimates of F

When a candidate variablex

l t + k j t

yields a zero

m i n

and is not added to the state

vector a linear combination of f

j

t

is uncorrelated with thep

t

Because of the method

used to construct thef

j

t

sequence the coefficient of x

l t + k j t

inl

can be taken as 1

Denote the coefficients of z

j

t

in this linear combination as l

This gives the relationship

x

l t + k j t

= l

0

z

j

t

The vectorl

is used as a preliminary estimate of the first r columns of the row of the

transition matrixF

corresponding tox

l t + k 1 j t

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Parameter Estimation

The model isz

t + 1

= F z

t

+ G e

t + 1

wheree

t

is a sequence of independent multivari-

ate normal innovations with mean vector 0 and variance

e e

The observed sequence

x

t

composes the first r components of z

t

and thusx

t

= H z

t

where H is ther s

matrix I

r

0

LetE

be ther n

matrix of innovations

E = e

1

e

n

If the number of observations n is reasonably large the log likelihood L can be

approximated up to an additive constant as follows

L =

n

2

l n j

e e

j

1

2

t r a c e

1

e e

E E

0

The elements of

e e

are taken as free parameters and are estimated as follows

S

0

=

1

n

E E

0

Replacing

e e

byS

0

in the likelihood equation the log likelihood up to an additive

constant is

L =

n

2

l n j S

0

j

Letting B be the backshift operator the formal relation betweenx

t

ande

t

is

x

t

= H I B F

1

G e

t

e

t

= H I B F

1

G

1

x

t

=

1

X

i = 0

i

x

t i

LettingC

i

be the ith lagged sample covariance of x

t

and neglecting end effects the

matrixS

0

is

S

0

=

1

X

i j = 0

i

C

i + j

0

j

For the computation of S

0

the infinite sum is truncated at the value of the KLAG=

option The value of the KLAG= option should be large enough that the sequence

i

is approximately 0 beyond that point

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Let

be the vector of free parameters in theF

andG

matrices The derivative of the

log likelihood with respect to the parameter

is

L

=

n

2

t r a c e

S

1

0

S

0

The second derivative is

2

L

0

=

n

2

t r a c e

S

1

0

S

0

0

S

1

0

S

0

t r a c e

S

1

0

2

S

0

0

Near the maximum the first term is unimportant and the second term can be approx-

imated to give the following second derivative approximation

2

L

0

=

n t r a c e

S

1

0

E

E

0

0

The first derivative matrix and this second derivative matrix approximation are com-

puted from the sample covariance matrixC

0

and the truncated sequence

i

The

approximate likelihood function is maximized by a modified Newton-Raphson algo-

rithm employing these derivative matrices

The matrixS

0

is used as the estimate of the innovation covariance matrix

e e

The

negative of the inverse of the second derivative matrix at the maximum is used as an

approximate covariance matrix for the parameter estimates The standard errors of

the parameter estimates printed in the parameter estimates tables are taken from the

diagonal of this covariance matrix The parameter covariance matrix is printed when

the COVB option is specified

If the data are nearly nonstationary a better estimate of

e e

and the other parameters

can sometimes be obtained by specifying the RESIDEST option The RESIDEST

option estimates the parameters using conditional least squares instead of maximum

likelihood

The residuals are computed using the state space equation and the sample mean val-

ues of the variables in the model as start-up values The estimate of S

0

is then com-

puted using the residuals from the ith observation on where i is the maximum number

of times any variable occurs in the state vector A multivariate Gauss-Marquardt al-

gorithm is used to minimizej S

0

j

Refer to Harvey (1981a) for a further description

of this method

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Forecasting

Given estimates of F

G

and

e e

forecasts of x

t

are computed from the conditional

expectation of z

t

In forecasting the parameters F G and

e e

are replaced with the estimates or by val-

ues specified in the RESTRICT statement One-step-ahead forecasting is performedfor the observation

x

t

wheret n b

Heren

is the number of observations and

b is the value of the BACK= option For the observationx

t

wheret n b

m-

step-ahead forecasting is performed form = t n + b

The forecasts are generated

recursively with the initial conditionz

0

= 0

The m-step-ahead forecast of z

t + m

isz

t + m j t

wherez

t + m j t

denotes the conditional

expectation of z

t + m

given the information available at time t The m-step-ahead

forecast of x

t + m

isx

t + m j t

= H z

t + m j t

where the matrixH = I

r

0

Let

i

= F

i

G

Note that the lasts r

elements of z

t

consist of the elements of x

u j t

foru t

The state vector z

t + m

can be represented as

z

t + m

= F

m

z

t

+

m 1

X

i = 0

i

e

t + m i

Sincee

t + i j t

= 0

fori 0

the m-step-ahead forecastz

t + m j t

is

z

t + m j t

= F

m

z

t

= F z

t + m 1 j t

Therefore the m-step-ahead forecast of x

t + m

is

x

t + m j t

= H z

t + m j t

The m-step-ahead forecast error is

z

t + m

z

t + m j t

=

m 1

X

i = 0

i

e

t + m i

The variance of the m-step-ahead forecast error is

V

z m

=

m 1

X

i = 0

i

e e

0

i

LettingV

z 0

= 0

the variance of the m-step-ahead forecast error of z

t + m

V

z m

can

be computed recursively as follows

V

z m

= V

z m 1

+

m 1

e e

0

m 1

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The variance of the m-step-ahead forecast error of x

t + m

is ther r

left upper sub-

matrix of V

z m

that is

V

x m

= H V

z m

H

0

Unless the NOCENTER option is specified the sample mean vector is added to the

forecast When differencing is specified the forecasts xt + m j t

plus the sample mean

vector are integrated back to produce forecasts for the original series

Lety

t

be the original series specified by the VAR statement with some 0 values

appended corresponding to the unobserved past observations Let B be the backshift

operator and let B

be thes s

matrix polynomial in the backshift operator

corresponding to the differencing specified by the VAR statement The off-diagonal

elements of

i

are 0 Note that

0

= I

s

whereI

s

is thes s

identity matrix Then

z

t

= B y

t

This gives the relationship

y

t

=

1

B z

t

=

1

X

i = 0

i

z

t i

where

1

B =

P

1

i = 0

i

B

i and

0

= I

s

The m-step-ahead forecast of y

t + m

is

y

t + m j t

=

m 1

X

i = 0

i

z

t + m i j t

+

1

X

i = m

i

z

t + m i

The m-step-ahead forecast error of y

t + m

is

m 1

X

i = 0

i

z

t + m i

z

t + m i j t

=

m 1

X

i = 0

i

X

u = 0

u

i u

e

t + m i

LettingV

y 0

= 0

the variance of the m-step-ahead forecast error of y

t + m

V

y m

is

V

y m

=

m 1

X

i = 0

i

X

u = 0

u

i u

e e

i

X

u = 0

u

i u

0

= V

y m 1

+

m 1

X

u = 0

u

m 1 u

e e

m 1

X

u = 0

u

m 1 u

0

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Chapter 18 Details

Relation of ARMA and State Space Forms

Every state space model has an ARMA representation and conversely every ARMA

model has a state space representation This section discusses this equivalence The

following material is adapted from Akaike (1974) where there is a more complete

discussion Pham-Dinh-Tuan (1978) also contains a discussion of this material

Suppose you are given the following ARMA model

B x

t

= B e

t

or in more detail

x

t

1

x

t 1

p

x

t p

= e

t

+

1

e

t 1

+ +

q

e

t q

(1)

wheree

t

is a sequence of independent multivariate normal random vectors with mean

0 and variance matrix

e e

B is the backshift operator (B x

t

= x

t 1

) B

and

B

are matrix polynomials in B andx

t

is the observed process

If the roots of the determinantial equationj B j = 0

are outside the unit circle in

the complex plane the model can also be written as

x

t

=

1

B B e

t

=

1

X

i = 0

i

e

t i

The

i

matrices are known as the impulse response matrices and can be computed

as

1

B B

You can assume p q

since if this is not initially true you can add more terms

i

that are identically 0 without changing the model

To write this set of equations in a state space form proceed as follows Letx

t + i j t

be

the conditional expectation of x

t + i

givenx

w

forw t

The following relations hold

x

t + i j t

=

1

X

j = i

j

e

t + i j

x

t + i j t + 1

= x

t + i j t

+

i 1

e

t + 1

However from equation (1) you can derive the following relationship

x

t + p j t

=

1

x

t + p 1 j t

+ +

p

x

t

(2)

Hence wheni = p

you can substitute forx

t + p j t

in the right-hand side of equation

(2) and close the system of equations

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This substitution results in the following model in the state space form

z

t + 1

= F z

t

+ G e

t + 1

2

6

6

6

4

x

t + 1

x

t + 2 j t + 1

x

t + p j t + 1

3

7

7

7

5

=

2

6

6

4

0 I 0 0

0 0 I 0

p

p 1

1

3

7

7

5

2

6

6

6

4

x

t

x

t + 1 j t

x

t + p 1 j t

3

7

7

7

5

+

2

6

6

4

I

1

p 1

3

7

7

5

e

t + 1

Note that the state vectorz

t

is composed of conditional expectations of x

t

and the

first r components of z

t

are equal tox

t

The state space form can be cast into an ARMA form by solving the system of dif-

ference equations for the first r components

When converting from an ARMA form to a state space form you can generate a

state vector larger than needed that is the state space model may not be a minimal

representation When going from a state space form to an ARMA form you can have

nontrivial common factors in the autoregressive and moving average operators that

yield an ARMA model larger than necessary

If the state space form used is not a minimal representation some but not all compo-

nents of x

t + i j t

may be linearly dependent This situation corresponds to

p

p 1

being of less than full rank when B

and B

have no common nontrivial

left factors In this casez

t

consists of a subset of the possible components of

x

t + i j t

i = 1 2 p 1

However once a component of x

t + i j t

(for example the

jth one) is linearly dependent on the previous conditional expectations then all sub-

sequent jth components of x

t + k j t

fork i

must also be linearly dependent Note

that in this case equivalent but seemingly different structures can arise if the order of

the components withinx

t

is changed

OUT= Data Set

The forecasts are contained in the output data set specified by the OUT= option on the

PROC STATESPACE statement The OUT= data set contains the following variables

the BY variables

the ID variable

the VAR statement variables These variables contain the actual values from

the input data set

FORi numeric variables containing the forecasts The variable FORi contains

the forecasts for the ith variable in the VAR statement list Forecasts are one-

step-ahead predictions until the end of the data or until the observation specified

by the BACK= option

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Chapter 18 Details

RESi numeric variables containing the residual for the forecast of the ith vari-

able in the VAR statement list For forecast observations the actual values are

missing and the RESi variables contain missing values

STDi numeric variables containing the standard deviation for the forecast of

the ith variable in the VAR statement list The values of the STDi variables can

be used to construct univariate confidence limits for the corresponding fore-casts However such confidence limits do not take into account the covariance

of the forecasts

OUTAR= Data Set

The OUTAR= data set contains the estimates of the preliminary autoregressive mod-

els The OUTAR= data set contains the following variables

ORDER a numeric variable containing the order p of the autoregressive model

that the observation represents

AIC a numeric variable containing the value of the information criterionA I C

p

SIGFl numeric variables containing the estimate of the innovation covariance

matrices for the forward autoregressive models The variable SIGFl contains

the lth column of b

p

in the observations with ORDER= p

SIGBl numeric variables containing the estimate of the innovation covariance

matrices for the backward autoregressive models The variable SIGBl contains

the lth column of b

p

in the observations with ORDER= p

FORk ndashl numeric variables containing the estimates of the autoregressive pa-

rameter matrices for the forward models The variable FORk ndashl contains the

lth column of the lag k autoregressive parameter matrix b

p

k

in the observations

with ORDER= p

BACk ndashl numeric variables containing the estimates of the autoregressive pa-

rameter matrices for the backward models The variable BACk ndashl contains the

lth column of the lag k autoregressive parameter matrix b

p

k

in the observations

with ORDER= p

The estimates for the order p autoregressive model can be selected as those observa-

tions with ORDER= p Within these observations the k lth element of

p

i

is given by

the value of the FORindashl variable in the k th observation The k lth element of

p

i

is

given by the value of BACindashl variable in the k th observation The k lth element of

p

is given by SIGFl in the k th observation The k lth element of

p

is given by SIGBlin the k th observation

Table 181 shows an example of the OUTAR= data set with ARMAX=3 andx

t

of

dimension 2 In Table 181 i j

indicate the ijth element of the matrix

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Part 2 General Information

Table 181 Values in the OUTAR= Data Set

Obs ORDER AIC SIGF1 SIGF2 SIGB1 SIGB2 FOR1ndash 1 FOR1ndash 2 FOR2ndash 1 FOR2ndash 2 FOR3ndash 1

1 0 AIC0

0 1 1

0 1 2

0 1 1

0 1 2

2 0 AIC0

0 2 1

0 2 2

0 2 1

0 2 2

3 1 AIC1

1 1 1

1 1 2

1 1 1

1 1 2

1

1

1 1

1

1

1 2

4 1 AIC1

1 2 1

1 1 2

1 2 1

1 1 2

1

1

2 1

1

1

2 2

5 2 AIC2

2 1 1

2 1 2

2 1 1

2 1 2

2

1

1 1

2

1

1 2

2

2

1 1

2

2

1 2

6 2 AIC2

2 2 1

2 1 2

2 2 1

2 1 2

2

1

2 1

2

1

2 2

2

2

2 1

2

2

2 2

7 3 AIC3

3 1 1

3 1 2

3 1 1

3 1 2

3

1

1 1

3

1

1 2

3

2

1 1

3

2

1 2

3

3

1 1

8 3 AIC3

3 2 1

3 1 2

3 2 1

3 1 2

3

1

2 1

3

1

2 2

3

2

2 1

3

2

2 2

3

3

2 1

Obs FOR3ndash 2 BACK1ndash 1 BACK1ndash 2 BACK2ndash 1 BACK2ndash 2 BACK3ndash 1 BACK3ndash 2

1

2

3

1

1

1 1

1

1

1 2

4

1

1

2 1

1

1

2 2

5

2

1

1 1

2

1

1 2

2

2

1 1

2

2

1 2

6

2

1

2 1

2

1

2 2

2

2

2 1

2

2

2 2

7

3

3

1 2

3

1

1 1

3

1

1 2

3

2

1 1

3

2

1 2

3

3

1 1

3

3

1 2

8

3

3

2 2

3

1

2 1

3

1

2 2

3

2

2 1

3

2

2 2

3

3

2 1

3

3

2 2

The estimated autoregressive parameters can be used in the IML procedure to obtain

autoregressive estimates of the spectral density function or forecasts based on the

autoregressive models

OUTMODEL= Data Set

The OUTMODEL= data set contains the estimates of theF

andG

matrices and their

standard errors the names of the components of the state vector and the estimates of

the innovation covariance matrix The variables contained in the OUTMODEL= data

set are as follows

the BY variables

STATEVEC a character variable containing the name of the component of the

state vector corresponding to the observation The STATEVEC variable has

the value STD for standard deviations observations which contain the standarderrors for the estimates given in the preceding observation

Fndash j numeric variables containing the columns of theF

matrix The variable

Fndash j contains the jth column of F

The number of Fndash j variables is equal to

the value of the DIMMAX= option If the model is of smaller dimension the

extraneous variables are set to missing

Gndash j numeric variables containing the columns of theG

matrix The variable

Gndash j contains the jth column of G

The number of Gndash j variables is equal to r

the dimension of x

t

given by the number of variables in the VAR statement

SIGndash j numeric variables containing the columns of the innovation covariance

matrix The variable SIGndash j contains the jth column of

e e

There are r vari-ables SIGndash j

Table 182 shows an example of the OUTMODEL= data set withx

t

= x

t

y

t

0

z

t

= x

t

y

t

x

t + 1 j t

0 and DIMMAX=4 In Table 182F

i j

andG

i j

are the ijth

elements of F

andG

respectively Note that all elements for Fndash4 are missing because

F

is a3 3

matrix

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Chapter 18 Details

Table 182 Value in the OUTMODEL= Data Set

Obs STATEVEC Fndash 1 Fndash 2 Fndash 3 Fndash 4 Gndash 1 Gndash 2 SIGndash 1 SIGndash 2

1 X(TT) 0 0 1 1 0

1 1

1 2

2 STD

3 Y(TT) F2 1

F2 2

F2 3

0 1

2 1

2 2

4 STD std F2 1

std F2 2

std F2 3

5 X(T+1T) F3 1

F3 2

F3 3

G3 1

G3 2

6 STD std F3 1

std F3 2

std F3 3

std G3 1

std G3 2

Printed Output

The printed output produced by the STATESPACE procedure is described in the fol-

lowing

1 descriptive statistics which include the number of observations used the

names of the variables their means and standard deviations (Std) and the dif-

ferencing operations used

2 the Akaike information criteria for the sequence of preliminary autoregressive

models

3 if the PRINTOUT=LONG option is specified the sample autocovariance ma-

trices of the input series at various lags

4 if the PRINTOUT=LONG option is specified the sample autocorrelation ma-

trices of the input series

5 a schematic representation of the autocorrelation matrices showing the signif-

icant autocorrelations

6 if the PRINTOUT=LONG option is specified the partial autoregressive ma-

trices (These are

p

p

as described in Preliminary Autoregressive Modelsearlier in this chapter)

7 a schematic representation of the partial autocorrelation matrices showing the

significant partial autocorrelations

8 the Yule-Walker estimates of the autoregressive parameters for the autoregres-

sive model with the minimum AIC

9 if the PRINTOUT=LONG option is specified the autocovariance matrices of

the residuals of the minimum AIC model This is the sequence of estimated

innovation variance matrices for the solutions of the Yule-Walker equations

10 if the PRINTOUT=LONG option is specified the autocorrelation matrices of the residuals of the minimum AIC model

11 If the CANCORR option is specified the canonical correlations analysis for

each potential state vector considered in the state vector selection process

This includes the potential state vector the canonical correlations the informa-

tion criterion for the smallest canonical correlation Bartlettrsquos

2 statistic (ldquoChi

Squarerdquo) for the smallest canonical correlation and the degrees of freedom of

Bartlettrsquos

2

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Part 2 General Information

12 the components of the chosen state vector

13 the preliminary estimate of the transition matrixF

the input matrixG

and

the variance matrix for the innovations

e e

14 if the ITPRINT option is specified the iteration history of the likelihood max-

imization For each iteration this shows the iteration number the number of step halvings the determinant of the innovation variance matrix the damping

factor Lambda and the values of the parameters

15 the state vector printed again to aid interpretation of the following listing of F

andG

16 the final estimate of the transition matrixF

17 the final estimate of the input matrixG

18 the final estimate of the variance matrix for the innovations

e e

19 a table listing the estimates of the free parameters inF

andG

and their standarderrors and t statistics

20 if the COVB option is specified the covariance matrix of the parameter esti-

mates

21 if the COVB option is specified the correlation matrix of the parameter esti-

mates

22 if the PRINT option is specified the forecasts and their standard errors

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Chapter 18 Details

ODS Table Names

PROC STATESPACE assigns a name to each table it creates You can use these

names to reference the table when using the Output Delivery System (ODS) to select

tables and create output data sets These names are listed in the following table For

more information on ODS see Chapter 6 ldquoUsing the Output Delivery Systemrdquo

Table 183 ODS Tables Produced in PROC STATESPACE

ODS Table Name Description Option

NObs Number of observations default

Summary Simple summary statistics table default

InfoCriterion Information criterion table default

CovLags Covariance Matrices of Input Series PRINTOUT=LONG

CorrLags Correlation Matrices of Input Series PRINTOUT=LONG

PartialAR Partial Autoregressive Matrices PRINTOUT=LONG

YWEstimates Yule-Walker Estimates for Minimum AIC default

CovResiduals Covariance of Residuals PRINTOUT=LONGCorrResiduals Residual Correlations from AR Models PRINTOUT=LONG

StateVector State vector table default

CorrGraph Schematic Representation of Correlations default

TransitionMatrix Transition Matrix default

InputMatrix Input Matrix default

VarInnov Variance Matrix for the Innovation default

CovB Covariance of Parameter Estimates COVB

CorrB Correlation of Parameter Estimates COVB

CanCorr Canonical Correlation Analysis CANCORR

IterHistory Iterative Fitting table ITPRINT

ParameterEstimates Parameter Estimates Table default

Forecasts Forecasts Table PRINT

ConvergenceStatus Convergence Status Table default

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Part 2 General Information

Example

Example 181 Series J from Box and Jenkins

This example analyzes the gas furnace data (series J) from Box and Jenkins (The

data are not shown Refer to Box and Jenkins (1976) for the data)

First a model is selected and fit automatically using the following statements

title1 rsquoGas Furnace Datarsquo

title2 rsquoBox amp Jenkins Series Jrsquo

title3 rsquoAutomatically Selected Modelrsquo

proc statespace data=seriesj cancorr

var x y

run

The results for the automatically selected model are shown in Output 1811

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Chapter 18 Example

Output 1811 Results for Automatically Selected Model

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Number of Observations 296

Standard

Variable Mean Error

x -005683 1072766

y 5350912 3202121

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Information Criterion for Autoregressive Models

Lag=0 Lag=1 Lag=2 Lag=3 Lag=4 Lag=5 Lag=6 Lag=7 Lag=8

6513862 -103357 -163296 -164512 -165152 -164891 -164934 -164315 -163856

Information

Criterion for

Autoregressive

Models

Lag=9 Lag=10

-16348 -163359

Schematic Representation of Correlations

NameLag 0 1 2 3 4 5 6 7 8 9 10

x +- +- +- +- +- +- +- +- +- +- +-

y -+ -+ -+ -+ -+ -+ -+ -+ -+ -+ -+

+ is gt 2std error - is lt -2std error is between

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Part 2 General Information

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Schematic Representation of Partial Autocorrelations

NameLag 1 2 3 4 5 6 7 8 9 10

x + - + -

y -+ -- - + +

+ is gt 2std error - is lt -2std error is between

Yule-Walker Estimates for Minimum AIC

------Lag=1------ ------Lag=2------ ------Lag=3------ ------Lag=4------

x y x y x y x y

x 1925887 -000124 -120166 0004224 0116918 -000867 0104236 0003268

y 0050496 1299793 -002046 -03277 -071182 -025701 0195411 0133417

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Canonical Correlations Analysis

Information Chi

x(TT) y(TT) x(T+1T) Criterion Square DF

1 1 0804883 2929228 3047481 8

Information Chix(TT) y(TT) x(T+1T) y(T+1T) Criterion Square DF

1 1 0906681 0607529 1223358 1347237 7

Information Chi

x(TT) y(TT) x (T+1T) y (T+1T) x (T+2T) Criterion Square DF

1 1 0909434 0610278 0186274 -154701 1034705 6

Information Chi

x(TT) y(TT) x (T+1T) y (T+1T) y (T+2T) Criterion Square DF

1 1 091014 0618937 0206823 0940392 1280924 6

Information Chi

x(TT) y(TT) x(T+1T) y(T+1T) y(T+2T) y(T+3T) Criterion Square DF

1 1 0912963 0628785 0226598 0083258 -794103 2041584 5

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Chapter 18 Example

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Selected Statespace Form and Preliminary Estimates

State Vector

x(TT) y(TT) x(T+1T) y(T+1T) y(T+2T)

Estimate of Transition Matrix

0 0 1 0 0

0 0 0 1 0

-084718 0026794 1711715 -005019 0

0 0 0 0 1

-019785 0334274 -018174 -123557 1787475

Input Matrix for Innovation

1 0

0 11925887 -000124

0050496 1299793

0142421 1361696

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Selected Statespace Form and Preliminary Estimates

Variance Matrix for Innovation

0035274 -000734

-000734 0097569

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Part 2 General Information

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Selected Statespace Form and Fitted Model

State Vector

x(TT) y(TT) x(T+1T) y(T+1T) y(T+2T)

Estimate of Transition Matrix

0 0 1 0 0

0 0 0 1 0

-086192 0030609 1724235 -005483 0

0 0 0 0 1

-034839 0292124 -009435 -109823 1671418

Input Matrix for Innovation

1 0

0 1192442 -000416

0015621 1258495

008058 1353204

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Selected Statespace Form and Fitted Model

Variance Matrix for Innovation

0035579 -000728

-000728 0095577

Parameter Estimates

Standard

Parameter Estimate Error t Value

F(31) -086192 0072961 -1181

F(32) 0030609 0026167 117

F(33) 1724235 0061599 2799

F(34) -005483 0030169 -182

F(51) -034839 0135253 -258

F(52) 0292124 0046299 631

F(53) -009435 0096527 -098F(54) -109823 0109525 -1003

F(55) 1671418 0083737 1996

G(31) 1924420 0058162 3309

G(32) -000416 0035255 -012

G(41) 0015621 0095771 016

G(42) 1258495 0055742 2258

G(51) 0080580 0151622 053

G(52) 1353204 0091388 1481

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Chapter 18 Example

The two series are believed to have a transfer function relation with the gas rate

(variable X) as the input and the CO2

concentration (variable Y) as the output Since

the parameter estimates shown in Output 1811 support this kind of model the model

is reestimated with the feedback parameters restricted to 0 The following statements

fit the transfer function (no feedback) model

title3 rsquoTransfer Function Modelrsquo

proc statespace data=seriesj printout=none

var x y

restrict f(32)=0 f(34)=0

g(32)=0 g(41)=0 g(51)=0

run

The last 2 pages of the output are shown in Output 1812

Output 1812 STATESPACE Output for Transfer Function Model

Gas Furnace Data

Box amp Jenkins Series J

Transfer Function Model

The STATESPACE Procedure

Selected Statespace Form and Fitted Model

State Vector

x(TT) y(TT) x(T+1T) y(T+1T) y(T+2T)

Estimate of Transition Matrix

0 0 1 0 0

0 0 0 1 0

-068882 0 1598717 0 0

0 0 0 0 1

-035944 0284179 -00963 -107313 1650047

Input Matrix for Innovation

1 0

0 1

1923446 0

0 1260856

0 1346332

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Part 2 General Information

Gas Furnace Data

Box amp Jenkins Series J

Transfer Function Model

The STATESPACE Procedure

Selected Statespace Form and Fitted Model

Variance Matrix for Innovation

0036995 -00072

-00072 0095712

Parameter Estimates

Standard

Parameter Estimate Error t Value

F(31) -068882 0050549 -1363

F(33) 1598717 0050924 3139

F(51) -035944 0229044 -157

F(52) 0284179 0096944 293

F(53) -009630 0140876 -068

F(54) -107313 0250385 -429

F(55) 1650047 0188533 875G(31) 1923446 0056328 3415

G(42) 1260856 0056464 2233

G(52) 1346332 0091086 1478

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Chapter 18 References

References

Akaike H (1974) Markovian Representation of Stochastic Processes and Its Ap-

plication to the Analysis of Autoregressive Moving Average Processes Annals

of the Institute of Statistical Mathematics 26 363-387

Akaike H (1976) Canonical Correlations Analysis of Time Series and the Use of

an Information Criterion in Advances and Case Studies in System Identification

eds R Mehra and DG Lainiotis New York Academic Press

Anderson TW (1971) The Statistical Analysis of Time Series New York John

Wiley amp Sons

Ansley CF and Newbold P (1979) Multivariate Partial Autocorrelations Pro-

ceedings of the Business and Economic Statistics Section American Statistical

Association 349-353

Box GEP and Jenkins G (1976) Time Series Analysis Forecasting and Control

San Francisco Holden-Day

Brockwell PJ and Davis RA (1991) Time Series Theory and Methods 2nd Edi-

tion Springer-Verlag

Hannan EJ (1970) Multiple Time Series New York John Wiley amp Sons

Hannan EJ (1976) The Identification and Parameterization of ARMAX and State

Space Forms Econometrica 44 713-722

Harvey AC (1981a) The Econometric Analysis of Time Series New York John

Wiley amp Sons

Harvey AC (1981b) Time Series Models New York John Wiley amp Sons

Jones RH (1974) Identification and Autoregressive Spectrum Estimation IEEE Transactions on Automatic Control AC-19 894-897

Pham-Dinh-Tuan (1978) On the Fitting of Multivariate Processes of the

Autoregressive-Moving Average Type Biometrika 65 99-107

Priestley MB (1980) System Identification Kalman Filtering and Stochastic Con-

trol in Directions in Time Series eds DR Brillinger and GC Tiao Institute

of Mathematical Statistics

Whittle P (1963) On the Fitting of Multivariate Autoregressions and the Approxi-

mate Canonical Factorization of a Spectral Density Matrix Biometrika 50 129-

134

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The correct bibliographic citation for this manual is as follows SAS Institute Inc SAS

ET S U serrsquos Gui de Version 8 Cary NC SAS Institute Inc 1999 1546 pp

SASET S Userrsquos Guide Version 8

Copyright copy 1999 by S AS I ns t it u t e I nc Ca ry N C U S A

ISBN 1ndash58025ndash489ndash6

All r ight s res erved P r int ed in t he U n it ed St a t es of A merica N o pa rt of t his p u blica t ionma y b e reprodu ced s t ored in a ret r ieva l s ys t em or t ra ns mit t ed in a ny form or b y a nymeans electronic mechanical photocopying or otherwise without the prior writtenpermission of the publisher SAS Institute Inc

US Government Restricted Rights Notice Use du plicat ion or disclosure of the

software by the government is subject to restrictions as set forth in FAR 52227ndash19Commercial Computer Softw ar e-Restricted Rights (J une 1987)

SAS Institute Inc SAS Campus Drive Cary North Carolina 27513

1st print ing October 1999

S AS reg a nd a ll ot her SA S I ns t it u t e I nc p rodu ct or s ervice na m es a re regis t ered t ra dema rkst d k f SA S I t it t I i t h U S A d t h t i reg i di t U SA

Page 5: Statespace Model

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Chapter 18 Overview

Following Akaike (1976) the procedure first fits a sequence of unrestricted vector

autoregressive (VAR) models and computes Akaikersquos information criterion (AIC) for

each model The vector autoregressive models are estimated using the sample au-

tocovariance matrices and the Yule-Walker equations The order of the VAR model

producing the smallest Akaike information criterion is chosen as the order (number

of lags into the past) to use in the canonical correlation analysis

The elements of the state vector are then determined via a sequence of canonical cor-

relation analyses of the sample autocovariance matrices through the selected order

This analysis computes the sample canonical correlations of the past with an increas-

ing number of steps into the future Variables that yield significant correlations are

added to the state vector those that yield insignificant correlations are excluded from

further consideration The importance of the correlation is judged on the basis of

another information criterion proposed by Akaike See the section Canonical Cor-

relation Analysis for details If you specify the state vector explicitly these model

identification steps are omitted

Once the state vector is determined the state space model is fit to the data The free

parameters in the F G and

e e

matrices are estimated by approximate maximumlikelihood By default the

F

andG

matrices are unrestricted except for identifia-

bility requirements Optionally conditional least-squares estimates can be computed

You can impose restrictions on elements of theF

andG

matrices

After the parameters are estimated forecasts are produced from the fitted state space

model using the Kalman filtering technique If differencing was specified the fore-

casts are integrated to produce forecasts of the original input variables

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Part 2 General Information

Getting Started

The following introductory example uses simulated data for two variables X and Y

The following statements generate the X and Y series

data inx=10 y=40

x1=0 y1=0

a1=0 b1=0

iseed=123

do t=-100 to 200

a=rannor(iseed)

b=rannor(iseed)

dx = 05x1 + 03y1 + a - 02a1 - 01b1

dy = 03x1 + 05y1 + b

x = x + d x + 2 5

y = y + d y + 2 5

if t gt= 0 then output

x1 = dx y1 = dy

a 1 = a b 1 = b

end

keep t x y

run

The simulated series X and Y are shown in Figure 181

Figure 181 Example Series

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Chapter 18 Getting Started

Automatic State Space Model Selection

The STATESPACE procedure is designed to automatically select the best state space

model for forecasting the series You can specify your own model if you wish and

you can use the output from PROC STATESPACE to help you identify a state space

model However the easiest way to use PROC STATESPACE is to let it choose the

model

Stationarity and Differencing

Although PROC STATESPACE selects the state space model automatically it does

assume that the input series are stationary If the series are nonstationary then the

process may fail Therefore the first step is to examine your data and test to see if

differencing is required (See the section Stationarity and Differencing later in this

chapter for further discussion of this issue)

The series shown in Figure 181 are nonstationary In order to forecast X and Y

with a state space model you must difference them (or use some other de-trending

method) If you fail to difference when needed and try to use PROC STATESPACE

with nonstationary data an inappropriate state space model may be selected and themodel estimation may fail to converge

The following statements identify and fit a state space model for the first differences

of X and Y and forecast X and Y 10 periods ahead

proc statespace data=in out=out lead=10

var x(1) y(1)

id t

run

The DATA= option specifies the input data set and the OUT= option specifies theoutput data set for the forecasts The LEAD= option specifies forecasting 10 obser-

vations past the end of the input data The VAR statement specifies the variables to

forecast and specifies differencing The notation X(1) Y(1) specifies that the state

space model analyzes the first differences of X and Y

Descriptive Statistics and Preliminary Autoregressions

The first page of the printed output produced by the preceding statements is shown in

Figure 182

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Part 2 General Information

The STATESPACE Procedure

Number of Observations 200

Standard

Variable Mean Error

x 0144316 1233457 Has been differenced With period(s) = 1

y 0164871 1304358 Has been differenced

With period(s) = 1

The STATESPACE Procedure

Information Criterion for Autoregressive Models

Lag=0 Lag=1 Lag=2 Lag=3 Lag=4 Lag=5 Lag=6 Lag=7 Lag=8

149697 8387786 5517099 1205986 1536952 2179538 2400638 2988874 3355708

Information

Criterion for Autoregressive

Models

Lag=9 Lag=10

4117606 4770222

Schematic Representation of Correlations

NameLag 0 1 2 3 4 5 6 7 8 9 10

x ++ ++ ++ ++ ++ ++ + + +

y ++ ++ ++ ++ ++ + + + +

+ is gt 2std error - is lt -2std error is between

Figure 182 Descriptive Statistics and VAR Order Selection

Descriptive statistics are printed first giving the number of nonmissing observations

after differencing and the sample means and standard deviations of the differenced

series The sample means are subtracted before the series are modeled (unless the

NOCENTER option is specified) and the sample means are added back when the

forecasts are produced

LetX

t

andY

t

be the observed values of X and Y and letx

t

andy

t

be the values of X

and Y after differencing and subtracting the mean difference The seriesx

t

modeled

by the STATEPSPACE procedure is

x

t

=

x

t

y

t

=

1 B X

t

0 1 4 4 3 1 6

1 B Y

t

0 1 6 4 8 7 1

where B represents the backshift operator

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Chapter 18 Getting Started

After the descriptive statistics PROC STATESPACE prints the Akaike information

criterion (AIC) values for the autoregressive models fit to the series The smallest AIC

value in this case 5517 at lag 2 determines the number of autocovariance matrices

analyzed in the canonical correlation phase

A schematic representation of the autocorrelations is printed next This indicates

which elements of the autocorrelation matrices at different lags are significantlygreater or less than 0

The second page of the STATESPACE printed output is shown in Figure 183

The STATESPACE Procedure

Schematic Representation of Partial Autocorrelations

NameLag 1 2 3 4 5 6 7 8 9 10

x ++ +

y ++

+ is gt 2std error - is lt -2std error is between

Yule-Walker Estimates for Minimum AIC

--------Lag=1------- --------Lag=2-------

x y x y

x 0257438 0202237 0170812 0133554

y 0292177 0469297 -000537 -000048

Figure 183 Partial Autocorrelations and VAR Model

Figure 183 shows a schematic representation of the partial autocorrelations similar

to the autocorrelations shown in Figure 182 The selection of a second order autore-

gressive model by the AIC statistic looks reasonable in this case because the partialautocorrelations for lags greater than 2 are not significant

Next the Yule-Walker estimates for the selected autoregressive model are printed

This output shows the coefficient matrices of the vector autoregressive model at each

lag

Selected State Space Model Form and Preliminary Estimates

After the autoregressive order selection process has determined the number of lags to

consider the canonical correlation analysis phase selects the state vector By default

output for this process is not printed You can use the CANCORR option to print

details of the canonical correlation analysis See the section Canonical Correlation

Analysis later in this chapter for an explanation of this process

Once the state vector is selected the state space model is estimated by approximate

maximum likelihood Information from the canonical correlation analysis and from

the preliminary autoregression is used to form preliminary estimates of the state space

model parameters These preliminary estimates are used as starting values for the

iterative estimation process

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Part 2 General Information

The form of the state vector and the preliminary estimates are printed next as shown

in Figure 184

The STATESPACE Procedure

Selected Statespace Form and Preliminary Estimates

State Vector

x(TT) y(TT) x(T+1T)

Estimate of Transition Matrix

0 0 1

0291536 0468762 -000411

024869 024484 0204257

Input Matrix for Innovation

1 0

0 1

0257438 0202237

Variance Matrix for Innovation

0945196 0100786

0100786 1014703

Figure 184 Preliminary Estimates of State Space Model

Figure 184 first prints the state vector as X[TT] Y[TT] X[T+1T] This notation

indicates that the state vector is

z

t

=

2

4

x

t j t

y

t j t

x

t + 1 j t

3

5

The notationx

t + 1 j t

indicates the conditional expectation or prediction of x

t + 1

based

on the information available at time t andx

t j t

andy

t j t

arex

t

andy

t

respectively

The remainder of Figure 184 shows the preliminary estimates of the transition matrix

F

the input matrixG

and the covariance matrix

e e

Estimated State Space Model

The next page of the STATESPACE output prints the final estimates of the fittedmodel as shown in Figure 185 This output has the same form as in Figure 184 but

shows the maximum likelihood estimates instead of the preliminary estimates

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Chapter 18 Getting Started

The STATESPACE Procedure

Selected Statespace Form and Fitted Model

State Vector

x(TT) y(TT) x(T+1T)

Estimate of Transition Matrix

0 0 1

0297273 047376 -001998

02301 0228425 0256031

Input Matrix for Innovation

1 0

0 1

0257284 0202273

Variance Matrix for Innovation

0945188 01007520100752 1014712

Figure 185 Fitted State Space Model

The estimated state space model shown in Figure 185 is

2

4

x

t + 1 j t + 1

y

t + 1 j t + 1

x

t + 2 j t + 1

3

5

=

2

4

0 0 1

0 2 9 7 0 4 7 4 0 0 2 0

0 2 3 0 0 2 2 8 0 2 5 6

3

5

2

4

x

t

y

t

x

t + 1 j t

3

5

+

2

4

1 0

0 1

0 2 5 7 0 2 0 2

3

5

e

t + 1

n

t + 1

v a r

e

t + 1

n

t + 1

=

0 9 4 5 0 1 0 1

0 1 0 1 1 0 1 5

The next page of the STATESPACE output lists the estimates of the free parameters

in theF

andG

matrices with standard errors and t statistics as shown in Figure 186

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Part 2 General Information

The STATESPACE Procedure

Parameter Estimates

Standard

Parameter Estimate Error t Value

F(21) 0297273 0129995 229

F(22) 0473760 0115688 410F(23) -001998 0313025 -006

F(31) 0230100 0126226 182

F(32) 0228425 0112978 202

F(33) 0256031 0305256 084

G(31) 0257284 0071060 362

G(32) 0202273 0068593 295

Figure 186 Final Parameter Estimates

Convergence Failures

The maximum likelihood estimates are computed by an iterative nonlinear maximiza-

tion algorithm which may not converge If the estimates fail to converge warning

messages are printed in the output

If you encounter convergence problems you should recheck the stationarity of the

data and ensure that the specified differencing orders are correct Attempting to fit

state space models to nonstationary data is a common cause of convergence failure

You can also use the MAXIT= option to increase the number of iterations allowed

or experiment with the convergence tolerance options DETTOL= and PARMTOL=

Forecast Data Set

The following statements print the output data set The WHERE statement excludes

the first 190 observations from the output so that only the forecasts and the last 10

actual observations are printed

proc print data=outid t

where t gt 190

run

The PROC PRINT output is shown in Figure 187

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Part 2 General Information

Figure 188 Plot of Forecasts

Controlling Printed Output

By default the STATESPACE procedure produces a large amount of printed output

The NOPRINT option suppresses all printed output You can suppress the printed

output for the autoregressive model selection process with the PRINTOUT=NONE

option The descriptive statistics and state space model estimation output are still

printed when PRINTOUT=NONE is specified You can produce more detailed output

with the PRINTOUT=LONG option and by specifying the printing control optionsCANCORR COVB and PRINT

Specifying the State Space Model

Instead of allowing the STATESPACE procedure to select the model automatically

you can use FORM and RESTRICT statements to specify a state space model

Specifying the State Vector

Use the FORM statement to control the form of the state vector You can use this

feature to force PROC STATESPACE to estimate and forecast a model different from

the model it would select automatically You can also use this feature to reestimate

the automatically selected model (possibly with restrictions) without repeating thecanonical correlation analysis

The FORM statement specifies the number of lags of each variable to include in

the state vector For example the statement FORM X 3 forces the state vector to

includex

t j t

x

t + 1 j t

andx

t + 2 j t

The following statement specifies the state vector

x

t j t

y

t j t

x

t + 1 j t

which is the same state vector selected in the preceding example

f o r m x 2 y 1

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Chapter 18 Getting Started

You can specify the form for only some of the variables and allow PROC STATES-

PACE to select the form for the other variables If only some of the variables are

specified in the FORM statement canonical correlation analysis is used to determine

the number of lags included in the state vector for the remaining variables not spec-

ified by the FORM statement If the FORM statement includes specifications for all

the variables listed in the VAR statement the state vector is completely defined and

the canonical correlation analysis is not performed

Restricting the F and G matrices

After you know the form of the state vector you can use the RESTRICT statement

to fix some parameters in theF

andG

matrices to specified values One use of this

feature is to remove insignificant parameters by restricting them to 0

In the introductory example shown in the preceding section the F[23] parameter is

not significant (The parameters estimation output shown in Figure 186 gives the t

statistic for F[23] as -006 F[33] and F[31] also have low significance witht 2

)

The following statements reestimate this model with F[23] restricted to 0 The

FORM statement is used to specify the state vector and thus bypass the canonicalcorrelation analysis

proc statespace data=in out=out lead=10

var x(1) y(1)

id t

form x 2 y 1

restrict f(23)=0

run

The final estimates produced by these statements are shown in Figure 189

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Part 2 General Information

The STATESPACE Procedure

Selected Statespace Form and Fitted Model

State Vector

x(TT) y(TT) x(T+1T)

Estimate of Transition Matrix

0 0 1

0290051 0467468 0

0227051 0226139 026436

Input Matrix for Innovation

1 0

0 1

0256826 0202022

Variance Matrix for Innovation

0945175 01006960100696 1014733

The STATESPACE Procedure

Parameter Estimates

Standard

Parameter Estimate Error t Value

F(21) 0290051 0063904 454

F(22) 0467468 0060430 774

F(31) 0227051 0125221 181

F(32) 0226139 0111711 202

F(33) 0264360 0299537 088G(31) 0256826 0070994 362

G(32) 0202022 0068507 295

Figure 189 Results using RESTRICT Statement

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Chapter 18 Syntax

Syntax

The STATESPACE procedure uses the following statements

PROC STATESPACE options

BY variable FORM variable value

ID variable

INITIAL F(rowcolumn)=value

G(rowcolumn)=value

RESTRICT F(rowcolumn)=value

G(rowcolumn)=value

VAR variable (difference difference

)

Functional Summary

The statements and options used by PROC STATESPACE are summarized in the

following table

Description Statement Option

Input Data Set Options

specify the input data set PROC STATESPACE DATA=

prevent subtraction of sample mean PROC STATESPACE NOCENTER

specify the ID variable ID

specify the observed series and differencing VAR

Options for Autoregressive Estimates

specify the maximum order PROC STATESPACE ARMAX=

specify maximum lag for autocovariances PROC STATESPACE LAGMAX=

output only minimum AIC model PROC STATESPACE MINIC

specify the amount of detail printed PROC STATESPACE PRINTOUT=

write preliminary AR models to a data set PROC STATESPACE OUTAR=

Options for Canonical Correlation Analysis

print the sequence of canonical correlations PROC STATESPACE CANCORR

specify upper limit of dimension of state

vector

PROC STATESPACE DIMMAX=

specify the minimum number of lags PROC STATESPACE PASTMIN=

specify the multiplier of the degrees of

freedom

PROC STATESPACE SIGCORR=

Options for State Space Model Estimation

specify starting values INITIAL

print covariance matrix of parameter estimates PROC STATESPACE COVB

specify the convergence criterion PROC STATESPACE DETTOL=

specify the convergence criterion PROC STATESPACE PARMTOL=

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Part 2 General Information

Description Statement Option

print the details of the iterations PROC STATESPACE ITPRINT

specify an upper limit of the number of lags PROC STATESPACE KLAG=

specify maximum number of iterations

allowed

PROC STATESPACE MAXIT=

suppress the final estimation PROC STATESPACE NOEST

write the state space model parameter esti-

mates to an output data set

PROC STATESPACE OUTMODEL=

use conditional least squares for final estimates PROC STATESPACE RESIDEST

specify criterion for testing for singularity PROC STATESPACE SINGULAR=

Options for Forecasting

start forecasting before end of the input data PROC STATESPACE BACK=

specify the time interval between observations PROC STATESPACE INTERVAL=

specify multiple periods in the time series PROC STATESPACE INTPER=specify how many periods to forecast PROC STATESPACE LEAD=

specify the output data set for forecasts PROC STATESPACE OUT=

print forecasts PROC STATESPACE PRINT

Options to Specify the State Space Model

specify the state vector FORM

specify the parameter values RESTRICT

BY Groups

specify BY-group processing BY

Printing

suppresses all printed output NOPRINT

PROC STATESPACE Statement

PROC STATESPACE options

The following options can be specified in the PROC STATESPACE statement

Printing Options NOPRINT

suppresses all printed output

Input Data Options

DATA= SAS-data-set

specifies the name of the SAS data set to be used by the procedure If the DATA=

option is omitted the most recently created SAS data set is used

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Chapter 18 Syntax

LAGMAX= k

specifies the number of lags for which the sample autocovariance matrix is computed

The LAGMAX= option controls the number of lags printed in the schematic repre-

sentation of the autocorrelations

The sample autocovariance matrix of lag i denoted asC

i

is computed as

C

i

=

1

N 1

N

X

t = 1 + i

x

t

x

0

t i

wherex

t

is the differenced and centered data andN

is the number of observations

(If the NOCENTER option is specified 1 is not subtracted fromN

) LAGMAX= k

specifies thatC

0

throughC

k

are computed The default is LAGMAX=10

NOCENTER

prevents subtraction of the sample mean from the input series (after any specified

differencing) before the analysis

Options for Preliminary Autoregressive Models ARMAX= n

specifies the maximum order of the preliminary autoregressive models The AR-

MAX= option controls the autoregressive orders for which information criteria are

printed and controls the number of lags printed in the schematic representation of

partial autocorrelations The default is ARMAX=10 See Preliminary Autoregres-

sive Models later in this chapter for details

MINIC

writes to the OUTAR= data set only the preliminary Yule-Walker estimates for the

VAR model producing the minimum AIC See OUTAR= Data Set later in this chap-

ter for details

OUTAR= SAS-data-set

writes the Yule-Walker estimates of the preliminary autoregressive models to a SAS

data set See OUTAR= Data Set later in this chapter for details

PRINTOUT= SHORT | LONG | NONE

determines the amount of detail printed PRINTOUT=LONG prints the lagged co-

variance matrices the partial autoregressive matrices and estimates of the resid-

ual covariance matrices from the sequence of autoregressive models PRINT-

OUT=NONE suppresses the output for the preliminary autoregressive models The

descriptive statistics and state space model estimation output are still printed when

PRINTOUT=NONE is specified PRINTOUT=SHORT is the default

Canonical Correlation Analysis Options

CANCORR

prints the canonical correlations and information criterion for each candidate state

vector considered See Canonical Correlation Analysis later in this chapter for

details

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Part 2 General Information

DIMMAX= n

specifies the upper limit to the dimension of the state vector The DIMMAX= option

can be used to limit the size of the model selected The default is DIMMAX=10

PASTMIN= n

specifies the minimum number of lags to include in the canonical correlation analy-

sis The default is PASTMIN=0 See Canonical Correlation Analysis later in thischapter for details

SIGCORR= value

specifies the multiplier of the degrees of freedom for the penalty term in the informa-

tion criterion used to select the state space form The default is SIGCORR=2 The

larger the value of the SIGCORR= option the smaller the state vector tends to be

Hence a large value causes a simpler model to be fit See Canonical Correlations

Analysis later in this chapter for details

State Space Model Estimation Options

COVB

prints the inverse of the observed information matrix for the parameter estimatesThis matrix is an estimate of the covariance matrix for the parameter estimates

DETTOL= value

specifies the convergence criterion The DETTOL= and PARMTOL= option values

are used together to test for convergence of the estimation process If during an

iteration the relative change of the parameter estimates is less than the PARMTOL=

value and the relative change of the determinant of the innovation variance matrix

is less than the DETTOL= value then iteration ceases and the current estimates are

accepted The default is DETTOL=1E-5

ITPRINT

prints the iterations during the estimation process

KLAG= n

sets an upper limit for the number of lags of the sample autocovariance matrix used

in computing the approximate likelihood function If the data have a strong moving

average character a larger KLAG= value may be necessary to obtain good estimates

The default is KLAG=15 See Parameter Estimation later in this chapter for details

MAXIT= n

sets an upper limit to the number of iterations in the maximum likelihood or condi-

tional least-squares estimation The default is MAXIT=50

NOEST

suppresses the final maximum likelihood estimation of the selected model

OUTMODEL= SAS-data-set

writes the parameter estimates and their standard errors to a SAS data set See OUT-

MODEL= Data Set later in this chapter for details

PARMTOL= value

specifies the convergence criterion The DETTOL= and PARMTOL= option values

are used together to test for convergence of the estimation process If during an

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Chapter 18 Syntax

iteration the relative change of the parameter estimates is less than the PARMTOL=

value and the relative change of the determinant of the innovation variance matrix

is less than the DETTOL= value then iteration ceases and the current estimates are

accepted The default is PARMTOL=001

RESIDEST

computes the final estimates using conditional least squares on the raw data Thistype of estimation may be more stable than the default maximum likelihood method

but is usually more computationally expensive See Parameter Estimation later in

this chapter for details of the conditional least squares method

SINGULAR= value

specifies the criterion for testing for singularity of a matrix A matrix is declared

singular if a scaled pivot is less than the SINGULAR= value when sweeping the

matrix The default is SINGULAR=1E-7

Forecasting Options

BACK= n

starts forecasting n periods before the end of the input data The BACK= option valuemust not be greater than the number of observations The default is BACK=0

INTERVAL= interval

specifies the time interval between observations The INTERVAL= value is used

in conjunction with the ID variable to check that the input data are in order and

have no missing periods The INTERVAL= option is also used to extrapolate the ID

values past the end of the input data See Chapter 3 ldquoDate Intervals Formats and

Functionsrdquo for details on the INTERVAL= values allowed

INTPER= n

specifies that each input observation corresponds to n time periods For example

the options INTERVAL=MONTH and INTPER=2 specify bimonthly data and are

equivalent to specifying INTERVAL=MONTH2 If the INTERVAL= option is not

specified the INTPER= option controls the increment used to generate ID values for

the forecast observations The default is INTPER=1

LEAD= n

specifies how many forecast observations are produced The forecasts start at the

point set by the BACK= option The default is LEAD=0 which produces no fore-

casts

OUT= SAS-data-set

writes the residuals actual values forecasts and forecast standard errors to a SAS

data set See OUT= Data Set later in this chapter for details

PRINT

prints the forecasts

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Part 2 General Information

BY Statement

BY variable

A BY statement can be used with the STATESPACE procedure to obtain separateanalyses on observations in groups defined by the BY variables

FORM Statement

FORM variable value

The FORM statement specifies the number of times a variable is included in the state

vector Values can be specified for any variable listed in the VAR statement If a

value is specified for each variable in the VAR statement the state vector for the state

space model is entirely specified and automatic selection of the state space model is

not performed

The FORM statement forces the state vectorz

t

to contain a specific variable a given

number of times For example if Y is one of the variables inx

t

then the statement

form y 3

forces the state vector to containY

t

Y

t + 1 j t

andY

t + 2 j t

possibly along with other

variables

The following statements illustrate the use of the FORM statement

proc statespace data=in

var x y

f o r m x 3 y 2

run

These statements fit a state space model with the following state vector

z

t

=

2

6

6

6

6

4

x

t j t

y

t j t

x

t + 1 j t

y

t + 1 j t

x

t + 2 j t

3

7

7

7

7

5

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Chapter 18 Syntax

ID Statement

ID variable

The ID statement specifies a variable that identifies observations in the input data setThe variable specified in the ID statement is included in the OUT= data set The

values of the ID variable are extrapolated for the forecast observations based on the

values of the INTERVAL= and INTPER= options

INITIAL Statement

INITIAL F (rowcolumn)= value G(row column)= value

The INITIAL statement gives initial values to the specified elements of theF

andG

matrices These initial values are used as starting values for the iterative estimation

Parts of theF

andG

matrices represent fixed structural identities If an element

specified is a fixed structural element instead of a free parameter the corresponding

initialization is ignored

The following is an example of an INITIAL statement

initial f(32)=0 g(41)=0 g(51)=0

RESTRICT Statement

RESTRICT F(rowcolumn)= value G(rowcolumn)= value

The RESTRICT statement restricts the specified elements of theF

andG

matrices

to the specified values

To use the restrict statement you need to know the form of the model Either specify

the form of the model with the FORM statement or do a preliminary run perhaps

with the NOEST option to find the form of the model that PROC STATESPACE

selects for the data

The following is an example of a RESTRICT statement

restrict f(32)=0 g(41)=0 g(51)=0

Parts of theF

andG

matrices represent fixed structural identities If a restriction is

specified for an element that is a fixed structural element instead of a free parameter

the restriction is ignored

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Part 2 General Information

VAR Statement

VAR variable (difference difference )

The VAR statement specifies the variables in the input data set to model and fore-cast The VAR statement also specifies differencing of the input variables The VAR

statement is required

Differencing is specified by following the variable name with a list of difference

periods separated by commas See the section Stationarity and Differencing for

more information on differencing of input variables

The order in which variables are listed in the VAR statement controls the order in

which variables are included in the state vector Usually potential inputs should be

listed before potential outputs

For example assuming the input data are monthly the following VAR statement

specifies modeling and forecasting of the one period and seasonal second differenceof X and Y

var x(112) y(112)

In this example the vector time series analyzed is

x

t

=

1 B 1 B

1 2

X

t

x

1 B 1 B

1 2

Y

t

y

where B represents the back shift operator and x and y represent the means of thedifferenced series If the NOCENTER option is specified the mean differences are

not subtracted

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Chapter 18 Details

Details

Missing Values

The STATESPACE procedure does not support missing values The procedure uses

the first contiguous group of observations with no missing values for any of the VARstatement variables Observations at the beginning of the data set with missing values

for any VAR statement variable are not used or included in the output data set

Stationarity and Differencing

The state space model used by the STATESPACE procedure assumes that the time

series are stationary Hence the data should be checked for stationarity One way

to check for stationarity is to plot the series A graph of series over time can show a

time trend or variability changes

You can also check stationarity by using the sample autocorrelation functions dis-

played by the ARIMA procedure The autocorrelation functions of nonstationaryseries tend to decay slowly See Chapter 7 ldquoThe ARIMA Procedurerdquo for more in-

formation

Another alternative is to use the STATIONARITY= option on the IDENTIFY state-

ment in PROC ARIMA to apply Dickey-Fuller tests for unit roots in the time series

See Chapter 7 ldquoThe ARIMA Procedurerdquo for more information on Dickey-Fuller unit

root tests

The most popular way to transform a nonstationary series to stationarity is by dif-

ferencing Differencing of the time series is specified in the VAR statement For

example to take a simple first difference of the series X use this statement

var x(1)

In this example the change in X from one period to the next is analyzed When the se-

ries has a seasonal pattern differencing at a period equal to the length of the seasonal

cycle may be desirable For example suppose the variable X is measured quarterly

and shows a seasonal cycle over the year You can use the following statement to

analyze the series of changes from the same quarter in the previous year

var x(4)

To difference twice add another differencing period to the list For ex-

ample the following statement analyzes the series of second differences

X

t

X

t 1

X

t 1

X

t 2

= X

t

2 X

t 1

+ X

t 2

var x(11)

The following statement analyzes the seasonal second difference series

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Part 2 General Information

var x(14)

The series modeled is the 1-period difference of the 4-period difference

X

t

X

t 4

X

t 1

X

t 5

= X

t

X

t 1

X

t 4

+ X

t 5

Another way to obtain stationary series is to use a regression on time to de-trend the

data If the time series has a deterministic linear trend regressing the series on time

produces residuals that should be stationary The following statements write residuals

of X and Y to the variable RX and RY in the output data set DETREND

data a

set a

t=_n_

run

proc reg data=a

model x y = t

output out=detrend r=rx ry

run

You then use PROC STATESPACE to forecast the de-trended series RX and RY A

disadvantage of this method is that you need to add the trend back to the forecast

series in an additional step A more serious disadvantage of the de-trending method

is that it assumes a deterministic trend In practice most time series appear to have a

stochastic rather than a deterministic trend Differencing is a more flexible and often

more appropriate method

There are several other methods to handle nonstationary time series For more infor-

mation and examples refer to Brockwell and Davis (1991)

Preliminary Autoregressive Models

After computing the sample autocovariance matrices PROC STATESPACE fits a

sequence of vector autoregressive models These preliminary autoregressive models

are used to estimate the autoregressive order of the process and limit the order of the

autocovariances considered in the state vector selection process

Yule-Walker Equations for Forward and Backward Models

Unlike a univariate autoregressive model a multivariate autoregressive model has

different forms depending on whether the present observation is being predicted from

the past observations or from the future observations

Letx

t

be the r -component stationary time series given by the VAR statement after

differencing and subtracting the vector of sample means (If the NOCENTER option

is specified the mean is not subtracted) Let n be the number of observations of x

t

from the input data set

Lete

t

be a vector white noise sequence with mean vector 0 and variance matrix

p

and letn

t

be a vector white noise sequence with mean vector 0 and variance matrix

p

Let p be the order of the vector autoregressive model forx

t

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The forward autoregressive form based on the past observations is written as follows

x

t

=

p

X

i = 1

p

i

x

t i

+ e

t

The backward autoregressive form based on the future observations is written as fol-

lows

x

t

=

p

X

i = 1

p

i

x

t + i

+ n

t

LettingE

denote the expected value operator the autocovariance sequence for thex

t

series

i

is

i

= E x

t

x

0

t i

The Yule-Walker equations for the autoregressive model that matches the first p ele-

ments of the autocovariance sequence are

2

6

6

4

0

1

p 1

0

1

0

p 2

0

p 1

0

p 2

0

3

7

7

5

2

6

6

4

p

1

p

2

p

p

3

7

7

5

=

2

6

6

4

1

2

p

3

7

7

5

and

2

6

6

6

4

0

0

1

0

p 1

1

0

0

p 2

p 1

p 2

0

3

7

7

7

5

2

6

6

4

p

1

p

2

p

p

3

7

7

5

=

2

6

6

4

0

1

0

2

0

p

3

7

7

5

Here

p

i

are the coefficient matrices for the past observation form of the vector au-toregressive model and

p

i

are the coefficient matrices for the future observation

form More information on the Yule-Walker equations in the multivariate setting can

be found in Whittle (1963) and Ansley and Newbold (1979)

The innovation variance matrices for the two forms can be written as follows

p

=

0

p

X

i = 1

p

i

0

i

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p

=

0

p

X

i = 1

p

i

i

The autoregressive models are fit to the data using the preceding Yule-Walker equa-

tions with

i

replaced by the sample covariance sequenceC

i

The covariance matri-

ces are calculated as

C

i

=

1

N 1

N

X

t = i + 1

x

t

x

0

t i

Let b

p

b

p

b

p

and b

p

represent the Yule-Walker estimates of

p

p

p

and

p

respectively These matrices are written to an output data set when the OUTAR=

option is specified

When the PRINTOUT=LONG option is specified the sequence of matrices b

p

and

the corresponding correlation matrices are printed The sequence of matricesb

p isused to compute Akaike information criteria for selection of the autoregressive order

of the process

Akaike Information Criterion

The Akaike information criterion or AIC is defined as -2( maximum of log likeli-

hood )+2(number of parameters) Since the vector autoregressive models are esti-

mates from the Yule-Walker equations not by maximum likelihood the exact likeli-

hood values are not available for computing the AIC However for the vector autore-

gressive model the maximum of the log likelihood can be approximated as

l n L

n

2

l n j

b

p

j

Thus the AIC for the order p model is computed as

A I C

p

= n l n j

b

p

j + 2 p r

2

You can use the printed AIC array to compute a likelihood ratio test of the autoregres-

sive order The log-likelihood ratio test statistic for testing the order p model against

the order p 1

model is

n l n j

b

p

j + n l n j

b

p 1

j

This quantity is asymptotically distributed as a

2 withr

2 degrees of freedom if the

series is autoregressive of order p 1

It can be computed from the AIC array as

A I C

p 1

A I C

p

+ 2 r

2

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You can evaluate the significance of these test statistics with the PROBCHI function

in a SAS DATA step or with a

2 table

Determining the Autoregressive Order

Although the autoregressive models can be used for prediction their primary value is

to aid in the selection of a suitable portion of the sample covariance matrix for use in

computing canonical correlations If the multivariate time series x

t is of autoregres-sive order p then the vector of past values to lag p is considered to contain essentially

all the information relevant for prediction of future values of the time series

By default PROC STATESPACE selects the order p producing the autoregressive

model with the smallestA I C

p

If the value p for the minimumA I C

p

is less than the

value of the PASTMIN= option then p is set to the PASTMIN= value Alternatively

you can use the ARMAX= and PASTMIN= options to force PROC STATESPACE to

use an order you select

Significance Limits for Partial Autocorrelations

The STATESPACE procedure prints a schematic representation of the partial autocor-

relation matrices indicating which partial autocorrelations are significantly greater orsignificantly less than 0 Figure 1810 shows an example of this table

The STATESPACE Procedure

Schematic Representation of Partial Autocorrelations

NameLag 1 2 3 4 5 6 7 8 9 10

x ++ +

y ++

+ is gt 2std error - is lt -2std error is between

Figure 1810 Significant Partial Autocorrelations

The partial autocorrelations are from the sample partial autoregressive matrices b

p

p

The standard errors used for the significance limits of the partial autocorrelations are

computed from the sequence of matrices

p

and

p

Under the assumption that the observed series arises from an autoregressive process

of order p 1

the pth sample partial autoregressive matrix b

p

p

has an asymptotic

variance matrix 1

n

1

p

p

The significance limits for b

p

p

used in the schematic plot of the sample partial autore-

gressive sequence are derived by replacing

p

and

p

with their sample estimators

to produce the variance estimate as follows

d

V a r

b

p

p

=

1

n r p

b

1

p

b

p

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Canonical Correlation Analysis

Given the order p letp

t

be the vector of current and past values relevant to prediction

of x

t + 1

p

t

= x

0

t

x

0

t 1

x

0

t p

0

Letf

t

be the vector of current and future values

f

t

= x

0

t

x

0

t + 1

x

0

t + p

0

In the canonical correlation analysis consider submatrices of the sample covariance

matrix of p

t

andf

t

This covariance matrixV

has a block Hankel form

V =

2

6

6

6

4

C

0

C

0

1

C

0

2

C

0

p

C

0

1

C

0

2

C

0

3

C

0

p + 1

C

0

p

C

0

p + 1

C

0

p + 2

C

0

2 p

3

7

7

7

5

State Vector Selection Process

The canonical correlation analysis forms a sequence of potential state vectorsz

j

t

Examine a sequencef

j

t

of subvectors of f

t

and form the submatrixV

j consisting

of the rows and columns of V

corresponding to the components of f

j

t

and compute

its canonical correlations

The smallest canonical correlation of V

j is then used in the selection of the com-

ponents of the state vector The selection process is described in the following For

more details about this process refer to Akaike (1976)

In the following discussion the notationx

t + k j t

denotes the wide sense conditional

expectation (best linear predictor) of x

t + k

given allx

s

with s less than or equal to t

In the notationx

i t + 1

the first subscript denotes the ith component of x

t + 1

The initial state vectorz

1

t

is set tox

t

The sequencef

j

t

is initialized by setting

f

1

t

= z

1

0

t

x

1 t + 1 j t

0

= x

0

t

x

1 t + 1 j t

0

That is start by considering whether to addx

1 t + 1 j t

to the initial state vectorz

1

t

The procedure forms the submatrixV

1 corresponding tof

1

t

and computes its canon-

ical correlations Denote the smallest canonical correlation of V

1 as

m i n

If

m i n

is

significantly greater than 0x

1 t + 1 j t

is added to the state vector

If the smallest canonical correlation of V

1 is not significantly greater than 0 then a

linear combination of f

1

t

is uncorrelated with the pastp

t

Assuming that the determi-

nant of C

0

is not 0 (that is no input series is a constant) you can take the coefficient

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Chapter 18 Details

of x

1 t + 1 j t

in this linear combination to be 1 Denote the coefficients of z

1

t

in this

linear combination as

This gives the relationship

x

1 t + 1 j t

=

0

x

t

Therefore the current state vector already contains all the past information useful forpredicting

x

1 t + 1

and any greater leads of x

1 t

The variablex

1 t + 1 j t

is not added to

the state vector nor are any termsx

1 t + k j t

considered as possible components of the

state vector The variablex

1

is no longer active for state vector selection

The process described forx

1 t + 1 j t

is repeated for the remaining elements of f

t

The

next candidate for inclusion in the state vector is the next component of f

t

corre-

sponding to an active variable Components of f

t

corresponding to inactive variables

that produced a zero

m i n

in a previous step are skipped

Denote the next candidate asx

l t + k j t

The vectorf

j

t

is formed from the current state

vector andx

l t + k j t

as follows

f

j

t

= z

j

0

t

x

l t + k j t

0

The matrixV

j is formed fromf

j

t

and its canonical correlations are computed The

smallest canonical correlation of V

j is judged to be either greater than or equal to 0

If it is judged to be greater than 0x

l t + k j t

is added to the state vector If it is judged

to be 0 then a linear combination of f

j

t

is uncorrelated with thep

t

and the variable

x

l

is now inactive

The state vector selection process continues until no active variables remain

Testing Significance of Canonical Correlations For each step in the canonical correlation sequence the significance of the small-

est canonical correlation

m i n

is judged by an information criterion from Akaike

(1976) This information criterion is

n l n 1

2

m i n

r p + 1 q + 1

where q is the dimension of f

j

t

at the current step r is the order of the state vector p

is the order of the vector autoregressive process and

is the value of the SIGCORR=

option The default is SIGCORR=2 If this information criterion is less than or equal

to 0

m i n

is taken to be 0 otherwise it is taken to be significantly greater than 0

(Do not confuse this information criterion with the AIC)

Variables inx

t + p j t

are not added in the model even with positive information crite-

rion because of the singularity of V

You can force the consideration of more candi-

date state variables by increasing the size of theV

matrix by specifying a PASTMIN=

option value larger than p

Printing the Canonical Correlations

To print the details of the canonical correlation analysis process specify the CAN-

CORR option in the PROC STATESPACE statement The CANCORR option prints

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the candidate state vectors the canonical correlations and the information criteria for

testing the significance of the smallest canonical correlation

Bartlettrsquos

2 and its degrees of freedom are also printed when the CANCORR option

is specified The formula used for Bartlettrsquos

2 is

2

= n 5 r p + 1 q + 1 l n 1

2

m i n

withr p + 1 q + 1

degrees of freedom

Figure 1811 shows the output of the CANCORR option for the introductory example

shown in the Getting Started section of this chapter

The STATESPACE Procedure

Canonical Correlations Analysis

Information Chi

x(TT) y(TT) x(T+1T) Criterion Square DF

1 1 0237045 3566167 114505 4

Information Chi

x(TT) y(TT) x(T+1T) y(T+1T) Criterion Square DF

1 1 0238244 0056565 -535906 0636134 3

Information Chi

x(TT) y(TT) x(T+1T) x(T+2T) Criterion Square DF

1 1 0237602 0087493 -446312 1525353 3

Figure 1811 Canonical Correlations Analysis

New variables are added to the state vector if the information criteria are positive Inthis example

y

t + 1 j t

andx

t + 2 j t

are not added to the state space vector because the

information criteria for these models are negative

If the information criterion is nearly 0 then you may want to investigate models

that arise if the opposite decision is made regarding

m i n

This investigation can be

accomplished by using a FORM statement to specify part or all of the state vector

Preliminary Estimates of F

When a candidate variablex

l t + k j t

yields a zero

m i n

and is not added to the state

vector a linear combination of f

j

t

is uncorrelated with thep

t

Because of the method

used to construct thef

j

t

sequence the coefficient of x

l t + k j t

inl

can be taken as 1

Denote the coefficients of z

j

t

in this linear combination as l

This gives the relationship

x

l t + k j t

= l

0

z

j

t

The vectorl

is used as a preliminary estimate of the first r columns of the row of the

transition matrixF

corresponding tox

l t + k 1 j t

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Parameter Estimation

The model isz

t + 1

= F z

t

+ G e

t + 1

wheree

t

is a sequence of independent multivari-

ate normal innovations with mean vector 0 and variance

e e

The observed sequence

x

t

composes the first r components of z

t

and thusx

t

= H z

t

where H is ther s

matrix I

r

0

LetE

be ther n

matrix of innovations

E = e

1

e

n

If the number of observations n is reasonably large the log likelihood L can be

approximated up to an additive constant as follows

L =

n

2

l n j

e e

j

1

2

t r a c e

1

e e

E E

0

The elements of

e e

are taken as free parameters and are estimated as follows

S

0

=

1

n

E E

0

Replacing

e e

byS

0

in the likelihood equation the log likelihood up to an additive

constant is

L =

n

2

l n j S

0

j

Letting B be the backshift operator the formal relation betweenx

t

ande

t

is

x

t

= H I B F

1

G e

t

e

t

= H I B F

1

G

1

x

t

=

1

X

i = 0

i

x

t i

LettingC

i

be the ith lagged sample covariance of x

t

and neglecting end effects the

matrixS

0

is

S

0

=

1

X

i j = 0

i

C

i + j

0

j

For the computation of S

0

the infinite sum is truncated at the value of the KLAG=

option The value of the KLAG= option should be large enough that the sequence

i

is approximately 0 beyond that point

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Let

be the vector of free parameters in theF

andG

matrices The derivative of the

log likelihood with respect to the parameter

is

L

=

n

2

t r a c e

S

1

0

S

0

The second derivative is

2

L

0

=

n

2

t r a c e

S

1

0

S

0

0

S

1

0

S

0

t r a c e

S

1

0

2

S

0

0

Near the maximum the first term is unimportant and the second term can be approx-

imated to give the following second derivative approximation

2

L

0

=

n t r a c e

S

1

0

E

E

0

0

The first derivative matrix and this second derivative matrix approximation are com-

puted from the sample covariance matrixC

0

and the truncated sequence

i

The

approximate likelihood function is maximized by a modified Newton-Raphson algo-

rithm employing these derivative matrices

The matrixS

0

is used as the estimate of the innovation covariance matrix

e e

The

negative of the inverse of the second derivative matrix at the maximum is used as an

approximate covariance matrix for the parameter estimates The standard errors of

the parameter estimates printed in the parameter estimates tables are taken from the

diagonal of this covariance matrix The parameter covariance matrix is printed when

the COVB option is specified

If the data are nearly nonstationary a better estimate of

e e

and the other parameters

can sometimes be obtained by specifying the RESIDEST option The RESIDEST

option estimates the parameters using conditional least squares instead of maximum

likelihood

The residuals are computed using the state space equation and the sample mean val-

ues of the variables in the model as start-up values The estimate of S

0

is then com-

puted using the residuals from the ith observation on where i is the maximum number

of times any variable occurs in the state vector A multivariate Gauss-Marquardt al-

gorithm is used to minimizej S

0

j

Refer to Harvey (1981a) for a further description

of this method

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Forecasting

Given estimates of F

G

and

e e

forecasts of x

t

are computed from the conditional

expectation of z

t

In forecasting the parameters F G and

e e

are replaced with the estimates or by val-

ues specified in the RESTRICT statement One-step-ahead forecasting is performedfor the observation

x

t

wheret n b

Heren

is the number of observations and

b is the value of the BACK= option For the observationx

t

wheret n b

m-

step-ahead forecasting is performed form = t n + b

The forecasts are generated

recursively with the initial conditionz

0

= 0

The m-step-ahead forecast of z

t + m

isz

t + m j t

wherez

t + m j t

denotes the conditional

expectation of z

t + m

given the information available at time t The m-step-ahead

forecast of x

t + m

isx

t + m j t

= H z

t + m j t

where the matrixH = I

r

0

Let

i

= F

i

G

Note that the lasts r

elements of z

t

consist of the elements of x

u j t

foru t

The state vector z

t + m

can be represented as

z

t + m

= F

m

z

t

+

m 1

X

i = 0

i

e

t + m i

Sincee

t + i j t

= 0

fori 0

the m-step-ahead forecastz

t + m j t

is

z

t + m j t

= F

m

z

t

= F z

t + m 1 j t

Therefore the m-step-ahead forecast of x

t + m

is

x

t + m j t

= H z

t + m j t

The m-step-ahead forecast error is

z

t + m

z

t + m j t

=

m 1

X

i = 0

i

e

t + m i

The variance of the m-step-ahead forecast error is

V

z m

=

m 1

X

i = 0

i

e e

0

i

LettingV

z 0

= 0

the variance of the m-step-ahead forecast error of z

t + m

V

z m

can

be computed recursively as follows

V

z m

= V

z m 1

+

m 1

e e

0

m 1

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The variance of the m-step-ahead forecast error of x

t + m

is ther r

left upper sub-

matrix of V

z m

that is

V

x m

= H V

z m

H

0

Unless the NOCENTER option is specified the sample mean vector is added to the

forecast When differencing is specified the forecasts xt + m j t

plus the sample mean

vector are integrated back to produce forecasts for the original series

Lety

t

be the original series specified by the VAR statement with some 0 values

appended corresponding to the unobserved past observations Let B be the backshift

operator and let B

be thes s

matrix polynomial in the backshift operator

corresponding to the differencing specified by the VAR statement The off-diagonal

elements of

i

are 0 Note that

0

= I

s

whereI

s

is thes s

identity matrix Then

z

t

= B y

t

This gives the relationship

y

t

=

1

B z

t

=

1

X

i = 0

i

z

t i

where

1

B =

P

1

i = 0

i

B

i and

0

= I

s

The m-step-ahead forecast of y

t + m

is

y

t + m j t

=

m 1

X

i = 0

i

z

t + m i j t

+

1

X

i = m

i

z

t + m i

The m-step-ahead forecast error of y

t + m

is

m 1

X

i = 0

i

z

t + m i

z

t + m i j t

=

m 1

X

i = 0

i

X

u = 0

u

i u

e

t + m i

LettingV

y 0

= 0

the variance of the m-step-ahead forecast error of y

t + m

V

y m

is

V

y m

=

m 1

X

i = 0

i

X

u = 0

u

i u

e e

i

X

u = 0

u

i u

0

= V

y m 1

+

m 1

X

u = 0

u

m 1 u

e e

m 1

X

u = 0

u

m 1 u

0

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Relation of ARMA and State Space Forms

Every state space model has an ARMA representation and conversely every ARMA

model has a state space representation This section discusses this equivalence The

following material is adapted from Akaike (1974) where there is a more complete

discussion Pham-Dinh-Tuan (1978) also contains a discussion of this material

Suppose you are given the following ARMA model

B x

t

= B e

t

or in more detail

x

t

1

x

t 1

p

x

t p

= e

t

+

1

e

t 1

+ +

q

e

t q

(1)

wheree

t

is a sequence of independent multivariate normal random vectors with mean

0 and variance matrix

e e

B is the backshift operator (B x

t

= x

t 1

) B

and

B

are matrix polynomials in B andx

t

is the observed process

If the roots of the determinantial equationj B j = 0

are outside the unit circle in

the complex plane the model can also be written as

x

t

=

1

B B e

t

=

1

X

i = 0

i

e

t i

The

i

matrices are known as the impulse response matrices and can be computed

as

1

B B

You can assume p q

since if this is not initially true you can add more terms

i

that are identically 0 without changing the model

To write this set of equations in a state space form proceed as follows Letx

t + i j t

be

the conditional expectation of x

t + i

givenx

w

forw t

The following relations hold

x

t + i j t

=

1

X

j = i

j

e

t + i j

x

t + i j t + 1

= x

t + i j t

+

i 1

e

t + 1

However from equation (1) you can derive the following relationship

x

t + p j t

=

1

x

t + p 1 j t

+ +

p

x

t

(2)

Hence wheni = p

you can substitute forx

t + p j t

in the right-hand side of equation

(2) and close the system of equations

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This substitution results in the following model in the state space form

z

t + 1

= F z

t

+ G e

t + 1

2

6

6

6

4

x

t + 1

x

t + 2 j t + 1

x

t + p j t + 1

3

7

7

7

5

=

2

6

6

4

0 I 0 0

0 0 I 0

p

p 1

1

3

7

7

5

2

6

6

6

4

x

t

x

t + 1 j t

x

t + p 1 j t

3

7

7

7

5

+

2

6

6

4

I

1

p 1

3

7

7

5

e

t + 1

Note that the state vectorz

t

is composed of conditional expectations of x

t

and the

first r components of z

t

are equal tox

t

The state space form can be cast into an ARMA form by solving the system of dif-

ference equations for the first r components

When converting from an ARMA form to a state space form you can generate a

state vector larger than needed that is the state space model may not be a minimal

representation When going from a state space form to an ARMA form you can have

nontrivial common factors in the autoregressive and moving average operators that

yield an ARMA model larger than necessary

If the state space form used is not a minimal representation some but not all compo-

nents of x

t + i j t

may be linearly dependent This situation corresponds to

p

p 1

being of less than full rank when B

and B

have no common nontrivial

left factors In this casez

t

consists of a subset of the possible components of

x

t + i j t

i = 1 2 p 1

However once a component of x

t + i j t

(for example the

jth one) is linearly dependent on the previous conditional expectations then all sub-

sequent jth components of x

t + k j t

fork i

must also be linearly dependent Note

that in this case equivalent but seemingly different structures can arise if the order of

the components withinx

t

is changed

OUT= Data Set

The forecasts are contained in the output data set specified by the OUT= option on the

PROC STATESPACE statement The OUT= data set contains the following variables

the BY variables

the ID variable

the VAR statement variables These variables contain the actual values from

the input data set

FORi numeric variables containing the forecasts The variable FORi contains

the forecasts for the ith variable in the VAR statement list Forecasts are one-

step-ahead predictions until the end of the data or until the observation specified

by the BACK= option

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Chapter 18 Details

RESi numeric variables containing the residual for the forecast of the ith vari-

able in the VAR statement list For forecast observations the actual values are

missing and the RESi variables contain missing values

STDi numeric variables containing the standard deviation for the forecast of

the ith variable in the VAR statement list The values of the STDi variables can

be used to construct univariate confidence limits for the corresponding fore-casts However such confidence limits do not take into account the covariance

of the forecasts

OUTAR= Data Set

The OUTAR= data set contains the estimates of the preliminary autoregressive mod-

els The OUTAR= data set contains the following variables

ORDER a numeric variable containing the order p of the autoregressive model

that the observation represents

AIC a numeric variable containing the value of the information criterionA I C

p

SIGFl numeric variables containing the estimate of the innovation covariance

matrices for the forward autoregressive models The variable SIGFl contains

the lth column of b

p

in the observations with ORDER= p

SIGBl numeric variables containing the estimate of the innovation covariance

matrices for the backward autoregressive models The variable SIGBl contains

the lth column of b

p

in the observations with ORDER= p

FORk ndashl numeric variables containing the estimates of the autoregressive pa-

rameter matrices for the forward models The variable FORk ndashl contains the

lth column of the lag k autoregressive parameter matrix b

p

k

in the observations

with ORDER= p

BACk ndashl numeric variables containing the estimates of the autoregressive pa-

rameter matrices for the backward models The variable BACk ndashl contains the

lth column of the lag k autoregressive parameter matrix b

p

k

in the observations

with ORDER= p

The estimates for the order p autoregressive model can be selected as those observa-

tions with ORDER= p Within these observations the k lth element of

p

i

is given by

the value of the FORindashl variable in the k th observation The k lth element of

p

i

is

given by the value of BACindashl variable in the k th observation The k lth element of

p

is given by SIGFl in the k th observation The k lth element of

p

is given by SIGBlin the k th observation

Table 181 shows an example of the OUTAR= data set with ARMAX=3 andx

t

of

dimension 2 In Table 181 i j

indicate the ijth element of the matrix

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Part 2 General Information

Table 181 Values in the OUTAR= Data Set

Obs ORDER AIC SIGF1 SIGF2 SIGB1 SIGB2 FOR1ndash 1 FOR1ndash 2 FOR2ndash 1 FOR2ndash 2 FOR3ndash 1

1 0 AIC0

0 1 1

0 1 2

0 1 1

0 1 2

2 0 AIC0

0 2 1

0 2 2

0 2 1

0 2 2

3 1 AIC1

1 1 1

1 1 2

1 1 1

1 1 2

1

1

1 1

1

1

1 2

4 1 AIC1

1 2 1

1 1 2

1 2 1

1 1 2

1

1

2 1

1

1

2 2

5 2 AIC2

2 1 1

2 1 2

2 1 1

2 1 2

2

1

1 1

2

1

1 2

2

2

1 1

2

2

1 2

6 2 AIC2

2 2 1

2 1 2

2 2 1

2 1 2

2

1

2 1

2

1

2 2

2

2

2 1

2

2

2 2

7 3 AIC3

3 1 1

3 1 2

3 1 1

3 1 2

3

1

1 1

3

1

1 2

3

2

1 1

3

2

1 2

3

3

1 1

8 3 AIC3

3 2 1

3 1 2

3 2 1

3 1 2

3

1

2 1

3

1

2 2

3

2

2 1

3

2

2 2

3

3

2 1

Obs FOR3ndash 2 BACK1ndash 1 BACK1ndash 2 BACK2ndash 1 BACK2ndash 2 BACK3ndash 1 BACK3ndash 2

1

2

3

1

1

1 1

1

1

1 2

4

1

1

2 1

1

1

2 2

5

2

1

1 1

2

1

1 2

2

2

1 1

2

2

1 2

6

2

1

2 1

2

1

2 2

2

2

2 1

2

2

2 2

7

3

3

1 2

3

1

1 1

3

1

1 2

3

2

1 1

3

2

1 2

3

3

1 1

3

3

1 2

8

3

3

2 2

3

1

2 1

3

1

2 2

3

2

2 1

3

2

2 2

3

3

2 1

3

3

2 2

The estimated autoregressive parameters can be used in the IML procedure to obtain

autoregressive estimates of the spectral density function or forecasts based on the

autoregressive models

OUTMODEL= Data Set

The OUTMODEL= data set contains the estimates of theF

andG

matrices and their

standard errors the names of the components of the state vector and the estimates of

the innovation covariance matrix The variables contained in the OUTMODEL= data

set are as follows

the BY variables

STATEVEC a character variable containing the name of the component of the

state vector corresponding to the observation The STATEVEC variable has

the value STD for standard deviations observations which contain the standarderrors for the estimates given in the preceding observation

Fndash j numeric variables containing the columns of theF

matrix The variable

Fndash j contains the jth column of F

The number of Fndash j variables is equal to

the value of the DIMMAX= option If the model is of smaller dimension the

extraneous variables are set to missing

Gndash j numeric variables containing the columns of theG

matrix The variable

Gndash j contains the jth column of G

The number of Gndash j variables is equal to r

the dimension of x

t

given by the number of variables in the VAR statement

SIGndash j numeric variables containing the columns of the innovation covariance

matrix The variable SIGndash j contains the jth column of

e e

There are r vari-ables SIGndash j

Table 182 shows an example of the OUTMODEL= data set withx

t

= x

t

y

t

0

z

t

= x

t

y

t

x

t + 1 j t

0 and DIMMAX=4 In Table 182F

i j

andG

i j

are the ijth

elements of F

andG

respectively Note that all elements for Fndash4 are missing because

F

is a3 3

matrix

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Chapter 18 Details

Table 182 Value in the OUTMODEL= Data Set

Obs STATEVEC Fndash 1 Fndash 2 Fndash 3 Fndash 4 Gndash 1 Gndash 2 SIGndash 1 SIGndash 2

1 X(TT) 0 0 1 1 0

1 1

1 2

2 STD

3 Y(TT) F2 1

F2 2

F2 3

0 1

2 1

2 2

4 STD std F2 1

std F2 2

std F2 3

5 X(T+1T) F3 1

F3 2

F3 3

G3 1

G3 2

6 STD std F3 1

std F3 2

std F3 3

std G3 1

std G3 2

Printed Output

The printed output produced by the STATESPACE procedure is described in the fol-

lowing

1 descriptive statistics which include the number of observations used the

names of the variables their means and standard deviations (Std) and the dif-

ferencing operations used

2 the Akaike information criteria for the sequence of preliminary autoregressive

models

3 if the PRINTOUT=LONG option is specified the sample autocovariance ma-

trices of the input series at various lags

4 if the PRINTOUT=LONG option is specified the sample autocorrelation ma-

trices of the input series

5 a schematic representation of the autocorrelation matrices showing the signif-

icant autocorrelations

6 if the PRINTOUT=LONG option is specified the partial autoregressive ma-

trices (These are

p

p

as described in Preliminary Autoregressive Modelsearlier in this chapter)

7 a schematic representation of the partial autocorrelation matrices showing the

significant partial autocorrelations

8 the Yule-Walker estimates of the autoregressive parameters for the autoregres-

sive model with the minimum AIC

9 if the PRINTOUT=LONG option is specified the autocovariance matrices of

the residuals of the minimum AIC model This is the sequence of estimated

innovation variance matrices for the solutions of the Yule-Walker equations

10 if the PRINTOUT=LONG option is specified the autocorrelation matrices of the residuals of the minimum AIC model

11 If the CANCORR option is specified the canonical correlations analysis for

each potential state vector considered in the state vector selection process

This includes the potential state vector the canonical correlations the informa-

tion criterion for the smallest canonical correlation Bartlettrsquos

2 statistic (ldquoChi

Squarerdquo) for the smallest canonical correlation and the degrees of freedom of

Bartlettrsquos

2

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Part 2 General Information

12 the components of the chosen state vector

13 the preliminary estimate of the transition matrixF

the input matrixG

and

the variance matrix for the innovations

e e

14 if the ITPRINT option is specified the iteration history of the likelihood max-

imization For each iteration this shows the iteration number the number of step halvings the determinant of the innovation variance matrix the damping

factor Lambda and the values of the parameters

15 the state vector printed again to aid interpretation of the following listing of F

andG

16 the final estimate of the transition matrixF

17 the final estimate of the input matrixG

18 the final estimate of the variance matrix for the innovations

e e

19 a table listing the estimates of the free parameters inF

andG

and their standarderrors and t statistics

20 if the COVB option is specified the covariance matrix of the parameter esti-

mates

21 if the COVB option is specified the correlation matrix of the parameter esti-

mates

22 if the PRINT option is specified the forecasts and their standard errors

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Chapter 18 Details

ODS Table Names

PROC STATESPACE assigns a name to each table it creates You can use these

names to reference the table when using the Output Delivery System (ODS) to select

tables and create output data sets These names are listed in the following table For

more information on ODS see Chapter 6 ldquoUsing the Output Delivery Systemrdquo

Table 183 ODS Tables Produced in PROC STATESPACE

ODS Table Name Description Option

NObs Number of observations default

Summary Simple summary statistics table default

InfoCriterion Information criterion table default

CovLags Covariance Matrices of Input Series PRINTOUT=LONG

CorrLags Correlation Matrices of Input Series PRINTOUT=LONG

PartialAR Partial Autoregressive Matrices PRINTOUT=LONG

YWEstimates Yule-Walker Estimates for Minimum AIC default

CovResiduals Covariance of Residuals PRINTOUT=LONGCorrResiduals Residual Correlations from AR Models PRINTOUT=LONG

StateVector State vector table default

CorrGraph Schematic Representation of Correlations default

TransitionMatrix Transition Matrix default

InputMatrix Input Matrix default

VarInnov Variance Matrix for the Innovation default

CovB Covariance of Parameter Estimates COVB

CorrB Correlation of Parameter Estimates COVB

CanCorr Canonical Correlation Analysis CANCORR

IterHistory Iterative Fitting table ITPRINT

ParameterEstimates Parameter Estimates Table default

Forecasts Forecasts Table PRINT

ConvergenceStatus Convergence Status Table default

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Part 2 General Information

Example

Example 181 Series J from Box and Jenkins

This example analyzes the gas furnace data (series J) from Box and Jenkins (The

data are not shown Refer to Box and Jenkins (1976) for the data)

First a model is selected and fit automatically using the following statements

title1 rsquoGas Furnace Datarsquo

title2 rsquoBox amp Jenkins Series Jrsquo

title3 rsquoAutomatically Selected Modelrsquo

proc statespace data=seriesj cancorr

var x y

run

The results for the automatically selected model are shown in Output 1811

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Chapter 18 Example

Output 1811 Results for Automatically Selected Model

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Number of Observations 296

Standard

Variable Mean Error

x -005683 1072766

y 5350912 3202121

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Information Criterion for Autoregressive Models

Lag=0 Lag=1 Lag=2 Lag=3 Lag=4 Lag=5 Lag=6 Lag=7 Lag=8

6513862 -103357 -163296 -164512 -165152 -164891 -164934 -164315 -163856

Information

Criterion for

Autoregressive

Models

Lag=9 Lag=10

-16348 -163359

Schematic Representation of Correlations

NameLag 0 1 2 3 4 5 6 7 8 9 10

x +- +- +- +- +- +- +- +- +- +- +-

y -+ -+ -+ -+ -+ -+ -+ -+ -+ -+ -+

+ is gt 2std error - is lt -2std error is between

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Part 2 General Information

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Schematic Representation of Partial Autocorrelations

NameLag 1 2 3 4 5 6 7 8 9 10

x + - + -

y -+ -- - + +

+ is gt 2std error - is lt -2std error is between

Yule-Walker Estimates for Minimum AIC

------Lag=1------ ------Lag=2------ ------Lag=3------ ------Lag=4------

x y x y x y x y

x 1925887 -000124 -120166 0004224 0116918 -000867 0104236 0003268

y 0050496 1299793 -002046 -03277 -071182 -025701 0195411 0133417

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Canonical Correlations Analysis

Information Chi

x(TT) y(TT) x(T+1T) Criterion Square DF

1 1 0804883 2929228 3047481 8

Information Chix(TT) y(TT) x(T+1T) y(T+1T) Criterion Square DF

1 1 0906681 0607529 1223358 1347237 7

Information Chi

x(TT) y(TT) x (T+1T) y (T+1T) x (T+2T) Criterion Square DF

1 1 0909434 0610278 0186274 -154701 1034705 6

Information Chi

x(TT) y(TT) x (T+1T) y (T+1T) y (T+2T) Criterion Square DF

1 1 091014 0618937 0206823 0940392 1280924 6

Information Chi

x(TT) y(TT) x(T+1T) y(T+1T) y(T+2T) y(T+3T) Criterion Square DF

1 1 0912963 0628785 0226598 0083258 -794103 2041584 5

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Chapter 18 Example

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Selected Statespace Form and Preliminary Estimates

State Vector

x(TT) y(TT) x(T+1T) y(T+1T) y(T+2T)

Estimate of Transition Matrix

0 0 1 0 0

0 0 0 1 0

-084718 0026794 1711715 -005019 0

0 0 0 0 1

-019785 0334274 -018174 -123557 1787475

Input Matrix for Innovation

1 0

0 11925887 -000124

0050496 1299793

0142421 1361696

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Selected Statespace Form and Preliminary Estimates

Variance Matrix for Innovation

0035274 -000734

-000734 0097569

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Part 2 General Information

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Selected Statespace Form and Fitted Model

State Vector

x(TT) y(TT) x(T+1T) y(T+1T) y(T+2T)

Estimate of Transition Matrix

0 0 1 0 0

0 0 0 1 0

-086192 0030609 1724235 -005483 0

0 0 0 0 1

-034839 0292124 -009435 -109823 1671418

Input Matrix for Innovation

1 0

0 1192442 -000416

0015621 1258495

008058 1353204

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Selected Statespace Form and Fitted Model

Variance Matrix for Innovation

0035579 -000728

-000728 0095577

Parameter Estimates

Standard

Parameter Estimate Error t Value

F(31) -086192 0072961 -1181

F(32) 0030609 0026167 117

F(33) 1724235 0061599 2799

F(34) -005483 0030169 -182

F(51) -034839 0135253 -258

F(52) 0292124 0046299 631

F(53) -009435 0096527 -098F(54) -109823 0109525 -1003

F(55) 1671418 0083737 1996

G(31) 1924420 0058162 3309

G(32) -000416 0035255 -012

G(41) 0015621 0095771 016

G(42) 1258495 0055742 2258

G(51) 0080580 0151622 053

G(52) 1353204 0091388 1481

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Chapter 18 Example

The two series are believed to have a transfer function relation with the gas rate

(variable X) as the input and the CO2

concentration (variable Y) as the output Since

the parameter estimates shown in Output 1811 support this kind of model the model

is reestimated with the feedback parameters restricted to 0 The following statements

fit the transfer function (no feedback) model

title3 rsquoTransfer Function Modelrsquo

proc statespace data=seriesj printout=none

var x y

restrict f(32)=0 f(34)=0

g(32)=0 g(41)=0 g(51)=0

run

The last 2 pages of the output are shown in Output 1812

Output 1812 STATESPACE Output for Transfer Function Model

Gas Furnace Data

Box amp Jenkins Series J

Transfer Function Model

The STATESPACE Procedure

Selected Statespace Form and Fitted Model

State Vector

x(TT) y(TT) x(T+1T) y(T+1T) y(T+2T)

Estimate of Transition Matrix

0 0 1 0 0

0 0 0 1 0

-068882 0 1598717 0 0

0 0 0 0 1

-035944 0284179 -00963 -107313 1650047

Input Matrix for Innovation

1 0

0 1

1923446 0

0 1260856

0 1346332

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Part 2 General Information

Gas Furnace Data

Box amp Jenkins Series J

Transfer Function Model

The STATESPACE Procedure

Selected Statespace Form and Fitted Model

Variance Matrix for Innovation

0036995 -00072

-00072 0095712

Parameter Estimates

Standard

Parameter Estimate Error t Value

F(31) -068882 0050549 -1363

F(33) 1598717 0050924 3139

F(51) -035944 0229044 -157

F(52) 0284179 0096944 293

F(53) -009630 0140876 -068

F(54) -107313 0250385 -429

F(55) 1650047 0188533 875G(31) 1923446 0056328 3415

G(42) 1260856 0056464 2233

G(52) 1346332 0091086 1478

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Chapter 18 References

References

Akaike H (1974) Markovian Representation of Stochastic Processes and Its Ap-

plication to the Analysis of Autoregressive Moving Average Processes Annals

of the Institute of Statistical Mathematics 26 363-387

Akaike H (1976) Canonical Correlations Analysis of Time Series and the Use of

an Information Criterion in Advances and Case Studies in System Identification

eds R Mehra and DG Lainiotis New York Academic Press

Anderson TW (1971) The Statistical Analysis of Time Series New York John

Wiley amp Sons

Ansley CF and Newbold P (1979) Multivariate Partial Autocorrelations Pro-

ceedings of the Business and Economic Statistics Section American Statistical

Association 349-353

Box GEP and Jenkins G (1976) Time Series Analysis Forecasting and Control

San Francisco Holden-Day

Brockwell PJ and Davis RA (1991) Time Series Theory and Methods 2nd Edi-

tion Springer-Verlag

Hannan EJ (1970) Multiple Time Series New York John Wiley amp Sons

Hannan EJ (1976) The Identification and Parameterization of ARMAX and State

Space Forms Econometrica 44 713-722

Harvey AC (1981a) The Econometric Analysis of Time Series New York John

Wiley amp Sons

Harvey AC (1981b) Time Series Models New York John Wiley amp Sons

Jones RH (1974) Identification and Autoregressive Spectrum Estimation IEEE Transactions on Automatic Control AC-19 894-897

Pham-Dinh-Tuan (1978) On the Fitting of Multivariate Processes of the

Autoregressive-Moving Average Type Biometrika 65 99-107

Priestley MB (1980) System Identification Kalman Filtering and Stochastic Con-

trol in Directions in Time Series eds DR Brillinger and GC Tiao Institute

of Mathematical Statistics

Whittle P (1963) On the Fitting of Multivariate Autoregressions and the Approxi-

mate Canonical Factorization of a Spectral Density Matrix Biometrika 50 129-

134

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The correct bibliographic citation for this manual is as follows SAS Institute Inc SAS

ET S U serrsquos Gui de Version 8 Cary NC SAS Institute Inc 1999 1546 pp

SASET S Userrsquos Guide Version 8

Copyright copy 1999 by S AS I ns t it u t e I nc Ca ry N C U S A

ISBN 1ndash58025ndash489ndash6

All r ight s res erved P r int ed in t he U n it ed St a t es of A merica N o pa rt of t his p u blica t ionma y b e reprodu ced s t ored in a ret r ieva l s ys t em or t ra ns mit t ed in a ny form or b y a nymeans electronic mechanical photocopying or otherwise without the prior writtenpermission of the publisher SAS Institute Inc

US Government Restricted Rights Notice Use du plicat ion or disclosure of the

software by the government is subject to restrictions as set forth in FAR 52227ndash19Commercial Computer Softw ar e-Restricted Rights (J une 1987)

SAS Institute Inc SAS Campus Drive Cary North Carolina 27513

1st print ing October 1999

S AS reg a nd a ll ot her SA S I ns t it u t e I nc p rodu ct or s ervice na m es a re regis t ered t ra dema rkst d k f SA S I t it t I i t h U S A d t h t i reg i di t U SA

Page 6: Statespace Model

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Part 2 General Information

Getting Started

The following introductory example uses simulated data for two variables X and Y

The following statements generate the X and Y series

data inx=10 y=40

x1=0 y1=0

a1=0 b1=0

iseed=123

do t=-100 to 200

a=rannor(iseed)

b=rannor(iseed)

dx = 05x1 + 03y1 + a - 02a1 - 01b1

dy = 03x1 + 05y1 + b

x = x + d x + 2 5

y = y + d y + 2 5

if t gt= 0 then output

x1 = dx y1 = dy

a 1 = a b 1 = b

end

keep t x y

run

The simulated series X and Y are shown in Figure 181

Figure 181 Example Series

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Chapter 18 Getting Started

Automatic State Space Model Selection

The STATESPACE procedure is designed to automatically select the best state space

model for forecasting the series You can specify your own model if you wish and

you can use the output from PROC STATESPACE to help you identify a state space

model However the easiest way to use PROC STATESPACE is to let it choose the

model

Stationarity and Differencing

Although PROC STATESPACE selects the state space model automatically it does

assume that the input series are stationary If the series are nonstationary then the

process may fail Therefore the first step is to examine your data and test to see if

differencing is required (See the section Stationarity and Differencing later in this

chapter for further discussion of this issue)

The series shown in Figure 181 are nonstationary In order to forecast X and Y

with a state space model you must difference them (or use some other de-trending

method) If you fail to difference when needed and try to use PROC STATESPACE

with nonstationary data an inappropriate state space model may be selected and themodel estimation may fail to converge

The following statements identify and fit a state space model for the first differences

of X and Y and forecast X and Y 10 periods ahead

proc statespace data=in out=out lead=10

var x(1) y(1)

id t

run

The DATA= option specifies the input data set and the OUT= option specifies theoutput data set for the forecasts The LEAD= option specifies forecasting 10 obser-

vations past the end of the input data The VAR statement specifies the variables to

forecast and specifies differencing The notation X(1) Y(1) specifies that the state

space model analyzes the first differences of X and Y

Descriptive Statistics and Preliminary Autoregressions

The first page of the printed output produced by the preceding statements is shown in

Figure 182

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Part 2 General Information

The STATESPACE Procedure

Number of Observations 200

Standard

Variable Mean Error

x 0144316 1233457 Has been differenced With period(s) = 1

y 0164871 1304358 Has been differenced

With period(s) = 1

The STATESPACE Procedure

Information Criterion for Autoregressive Models

Lag=0 Lag=1 Lag=2 Lag=3 Lag=4 Lag=5 Lag=6 Lag=7 Lag=8

149697 8387786 5517099 1205986 1536952 2179538 2400638 2988874 3355708

Information

Criterion for Autoregressive

Models

Lag=9 Lag=10

4117606 4770222

Schematic Representation of Correlations

NameLag 0 1 2 3 4 5 6 7 8 9 10

x ++ ++ ++ ++ ++ ++ + + +

y ++ ++ ++ ++ ++ + + + +

+ is gt 2std error - is lt -2std error is between

Figure 182 Descriptive Statistics and VAR Order Selection

Descriptive statistics are printed first giving the number of nonmissing observations

after differencing and the sample means and standard deviations of the differenced

series The sample means are subtracted before the series are modeled (unless the

NOCENTER option is specified) and the sample means are added back when the

forecasts are produced

LetX

t

andY

t

be the observed values of X and Y and letx

t

andy

t

be the values of X

and Y after differencing and subtracting the mean difference The seriesx

t

modeled

by the STATEPSPACE procedure is

x

t

=

x

t

y

t

=

1 B X

t

0 1 4 4 3 1 6

1 B Y

t

0 1 6 4 8 7 1

where B represents the backshift operator

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Chapter 18 Getting Started

After the descriptive statistics PROC STATESPACE prints the Akaike information

criterion (AIC) values for the autoregressive models fit to the series The smallest AIC

value in this case 5517 at lag 2 determines the number of autocovariance matrices

analyzed in the canonical correlation phase

A schematic representation of the autocorrelations is printed next This indicates

which elements of the autocorrelation matrices at different lags are significantlygreater or less than 0

The second page of the STATESPACE printed output is shown in Figure 183

The STATESPACE Procedure

Schematic Representation of Partial Autocorrelations

NameLag 1 2 3 4 5 6 7 8 9 10

x ++ +

y ++

+ is gt 2std error - is lt -2std error is between

Yule-Walker Estimates for Minimum AIC

--------Lag=1------- --------Lag=2-------

x y x y

x 0257438 0202237 0170812 0133554

y 0292177 0469297 -000537 -000048

Figure 183 Partial Autocorrelations and VAR Model

Figure 183 shows a schematic representation of the partial autocorrelations similar

to the autocorrelations shown in Figure 182 The selection of a second order autore-

gressive model by the AIC statistic looks reasonable in this case because the partialautocorrelations for lags greater than 2 are not significant

Next the Yule-Walker estimates for the selected autoregressive model are printed

This output shows the coefficient matrices of the vector autoregressive model at each

lag

Selected State Space Model Form and Preliminary Estimates

After the autoregressive order selection process has determined the number of lags to

consider the canonical correlation analysis phase selects the state vector By default

output for this process is not printed You can use the CANCORR option to print

details of the canonical correlation analysis See the section Canonical Correlation

Analysis later in this chapter for an explanation of this process

Once the state vector is selected the state space model is estimated by approximate

maximum likelihood Information from the canonical correlation analysis and from

the preliminary autoregression is used to form preliminary estimates of the state space

model parameters These preliminary estimates are used as starting values for the

iterative estimation process

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Part 2 General Information

The form of the state vector and the preliminary estimates are printed next as shown

in Figure 184

The STATESPACE Procedure

Selected Statespace Form and Preliminary Estimates

State Vector

x(TT) y(TT) x(T+1T)

Estimate of Transition Matrix

0 0 1

0291536 0468762 -000411

024869 024484 0204257

Input Matrix for Innovation

1 0

0 1

0257438 0202237

Variance Matrix for Innovation

0945196 0100786

0100786 1014703

Figure 184 Preliminary Estimates of State Space Model

Figure 184 first prints the state vector as X[TT] Y[TT] X[T+1T] This notation

indicates that the state vector is

z

t

=

2

4

x

t j t

y

t j t

x

t + 1 j t

3

5

The notationx

t + 1 j t

indicates the conditional expectation or prediction of x

t + 1

based

on the information available at time t andx

t j t

andy

t j t

arex

t

andy

t

respectively

The remainder of Figure 184 shows the preliminary estimates of the transition matrix

F

the input matrixG

and the covariance matrix

e e

Estimated State Space Model

The next page of the STATESPACE output prints the final estimates of the fittedmodel as shown in Figure 185 This output has the same form as in Figure 184 but

shows the maximum likelihood estimates instead of the preliminary estimates

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Chapter 18 Getting Started

The STATESPACE Procedure

Selected Statespace Form and Fitted Model

State Vector

x(TT) y(TT) x(T+1T)

Estimate of Transition Matrix

0 0 1

0297273 047376 -001998

02301 0228425 0256031

Input Matrix for Innovation

1 0

0 1

0257284 0202273

Variance Matrix for Innovation

0945188 01007520100752 1014712

Figure 185 Fitted State Space Model

The estimated state space model shown in Figure 185 is

2

4

x

t + 1 j t + 1

y

t + 1 j t + 1

x

t + 2 j t + 1

3

5

=

2

4

0 0 1

0 2 9 7 0 4 7 4 0 0 2 0

0 2 3 0 0 2 2 8 0 2 5 6

3

5

2

4

x

t

y

t

x

t + 1 j t

3

5

+

2

4

1 0

0 1

0 2 5 7 0 2 0 2

3

5

e

t + 1

n

t + 1

v a r

e

t + 1

n

t + 1

=

0 9 4 5 0 1 0 1

0 1 0 1 1 0 1 5

The next page of the STATESPACE output lists the estimates of the free parameters

in theF

andG

matrices with standard errors and t statistics as shown in Figure 186

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Part 2 General Information

The STATESPACE Procedure

Parameter Estimates

Standard

Parameter Estimate Error t Value

F(21) 0297273 0129995 229

F(22) 0473760 0115688 410F(23) -001998 0313025 -006

F(31) 0230100 0126226 182

F(32) 0228425 0112978 202

F(33) 0256031 0305256 084

G(31) 0257284 0071060 362

G(32) 0202273 0068593 295

Figure 186 Final Parameter Estimates

Convergence Failures

The maximum likelihood estimates are computed by an iterative nonlinear maximiza-

tion algorithm which may not converge If the estimates fail to converge warning

messages are printed in the output

If you encounter convergence problems you should recheck the stationarity of the

data and ensure that the specified differencing orders are correct Attempting to fit

state space models to nonstationary data is a common cause of convergence failure

You can also use the MAXIT= option to increase the number of iterations allowed

or experiment with the convergence tolerance options DETTOL= and PARMTOL=

Forecast Data Set

The following statements print the output data set The WHERE statement excludes

the first 190 observations from the output so that only the forecasts and the last 10

actual observations are printed

proc print data=outid t

where t gt 190

run

The PROC PRINT output is shown in Figure 187

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Part 2 General Information

Figure 188 Plot of Forecasts

Controlling Printed Output

By default the STATESPACE procedure produces a large amount of printed output

The NOPRINT option suppresses all printed output You can suppress the printed

output for the autoregressive model selection process with the PRINTOUT=NONE

option The descriptive statistics and state space model estimation output are still

printed when PRINTOUT=NONE is specified You can produce more detailed output

with the PRINTOUT=LONG option and by specifying the printing control optionsCANCORR COVB and PRINT

Specifying the State Space Model

Instead of allowing the STATESPACE procedure to select the model automatically

you can use FORM and RESTRICT statements to specify a state space model

Specifying the State Vector

Use the FORM statement to control the form of the state vector You can use this

feature to force PROC STATESPACE to estimate and forecast a model different from

the model it would select automatically You can also use this feature to reestimate

the automatically selected model (possibly with restrictions) without repeating thecanonical correlation analysis

The FORM statement specifies the number of lags of each variable to include in

the state vector For example the statement FORM X 3 forces the state vector to

includex

t j t

x

t + 1 j t

andx

t + 2 j t

The following statement specifies the state vector

x

t j t

y

t j t

x

t + 1 j t

which is the same state vector selected in the preceding example

f o r m x 2 y 1

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Chapter 18 Getting Started

You can specify the form for only some of the variables and allow PROC STATES-

PACE to select the form for the other variables If only some of the variables are

specified in the FORM statement canonical correlation analysis is used to determine

the number of lags included in the state vector for the remaining variables not spec-

ified by the FORM statement If the FORM statement includes specifications for all

the variables listed in the VAR statement the state vector is completely defined and

the canonical correlation analysis is not performed

Restricting the F and G matrices

After you know the form of the state vector you can use the RESTRICT statement

to fix some parameters in theF

andG

matrices to specified values One use of this

feature is to remove insignificant parameters by restricting them to 0

In the introductory example shown in the preceding section the F[23] parameter is

not significant (The parameters estimation output shown in Figure 186 gives the t

statistic for F[23] as -006 F[33] and F[31] also have low significance witht 2

)

The following statements reestimate this model with F[23] restricted to 0 The

FORM statement is used to specify the state vector and thus bypass the canonicalcorrelation analysis

proc statespace data=in out=out lead=10

var x(1) y(1)

id t

form x 2 y 1

restrict f(23)=0

run

The final estimates produced by these statements are shown in Figure 189

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Part 2 General Information

The STATESPACE Procedure

Selected Statespace Form and Fitted Model

State Vector

x(TT) y(TT) x(T+1T)

Estimate of Transition Matrix

0 0 1

0290051 0467468 0

0227051 0226139 026436

Input Matrix for Innovation

1 0

0 1

0256826 0202022

Variance Matrix for Innovation

0945175 01006960100696 1014733

The STATESPACE Procedure

Parameter Estimates

Standard

Parameter Estimate Error t Value

F(21) 0290051 0063904 454

F(22) 0467468 0060430 774

F(31) 0227051 0125221 181

F(32) 0226139 0111711 202

F(33) 0264360 0299537 088G(31) 0256826 0070994 362

G(32) 0202022 0068507 295

Figure 189 Results using RESTRICT Statement

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Chapter 18 Syntax

Syntax

The STATESPACE procedure uses the following statements

PROC STATESPACE options

BY variable FORM variable value

ID variable

INITIAL F(rowcolumn)=value

G(rowcolumn)=value

RESTRICT F(rowcolumn)=value

G(rowcolumn)=value

VAR variable (difference difference

)

Functional Summary

The statements and options used by PROC STATESPACE are summarized in the

following table

Description Statement Option

Input Data Set Options

specify the input data set PROC STATESPACE DATA=

prevent subtraction of sample mean PROC STATESPACE NOCENTER

specify the ID variable ID

specify the observed series and differencing VAR

Options for Autoregressive Estimates

specify the maximum order PROC STATESPACE ARMAX=

specify maximum lag for autocovariances PROC STATESPACE LAGMAX=

output only minimum AIC model PROC STATESPACE MINIC

specify the amount of detail printed PROC STATESPACE PRINTOUT=

write preliminary AR models to a data set PROC STATESPACE OUTAR=

Options for Canonical Correlation Analysis

print the sequence of canonical correlations PROC STATESPACE CANCORR

specify upper limit of dimension of state

vector

PROC STATESPACE DIMMAX=

specify the minimum number of lags PROC STATESPACE PASTMIN=

specify the multiplier of the degrees of

freedom

PROC STATESPACE SIGCORR=

Options for State Space Model Estimation

specify starting values INITIAL

print covariance matrix of parameter estimates PROC STATESPACE COVB

specify the convergence criterion PROC STATESPACE DETTOL=

specify the convergence criterion PROC STATESPACE PARMTOL=

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Part 2 General Information

Description Statement Option

print the details of the iterations PROC STATESPACE ITPRINT

specify an upper limit of the number of lags PROC STATESPACE KLAG=

specify maximum number of iterations

allowed

PROC STATESPACE MAXIT=

suppress the final estimation PROC STATESPACE NOEST

write the state space model parameter esti-

mates to an output data set

PROC STATESPACE OUTMODEL=

use conditional least squares for final estimates PROC STATESPACE RESIDEST

specify criterion for testing for singularity PROC STATESPACE SINGULAR=

Options for Forecasting

start forecasting before end of the input data PROC STATESPACE BACK=

specify the time interval between observations PROC STATESPACE INTERVAL=

specify multiple periods in the time series PROC STATESPACE INTPER=specify how many periods to forecast PROC STATESPACE LEAD=

specify the output data set for forecasts PROC STATESPACE OUT=

print forecasts PROC STATESPACE PRINT

Options to Specify the State Space Model

specify the state vector FORM

specify the parameter values RESTRICT

BY Groups

specify BY-group processing BY

Printing

suppresses all printed output NOPRINT

PROC STATESPACE Statement

PROC STATESPACE options

The following options can be specified in the PROC STATESPACE statement

Printing Options NOPRINT

suppresses all printed output

Input Data Options

DATA= SAS-data-set

specifies the name of the SAS data set to be used by the procedure If the DATA=

option is omitted the most recently created SAS data set is used

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Chapter 18 Syntax

LAGMAX= k

specifies the number of lags for which the sample autocovariance matrix is computed

The LAGMAX= option controls the number of lags printed in the schematic repre-

sentation of the autocorrelations

The sample autocovariance matrix of lag i denoted asC

i

is computed as

C

i

=

1

N 1

N

X

t = 1 + i

x

t

x

0

t i

wherex

t

is the differenced and centered data andN

is the number of observations

(If the NOCENTER option is specified 1 is not subtracted fromN

) LAGMAX= k

specifies thatC

0

throughC

k

are computed The default is LAGMAX=10

NOCENTER

prevents subtraction of the sample mean from the input series (after any specified

differencing) before the analysis

Options for Preliminary Autoregressive Models ARMAX= n

specifies the maximum order of the preliminary autoregressive models The AR-

MAX= option controls the autoregressive orders for which information criteria are

printed and controls the number of lags printed in the schematic representation of

partial autocorrelations The default is ARMAX=10 See Preliminary Autoregres-

sive Models later in this chapter for details

MINIC

writes to the OUTAR= data set only the preliminary Yule-Walker estimates for the

VAR model producing the minimum AIC See OUTAR= Data Set later in this chap-

ter for details

OUTAR= SAS-data-set

writes the Yule-Walker estimates of the preliminary autoregressive models to a SAS

data set See OUTAR= Data Set later in this chapter for details

PRINTOUT= SHORT | LONG | NONE

determines the amount of detail printed PRINTOUT=LONG prints the lagged co-

variance matrices the partial autoregressive matrices and estimates of the resid-

ual covariance matrices from the sequence of autoregressive models PRINT-

OUT=NONE suppresses the output for the preliminary autoregressive models The

descriptive statistics and state space model estimation output are still printed when

PRINTOUT=NONE is specified PRINTOUT=SHORT is the default

Canonical Correlation Analysis Options

CANCORR

prints the canonical correlations and information criterion for each candidate state

vector considered See Canonical Correlation Analysis later in this chapter for

details

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Part 2 General Information

DIMMAX= n

specifies the upper limit to the dimension of the state vector The DIMMAX= option

can be used to limit the size of the model selected The default is DIMMAX=10

PASTMIN= n

specifies the minimum number of lags to include in the canonical correlation analy-

sis The default is PASTMIN=0 See Canonical Correlation Analysis later in thischapter for details

SIGCORR= value

specifies the multiplier of the degrees of freedom for the penalty term in the informa-

tion criterion used to select the state space form The default is SIGCORR=2 The

larger the value of the SIGCORR= option the smaller the state vector tends to be

Hence a large value causes a simpler model to be fit See Canonical Correlations

Analysis later in this chapter for details

State Space Model Estimation Options

COVB

prints the inverse of the observed information matrix for the parameter estimatesThis matrix is an estimate of the covariance matrix for the parameter estimates

DETTOL= value

specifies the convergence criterion The DETTOL= and PARMTOL= option values

are used together to test for convergence of the estimation process If during an

iteration the relative change of the parameter estimates is less than the PARMTOL=

value and the relative change of the determinant of the innovation variance matrix

is less than the DETTOL= value then iteration ceases and the current estimates are

accepted The default is DETTOL=1E-5

ITPRINT

prints the iterations during the estimation process

KLAG= n

sets an upper limit for the number of lags of the sample autocovariance matrix used

in computing the approximate likelihood function If the data have a strong moving

average character a larger KLAG= value may be necessary to obtain good estimates

The default is KLAG=15 See Parameter Estimation later in this chapter for details

MAXIT= n

sets an upper limit to the number of iterations in the maximum likelihood or condi-

tional least-squares estimation The default is MAXIT=50

NOEST

suppresses the final maximum likelihood estimation of the selected model

OUTMODEL= SAS-data-set

writes the parameter estimates and their standard errors to a SAS data set See OUT-

MODEL= Data Set later in this chapter for details

PARMTOL= value

specifies the convergence criterion The DETTOL= and PARMTOL= option values

are used together to test for convergence of the estimation process If during an

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Chapter 18 Syntax

iteration the relative change of the parameter estimates is less than the PARMTOL=

value and the relative change of the determinant of the innovation variance matrix

is less than the DETTOL= value then iteration ceases and the current estimates are

accepted The default is PARMTOL=001

RESIDEST

computes the final estimates using conditional least squares on the raw data Thistype of estimation may be more stable than the default maximum likelihood method

but is usually more computationally expensive See Parameter Estimation later in

this chapter for details of the conditional least squares method

SINGULAR= value

specifies the criterion for testing for singularity of a matrix A matrix is declared

singular if a scaled pivot is less than the SINGULAR= value when sweeping the

matrix The default is SINGULAR=1E-7

Forecasting Options

BACK= n

starts forecasting n periods before the end of the input data The BACK= option valuemust not be greater than the number of observations The default is BACK=0

INTERVAL= interval

specifies the time interval between observations The INTERVAL= value is used

in conjunction with the ID variable to check that the input data are in order and

have no missing periods The INTERVAL= option is also used to extrapolate the ID

values past the end of the input data See Chapter 3 ldquoDate Intervals Formats and

Functionsrdquo for details on the INTERVAL= values allowed

INTPER= n

specifies that each input observation corresponds to n time periods For example

the options INTERVAL=MONTH and INTPER=2 specify bimonthly data and are

equivalent to specifying INTERVAL=MONTH2 If the INTERVAL= option is not

specified the INTPER= option controls the increment used to generate ID values for

the forecast observations The default is INTPER=1

LEAD= n

specifies how many forecast observations are produced The forecasts start at the

point set by the BACK= option The default is LEAD=0 which produces no fore-

casts

OUT= SAS-data-set

writes the residuals actual values forecasts and forecast standard errors to a SAS

data set See OUT= Data Set later in this chapter for details

PRINT

prints the forecasts

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Part 2 General Information

BY Statement

BY variable

A BY statement can be used with the STATESPACE procedure to obtain separateanalyses on observations in groups defined by the BY variables

FORM Statement

FORM variable value

The FORM statement specifies the number of times a variable is included in the state

vector Values can be specified for any variable listed in the VAR statement If a

value is specified for each variable in the VAR statement the state vector for the state

space model is entirely specified and automatic selection of the state space model is

not performed

The FORM statement forces the state vectorz

t

to contain a specific variable a given

number of times For example if Y is one of the variables inx

t

then the statement

form y 3

forces the state vector to containY

t

Y

t + 1 j t

andY

t + 2 j t

possibly along with other

variables

The following statements illustrate the use of the FORM statement

proc statespace data=in

var x y

f o r m x 3 y 2

run

These statements fit a state space model with the following state vector

z

t

=

2

6

6

6

6

4

x

t j t

y

t j t

x

t + 1 j t

y

t + 1 j t

x

t + 2 j t

3

7

7

7

7

5

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Chapter 18 Syntax

ID Statement

ID variable

The ID statement specifies a variable that identifies observations in the input data setThe variable specified in the ID statement is included in the OUT= data set The

values of the ID variable are extrapolated for the forecast observations based on the

values of the INTERVAL= and INTPER= options

INITIAL Statement

INITIAL F (rowcolumn)= value G(row column)= value

The INITIAL statement gives initial values to the specified elements of theF

andG

matrices These initial values are used as starting values for the iterative estimation

Parts of theF

andG

matrices represent fixed structural identities If an element

specified is a fixed structural element instead of a free parameter the corresponding

initialization is ignored

The following is an example of an INITIAL statement

initial f(32)=0 g(41)=0 g(51)=0

RESTRICT Statement

RESTRICT F(rowcolumn)= value G(rowcolumn)= value

The RESTRICT statement restricts the specified elements of theF

andG

matrices

to the specified values

To use the restrict statement you need to know the form of the model Either specify

the form of the model with the FORM statement or do a preliminary run perhaps

with the NOEST option to find the form of the model that PROC STATESPACE

selects for the data

The following is an example of a RESTRICT statement

restrict f(32)=0 g(41)=0 g(51)=0

Parts of theF

andG

matrices represent fixed structural identities If a restriction is

specified for an element that is a fixed structural element instead of a free parameter

the restriction is ignored

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Part 2 General Information

VAR Statement

VAR variable (difference difference )

The VAR statement specifies the variables in the input data set to model and fore-cast The VAR statement also specifies differencing of the input variables The VAR

statement is required

Differencing is specified by following the variable name with a list of difference

periods separated by commas See the section Stationarity and Differencing for

more information on differencing of input variables

The order in which variables are listed in the VAR statement controls the order in

which variables are included in the state vector Usually potential inputs should be

listed before potential outputs

For example assuming the input data are monthly the following VAR statement

specifies modeling and forecasting of the one period and seasonal second differenceof X and Y

var x(112) y(112)

In this example the vector time series analyzed is

x

t

=

1 B 1 B

1 2

X

t

x

1 B 1 B

1 2

Y

t

y

where B represents the back shift operator and x and y represent the means of thedifferenced series If the NOCENTER option is specified the mean differences are

not subtracted

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Chapter 18 Details

Details

Missing Values

The STATESPACE procedure does not support missing values The procedure uses

the first contiguous group of observations with no missing values for any of the VARstatement variables Observations at the beginning of the data set with missing values

for any VAR statement variable are not used or included in the output data set

Stationarity and Differencing

The state space model used by the STATESPACE procedure assumes that the time

series are stationary Hence the data should be checked for stationarity One way

to check for stationarity is to plot the series A graph of series over time can show a

time trend or variability changes

You can also check stationarity by using the sample autocorrelation functions dis-

played by the ARIMA procedure The autocorrelation functions of nonstationaryseries tend to decay slowly See Chapter 7 ldquoThe ARIMA Procedurerdquo for more in-

formation

Another alternative is to use the STATIONARITY= option on the IDENTIFY state-

ment in PROC ARIMA to apply Dickey-Fuller tests for unit roots in the time series

See Chapter 7 ldquoThe ARIMA Procedurerdquo for more information on Dickey-Fuller unit

root tests

The most popular way to transform a nonstationary series to stationarity is by dif-

ferencing Differencing of the time series is specified in the VAR statement For

example to take a simple first difference of the series X use this statement

var x(1)

In this example the change in X from one period to the next is analyzed When the se-

ries has a seasonal pattern differencing at a period equal to the length of the seasonal

cycle may be desirable For example suppose the variable X is measured quarterly

and shows a seasonal cycle over the year You can use the following statement to

analyze the series of changes from the same quarter in the previous year

var x(4)

To difference twice add another differencing period to the list For ex-

ample the following statement analyzes the series of second differences

X

t

X

t 1

X

t 1

X

t 2

= X

t

2 X

t 1

+ X

t 2

var x(11)

The following statement analyzes the seasonal second difference series

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Part 2 General Information

var x(14)

The series modeled is the 1-period difference of the 4-period difference

X

t

X

t 4

X

t 1

X

t 5

= X

t

X

t 1

X

t 4

+ X

t 5

Another way to obtain stationary series is to use a regression on time to de-trend the

data If the time series has a deterministic linear trend regressing the series on time

produces residuals that should be stationary The following statements write residuals

of X and Y to the variable RX and RY in the output data set DETREND

data a

set a

t=_n_

run

proc reg data=a

model x y = t

output out=detrend r=rx ry

run

You then use PROC STATESPACE to forecast the de-trended series RX and RY A

disadvantage of this method is that you need to add the trend back to the forecast

series in an additional step A more serious disadvantage of the de-trending method

is that it assumes a deterministic trend In practice most time series appear to have a

stochastic rather than a deterministic trend Differencing is a more flexible and often

more appropriate method

There are several other methods to handle nonstationary time series For more infor-

mation and examples refer to Brockwell and Davis (1991)

Preliminary Autoregressive Models

After computing the sample autocovariance matrices PROC STATESPACE fits a

sequence of vector autoregressive models These preliminary autoregressive models

are used to estimate the autoregressive order of the process and limit the order of the

autocovariances considered in the state vector selection process

Yule-Walker Equations for Forward and Backward Models

Unlike a univariate autoregressive model a multivariate autoregressive model has

different forms depending on whether the present observation is being predicted from

the past observations or from the future observations

Letx

t

be the r -component stationary time series given by the VAR statement after

differencing and subtracting the vector of sample means (If the NOCENTER option

is specified the mean is not subtracted) Let n be the number of observations of x

t

from the input data set

Lete

t

be a vector white noise sequence with mean vector 0 and variance matrix

p

and letn

t

be a vector white noise sequence with mean vector 0 and variance matrix

p

Let p be the order of the vector autoregressive model forx

t

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The forward autoregressive form based on the past observations is written as follows

x

t

=

p

X

i = 1

p

i

x

t i

+ e

t

The backward autoregressive form based on the future observations is written as fol-

lows

x

t

=

p

X

i = 1

p

i

x

t + i

+ n

t

LettingE

denote the expected value operator the autocovariance sequence for thex

t

series

i

is

i

= E x

t

x

0

t i

The Yule-Walker equations for the autoregressive model that matches the first p ele-

ments of the autocovariance sequence are

2

6

6

4

0

1

p 1

0

1

0

p 2

0

p 1

0

p 2

0

3

7

7

5

2

6

6

4

p

1

p

2

p

p

3

7

7

5

=

2

6

6

4

1

2

p

3

7

7

5

and

2

6

6

6

4

0

0

1

0

p 1

1

0

0

p 2

p 1

p 2

0

3

7

7

7

5

2

6

6

4

p

1

p

2

p

p

3

7

7

5

=

2

6

6

4

0

1

0

2

0

p

3

7

7

5

Here

p

i

are the coefficient matrices for the past observation form of the vector au-toregressive model and

p

i

are the coefficient matrices for the future observation

form More information on the Yule-Walker equations in the multivariate setting can

be found in Whittle (1963) and Ansley and Newbold (1979)

The innovation variance matrices for the two forms can be written as follows

p

=

0

p

X

i = 1

p

i

0

i

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p

=

0

p

X

i = 1

p

i

i

The autoregressive models are fit to the data using the preceding Yule-Walker equa-

tions with

i

replaced by the sample covariance sequenceC

i

The covariance matri-

ces are calculated as

C

i

=

1

N 1

N

X

t = i + 1

x

t

x

0

t i

Let b

p

b

p

b

p

and b

p

represent the Yule-Walker estimates of

p

p

p

and

p

respectively These matrices are written to an output data set when the OUTAR=

option is specified

When the PRINTOUT=LONG option is specified the sequence of matrices b

p

and

the corresponding correlation matrices are printed The sequence of matricesb

p isused to compute Akaike information criteria for selection of the autoregressive order

of the process

Akaike Information Criterion

The Akaike information criterion or AIC is defined as -2( maximum of log likeli-

hood )+2(number of parameters) Since the vector autoregressive models are esti-

mates from the Yule-Walker equations not by maximum likelihood the exact likeli-

hood values are not available for computing the AIC However for the vector autore-

gressive model the maximum of the log likelihood can be approximated as

l n L

n

2

l n j

b

p

j

Thus the AIC for the order p model is computed as

A I C

p

= n l n j

b

p

j + 2 p r

2

You can use the printed AIC array to compute a likelihood ratio test of the autoregres-

sive order The log-likelihood ratio test statistic for testing the order p model against

the order p 1

model is

n l n j

b

p

j + n l n j

b

p 1

j

This quantity is asymptotically distributed as a

2 withr

2 degrees of freedom if the

series is autoregressive of order p 1

It can be computed from the AIC array as

A I C

p 1

A I C

p

+ 2 r

2

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You can evaluate the significance of these test statistics with the PROBCHI function

in a SAS DATA step or with a

2 table

Determining the Autoregressive Order

Although the autoregressive models can be used for prediction their primary value is

to aid in the selection of a suitable portion of the sample covariance matrix for use in

computing canonical correlations If the multivariate time series x

t is of autoregres-sive order p then the vector of past values to lag p is considered to contain essentially

all the information relevant for prediction of future values of the time series

By default PROC STATESPACE selects the order p producing the autoregressive

model with the smallestA I C

p

If the value p for the minimumA I C

p

is less than the

value of the PASTMIN= option then p is set to the PASTMIN= value Alternatively

you can use the ARMAX= and PASTMIN= options to force PROC STATESPACE to

use an order you select

Significance Limits for Partial Autocorrelations

The STATESPACE procedure prints a schematic representation of the partial autocor-

relation matrices indicating which partial autocorrelations are significantly greater orsignificantly less than 0 Figure 1810 shows an example of this table

The STATESPACE Procedure

Schematic Representation of Partial Autocorrelations

NameLag 1 2 3 4 5 6 7 8 9 10

x ++ +

y ++

+ is gt 2std error - is lt -2std error is between

Figure 1810 Significant Partial Autocorrelations

The partial autocorrelations are from the sample partial autoregressive matrices b

p

p

The standard errors used for the significance limits of the partial autocorrelations are

computed from the sequence of matrices

p

and

p

Under the assumption that the observed series arises from an autoregressive process

of order p 1

the pth sample partial autoregressive matrix b

p

p

has an asymptotic

variance matrix 1

n

1

p

p

The significance limits for b

p

p

used in the schematic plot of the sample partial autore-

gressive sequence are derived by replacing

p

and

p

with their sample estimators

to produce the variance estimate as follows

d

V a r

b

p

p

=

1

n r p

b

1

p

b

p

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Canonical Correlation Analysis

Given the order p letp

t

be the vector of current and past values relevant to prediction

of x

t + 1

p

t

= x

0

t

x

0

t 1

x

0

t p

0

Letf

t

be the vector of current and future values

f

t

= x

0

t

x

0

t + 1

x

0

t + p

0

In the canonical correlation analysis consider submatrices of the sample covariance

matrix of p

t

andf

t

This covariance matrixV

has a block Hankel form

V =

2

6

6

6

4

C

0

C

0

1

C

0

2

C

0

p

C

0

1

C

0

2

C

0

3

C

0

p + 1

C

0

p

C

0

p + 1

C

0

p + 2

C

0

2 p

3

7

7

7

5

State Vector Selection Process

The canonical correlation analysis forms a sequence of potential state vectorsz

j

t

Examine a sequencef

j

t

of subvectors of f

t

and form the submatrixV

j consisting

of the rows and columns of V

corresponding to the components of f

j

t

and compute

its canonical correlations

The smallest canonical correlation of V

j is then used in the selection of the com-

ponents of the state vector The selection process is described in the following For

more details about this process refer to Akaike (1976)

In the following discussion the notationx

t + k j t

denotes the wide sense conditional

expectation (best linear predictor) of x

t + k

given allx

s

with s less than or equal to t

In the notationx

i t + 1

the first subscript denotes the ith component of x

t + 1

The initial state vectorz

1

t

is set tox

t

The sequencef

j

t

is initialized by setting

f

1

t

= z

1

0

t

x

1 t + 1 j t

0

= x

0

t

x

1 t + 1 j t

0

That is start by considering whether to addx

1 t + 1 j t

to the initial state vectorz

1

t

The procedure forms the submatrixV

1 corresponding tof

1

t

and computes its canon-

ical correlations Denote the smallest canonical correlation of V

1 as

m i n

If

m i n

is

significantly greater than 0x

1 t + 1 j t

is added to the state vector

If the smallest canonical correlation of V

1 is not significantly greater than 0 then a

linear combination of f

1

t

is uncorrelated with the pastp

t

Assuming that the determi-

nant of C

0

is not 0 (that is no input series is a constant) you can take the coefficient

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of x

1 t + 1 j t

in this linear combination to be 1 Denote the coefficients of z

1

t

in this

linear combination as

This gives the relationship

x

1 t + 1 j t

=

0

x

t

Therefore the current state vector already contains all the past information useful forpredicting

x

1 t + 1

and any greater leads of x

1 t

The variablex

1 t + 1 j t

is not added to

the state vector nor are any termsx

1 t + k j t

considered as possible components of the

state vector The variablex

1

is no longer active for state vector selection

The process described forx

1 t + 1 j t

is repeated for the remaining elements of f

t

The

next candidate for inclusion in the state vector is the next component of f

t

corre-

sponding to an active variable Components of f

t

corresponding to inactive variables

that produced a zero

m i n

in a previous step are skipped

Denote the next candidate asx

l t + k j t

The vectorf

j

t

is formed from the current state

vector andx

l t + k j t

as follows

f

j

t

= z

j

0

t

x

l t + k j t

0

The matrixV

j is formed fromf

j

t

and its canonical correlations are computed The

smallest canonical correlation of V

j is judged to be either greater than or equal to 0

If it is judged to be greater than 0x

l t + k j t

is added to the state vector If it is judged

to be 0 then a linear combination of f

j

t

is uncorrelated with thep

t

and the variable

x

l

is now inactive

The state vector selection process continues until no active variables remain

Testing Significance of Canonical Correlations For each step in the canonical correlation sequence the significance of the small-

est canonical correlation

m i n

is judged by an information criterion from Akaike

(1976) This information criterion is

n l n 1

2

m i n

r p + 1 q + 1

where q is the dimension of f

j

t

at the current step r is the order of the state vector p

is the order of the vector autoregressive process and

is the value of the SIGCORR=

option The default is SIGCORR=2 If this information criterion is less than or equal

to 0

m i n

is taken to be 0 otherwise it is taken to be significantly greater than 0

(Do not confuse this information criterion with the AIC)

Variables inx

t + p j t

are not added in the model even with positive information crite-

rion because of the singularity of V

You can force the consideration of more candi-

date state variables by increasing the size of theV

matrix by specifying a PASTMIN=

option value larger than p

Printing the Canonical Correlations

To print the details of the canonical correlation analysis process specify the CAN-

CORR option in the PROC STATESPACE statement The CANCORR option prints

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the candidate state vectors the canonical correlations and the information criteria for

testing the significance of the smallest canonical correlation

Bartlettrsquos

2 and its degrees of freedom are also printed when the CANCORR option

is specified The formula used for Bartlettrsquos

2 is

2

= n 5 r p + 1 q + 1 l n 1

2

m i n

withr p + 1 q + 1

degrees of freedom

Figure 1811 shows the output of the CANCORR option for the introductory example

shown in the Getting Started section of this chapter

The STATESPACE Procedure

Canonical Correlations Analysis

Information Chi

x(TT) y(TT) x(T+1T) Criterion Square DF

1 1 0237045 3566167 114505 4

Information Chi

x(TT) y(TT) x(T+1T) y(T+1T) Criterion Square DF

1 1 0238244 0056565 -535906 0636134 3

Information Chi

x(TT) y(TT) x(T+1T) x(T+2T) Criterion Square DF

1 1 0237602 0087493 -446312 1525353 3

Figure 1811 Canonical Correlations Analysis

New variables are added to the state vector if the information criteria are positive Inthis example

y

t + 1 j t

andx

t + 2 j t

are not added to the state space vector because the

information criteria for these models are negative

If the information criterion is nearly 0 then you may want to investigate models

that arise if the opposite decision is made regarding

m i n

This investigation can be

accomplished by using a FORM statement to specify part or all of the state vector

Preliminary Estimates of F

When a candidate variablex

l t + k j t

yields a zero

m i n

and is not added to the state

vector a linear combination of f

j

t

is uncorrelated with thep

t

Because of the method

used to construct thef

j

t

sequence the coefficient of x

l t + k j t

inl

can be taken as 1

Denote the coefficients of z

j

t

in this linear combination as l

This gives the relationship

x

l t + k j t

= l

0

z

j

t

The vectorl

is used as a preliminary estimate of the first r columns of the row of the

transition matrixF

corresponding tox

l t + k 1 j t

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Parameter Estimation

The model isz

t + 1

= F z

t

+ G e

t + 1

wheree

t

is a sequence of independent multivari-

ate normal innovations with mean vector 0 and variance

e e

The observed sequence

x

t

composes the first r components of z

t

and thusx

t

= H z

t

where H is ther s

matrix I

r

0

LetE

be ther n

matrix of innovations

E = e

1

e

n

If the number of observations n is reasonably large the log likelihood L can be

approximated up to an additive constant as follows

L =

n

2

l n j

e e

j

1

2

t r a c e

1

e e

E E

0

The elements of

e e

are taken as free parameters and are estimated as follows

S

0

=

1

n

E E

0

Replacing

e e

byS

0

in the likelihood equation the log likelihood up to an additive

constant is

L =

n

2

l n j S

0

j

Letting B be the backshift operator the formal relation betweenx

t

ande

t

is

x

t

= H I B F

1

G e

t

e

t

= H I B F

1

G

1

x

t

=

1

X

i = 0

i

x

t i

LettingC

i

be the ith lagged sample covariance of x

t

and neglecting end effects the

matrixS

0

is

S

0

=

1

X

i j = 0

i

C

i + j

0

j

For the computation of S

0

the infinite sum is truncated at the value of the KLAG=

option The value of the KLAG= option should be large enough that the sequence

i

is approximately 0 beyond that point

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Let

be the vector of free parameters in theF

andG

matrices The derivative of the

log likelihood with respect to the parameter

is

L

=

n

2

t r a c e

S

1

0

S

0

The second derivative is

2

L

0

=

n

2

t r a c e

S

1

0

S

0

0

S

1

0

S

0

t r a c e

S

1

0

2

S

0

0

Near the maximum the first term is unimportant and the second term can be approx-

imated to give the following second derivative approximation

2

L

0

=

n t r a c e

S

1

0

E

E

0

0

The first derivative matrix and this second derivative matrix approximation are com-

puted from the sample covariance matrixC

0

and the truncated sequence

i

The

approximate likelihood function is maximized by a modified Newton-Raphson algo-

rithm employing these derivative matrices

The matrixS

0

is used as the estimate of the innovation covariance matrix

e e

The

negative of the inverse of the second derivative matrix at the maximum is used as an

approximate covariance matrix for the parameter estimates The standard errors of

the parameter estimates printed in the parameter estimates tables are taken from the

diagonal of this covariance matrix The parameter covariance matrix is printed when

the COVB option is specified

If the data are nearly nonstationary a better estimate of

e e

and the other parameters

can sometimes be obtained by specifying the RESIDEST option The RESIDEST

option estimates the parameters using conditional least squares instead of maximum

likelihood

The residuals are computed using the state space equation and the sample mean val-

ues of the variables in the model as start-up values The estimate of S

0

is then com-

puted using the residuals from the ith observation on where i is the maximum number

of times any variable occurs in the state vector A multivariate Gauss-Marquardt al-

gorithm is used to minimizej S

0

j

Refer to Harvey (1981a) for a further description

of this method

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Forecasting

Given estimates of F

G

and

e e

forecasts of x

t

are computed from the conditional

expectation of z

t

In forecasting the parameters F G and

e e

are replaced with the estimates or by val-

ues specified in the RESTRICT statement One-step-ahead forecasting is performedfor the observation

x

t

wheret n b

Heren

is the number of observations and

b is the value of the BACK= option For the observationx

t

wheret n b

m-

step-ahead forecasting is performed form = t n + b

The forecasts are generated

recursively with the initial conditionz

0

= 0

The m-step-ahead forecast of z

t + m

isz

t + m j t

wherez

t + m j t

denotes the conditional

expectation of z

t + m

given the information available at time t The m-step-ahead

forecast of x

t + m

isx

t + m j t

= H z

t + m j t

where the matrixH = I

r

0

Let

i

= F

i

G

Note that the lasts r

elements of z

t

consist of the elements of x

u j t

foru t

The state vector z

t + m

can be represented as

z

t + m

= F

m

z

t

+

m 1

X

i = 0

i

e

t + m i

Sincee

t + i j t

= 0

fori 0

the m-step-ahead forecastz

t + m j t

is

z

t + m j t

= F

m

z

t

= F z

t + m 1 j t

Therefore the m-step-ahead forecast of x

t + m

is

x

t + m j t

= H z

t + m j t

The m-step-ahead forecast error is

z

t + m

z

t + m j t

=

m 1

X

i = 0

i

e

t + m i

The variance of the m-step-ahead forecast error is

V

z m

=

m 1

X

i = 0

i

e e

0

i

LettingV

z 0

= 0

the variance of the m-step-ahead forecast error of z

t + m

V

z m

can

be computed recursively as follows

V

z m

= V

z m 1

+

m 1

e e

0

m 1

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The variance of the m-step-ahead forecast error of x

t + m

is ther r

left upper sub-

matrix of V

z m

that is

V

x m

= H V

z m

H

0

Unless the NOCENTER option is specified the sample mean vector is added to the

forecast When differencing is specified the forecasts xt + m j t

plus the sample mean

vector are integrated back to produce forecasts for the original series

Lety

t

be the original series specified by the VAR statement with some 0 values

appended corresponding to the unobserved past observations Let B be the backshift

operator and let B

be thes s

matrix polynomial in the backshift operator

corresponding to the differencing specified by the VAR statement The off-diagonal

elements of

i

are 0 Note that

0

= I

s

whereI

s

is thes s

identity matrix Then

z

t

= B y

t

This gives the relationship

y

t

=

1

B z

t

=

1

X

i = 0

i

z

t i

where

1

B =

P

1

i = 0

i

B

i and

0

= I

s

The m-step-ahead forecast of y

t + m

is

y

t + m j t

=

m 1

X

i = 0

i

z

t + m i j t

+

1

X

i = m

i

z

t + m i

The m-step-ahead forecast error of y

t + m

is

m 1

X

i = 0

i

z

t + m i

z

t + m i j t

=

m 1

X

i = 0

i

X

u = 0

u

i u

e

t + m i

LettingV

y 0

= 0

the variance of the m-step-ahead forecast error of y

t + m

V

y m

is

V

y m

=

m 1

X

i = 0

i

X

u = 0

u

i u

e e

i

X

u = 0

u

i u

0

= V

y m 1

+

m 1

X

u = 0

u

m 1 u

e e

m 1

X

u = 0

u

m 1 u

0

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Chapter 18 Details

Relation of ARMA and State Space Forms

Every state space model has an ARMA representation and conversely every ARMA

model has a state space representation This section discusses this equivalence The

following material is adapted from Akaike (1974) where there is a more complete

discussion Pham-Dinh-Tuan (1978) also contains a discussion of this material

Suppose you are given the following ARMA model

B x

t

= B e

t

or in more detail

x

t

1

x

t 1

p

x

t p

= e

t

+

1

e

t 1

+ +

q

e

t q

(1)

wheree

t

is a sequence of independent multivariate normal random vectors with mean

0 and variance matrix

e e

B is the backshift operator (B x

t

= x

t 1

) B

and

B

are matrix polynomials in B andx

t

is the observed process

If the roots of the determinantial equationj B j = 0

are outside the unit circle in

the complex plane the model can also be written as

x

t

=

1

B B e

t

=

1

X

i = 0

i

e

t i

The

i

matrices are known as the impulse response matrices and can be computed

as

1

B B

You can assume p q

since if this is not initially true you can add more terms

i

that are identically 0 without changing the model

To write this set of equations in a state space form proceed as follows Letx

t + i j t

be

the conditional expectation of x

t + i

givenx

w

forw t

The following relations hold

x

t + i j t

=

1

X

j = i

j

e

t + i j

x

t + i j t + 1

= x

t + i j t

+

i 1

e

t + 1

However from equation (1) you can derive the following relationship

x

t + p j t

=

1

x

t + p 1 j t

+ +

p

x

t

(2)

Hence wheni = p

you can substitute forx

t + p j t

in the right-hand side of equation

(2) and close the system of equations

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This substitution results in the following model in the state space form

z

t + 1

= F z

t

+ G e

t + 1

2

6

6

6

4

x

t + 1

x

t + 2 j t + 1

x

t + p j t + 1

3

7

7

7

5

=

2

6

6

4

0 I 0 0

0 0 I 0

p

p 1

1

3

7

7

5

2

6

6

6

4

x

t

x

t + 1 j t

x

t + p 1 j t

3

7

7

7

5

+

2

6

6

4

I

1

p 1

3

7

7

5

e

t + 1

Note that the state vectorz

t

is composed of conditional expectations of x

t

and the

first r components of z

t

are equal tox

t

The state space form can be cast into an ARMA form by solving the system of dif-

ference equations for the first r components

When converting from an ARMA form to a state space form you can generate a

state vector larger than needed that is the state space model may not be a minimal

representation When going from a state space form to an ARMA form you can have

nontrivial common factors in the autoregressive and moving average operators that

yield an ARMA model larger than necessary

If the state space form used is not a minimal representation some but not all compo-

nents of x

t + i j t

may be linearly dependent This situation corresponds to

p

p 1

being of less than full rank when B

and B

have no common nontrivial

left factors In this casez

t

consists of a subset of the possible components of

x

t + i j t

i = 1 2 p 1

However once a component of x

t + i j t

(for example the

jth one) is linearly dependent on the previous conditional expectations then all sub-

sequent jth components of x

t + k j t

fork i

must also be linearly dependent Note

that in this case equivalent but seemingly different structures can arise if the order of

the components withinx

t

is changed

OUT= Data Set

The forecasts are contained in the output data set specified by the OUT= option on the

PROC STATESPACE statement The OUT= data set contains the following variables

the BY variables

the ID variable

the VAR statement variables These variables contain the actual values from

the input data set

FORi numeric variables containing the forecasts The variable FORi contains

the forecasts for the ith variable in the VAR statement list Forecasts are one-

step-ahead predictions until the end of the data or until the observation specified

by the BACK= option

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Chapter 18 Details

RESi numeric variables containing the residual for the forecast of the ith vari-

able in the VAR statement list For forecast observations the actual values are

missing and the RESi variables contain missing values

STDi numeric variables containing the standard deviation for the forecast of

the ith variable in the VAR statement list The values of the STDi variables can

be used to construct univariate confidence limits for the corresponding fore-casts However such confidence limits do not take into account the covariance

of the forecasts

OUTAR= Data Set

The OUTAR= data set contains the estimates of the preliminary autoregressive mod-

els The OUTAR= data set contains the following variables

ORDER a numeric variable containing the order p of the autoregressive model

that the observation represents

AIC a numeric variable containing the value of the information criterionA I C

p

SIGFl numeric variables containing the estimate of the innovation covariance

matrices for the forward autoregressive models The variable SIGFl contains

the lth column of b

p

in the observations with ORDER= p

SIGBl numeric variables containing the estimate of the innovation covariance

matrices for the backward autoregressive models The variable SIGBl contains

the lth column of b

p

in the observations with ORDER= p

FORk ndashl numeric variables containing the estimates of the autoregressive pa-

rameter matrices for the forward models The variable FORk ndashl contains the

lth column of the lag k autoregressive parameter matrix b

p

k

in the observations

with ORDER= p

BACk ndashl numeric variables containing the estimates of the autoregressive pa-

rameter matrices for the backward models The variable BACk ndashl contains the

lth column of the lag k autoregressive parameter matrix b

p

k

in the observations

with ORDER= p

The estimates for the order p autoregressive model can be selected as those observa-

tions with ORDER= p Within these observations the k lth element of

p

i

is given by

the value of the FORindashl variable in the k th observation The k lth element of

p

i

is

given by the value of BACindashl variable in the k th observation The k lth element of

p

is given by SIGFl in the k th observation The k lth element of

p

is given by SIGBlin the k th observation

Table 181 shows an example of the OUTAR= data set with ARMAX=3 andx

t

of

dimension 2 In Table 181 i j

indicate the ijth element of the matrix

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Part 2 General Information

Table 181 Values in the OUTAR= Data Set

Obs ORDER AIC SIGF1 SIGF2 SIGB1 SIGB2 FOR1ndash 1 FOR1ndash 2 FOR2ndash 1 FOR2ndash 2 FOR3ndash 1

1 0 AIC0

0 1 1

0 1 2

0 1 1

0 1 2

2 0 AIC0

0 2 1

0 2 2

0 2 1

0 2 2

3 1 AIC1

1 1 1

1 1 2

1 1 1

1 1 2

1

1

1 1

1

1

1 2

4 1 AIC1

1 2 1

1 1 2

1 2 1

1 1 2

1

1

2 1

1

1

2 2

5 2 AIC2

2 1 1

2 1 2

2 1 1

2 1 2

2

1

1 1

2

1

1 2

2

2

1 1

2

2

1 2

6 2 AIC2

2 2 1

2 1 2

2 2 1

2 1 2

2

1

2 1

2

1

2 2

2

2

2 1

2

2

2 2

7 3 AIC3

3 1 1

3 1 2

3 1 1

3 1 2

3

1

1 1

3

1

1 2

3

2

1 1

3

2

1 2

3

3

1 1

8 3 AIC3

3 2 1

3 1 2

3 2 1

3 1 2

3

1

2 1

3

1

2 2

3

2

2 1

3

2

2 2

3

3

2 1

Obs FOR3ndash 2 BACK1ndash 1 BACK1ndash 2 BACK2ndash 1 BACK2ndash 2 BACK3ndash 1 BACK3ndash 2

1

2

3

1

1

1 1

1

1

1 2

4

1

1

2 1

1

1

2 2

5

2

1

1 1

2

1

1 2

2

2

1 1

2

2

1 2

6

2

1

2 1

2

1

2 2

2

2

2 1

2

2

2 2

7

3

3

1 2

3

1

1 1

3

1

1 2

3

2

1 1

3

2

1 2

3

3

1 1

3

3

1 2

8

3

3

2 2

3

1

2 1

3

1

2 2

3

2

2 1

3

2

2 2

3

3

2 1

3

3

2 2

The estimated autoregressive parameters can be used in the IML procedure to obtain

autoregressive estimates of the spectral density function or forecasts based on the

autoregressive models

OUTMODEL= Data Set

The OUTMODEL= data set contains the estimates of theF

andG

matrices and their

standard errors the names of the components of the state vector and the estimates of

the innovation covariance matrix The variables contained in the OUTMODEL= data

set are as follows

the BY variables

STATEVEC a character variable containing the name of the component of the

state vector corresponding to the observation The STATEVEC variable has

the value STD for standard deviations observations which contain the standarderrors for the estimates given in the preceding observation

Fndash j numeric variables containing the columns of theF

matrix The variable

Fndash j contains the jth column of F

The number of Fndash j variables is equal to

the value of the DIMMAX= option If the model is of smaller dimension the

extraneous variables are set to missing

Gndash j numeric variables containing the columns of theG

matrix The variable

Gndash j contains the jth column of G

The number of Gndash j variables is equal to r

the dimension of x

t

given by the number of variables in the VAR statement

SIGndash j numeric variables containing the columns of the innovation covariance

matrix The variable SIGndash j contains the jth column of

e e

There are r vari-ables SIGndash j

Table 182 shows an example of the OUTMODEL= data set withx

t

= x

t

y

t

0

z

t

= x

t

y

t

x

t + 1 j t

0 and DIMMAX=4 In Table 182F

i j

andG

i j

are the ijth

elements of F

andG

respectively Note that all elements for Fndash4 are missing because

F

is a3 3

matrix

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Chapter 18 Details

Table 182 Value in the OUTMODEL= Data Set

Obs STATEVEC Fndash 1 Fndash 2 Fndash 3 Fndash 4 Gndash 1 Gndash 2 SIGndash 1 SIGndash 2

1 X(TT) 0 0 1 1 0

1 1

1 2

2 STD

3 Y(TT) F2 1

F2 2

F2 3

0 1

2 1

2 2

4 STD std F2 1

std F2 2

std F2 3

5 X(T+1T) F3 1

F3 2

F3 3

G3 1

G3 2

6 STD std F3 1

std F3 2

std F3 3

std G3 1

std G3 2

Printed Output

The printed output produced by the STATESPACE procedure is described in the fol-

lowing

1 descriptive statistics which include the number of observations used the

names of the variables their means and standard deviations (Std) and the dif-

ferencing operations used

2 the Akaike information criteria for the sequence of preliminary autoregressive

models

3 if the PRINTOUT=LONG option is specified the sample autocovariance ma-

trices of the input series at various lags

4 if the PRINTOUT=LONG option is specified the sample autocorrelation ma-

trices of the input series

5 a schematic representation of the autocorrelation matrices showing the signif-

icant autocorrelations

6 if the PRINTOUT=LONG option is specified the partial autoregressive ma-

trices (These are

p

p

as described in Preliminary Autoregressive Modelsearlier in this chapter)

7 a schematic representation of the partial autocorrelation matrices showing the

significant partial autocorrelations

8 the Yule-Walker estimates of the autoregressive parameters for the autoregres-

sive model with the minimum AIC

9 if the PRINTOUT=LONG option is specified the autocovariance matrices of

the residuals of the minimum AIC model This is the sequence of estimated

innovation variance matrices for the solutions of the Yule-Walker equations

10 if the PRINTOUT=LONG option is specified the autocorrelation matrices of the residuals of the minimum AIC model

11 If the CANCORR option is specified the canonical correlations analysis for

each potential state vector considered in the state vector selection process

This includes the potential state vector the canonical correlations the informa-

tion criterion for the smallest canonical correlation Bartlettrsquos

2 statistic (ldquoChi

Squarerdquo) for the smallest canonical correlation and the degrees of freedom of

Bartlettrsquos

2

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Part 2 General Information

12 the components of the chosen state vector

13 the preliminary estimate of the transition matrixF

the input matrixG

and

the variance matrix for the innovations

e e

14 if the ITPRINT option is specified the iteration history of the likelihood max-

imization For each iteration this shows the iteration number the number of step halvings the determinant of the innovation variance matrix the damping

factor Lambda and the values of the parameters

15 the state vector printed again to aid interpretation of the following listing of F

andG

16 the final estimate of the transition matrixF

17 the final estimate of the input matrixG

18 the final estimate of the variance matrix for the innovations

e e

19 a table listing the estimates of the free parameters inF

andG

and their standarderrors and t statistics

20 if the COVB option is specified the covariance matrix of the parameter esti-

mates

21 if the COVB option is specified the correlation matrix of the parameter esti-

mates

22 if the PRINT option is specified the forecasts and their standard errors

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Chapter 18 Details

ODS Table Names

PROC STATESPACE assigns a name to each table it creates You can use these

names to reference the table when using the Output Delivery System (ODS) to select

tables and create output data sets These names are listed in the following table For

more information on ODS see Chapter 6 ldquoUsing the Output Delivery Systemrdquo

Table 183 ODS Tables Produced in PROC STATESPACE

ODS Table Name Description Option

NObs Number of observations default

Summary Simple summary statistics table default

InfoCriterion Information criterion table default

CovLags Covariance Matrices of Input Series PRINTOUT=LONG

CorrLags Correlation Matrices of Input Series PRINTOUT=LONG

PartialAR Partial Autoregressive Matrices PRINTOUT=LONG

YWEstimates Yule-Walker Estimates for Minimum AIC default

CovResiduals Covariance of Residuals PRINTOUT=LONGCorrResiduals Residual Correlations from AR Models PRINTOUT=LONG

StateVector State vector table default

CorrGraph Schematic Representation of Correlations default

TransitionMatrix Transition Matrix default

InputMatrix Input Matrix default

VarInnov Variance Matrix for the Innovation default

CovB Covariance of Parameter Estimates COVB

CorrB Correlation of Parameter Estimates COVB

CanCorr Canonical Correlation Analysis CANCORR

IterHistory Iterative Fitting table ITPRINT

ParameterEstimates Parameter Estimates Table default

Forecasts Forecasts Table PRINT

ConvergenceStatus Convergence Status Table default

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Part 2 General Information

Example

Example 181 Series J from Box and Jenkins

This example analyzes the gas furnace data (series J) from Box and Jenkins (The

data are not shown Refer to Box and Jenkins (1976) for the data)

First a model is selected and fit automatically using the following statements

title1 rsquoGas Furnace Datarsquo

title2 rsquoBox amp Jenkins Series Jrsquo

title3 rsquoAutomatically Selected Modelrsquo

proc statespace data=seriesj cancorr

var x y

run

The results for the automatically selected model are shown in Output 1811

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Chapter 18 Example

Output 1811 Results for Automatically Selected Model

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Number of Observations 296

Standard

Variable Mean Error

x -005683 1072766

y 5350912 3202121

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Information Criterion for Autoregressive Models

Lag=0 Lag=1 Lag=2 Lag=3 Lag=4 Lag=5 Lag=6 Lag=7 Lag=8

6513862 -103357 -163296 -164512 -165152 -164891 -164934 -164315 -163856

Information

Criterion for

Autoregressive

Models

Lag=9 Lag=10

-16348 -163359

Schematic Representation of Correlations

NameLag 0 1 2 3 4 5 6 7 8 9 10

x +- +- +- +- +- +- +- +- +- +- +-

y -+ -+ -+ -+ -+ -+ -+ -+ -+ -+ -+

+ is gt 2std error - is lt -2std error is between

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Part 2 General Information

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Schematic Representation of Partial Autocorrelations

NameLag 1 2 3 4 5 6 7 8 9 10

x + - + -

y -+ -- - + +

+ is gt 2std error - is lt -2std error is between

Yule-Walker Estimates for Minimum AIC

------Lag=1------ ------Lag=2------ ------Lag=3------ ------Lag=4------

x y x y x y x y

x 1925887 -000124 -120166 0004224 0116918 -000867 0104236 0003268

y 0050496 1299793 -002046 -03277 -071182 -025701 0195411 0133417

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Canonical Correlations Analysis

Information Chi

x(TT) y(TT) x(T+1T) Criterion Square DF

1 1 0804883 2929228 3047481 8

Information Chix(TT) y(TT) x(T+1T) y(T+1T) Criterion Square DF

1 1 0906681 0607529 1223358 1347237 7

Information Chi

x(TT) y(TT) x (T+1T) y (T+1T) x (T+2T) Criterion Square DF

1 1 0909434 0610278 0186274 -154701 1034705 6

Information Chi

x(TT) y(TT) x (T+1T) y (T+1T) y (T+2T) Criterion Square DF

1 1 091014 0618937 0206823 0940392 1280924 6

Information Chi

x(TT) y(TT) x(T+1T) y(T+1T) y(T+2T) y(T+3T) Criterion Square DF

1 1 0912963 0628785 0226598 0083258 -794103 2041584 5

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Chapter 18 Example

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Selected Statespace Form and Preliminary Estimates

State Vector

x(TT) y(TT) x(T+1T) y(T+1T) y(T+2T)

Estimate of Transition Matrix

0 0 1 0 0

0 0 0 1 0

-084718 0026794 1711715 -005019 0

0 0 0 0 1

-019785 0334274 -018174 -123557 1787475

Input Matrix for Innovation

1 0

0 11925887 -000124

0050496 1299793

0142421 1361696

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Selected Statespace Form and Preliminary Estimates

Variance Matrix for Innovation

0035274 -000734

-000734 0097569

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Part 2 General Information

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Selected Statespace Form and Fitted Model

State Vector

x(TT) y(TT) x(T+1T) y(T+1T) y(T+2T)

Estimate of Transition Matrix

0 0 1 0 0

0 0 0 1 0

-086192 0030609 1724235 -005483 0

0 0 0 0 1

-034839 0292124 -009435 -109823 1671418

Input Matrix for Innovation

1 0

0 1192442 -000416

0015621 1258495

008058 1353204

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Selected Statespace Form and Fitted Model

Variance Matrix for Innovation

0035579 -000728

-000728 0095577

Parameter Estimates

Standard

Parameter Estimate Error t Value

F(31) -086192 0072961 -1181

F(32) 0030609 0026167 117

F(33) 1724235 0061599 2799

F(34) -005483 0030169 -182

F(51) -034839 0135253 -258

F(52) 0292124 0046299 631

F(53) -009435 0096527 -098F(54) -109823 0109525 -1003

F(55) 1671418 0083737 1996

G(31) 1924420 0058162 3309

G(32) -000416 0035255 -012

G(41) 0015621 0095771 016

G(42) 1258495 0055742 2258

G(51) 0080580 0151622 053

G(52) 1353204 0091388 1481

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Chapter 18 Example

The two series are believed to have a transfer function relation with the gas rate

(variable X) as the input and the CO2

concentration (variable Y) as the output Since

the parameter estimates shown in Output 1811 support this kind of model the model

is reestimated with the feedback parameters restricted to 0 The following statements

fit the transfer function (no feedback) model

title3 rsquoTransfer Function Modelrsquo

proc statespace data=seriesj printout=none

var x y

restrict f(32)=0 f(34)=0

g(32)=0 g(41)=0 g(51)=0

run

The last 2 pages of the output are shown in Output 1812

Output 1812 STATESPACE Output for Transfer Function Model

Gas Furnace Data

Box amp Jenkins Series J

Transfer Function Model

The STATESPACE Procedure

Selected Statespace Form and Fitted Model

State Vector

x(TT) y(TT) x(T+1T) y(T+1T) y(T+2T)

Estimate of Transition Matrix

0 0 1 0 0

0 0 0 1 0

-068882 0 1598717 0 0

0 0 0 0 1

-035944 0284179 -00963 -107313 1650047

Input Matrix for Innovation

1 0

0 1

1923446 0

0 1260856

0 1346332

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Part 2 General Information

Gas Furnace Data

Box amp Jenkins Series J

Transfer Function Model

The STATESPACE Procedure

Selected Statespace Form and Fitted Model

Variance Matrix for Innovation

0036995 -00072

-00072 0095712

Parameter Estimates

Standard

Parameter Estimate Error t Value

F(31) -068882 0050549 -1363

F(33) 1598717 0050924 3139

F(51) -035944 0229044 -157

F(52) 0284179 0096944 293

F(53) -009630 0140876 -068

F(54) -107313 0250385 -429

F(55) 1650047 0188533 875G(31) 1923446 0056328 3415

G(42) 1260856 0056464 2233

G(52) 1346332 0091086 1478

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Chapter 18 References

References

Akaike H (1974) Markovian Representation of Stochastic Processes and Its Ap-

plication to the Analysis of Autoregressive Moving Average Processes Annals

of the Institute of Statistical Mathematics 26 363-387

Akaike H (1976) Canonical Correlations Analysis of Time Series and the Use of

an Information Criterion in Advances and Case Studies in System Identification

eds R Mehra and DG Lainiotis New York Academic Press

Anderson TW (1971) The Statistical Analysis of Time Series New York John

Wiley amp Sons

Ansley CF and Newbold P (1979) Multivariate Partial Autocorrelations Pro-

ceedings of the Business and Economic Statistics Section American Statistical

Association 349-353

Box GEP and Jenkins G (1976) Time Series Analysis Forecasting and Control

San Francisco Holden-Day

Brockwell PJ and Davis RA (1991) Time Series Theory and Methods 2nd Edi-

tion Springer-Verlag

Hannan EJ (1970) Multiple Time Series New York John Wiley amp Sons

Hannan EJ (1976) The Identification and Parameterization of ARMAX and State

Space Forms Econometrica 44 713-722

Harvey AC (1981a) The Econometric Analysis of Time Series New York John

Wiley amp Sons

Harvey AC (1981b) Time Series Models New York John Wiley amp Sons

Jones RH (1974) Identification and Autoregressive Spectrum Estimation IEEE Transactions on Automatic Control AC-19 894-897

Pham-Dinh-Tuan (1978) On the Fitting of Multivariate Processes of the

Autoregressive-Moving Average Type Biometrika 65 99-107

Priestley MB (1980) System Identification Kalman Filtering and Stochastic Con-

trol in Directions in Time Series eds DR Brillinger and GC Tiao Institute

of Mathematical Statistics

Whittle P (1963) On the Fitting of Multivariate Autoregressions and the Approxi-

mate Canonical Factorization of a Spectral Density Matrix Biometrika 50 129-

134

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The correct bibliographic citation for this manual is as follows SAS Institute Inc SAS

ET S U serrsquos Gui de Version 8 Cary NC SAS Institute Inc 1999 1546 pp

SASET S Userrsquos Guide Version 8

Copyright copy 1999 by S AS I ns t it u t e I nc Ca ry N C U S A

ISBN 1ndash58025ndash489ndash6

All r ight s res erved P r int ed in t he U n it ed St a t es of A merica N o pa rt of t his p u blica t ionma y b e reprodu ced s t ored in a ret r ieva l s ys t em or t ra ns mit t ed in a ny form or b y a nymeans electronic mechanical photocopying or otherwise without the prior writtenpermission of the publisher SAS Institute Inc

US Government Restricted Rights Notice Use du plicat ion or disclosure of the

software by the government is subject to restrictions as set forth in FAR 52227ndash19Commercial Computer Softw ar e-Restricted Rights (J une 1987)

SAS Institute Inc SAS Campus Drive Cary North Carolina 27513

1st print ing October 1999

S AS reg a nd a ll ot her SA S I ns t it u t e I nc p rodu ct or s ervice na m es a re regis t ered t ra dema rkst d k f SA S I t it t I i t h U S A d t h t i reg i di t U SA

Page 7: Statespace Model

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Chapter 18 Getting Started

Automatic State Space Model Selection

The STATESPACE procedure is designed to automatically select the best state space

model for forecasting the series You can specify your own model if you wish and

you can use the output from PROC STATESPACE to help you identify a state space

model However the easiest way to use PROC STATESPACE is to let it choose the

model

Stationarity and Differencing

Although PROC STATESPACE selects the state space model automatically it does

assume that the input series are stationary If the series are nonstationary then the

process may fail Therefore the first step is to examine your data and test to see if

differencing is required (See the section Stationarity and Differencing later in this

chapter for further discussion of this issue)

The series shown in Figure 181 are nonstationary In order to forecast X and Y

with a state space model you must difference them (or use some other de-trending

method) If you fail to difference when needed and try to use PROC STATESPACE

with nonstationary data an inappropriate state space model may be selected and themodel estimation may fail to converge

The following statements identify and fit a state space model for the first differences

of X and Y and forecast X and Y 10 periods ahead

proc statespace data=in out=out lead=10

var x(1) y(1)

id t

run

The DATA= option specifies the input data set and the OUT= option specifies theoutput data set for the forecasts The LEAD= option specifies forecasting 10 obser-

vations past the end of the input data The VAR statement specifies the variables to

forecast and specifies differencing The notation X(1) Y(1) specifies that the state

space model analyzes the first differences of X and Y

Descriptive Statistics and Preliminary Autoregressions

The first page of the printed output produced by the preceding statements is shown in

Figure 182

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Part 2 General Information

The STATESPACE Procedure

Number of Observations 200

Standard

Variable Mean Error

x 0144316 1233457 Has been differenced With period(s) = 1

y 0164871 1304358 Has been differenced

With period(s) = 1

The STATESPACE Procedure

Information Criterion for Autoregressive Models

Lag=0 Lag=1 Lag=2 Lag=3 Lag=4 Lag=5 Lag=6 Lag=7 Lag=8

149697 8387786 5517099 1205986 1536952 2179538 2400638 2988874 3355708

Information

Criterion for Autoregressive

Models

Lag=9 Lag=10

4117606 4770222

Schematic Representation of Correlations

NameLag 0 1 2 3 4 5 6 7 8 9 10

x ++ ++ ++ ++ ++ ++ + + +

y ++ ++ ++ ++ ++ + + + +

+ is gt 2std error - is lt -2std error is between

Figure 182 Descriptive Statistics and VAR Order Selection

Descriptive statistics are printed first giving the number of nonmissing observations

after differencing and the sample means and standard deviations of the differenced

series The sample means are subtracted before the series are modeled (unless the

NOCENTER option is specified) and the sample means are added back when the

forecasts are produced

LetX

t

andY

t

be the observed values of X and Y and letx

t

andy

t

be the values of X

and Y after differencing and subtracting the mean difference The seriesx

t

modeled

by the STATEPSPACE procedure is

x

t

=

x

t

y

t

=

1 B X

t

0 1 4 4 3 1 6

1 B Y

t

0 1 6 4 8 7 1

where B represents the backshift operator

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Chapter 18 Getting Started

After the descriptive statistics PROC STATESPACE prints the Akaike information

criterion (AIC) values for the autoregressive models fit to the series The smallest AIC

value in this case 5517 at lag 2 determines the number of autocovariance matrices

analyzed in the canonical correlation phase

A schematic representation of the autocorrelations is printed next This indicates

which elements of the autocorrelation matrices at different lags are significantlygreater or less than 0

The second page of the STATESPACE printed output is shown in Figure 183

The STATESPACE Procedure

Schematic Representation of Partial Autocorrelations

NameLag 1 2 3 4 5 6 7 8 9 10

x ++ +

y ++

+ is gt 2std error - is lt -2std error is between

Yule-Walker Estimates for Minimum AIC

--------Lag=1------- --------Lag=2-------

x y x y

x 0257438 0202237 0170812 0133554

y 0292177 0469297 -000537 -000048

Figure 183 Partial Autocorrelations and VAR Model

Figure 183 shows a schematic representation of the partial autocorrelations similar

to the autocorrelations shown in Figure 182 The selection of a second order autore-

gressive model by the AIC statistic looks reasonable in this case because the partialautocorrelations for lags greater than 2 are not significant

Next the Yule-Walker estimates for the selected autoregressive model are printed

This output shows the coefficient matrices of the vector autoregressive model at each

lag

Selected State Space Model Form and Preliminary Estimates

After the autoregressive order selection process has determined the number of lags to

consider the canonical correlation analysis phase selects the state vector By default

output for this process is not printed You can use the CANCORR option to print

details of the canonical correlation analysis See the section Canonical Correlation

Analysis later in this chapter for an explanation of this process

Once the state vector is selected the state space model is estimated by approximate

maximum likelihood Information from the canonical correlation analysis and from

the preliminary autoregression is used to form preliminary estimates of the state space

model parameters These preliminary estimates are used as starting values for the

iterative estimation process

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Part 2 General Information

The form of the state vector and the preliminary estimates are printed next as shown

in Figure 184

The STATESPACE Procedure

Selected Statespace Form and Preliminary Estimates

State Vector

x(TT) y(TT) x(T+1T)

Estimate of Transition Matrix

0 0 1

0291536 0468762 -000411

024869 024484 0204257

Input Matrix for Innovation

1 0

0 1

0257438 0202237

Variance Matrix for Innovation

0945196 0100786

0100786 1014703

Figure 184 Preliminary Estimates of State Space Model

Figure 184 first prints the state vector as X[TT] Y[TT] X[T+1T] This notation

indicates that the state vector is

z

t

=

2

4

x

t j t

y

t j t

x

t + 1 j t

3

5

The notationx

t + 1 j t

indicates the conditional expectation or prediction of x

t + 1

based

on the information available at time t andx

t j t

andy

t j t

arex

t

andy

t

respectively

The remainder of Figure 184 shows the preliminary estimates of the transition matrix

F

the input matrixG

and the covariance matrix

e e

Estimated State Space Model

The next page of the STATESPACE output prints the final estimates of the fittedmodel as shown in Figure 185 This output has the same form as in Figure 184 but

shows the maximum likelihood estimates instead of the preliminary estimates

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Chapter 18 Getting Started

The STATESPACE Procedure

Selected Statespace Form and Fitted Model

State Vector

x(TT) y(TT) x(T+1T)

Estimate of Transition Matrix

0 0 1

0297273 047376 -001998

02301 0228425 0256031

Input Matrix for Innovation

1 0

0 1

0257284 0202273

Variance Matrix for Innovation

0945188 01007520100752 1014712

Figure 185 Fitted State Space Model

The estimated state space model shown in Figure 185 is

2

4

x

t + 1 j t + 1

y

t + 1 j t + 1

x

t + 2 j t + 1

3

5

=

2

4

0 0 1

0 2 9 7 0 4 7 4 0 0 2 0

0 2 3 0 0 2 2 8 0 2 5 6

3

5

2

4

x

t

y

t

x

t + 1 j t

3

5

+

2

4

1 0

0 1

0 2 5 7 0 2 0 2

3

5

e

t + 1

n

t + 1

v a r

e

t + 1

n

t + 1

=

0 9 4 5 0 1 0 1

0 1 0 1 1 0 1 5

The next page of the STATESPACE output lists the estimates of the free parameters

in theF

andG

matrices with standard errors and t statistics as shown in Figure 186

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Part 2 General Information

The STATESPACE Procedure

Parameter Estimates

Standard

Parameter Estimate Error t Value

F(21) 0297273 0129995 229

F(22) 0473760 0115688 410F(23) -001998 0313025 -006

F(31) 0230100 0126226 182

F(32) 0228425 0112978 202

F(33) 0256031 0305256 084

G(31) 0257284 0071060 362

G(32) 0202273 0068593 295

Figure 186 Final Parameter Estimates

Convergence Failures

The maximum likelihood estimates are computed by an iterative nonlinear maximiza-

tion algorithm which may not converge If the estimates fail to converge warning

messages are printed in the output

If you encounter convergence problems you should recheck the stationarity of the

data and ensure that the specified differencing orders are correct Attempting to fit

state space models to nonstationary data is a common cause of convergence failure

You can also use the MAXIT= option to increase the number of iterations allowed

or experiment with the convergence tolerance options DETTOL= and PARMTOL=

Forecast Data Set

The following statements print the output data set The WHERE statement excludes

the first 190 observations from the output so that only the forecasts and the last 10

actual observations are printed

proc print data=outid t

where t gt 190

run

The PROC PRINT output is shown in Figure 187

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Part 2 General Information

Figure 188 Plot of Forecasts

Controlling Printed Output

By default the STATESPACE procedure produces a large amount of printed output

The NOPRINT option suppresses all printed output You can suppress the printed

output for the autoregressive model selection process with the PRINTOUT=NONE

option The descriptive statistics and state space model estimation output are still

printed when PRINTOUT=NONE is specified You can produce more detailed output

with the PRINTOUT=LONG option and by specifying the printing control optionsCANCORR COVB and PRINT

Specifying the State Space Model

Instead of allowing the STATESPACE procedure to select the model automatically

you can use FORM and RESTRICT statements to specify a state space model

Specifying the State Vector

Use the FORM statement to control the form of the state vector You can use this

feature to force PROC STATESPACE to estimate and forecast a model different from

the model it would select automatically You can also use this feature to reestimate

the automatically selected model (possibly with restrictions) without repeating thecanonical correlation analysis

The FORM statement specifies the number of lags of each variable to include in

the state vector For example the statement FORM X 3 forces the state vector to

includex

t j t

x

t + 1 j t

andx

t + 2 j t

The following statement specifies the state vector

x

t j t

y

t j t

x

t + 1 j t

which is the same state vector selected in the preceding example

f o r m x 2 y 1

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Chapter 18 Getting Started

You can specify the form for only some of the variables and allow PROC STATES-

PACE to select the form for the other variables If only some of the variables are

specified in the FORM statement canonical correlation analysis is used to determine

the number of lags included in the state vector for the remaining variables not spec-

ified by the FORM statement If the FORM statement includes specifications for all

the variables listed in the VAR statement the state vector is completely defined and

the canonical correlation analysis is not performed

Restricting the F and G matrices

After you know the form of the state vector you can use the RESTRICT statement

to fix some parameters in theF

andG

matrices to specified values One use of this

feature is to remove insignificant parameters by restricting them to 0

In the introductory example shown in the preceding section the F[23] parameter is

not significant (The parameters estimation output shown in Figure 186 gives the t

statistic for F[23] as -006 F[33] and F[31] also have low significance witht 2

)

The following statements reestimate this model with F[23] restricted to 0 The

FORM statement is used to specify the state vector and thus bypass the canonicalcorrelation analysis

proc statespace data=in out=out lead=10

var x(1) y(1)

id t

form x 2 y 1

restrict f(23)=0

run

The final estimates produced by these statements are shown in Figure 189

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Part 2 General Information

The STATESPACE Procedure

Selected Statespace Form and Fitted Model

State Vector

x(TT) y(TT) x(T+1T)

Estimate of Transition Matrix

0 0 1

0290051 0467468 0

0227051 0226139 026436

Input Matrix for Innovation

1 0

0 1

0256826 0202022

Variance Matrix for Innovation

0945175 01006960100696 1014733

The STATESPACE Procedure

Parameter Estimates

Standard

Parameter Estimate Error t Value

F(21) 0290051 0063904 454

F(22) 0467468 0060430 774

F(31) 0227051 0125221 181

F(32) 0226139 0111711 202

F(33) 0264360 0299537 088G(31) 0256826 0070994 362

G(32) 0202022 0068507 295

Figure 189 Results using RESTRICT Statement

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Chapter 18 Syntax

Syntax

The STATESPACE procedure uses the following statements

PROC STATESPACE options

BY variable FORM variable value

ID variable

INITIAL F(rowcolumn)=value

G(rowcolumn)=value

RESTRICT F(rowcolumn)=value

G(rowcolumn)=value

VAR variable (difference difference

)

Functional Summary

The statements and options used by PROC STATESPACE are summarized in the

following table

Description Statement Option

Input Data Set Options

specify the input data set PROC STATESPACE DATA=

prevent subtraction of sample mean PROC STATESPACE NOCENTER

specify the ID variable ID

specify the observed series and differencing VAR

Options for Autoregressive Estimates

specify the maximum order PROC STATESPACE ARMAX=

specify maximum lag for autocovariances PROC STATESPACE LAGMAX=

output only minimum AIC model PROC STATESPACE MINIC

specify the amount of detail printed PROC STATESPACE PRINTOUT=

write preliminary AR models to a data set PROC STATESPACE OUTAR=

Options for Canonical Correlation Analysis

print the sequence of canonical correlations PROC STATESPACE CANCORR

specify upper limit of dimension of state

vector

PROC STATESPACE DIMMAX=

specify the minimum number of lags PROC STATESPACE PASTMIN=

specify the multiplier of the degrees of

freedom

PROC STATESPACE SIGCORR=

Options for State Space Model Estimation

specify starting values INITIAL

print covariance matrix of parameter estimates PROC STATESPACE COVB

specify the convergence criterion PROC STATESPACE DETTOL=

specify the convergence criterion PROC STATESPACE PARMTOL=

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Part 2 General Information

Description Statement Option

print the details of the iterations PROC STATESPACE ITPRINT

specify an upper limit of the number of lags PROC STATESPACE KLAG=

specify maximum number of iterations

allowed

PROC STATESPACE MAXIT=

suppress the final estimation PROC STATESPACE NOEST

write the state space model parameter esti-

mates to an output data set

PROC STATESPACE OUTMODEL=

use conditional least squares for final estimates PROC STATESPACE RESIDEST

specify criterion for testing for singularity PROC STATESPACE SINGULAR=

Options for Forecasting

start forecasting before end of the input data PROC STATESPACE BACK=

specify the time interval between observations PROC STATESPACE INTERVAL=

specify multiple periods in the time series PROC STATESPACE INTPER=specify how many periods to forecast PROC STATESPACE LEAD=

specify the output data set for forecasts PROC STATESPACE OUT=

print forecasts PROC STATESPACE PRINT

Options to Specify the State Space Model

specify the state vector FORM

specify the parameter values RESTRICT

BY Groups

specify BY-group processing BY

Printing

suppresses all printed output NOPRINT

PROC STATESPACE Statement

PROC STATESPACE options

The following options can be specified in the PROC STATESPACE statement

Printing Options NOPRINT

suppresses all printed output

Input Data Options

DATA= SAS-data-set

specifies the name of the SAS data set to be used by the procedure If the DATA=

option is omitted the most recently created SAS data set is used

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Chapter 18 Syntax

LAGMAX= k

specifies the number of lags for which the sample autocovariance matrix is computed

The LAGMAX= option controls the number of lags printed in the schematic repre-

sentation of the autocorrelations

The sample autocovariance matrix of lag i denoted asC

i

is computed as

C

i

=

1

N 1

N

X

t = 1 + i

x

t

x

0

t i

wherex

t

is the differenced and centered data andN

is the number of observations

(If the NOCENTER option is specified 1 is not subtracted fromN

) LAGMAX= k

specifies thatC

0

throughC

k

are computed The default is LAGMAX=10

NOCENTER

prevents subtraction of the sample mean from the input series (after any specified

differencing) before the analysis

Options for Preliminary Autoregressive Models ARMAX= n

specifies the maximum order of the preliminary autoregressive models The AR-

MAX= option controls the autoregressive orders for which information criteria are

printed and controls the number of lags printed in the schematic representation of

partial autocorrelations The default is ARMAX=10 See Preliminary Autoregres-

sive Models later in this chapter for details

MINIC

writes to the OUTAR= data set only the preliminary Yule-Walker estimates for the

VAR model producing the minimum AIC See OUTAR= Data Set later in this chap-

ter for details

OUTAR= SAS-data-set

writes the Yule-Walker estimates of the preliminary autoregressive models to a SAS

data set See OUTAR= Data Set later in this chapter for details

PRINTOUT= SHORT | LONG | NONE

determines the amount of detail printed PRINTOUT=LONG prints the lagged co-

variance matrices the partial autoregressive matrices and estimates of the resid-

ual covariance matrices from the sequence of autoregressive models PRINT-

OUT=NONE suppresses the output for the preliminary autoregressive models The

descriptive statistics and state space model estimation output are still printed when

PRINTOUT=NONE is specified PRINTOUT=SHORT is the default

Canonical Correlation Analysis Options

CANCORR

prints the canonical correlations and information criterion for each candidate state

vector considered See Canonical Correlation Analysis later in this chapter for

details

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Part 2 General Information

DIMMAX= n

specifies the upper limit to the dimension of the state vector The DIMMAX= option

can be used to limit the size of the model selected The default is DIMMAX=10

PASTMIN= n

specifies the minimum number of lags to include in the canonical correlation analy-

sis The default is PASTMIN=0 See Canonical Correlation Analysis later in thischapter for details

SIGCORR= value

specifies the multiplier of the degrees of freedom for the penalty term in the informa-

tion criterion used to select the state space form The default is SIGCORR=2 The

larger the value of the SIGCORR= option the smaller the state vector tends to be

Hence a large value causes a simpler model to be fit See Canonical Correlations

Analysis later in this chapter for details

State Space Model Estimation Options

COVB

prints the inverse of the observed information matrix for the parameter estimatesThis matrix is an estimate of the covariance matrix for the parameter estimates

DETTOL= value

specifies the convergence criterion The DETTOL= and PARMTOL= option values

are used together to test for convergence of the estimation process If during an

iteration the relative change of the parameter estimates is less than the PARMTOL=

value and the relative change of the determinant of the innovation variance matrix

is less than the DETTOL= value then iteration ceases and the current estimates are

accepted The default is DETTOL=1E-5

ITPRINT

prints the iterations during the estimation process

KLAG= n

sets an upper limit for the number of lags of the sample autocovariance matrix used

in computing the approximate likelihood function If the data have a strong moving

average character a larger KLAG= value may be necessary to obtain good estimates

The default is KLAG=15 See Parameter Estimation later in this chapter for details

MAXIT= n

sets an upper limit to the number of iterations in the maximum likelihood or condi-

tional least-squares estimation The default is MAXIT=50

NOEST

suppresses the final maximum likelihood estimation of the selected model

OUTMODEL= SAS-data-set

writes the parameter estimates and their standard errors to a SAS data set See OUT-

MODEL= Data Set later in this chapter for details

PARMTOL= value

specifies the convergence criterion The DETTOL= and PARMTOL= option values

are used together to test for convergence of the estimation process If during an

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Chapter 18 Syntax

iteration the relative change of the parameter estimates is less than the PARMTOL=

value and the relative change of the determinant of the innovation variance matrix

is less than the DETTOL= value then iteration ceases and the current estimates are

accepted The default is PARMTOL=001

RESIDEST

computes the final estimates using conditional least squares on the raw data Thistype of estimation may be more stable than the default maximum likelihood method

but is usually more computationally expensive See Parameter Estimation later in

this chapter for details of the conditional least squares method

SINGULAR= value

specifies the criterion for testing for singularity of a matrix A matrix is declared

singular if a scaled pivot is less than the SINGULAR= value when sweeping the

matrix The default is SINGULAR=1E-7

Forecasting Options

BACK= n

starts forecasting n periods before the end of the input data The BACK= option valuemust not be greater than the number of observations The default is BACK=0

INTERVAL= interval

specifies the time interval between observations The INTERVAL= value is used

in conjunction with the ID variable to check that the input data are in order and

have no missing periods The INTERVAL= option is also used to extrapolate the ID

values past the end of the input data See Chapter 3 ldquoDate Intervals Formats and

Functionsrdquo for details on the INTERVAL= values allowed

INTPER= n

specifies that each input observation corresponds to n time periods For example

the options INTERVAL=MONTH and INTPER=2 specify bimonthly data and are

equivalent to specifying INTERVAL=MONTH2 If the INTERVAL= option is not

specified the INTPER= option controls the increment used to generate ID values for

the forecast observations The default is INTPER=1

LEAD= n

specifies how many forecast observations are produced The forecasts start at the

point set by the BACK= option The default is LEAD=0 which produces no fore-

casts

OUT= SAS-data-set

writes the residuals actual values forecasts and forecast standard errors to a SAS

data set See OUT= Data Set later in this chapter for details

PRINT

prints the forecasts

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Part 2 General Information

BY Statement

BY variable

A BY statement can be used with the STATESPACE procedure to obtain separateanalyses on observations in groups defined by the BY variables

FORM Statement

FORM variable value

The FORM statement specifies the number of times a variable is included in the state

vector Values can be specified for any variable listed in the VAR statement If a

value is specified for each variable in the VAR statement the state vector for the state

space model is entirely specified and automatic selection of the state space model is

not performed

The FORM statement forces the state vectorz

t

to contain a specific variable a given

number of times For example if Y is one of the variables inx

t

then the statement

form y 3

forces the state vector to containY

t

Y

t + 1 j t

andY

t + 2 j t

possibly along with other

variables

The following statements illustrate the use of the FORM statement

proc statespace data=in

var x y

f o r m x 3 y 2

run

These statements fit a state space model with the following state vector

z

t

=

2

6

6

6

6

4

x

t j t

y

t j t

x

t + 1 j t

y

t + 1 j t

x

t + 2 j t

3

7

7

7

7

5

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Chapter 18 Syntax

ID Statement

ID variable

The ID statement specifies a variable that identifies observations in the input data setThe variable specified in the ID statement is included in the OUT= data set The

values of the ID variable are extrapolated for the forecast observations based on the

values of the INTERVAL= and INTPER= options

INITIAL Statement

INITIAL F (rowcolumn)= value G(row column)= value

The INITIAL statement gives initial values to the specified elements of theF

andG

matrices These initial values are used as starting values for the iterative estimation

Parts of theF

andG

matrices represent fixed structural identities If an element

specified is a fixed structural element instead of a free parameter the corresponding

initialization is ignored

The following is an example of an INITIAL statement

initial f(32)=0 g(41)=0 g(51)=0

RESTRICT Statement

RESTRICT F(rowcolumn)= value G(rowcolumn)= value

The RESTRICT statement restricts the specified elements of theF

andG

matrices

to the specified values

To use the restrict statement you need to know the form of the model Either specify

the form of the model with the FORM statement or do a preliminary run perhaps

with the NOEST option to find the form of the model that PROC STATESPACE

selects for the data

The following is an example of a RESTRICT statement

restrict f(32)=0 g(41)=0 g(51)=0

Parts of theF

andG

matrices represent fixed structural identities If a restriction is

specified for an element that is a fixed structural element instead of a free parameter

the restriction is ignored

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Part 2 General Information

VAR Statement

VAR variable (difference difference )

The VAR statement specifies the variables in the input data set to model and fore-cast The VAR statement also specifies differencing of the input variables The VAR

statement is required

Differencing is specified by following the variable name with a list of difference

periods separated by commas See the section Stationarity and Differencing for

more information on differencing of input variables

The order in which variables are listed in the VAR statement controls the order in

which variables are included in the state vector Usually potential inputs should be

listed before potential outputs

For example assuming the input data are monthly the following VAR statement

specifies modeling and forecasting of the one period and seasonal second differenceof X and Y

var x(112) y(112)

In this example the vector time series analyzed is

x

t

=

1 B 1 B

1 2

X

t

x

1 B 1 B

1 2

Y

t

y

where B represents the back shift operator and x and y represent the means of thedifferenced series If the NOCENTER option is specified the mean differences are

not subtracted

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Chapter 18 Details

Details

Missing Values

The STATESPACE procedure does not support missing values The procedure uses

the first contiguous group of observations with no missing values for any of the VARstatement variables Observations at the beginning of the data set with missing values

for any VAR statement variable are not used or included in the output data set

Stationarity and Differencing

The state space model used by the STATESPACE procedure assumes that the time

series are stationary Hence the data should be checked for stationarity One way

to check for stationarity is to plot the series A graph of series over time can show a

time trend or variability changes

You can also check stationarity by using the sample autocorrelation functions dis-

played by the ARIMA procedure The autocorrelation functions of nonstationaryseries tend to decay slowly See Chapter 7 ldquoThe ARIMA Procedurerdquo for more in-

formation

Another alternative is to use the STATIONARITY= option on the IDENTIFY state-

ment in PROC ARIMA to apply Dickey-Fuller tests for unit roots in the time series

See Chapter 7 ldquoThe ARIMA Procedurerdquo for more information on Dickey-Fuller unit

root tests

The most popular way to transform a nonstationary series to stationarity is by dif-

ferencing Differencing of the time series is specified in the VAR statement For

example to take a simple first difference of the series X use this statement

var x(1)

In this example the change in X from one period to the next is analyzed When the se-

ries has a seasonal pattern differencing at a period equal to the length of the seasonal

cycle may be desirable For example suppose the variable X is measured quarterly

and shows a seasonal cycle over the year You can use the following statement to

analyze the series of changes from the same quarter in the previous year

var x(4)

To difference twice add another differencing period to the list For ex-

ample the following statement analyzes the series of second differences

X

t

X

t 1

X

t 1

X

t 2

= X

t

2 X

t 1

+ X

t 2

var x(11)

The following statement analyzes the seasonal second difference series

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Part 2 General Information

var x(14)

The series modeled is the 1-period difference of the 4-period difference

X

t

X

t 4

X

t 1

X

t 5

= X

t

X

t 1

X

t 4

+ X

t 5

Another way to obtain stationary series is to use a regression on time to de-trend the

data If the time series has a deterministic linear trend regressing the series on time

produces residuals that should be stationary The following statements write residuals

of X and Y to the variable RX and RY in the output data set DETREND

data a

set a

t=_n_

run

proc reg data=a

model x y = t

output out=detrend r=rx ry

run

You then use PROC STATESPACE to forecast the de-trended series RX and RY A

disadvantage of this method is that you need to add the trend back to the forecast

series in an additional step A more serious disadvantage of the de-trending method

is that it assumes a deterministic trend In practice most time series appear to have a

stochastic rather than a deterministic trend Differencing is a more flexible and often

more appropriate method

There are several other methods to handle nonstationary time series For more infor-

mation and examples refer to Brockwell and Davis (1991)

Preliminary Autoregressive Models

After computing the sample autocovariance matrices PROC STATESPACE fits a

sequence of vector autoregressive models These preliminary autoregressive models

are used to estimate the autoregressive order of the process and limit the order of the

autocovariances considered in the state vector selection process

Yule-Walker Equations for Forward and Backward Models

Unlike a univariate autoregressive model a multivariate autoregressive model has

different forms depending on whether the present observation is being predicted from

the past observations or from the future observations

Letx

t

be the r -component stationary time series given by the VAR statement after

differencing and subtracting the vector of sample means (If the NOCENTER option

is specified the mean is not subtracted) Let n be the number of observations of x

t

from the input data set

Lete

t

be a vector white noise sequence with mean vector 0 and variance matrix

p

and letn

t

be a vector white noise sequence with mean vector 0 and variance matrix

p

Let p be the order of the vector autoregressive model forx

t

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Chapter 18 Details

The forward autoregressive form based on the past observations is written as follows

x

t

=

p

X

i = 1

p

i

x

t i

+ e

t

The backward autoregressive form based on the future observations is written as fol-

lows

x

t

=

p

X

i = 1

p

i

x

t + i

+ n

t

LettingE

denote the expected value operator the autocovariance sequence for thex

t

series

i

is

i

= E x

t

x

0

t i

The Yule-Walker equations for the autoregressive model that matches the first p ele-

ments of the autocovariance sequence are

2

6

6

4

0

1

p 1

0

1

0

p 2

0

p 1

0

p 2

0

3

7

7

5

2

6

6

4

p

1

p

2

p

p

3

7

7

5

=

2

6

6

4

1

2

p

3

7

7

5

and

2

6

6

6

4

0

0

1

0

p 1

1

0

0

p 2

p 1

p 2

0

3

7

7

7

5

2

6

6

4

p

1

p

2

p

p

3

7

7

5

=

2

6

6

4

0

1

0

2

0

p

3

7

7

5

Here

p

i

are the coefficient matrices for the past observation form of the vector au-toregressive model and

p

i

are the coefficient matrices for the future observation

form More information on the Yule-Walker equations in the multivariate setting can

be found in Whittle (1963) and Ansley and Newbold (1979)

The innovation variance matrices for the two forms can be written as follows

p

=

0

p

X

i = 1

p

i

0

i

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p

=

0

p

X

i = 1

p

i

i

The autoregressive models are fit to the data using the preceding Yule-Walker equa-

tions with

i

replaced by the sample covariance sequenceC

i

The covariance matri-

ces are calculated as

C

i

=

1

N 1

N

X

t = i + 1

x

t

x

0

t i

Let b

p

b

p

b

p

and b

p

represent the Yule-Walker estimates of

p

p

p

and

p

respectively These matrices are written to an output data set when the OUTAR=

option is specified

When the PRINTOUT=LONG option is specified the sequence of matrices b

p

and

the corresponding correlation matrices are printed The sequence of matricesb

p isused to compute Akaike information criteria for selection of the autoregressive order

of the process

Akaike Information Criterion

The Akaike information criterion or AIC is defined as -2( maximum of log likeli-

hood )+2(number of parameters) Since the vector autoregressive models are esti-

mates from the Yule-Walker equations not by maximum likelihood the exact likeli-

hood values are not available for computing the AIC However for the vector autore-

gressive model the maximum of the log likelihood can be approximated as

l n L

n

2

l n j

b

p

j

Thus the AIC for the order p model is computed as

A I C

p

= n l n j

b

p

j + 2 p r

2

You can use the printed AIC array to compute a likelihood ratio test of the autoregres-

sive order The log-likelihood ratio test statistic for testing the order p model against

the order p 1

model is

n l n j

b

p

j + n l n j

b

p 1

j

This quantity is asymptotically distributed as a

2 withr

2 degrees of freedom if the

series is autoregressive of order p 1

It can be computed from the AIC array as

A I C

p 1

A I C

p

+ 2 r

2

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You can evaluate the significance of these test statistics with the PROBCHI function

in a SAS DATA step or with a

2 table

Determining the Autoregressive Order

Although the autoregressive models can be used for prediction their primary value is

to aid in the selection of a suitable portion of the sample covariance matrix for use in

computing canonical correlations If the multivariate time series x

t is of autoregres-sive order p then the vector of past values to lag p is considered to contain essentially

all the information relevant for prediction of future values of the time series

By default PROC STATESPACE selects the order p producing the autoregressive

model with the smallestA I C

p

If the value p for the minimumA I C

p

is less than the

value of the PASTMIN= option then p is set to the PASTMIN= value Alternatively

you can use the ARMAX= and PASTMIN= options to force PROC STATESPACE to

use an order you select

Significance Limits for Partial Autocorrelations

The STATESPACE procedure prints a schematic representation of the partial autocor-

relation matrices indicating which partial autocorrelations are significantly greater orsignificantly less than 0 Figure 1810 shows an example of this table

The STATESPACE Procedure

Schematic Representation of Partial Autocorrelations

NameLag 1 2 3 4 5 6 7 8 9 10

x ++ +

y ++

+ is gt 2std error - is lt -2std error is between

Figure 1810 Significant Partial Autocorrelations

The partial autocorrelations are from the sample partial autoregressive matrices b

p

p

The standard errors used for the significance limits of the partial autocorrelations are

computed from the sequence of matrices

p

and

p

Under the assumption that the observed series arises from an autoregressive process

of order p 1

the pth sample partial autoregressive matrix b

p

p

has an asymptotic

variance matrix 1

n

1

p

p

The significance limits for b

p

p

used in the schematic plot of the sample partial autore-

gressive sequence are derived by replacing

p

and

p

with their sample estimators

to produce the variance estimate as follows

d

V a r

b

p

p

=

1

n r p

b

1

p

b

p

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Canonical Correlation Analysis

Given the order p letp

t

be the vector of current and past values relevant to prediction

of x

t + 1

p

t

= x

0

t

x

0

t 1

x

0

t p

0

Letf

t

be the vector of current and future values

f

t

= x

0

t

x

0

t + 1

x

0

t + p

0

In the canonical correlation analysis consider submatrices of the sample covariance

matrix of p

t

andf

t

This covariance matrixV

has a block Hankel form

V =

2

6

6

6

4

C

0

C

0

1

C

0

2

C

0

p

C

0

1

C

0

2

C

0

3

C

0

p + 1

C

0

p

C

0

p + 1

C

0

p + 2

C

0

2 p

3

7

7

7

5

State Vector Selection Process

The canonical correlation analysis forms a sequence of potential state vectorsz

j

t

Examine a sequencef

j

t

of subvectors of f

t

and form the submatrixV

j consisting

of the rows and columns of V

corresponding to the components of f

j

t

and compute

its canonical correlations

The smallest canonical correlation of V

j is then used in the selection of the com-

ponents of the state vector The selection process is described in the following For

more details about this process refer to Akaike (1976)

In the following discussion the notationx

t + k j t

denotes the wide sense conditional

expectation (best linear predictor) of x

t + k

given allx

s

with s less than or equal to t

In the notationx

i t + 1

the first subscript denotes the ith component of x

t + 1

The initial state vectorz

1

t

is set tox

t

The sequencef

j

t

is initialized by setting

f

1

t

= z

1

0

t

x

1 t + 1 j t

0

= x

0

t

x

1 t + 1 j t

0

That is start by considering whether to addx

1 t + 1 j t

to the initial state vectorz

1

t

The procedure forms the submatrixV

1 corresponding tof

1

t

and computes its canon-

ical correlations Denote the smallest canonical correlation of V

1 as

m i n

If

m i n

is

significantly greater than 0x

1 t + 1 j t

is added to the state vector

If the smallest canonical correlation of V

1 is not significantly greater than 0 then a

linear combination of f

1

t

is uncorrelated with the pastp

t

Assuming that the determi-

nant of C

0

is not 0 (that is no input series is a constant) you can take the coefficient

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Chapter 18 Details

of x

1 t + 1 j t

in this linear combination to be 1 Denote the coefficients of z

1

t

in this

linear combination as

This gives the relationship

x

1 t + 1 j t

=

0

x

t

Therefore the current state vector already contains all the past information useful forpredicting

x

1 t + 1

and any greater leads of x

1 t

The variablex

1 t + 1 j t

is not added to

the state vector nor are any termsx

1 t + k j t

considered as possible components of the

state vector The variablex

1

is no longer active for state vector selection

The process described forx

1 t + 1 j t

is repeated for the remaining elements of f

t

The

next candidate for inclusion in the state vector is the next component of f

t

corre-

sponding to an active variable Components of f

t

corresponding to inactive variables

that produced a zero

m i n

in a previous step are skipped

Denote the next candidate asx

l t + k j t

The vectorf

j

t

is formed from the current state

vector andx

l t + k j t

as follows

f

j

t

= z

j

0

t

x

l t + k j t

0

The matrixV

j is formed fromf

j

t

and its canonical correlations are computed The

smallest canonical correlation of V

j is judged to be either greater than or equal to 0

If it is judged to be greater than 0x

l t + k j t

is added to the state vector If it is judged

to be 0 then a linear combination of f

j

t

is uncorrelated with thep

t

and the variable

x

l

is now inactive

The state vector selection process continues until no active variables remain

Testing Significance of Canonical Correlations For each step in the canonical correlation sequence the significance of the small-

est canonical correlation

m i n

is judged by an information criterion from Akaike

(1976) This information criterion is

n l n 1

2

m i n

r p + 1 q + 1

where q is the dimension of f

j

t

at the current step r is the order of the state vector p

is the order of the vector autoregressive process and

is the value of the SIGCORR=

option The default is SIGCORR=2 If this information criterion is less than or equal

to 0

m i n

is taken to be 0 otherwise it is taken to be significantly greater than 0

(Do not confuse this information criterion with the AIC)

Variables inx

t + p j t

are not added in the model even with positive information crite-

rion because of the singularity of V

You can force the consideration of more candi-

date state variables by increasing the size of theV

matrix by specifying a PASTMIN=

option value larger than p

Printing the Canonical Correlations

To print the details of the canonical correlation analysis process specify the CAN-

CORR option in the PROC STATESPACE statement The CANCORR option prints

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the candidate state vectors the canonical correlations and the information criteria for

testing the significance of the smallest canonical correlation

Bartlettrsquos

2 and its degrees of freedom are also printed when the CANCORR option

is specified The formula used for Bartlettrsquos

2 is

2

= n 5 r p + 1 q + 1 l n 1

2

m i n

withr p + 1 q + 1

degrees of freedom

Figure 1811 shows the output of the CANCORR option for the introductory example

shown in the Getting Started section of this chapter

The STATESPACE Procedure

Canonical Correlations Analysis

Information Chi

x(TT) y(TT) x(T+1T) Criterion Square DF

1 1 0237045 3566167 114505 4

Information Chi

x(TT) y(TT) x(T+1T) y(T+1T) Criterion Square DF

1 1 0238244 0056565 -535906 0636134 3

Information Chi

x(TT) y(TT) x(T+1T) x(T+2T) Criterion Square DF

1 1 0237602 0087493 -446312 1525353 3

Figure 1811 Canonical Correlations Analysis

New variables are added to the state vector if the information criteria are positive Inthis example

y

t + 1 j t

andx

t + 2 j t

are not added to the state space vector because the

information criteria for these models are negative

If the information criterion is nearly 0 then you may want to investigate models

that arise if the opposite decision is made regarding

m i n

This investigation can be

accomplished by using a FORM statement to specify part or all of the state vector

Preliminary Estimates of F

When a candidate variablex

l t + k j t

yields a zero

m i n

and is not added to the state

vector a linear combination of f

j

t

is uncorrelated with thep

t

Because of the method

used to construct thef

j

t

sequence the coefficient of x

l t + k j t

inl

can be taken as 1

Denote the coefficients of z

j

t

in this linear combination as l

This gives the relationship

x

l t + k j t

= l

0

z

j

t

The vectorl

is used as a preliminary estimate of the first r columns of the row of the

transition matrixF

corresponding tox

l t + k 1 j t

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Chapter 18 Details

Parameter Estimation

The model isz

t + 1

= F z

t

+ G e

t + 1

wheree

t

is a sequence of independent multivari-

ate normal innovations with mean vector 0 and variance

e e

The observed sequence

x

t

composes the first r components of z

t

and thusx

t

= H z

t

where H is ther s

matrix I

r

0

LetE

be ther n

matrix of innovations

E = e

1

e

n

If the number of observations n is reasonably large the log likelihood L can be

approximated up to an additive constant as follows

L =

n

2

l n j

e e

j

1

2

t r a c e

1

e e

E E

0

The elements of

e e

are taken as free parameters and are estimated as follows

S

0

=

1

n

E E

0

Replacing

e e

byS

0

in the likelihood equation the log likelihood up to an additive

constant is

L =

n

2

l n j S

0

j

Letting B be the backshift operator the formal relation betweenx

t

ande

t

is

x

t

= H I B F

1

G e

t

e

t

= H I B F

1

G

1

x

t

=

1

X

i = 0

i

x

t i

LettingC

i

be the ith lagged sample covariance of x

t

and neglecting end effects the

matrixS

0

is

S

0

=

1

X

i j = 0

i

C

i + j

0

j

For the computation of S

0

the infinite sum is truncated at the value of the KLAG=

option The value of the KLAG= option should be large enough that the sequence

i

is approximately 0 beyond that point

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Let

be the vector of free parameters in theF

andG

matrices The derivative of the

log likelihood with respect to the parameter

is

L

=

n

2

t r a c e

S

1

0

S

0

The second derivative is

2

L

0

=

n

2

t r a c e

S

1

0

S

0

0

S

1

0

S

0

t r a c e

S

1

0

2

S

0

0

Near the maximum the first term is unimportant and the second term can be approx-

imated to give the following second derivative approximation

2

L

0

=

n t r a c e

S

1

0

E

E

0

0

The first derivative matrix and this second derivative matrix approximation are com-

puted from the sample covariance matrixC

0

and the truncated sequence

i

The

approximate likelihood function is maximized by a modified Newton-Raphson algo-

rithm employing these derivative matrices

The matrixS

0

is used as the estimate of the innovation covariance matrix

e e

The

negative of the inverse of the second derivative matrix at the maximum is used as an

approximate covariance matrix for the parameter estimates The standard errors of

the parameter estimates printed in the parameter estimates tables are taken from the

diagonal of this covariance matrix The parameter covariance matrix is printed when

the COVB option is specified

If the data are nearly nonstationary a better estimate of

e e

and the other parameters

can sometimes be obtained by specifying the RESIDEST option The RESIDEST

option estimates the parameters using conditional least squares instead of maximum

likelihood

The residuals are computed using the state space equation and the sample mean val-

ues of the variables in the model as start-up values The estimate of S

0

is then com-

puted using the residuals from the ith observation on where i is the maximum number

of times any variable occurs in the state vector A multivariate Gauss-Marquardt al-

gorithm is used to minimizej S

0

j

Refer to Harvey (1981a) for a further description

of this method

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Forecasting

Given estimates of F

G

and

e e

forecasts of x

t

are computed from the conditional

expectation of z

t

In forecasting the parameters F G and

e e

are replaced with the estimates or by val-

ues specified in the RESTRICT statement One-step-ahead forecasting is performedfor the observation

x

t

wheret n b

Heren

is the number of observations and

b is the value of the BACK= option For the observationx

t

wheret n b

m-

step-ahead forecasting is performed form = t n + b

The forecasts are generated

recursively with the initial conditionz

0

= 0

The m-step-ahead forecast of z

t + m

isz

t + m j t

wherez

t + m j t

denotes the conditional

expectation of z

t + m

given the information available at time t The m-step-ahead

forecast of x

t + m

isx

t + m j t

= H z

t + m j t

where the matrixH = I

r

0

Let

i

= F

i

G

Note that the lasts r

elements of z

t

consist of the elements of x

u j t

foru t

The state vector z

t + m

can be represented as

z

t + m

= F

m

z

t

+

m 1

X

i = 0

i

e

t + m i

Sincee

t + i j t

= 0

fori 0

the m-step-ahead forecastz

t + m j t

is

z

t + m j t

= F

m

z

t

= F z

t + m 1 j t

Therefore the m-step-ahead forecast of x

t + m

is

x

t + m j t

= H z

t + m j t

The m-step-ahead forecast error is

z

t + m

z

t + m j t

=

m 1

X

i = 0

i

e

t + m i

The variance of the m-step-ahead forecast error is

V

z m

=

m 1

X

i = 0

i

e e

0

i

LettingV

z 0

= 0

the variance of the m-step-ahead forecast error of z

t + m

V

z m

can

be computed recursively as follows

V

z m

= V

z m 1

+

m 1

e e

0

m 1

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The variance of the m-step-ahead forecast error of x

t + m

is ther r

left upper sub-

matrix of V

z m

that is

V

x m

= H V

z m

H

0

Unless the NOCENTER option is specified the sample mean vector is added to the

forecast When differencing is specified the forecasts xt + m j t

plus the sample mean

vector are integrated back to produce forecasts for the original series

Lety

t

be the original series specified by the VAR statement with some 0 values

appended corresponding to the unobserved past observations Let B be the backshift

operator and let B

be thes s

matrix polynomial in the backshift operator

corresponding to the differencing specified by the VAR statement The off-diagonal

elements of

i

are 0 Note that

0

= I

s

whereI

s

is thes s

identity matrix Then

z

t

= B y

t

This gives the relationship

y

t

=

1

B z

t

=

1

X

i = 0

i

z

t i

where

1

B =

P

1

i = 0

i

B

i and

0

= I

s

The m-step-ahead forecast of y

t + m

is

y

t + m j t

=

m 1

X

i = 0

i

z

t + m i j t

+

1

X

i = m

i

z

t + m i

The m-step-ahead forecast error of y

t + m

is

m 1

X

i = 0

i

z

t + m i

z

t + m i j t

=

m 1

X

i = 0

i

X

u = 0

u

i u

e

t + m i

LettingV

y 0

= 0

the variance of the m-step-ahead forecast error of y

t + m

V

y m

is

V

y m

=

m 1

X

i = 0

i

X

u = 0

u

i u

e e

i

X

u = 0

u

i u

0

= V

y m 1

+

m 1

X

u = 0

u

m 1 u

e e

m 1

X

u = 0

u

m 1 u

0

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Chapter 18 Details

Relation of ARMA and State Space Forms

Every state space model has an ARMA representation and conversely every ARMA

model has a state space representation This section discusses this equivalence The

following material is adapted from Akaike (1974) where there is a more complete

discussion Pham-Dinh-Tuan (1978) also contains a discussion of this material

Suppose you are given the following ARMA model

B x

t

= B e

t

or in more detail

x

t

1

x

t 1

p

x

t p

= e

t

+

1

e

t 1

+ +

q

e

t q

(1)

wheree

t

is a sequence of independent multivariate normal random vectors with mean

0 and variance matrix

e e

B is the backshift operator (B x

t

= x

t 1

) B

and

B

are matrix polynomials in B andx

t

is the observed process

If the roots of the determinantial equationj B j = 0

are outside the unit circle in

the complex plane the model can also be written as

x

t

=

1

B B e

t

=

1

X

i = 0

i

e

t i

The

i

matrices are known as the impulse response matrices and can be computed

as

1

B B

You can assume p q

since if this is not initially true you can add more terms

i

that are identically 0 without changing the model

To write this set of equations in a state space form proceed as follows Letx

t + i j t

be

the conditional expectation of x

t + i

givenx

w

forw t

The following relations hold

x

t + i j t

=

1

X

j = i

j

e

t + i j

x

t + i j t + 1

= x

t + i j t

+

i 1

e

t + 1

However from equation (1) you can derive the following relationship

x

t + p j t

=

1

x

t + p 1 j t

+ +

p

x

t

(2)

Hence wheni = p

you can substitute forx

t + p j t

in the right-hand side of equation

(2) and close the system of equations

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This substitution results in the following model in the state space form

z

t + 1

= F z

t

+ G e

t + 1

2

6

6

6

4

x

t + 1

x

t + 2 j t + 1

x

t + p j t + 1

3

7

7

7

5

=

2

6

6

4

0 I 0 0

0 0 I 0

p

p 1

1

3

7

7

5

2

6

6

6

4

x

t

x

t + 1 j t

x

t + p 1 j t

3

7

7

7

5

+

2

6

6

4

I

1

p 1

3

7

7

5

e

t + 1

Note that the state vectorz

t

is composed of conditional expectations of x

t

and the

first r components of z

t

are equal tox

t

The state space form can be cast into an ARMA form by solving the system of dif-

ference equations for the first r components

When converting from an ARMA form to a state space form you can generate a

state vector larger than needed that is the state space model may not be a minimal

representation When going from a state space form to an ARMA form you can have

nontrivial common factors in the autoregressive and moving average operators that

yield an ARMA model larger than necessary

If the state space form used is not a minimal representation some but not all compo-

nents of x

t + i j t

may be linearly dependent This situation corresponds to

p

p 1

being of less than full rank when B

and B

have no common nontrivial

left factors In this casez

t

consists of a subset of the possible components of

x

t + i j t

i = 1 2 p 1

However once a component of x

t + i j t

(for example the

jth one) is linearly dependent on the previous conditional expectations then all sub-

sequent jth components of x

t + k j t

fork i

must also be linearly dependent Note

that in this case equivalent but seemingly different structures can arise if the order of

the components withinx

t

is changed

OUT= Data Set

The forecasts are contained in the output data set specified by the OUT= option on the

PROC STATESPACE statement The OUT= data set contains the following variables

the BY variables

the ID variable

the VAR statement variables These variables contain the actual values from

the input data set

FORi numeric variables containing the forecasts The variable FORi contains

the forecasts for the ith variable in the VAR statement list Forecasts are one-

step-ahead predictions until the end of the data or until the observation specified

by the BACK= option

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Chapter 18 Details

RESi numeric variables containing the residual for the forecast of the ith vari-

able in the VAR statement list For forecast observations the actual values are

missing and the RESi variables contain missing values

STDi numeric variables containing the standard deviation for the forecast of

the ith variable in the VAR statement list The values of the STDi variables can

be used to construct univariate confidence limits for the corresponding fore-casts However such confidence limits do not take into account the covariance

of the forecasts

OUTAR= Data Set

The OUTAR= data set contains the estimates of the preliminary autoregressive mod-

els The OUTAR= data set contains the following variables

ORDER a numeric variable containing the order p of the autoregressive model

that the observation represents

AIC a numeric variable containing the value of the information criterionA I C

p

SIGFl numeric variables containing the estimate of the innovation covariance

matrices for the forward autoregressive models The variable SIGFl contains

the lth column of b

p

in the observations with ORDER= p

SIGBl numeric variables containing the estimate of the innovation covariance

matrices for the backward autoregressive models The variable SIGBl contains

the lth column of b

p

in the observations with ORDER= p

FORk ndashl numeric variables containing the estimates of the autoregressive pa-

rameter matrices for the forward models The variable FORk ndashl contains the

lth column of the lag k autoregressive parameter matrix b

p

k

in the observations

with ORDER= p

BACk ndashl numeric variables containing the estimates of the autoregressive pa-

rameter matrices for the backward models The variable BACk ndashl contains the

lth column of the lag k autoregressive parameter matrix b

p

k

in the observations

with ORDER= p

The estimates for the order p autoregressive model can be selected as those observa-

tions with ORDER= p Within these observations the k lth element of

p

i

is given by

the value of the FORindashl variable in the k th observation The k lth element of

p

i

is

given by the value of BACindashl variable in the k th observation The k lth element of

p

is given by SIGFl in the k th observation The k lth element of

p

is given by SIGBlin the k th observation

Table 181 shows an example of the OUTAR= data set with ARMAX=3 andx

t

of

dimension 2 In Table 181 i j

indicate the ijth element of the matrix

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Part 2 General Information

Table 181 Values in the OUTAR= Data Set

Obs ORDER AIC SIGF1 SIGF2 SIGB1 SIGB2 FOR1ndash 1 FOR1ndash 2 FOR2ndash 1 FOR2ndash 2 FOR3ndash 1

1 0 AIC0

0 1 1

0 1 2

0 1 1

0 1 2

2 0 AIC0

0 2 1

0 2 2

0 2 1

0 2 2

3 1 AIC1

1 1 1

1 1 2

1 1 1

1 1 2

1

1

1 1

1

1

1 2

4 1 AIC1

1 2 1

1 1 2

1 2 1

1 1 2

1

1

2 1

1

1

2 2

5 2 AIC2

2 1 1

2 1 2

2 1 1

2 1 2

2

1

1 1

2

1

1 2

2

2

1 1

2

2

1 2

6 2 AIC2

2 2 1

2 1 2

2 2 1

2 1 2

2

1

2 1

2

1

2 2

2

2

2 1

2

2

2 2

7 3 AIC3

3 1 1

3 1 2

3 1 1

3 1 2

3

1

1 1

3

1

1 2

3

2

1 1

3

2

1 2

3

3

1 1

8 3 AIC3

3 2 1

3 1 2

3 2 1

3 1 2

3

1

2 1

3

1

2 2

3

2

2 1

3

2

2 2

3

3

2 1

Obs FOR3ndash 2 BACK1ndash 1 BACK1ndash 2 BACK2ndash 1 BACK2ndash 2 BACK3ndash 1 BACK3ndash 2

1

2

3

1

1

1 1

1

1

1 2

4

1

1

2 1

1

1

2 2

5

2

1

1 1

2

1

1 2

2

2

1 1

2

2

1 2

6

2

1

2 1

2

1

2 2

2

2

2 1

2

2

2 2

7

3

3

1 2

3

1

1 1

3

1

1 2

3

2

1 1

3

2

1 2

3

3

1 1

3

3

1 2

8

3

3

2 2

3

1

2 1

3

1

2 2

3

2

2 1

3

2

2 2

3

3

2 1

3

3

2 2

The estimated autoregressive parameters can be used in the IML procedure to obtain

autoregressive estimates of the spectral density function or forecasts based on the

autoregressive models

OUTMODEL= Data Set

The OUTMODEL= data set contains the estimates of theF

andG

matrices and their

standard errors the names of the components of the state vector and the estimates of

the innovation covariance matrix The variables contained in the OUTMODEL= data

set are as follows

the BY variables

STATEVEC a character variable containing the name of the component of the

state vector corresponding to the observation The STATEVEC variable has

the value STD for standard deviations observations which contain the standarderrors for the estimates given in the preceding observation

Fndash j numeric variables containing the columns of theF

matrix The variable

Fndash j contains the jth column of F

The number of Fndash j variables is equal to

the value of the DIMMAX= option If the model is of smaller dimension the

extraneous variables are set to missing

Gndash j numeric variables containing the columns of theG

matrix The variable

Gndash j contains the jth column of G

The number of Gndash j variables is equal to r

the dimension of x

t

given by the number of variables in the VAR statement

SIGndash j numeric variables containing the columns of the innovation covariance

matrix The variable SIGndash j contains the jth column of

e e

There are r vari-ables SIGndash j

Table 182 shows an example of the OUTMODEL= data set withx

t

= x

t

y

t

0

z

t

= x

t

y

t

x

t + 1 j t

0 and DIMMAX=4 In Table 182F

i j

andG

i j

are the ijth

elements of F

andG

respectively Note that all elements for Fndash4 are missing because

F

is a3 3

matrix

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Chapter 18 Details

Table 182 Value in the OUTMODEL= Data Set

Obs STATEVEC Fndash 1 Fndash 2 Fndash 3 Fndash 4 Gndash 1 Gndash 2 SIGndash 1 SIGndash 2

1 X(TT) 0 0 1 1 0

1 1

1 2

2 STD

3 Y(TT) F2 1

F2 2

F2 3

0 1

2 1

2 2

4 STD std F2 1

std F2 2

std F2 3

5 X(T+1T) F3 1

F3 2

F3 3

G3 1

G3 2

6 STD std F3 1

std F3 2

std F3 3

std G3 1

std G3 2

Printed Output

The printed output produced by the STATESPACE procedure is described in the fol-

lowing

1 descriptive statistics which include the number of observations used the

names of the variables their means and standard deviations (Std) and the dif-

ferencing operations used

2 the Akaike information criteria for the sequence of preliminary autoregressive

models

3 if the PRINTOUT=LONG option is specified the sample autocovariance ma-

trices of the input series at various lags

4 if the PRINTOUT=LONG option is specified the sample autocorrelation ma-

trices of the input series

5 a schematic representation of the autocorrelation matrices showing the signif-

icant autocorrelations

6 if the PRINTOUT=LONG option is specified the partial autoregressive ma-

trices (These are

p

p

as described in Preliminary Autoregressive Modelsearlier in this chapter)

7 a schematic representation of the partial autocorrelation matrices showing the

significant partial autocorrelations

8 the Yule-Walker estimates of the autoregressive parameters for the autoregres-

sive model with the minimum AIC

9 if the PRINTOUT=LONG option is specified the autocovariance matrices of

the residuals of the minimum AIC model This is the sequence of estimated

innovation variance matrices for the solutions of the Yule-Walker equations

10 if the PRINTOUT=LONG option is specified the autocorrelation matrices of the residuals of the minimum AIC model

11 If the CANCORR option is specified the canonical correlations analysis for

each potential state vector considered in the state vector selection process

This includes the potential state vector the canonical correlations the informa-

tion criterion for the smallest canonical correlation Bartlettrsquos

2 statistic (ldquoChi

Squarerdquo) for the smallest canonical correlation and the degrees of freedom of

Bartlettrsquos

2

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Part 2 General Information

12 the components of the chosen state vector

13 the preliminary estimate of the transition matrixF

the input matrixG

and

the variance matrix for the innovations

e e

14 if the ITPRINT option is specified the iteration history of the likelihood max-

imization For each iteration this shows the iteration number the number of step halvings the determinant of the innovation variance matrix the damping

factor Lambda and the values of the parameters

15 the state vector printed again to aid interpretation of the following listing of F

andG

16 the final estimate of the transition matrixF

17 the final estimate of the input matrixG

18 the final estimate of the variance matrix for the innovations

e e

19 a table listing the estimates of the free parameters inF

andG

and their standarderrors and t statistics

20 if the COVB option is specified the covariance matrix of the parameter esti-

mates

21 if the COVB option is specified the correlation matrix of the parameter esti-

mates

22 if the PRINT option is specified the forecasts and their standard errors

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Chapter 18 Details

ODS Table Names

PROC STATESPACE assigns a name to each table it creates You can use these

names to reference the table when using the Output Delivery System (ODS) to select

tables and create output data sets These names are listed in the following table For

more information on ODS see Chapter 6 ldquoUsing the Output Delivery Systemrdquo

Table 183 ODS Tables Produced in PROC STATESPACE

ODS Table Name Description Option

NObs Number of observations default

Summary Simple summary statistics table default

InfoCriterion Information criterion table default

CovLags Covariance Matrices of Input Series PRINTOUT=LONG

CorrLags Correlation Matrices of Input Series PRINTOUT=LONG

PartialAR Partial Autoregressive Matrices PRINTOUT=LONG

YWEstimates Yule-Walker Estimates for Minimum AIC default

CovResiduals Covariance of Residuals PRINTOUT=LONGCorrResiduals Residual Correlations from AR Models PRINTOUT=LONG

StateVector State vector table default

CorrGraph Schematic Representation of Correlations default

TransitionMatrix Transition Matrix default

InputMatrix Input Matrix default

VarInnov Variance Matrix for the Innovation default

CovB Covariance of Parameter Estimates COVB

CorrB Correlation of Parameter Estimates COVB

CanCorr Canonical Correlation Analysis CANCORR

IterHistory Iterative Fitting table ITPRINT

ParameterEstimates Parameter Estimates Table default

Forecasts Forecasts Table PRINT

ConvergenceStatus Convergence Status Table default

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Part 2 General Information

Example

Example 181 Series J from Box and Jenkins

This example analyzes the gas furnace data (series J) from Box and Jenkins (The

data are not shown Refer to Box and Jenkins (1976) for the data)

First a model is selected and fit automatically using the following statements

title1 rsquoGas Furnace Datarsquo

title2 rsquoBox amp Jenkins Series Jrsquo

title3 rsquoAutomatically Selected Modelrsquo

proc statespace data=seriesj cancorr

var x y

run

The results for the automatically selected model are shown in Output 1811

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Chapter 18 Example

Output 1811 Results for Automatically Selected Model

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Number of Observations 296

Standard

Variable Mean Error

x -005683 1072766

y 5350912 3202121

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Information Criterion for Autoregressive Models

Lag=0 Lag=1 Lag=2 Lag=3 Lag=4 Lag=5 Lag=6 Lag=7 Lag=8

6513862 -103357 -163296 -164512 -165152 -164891 -164934 -164315 -163856

Information

Criterion for

Autoregressive

Models

Lag=9 Lag=10

-16348 -163359

Schematic Representation of Correlations

NameLag 0 1 2 3 4 5 6 7 8 9 10

x +- +- +- +- +- +- +- +- +- +- +-

y -+ -+ -+ -+ -+ -+ -+ -+ -+ -+ -+

+ is gt 2std error - is lt -2std error is between

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Part 2 General Information

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Schematic Representation of Partial Autocorrelations

NameLag 1 2 3 4 5 6 7 8 9 10

x + - + -

y -+ -- - + +

+ is gt 2std error - is lt -2std error is between

Yule-Walker Estimates for Minimum AIC

------Lag=1------ ------Lag=2------ ------Lag=3------ ------Lag=4------

x y x y x y x y

x 1925887 -000124 -120166 0004224 0116918 -000867 0104236 0003268

y 0050496 1299793 -002046 -03277 -071182 -025701 0195411 0133417

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Canonical Correlations Analysis

Information Chi

x(TT) y(TT) x(T+1T) Criterion Square DF

1 1 0804883 2929228 3047481 8

Information Chix(TT) y(TT) x(T+1T) y(T+1T) Criterion Square DF

1 1 0906681 0607529 1223358 1347237 7

Information Chi

x(TT) y(TT) x (T+1T) y (T+1T) x (T+2T) Criterion Square DF

1 1 0909434 0610278 0186274 -154701 1034705 6

Information Chi

x(TT) y(TT) x (T+1T) y (T+1T) y (T+2T) Criterion Square DF

1 1 091014 0618937 0206823 0940392 1280924 6

Information Chi

x(TT) y(TT) x(T+1T) y(T+1T) y(T+2T) y(T+3T) Criterion Square DF

1 1 0912963 0628785 0226598 0083258 -794103 2041584 5

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Chapter 18 Example

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Selected Statespace Form and Preliminary Estimates

State Vector

x(TT) y(TT) x(T+1T) y(T+1T) y(T+2T)

Estimate of Transition Matrix

0 0 1 0 0

0 0 0 1 0

-084718 0026794 1711715 -005019 0

0 0 0 0 1

-019785 0334274 -018174 -123557 1787475

Input Matrix for Innovation

1 0

0 11925887 -000124

0050496 1299793

0142421 1361696

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Selected Statespace Form and Preliminary Estimates

Variance Matrix for Innovation

0035274 -000734

-000734 0097569

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Part 2 General Information

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Selected Statespace Form and Fitted Model

State Vector

x(TT) y(TT) x(T+1T) y(T+1T) y(T+2T)

Estimate of Transition Matrix

0 0 1 0 0

0 0 0 1 0

-086192 0030609 1724235 -005483 0

0 0 0 0 1

-034839 0292124 -009435 -109823 1671418

Input Matrix for Innovation

1 0

0 1192442 -000416

0015621 1258495

008058 1353204

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Selected Statespace Form and Fitted Model

Variance Matrix for Innovation

0035579 -000728

-000728 0095577

Parameter Estimates

Standard

Parameter Estimate Error t Value

F(31) -086192 0072961 -1181

F(32) 0030609 0026167 117

F(33) 1724235 0061599 2799

F(34) -005483 0030169 -182

F(51) -034839 0135253 -258

F(52) 0292124 0046299 631

F(53) -009435 0096527 -098F(54) -109823 0109525 -1003

F(55) 1671418 0083737 1996

G(31) 1924420 0058162 3309

G(32) -000416 0035255 -012

G(41) 0015621 0095771 016

G(42) 1258495 0055742 2258

G(51) 0080580 0151622 053

G(52) 1353204 0091388 1481

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Chapter 18 Example

The two series are believed to have a transfer function relation with the gas rate

(variable X) as the input and the CO2

concentration (variable Y) as the output Since

the parameter estimates shown in Output 1811 support this kind of model the model

is reestimated with the feedback parameters restricted to 0 The following statements

fit the transfer function (no feedback) model

title3 rsquoTransfer Function Modelrsquo

proc statespace data=seriesj printout=none

var x y

restrict f(32)=0 f(34)=0

g(32)=0 g(41)=0 g(51)=0

run

The last 2 pages of the output are shown in Output 1812

Output 1812 STATESPACE Output for Transfer Function Model

Gas Furnace Data

Box amp Jenkins Series J

Transfer Function Model

The STATESPACE Procedure

Selected Statespace Form and Fitted Model

State Vector

x(TT) y(TT) x(T+1T) y(T+1T) y(T+2T)

Estimate of Transition Matrix

0 0 1 0 0

0 0 0 1 0

-068882 0 1598717 0 0

0 0 0 0 1

-035944 0284179 -00963 -107313 1650047

Input Matrix for Innovation

1 0

0 1

1923446 0

0 1260856

0 1346332

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Part 2 General Information

Gas Furnace Data

Box amp Jenkins Series J

Transfer Function Model

The STATESPACE Procedure

Selected Statespace Form and Fitted Model

Variance Matrix for Innovation

0036995 -00072

-00072 0095712

Parameter Estimates

Standard

Parameter Estimate Error t Value

F(31) -068882 0050549 -1363

F(33) 1598717 0050924 3139

F(51) -035944 0229044 -157

F(52) 0284179 0096944 293

F(53) -009630 0140876 -068

F(54) -107313 0250385 -429

F(55) 1650047 0188533 875G(31) 1923446 0056328 3415

G(42) 1260856 0056464 2233

G(52) 1346332 0091086 1478

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Chapter 18 References

References

Akaike H (1974) Markovian Representation of Stochastic Processes and Its Ap-

plication to the Analysis of Autoregressive Moving Average Processes Annals

of the Institute of Statistical Mathematics 26 363-387

Akaike H (1976) Canonical Correlations Analysis of Time Series and the Use of

an Information Criterion in Advances and Case Studies in System Identification

eds R Mehra and DG Lainiotis New York Academic Press

Anderson TW (1971) The Statistical Analysis of Time Series New York John

Wiley amp Sons

Ansley CF and Newbold P (1979) Multivariate Partial Autocorrelations Pro-

ceedings of the Business and Economic Statistics Section American Statistical

Association 349-353

Box GEP and Jenkins G (1976) Time Series Analysis Forecasting and Control

San Francisco Holden-Day

Brockwell PJ and Davis RA (1991) Time Series Theory and Methods 2nd Edi-

tion Springer-Verlag

Hannan EJ (1970) Multiple Time Series New York John Wiley amp Sons

Hannan EJ (1976) The Identification and Parameterization of ARMAX and State

Space Forms Econometrica 44 713-722

Harvey AC (1981a) The Econometric Analysis of Time Series New York John

Wiley amp Sons

Harvey AC (1981b) Time Series Models New York John Wiley amp Sons

Jones RH (1974) Identification and Autoregressive Spectrum Estimation IEEE Transactions on Automatic Control AC-19 894-897

Pham-Dinh-Tuan (1978) On the Fitting of Multivariate Processes of the

Autoregressive-Moving Average Type Biometrika 65 99-107

Priestley MB (1980) System Identification Kalman Filtering and Stochastic Con-

trol in Directions in Time Series eds DR Brillinger and GC Tiao Institute

of Mathematical Statistics

Whittle P (1963) On the Fitting of Multivariate Autoregressions and the Approxi-

mate Canonical Factorization of a Spectral Density Matrix Biometrika 50 129-

134

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The correct bibliographic citation for this manual is as follows SAS Institute Inc SAS

ET S U serrsquos Gui de Version 8 Cary NC SAS Institute Inc 1999 1546 pp

SASET S Userrsquos Guide Version 8

Copyright copy 1999 by S AS I ns t it u t e I nc Ca ry N C U S A

ISBN 1ndash58025ndash489ndash6

All r ight s res erved P r int ed in t he U n it ed St a t es of A merica N o pa rt of t his p u blica t ionma y b e reprodu ced s t ored in a ret r ieva l s ys t em or t ra ns mit t ed in a ny form or b y a nymeans electronic mechanical photocopying or otherwise without the prior writtenpermission of the publisher SAS Institute Inc

US Government Restricted Rights Notice Use du plicat ion or disclosure of the

software by the government is subject to restrictions as set forth in FAR 52227ndash19Commercial Computer Softw ar e-Restricted Rights (J une 1987)

SAS Institute Inc SAS Campus Drive Cary North Carolina 27513

1st print ing October 1999

S AS reg a nd a ll ot her SA S I ns t it u t e I nc p rodu ct or s ervice na m es a re regis t ered t ra dema rkst d k f SA S I t it t I i t h U S A d t h t i reg i di t U SA

Page 8: Statespace Model

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Part 2 General Information

The STATESPACE Procedure

Number of Observations 200

Standard

Variable Mean Error

x 0144316 1233457 Has been differenced With period(s) = 1

y 0164871 1304358 Has been differenced

With period(s) = 1

The STATESPACE Procedure

Information Criterion for Autoregressive Models

Lag=0 Lag=1 Lag=2 Lag=3 Lag=4 Lag=5 Lag=6 Lag=7 Lag=8

149697 8387786 5517099 1205986 1536952 2179538 2400638 2988874 3355708

Information

Criterion for Autoregressive

Models

Lag=9 Lag=10

4117606 4770222

Schematic Representation of Correlations

NameLag 0 1 2 3 4 5 6 7 8 9 10

x ++ ++ ++ ++ ++ ++ + + +

y ++ ++ ++ ++ ++ + + + +

+ is gt 2std error - is lt -2std error is between

Figure 182 Descriptive Statistics and VAR Order Selection

Descriptive statistics are printed first giving the number of nonmissing observations

after differencing and the sample means and standard deviations of the differenced

series The sample means are subtracted before the series are modeled (unless the

NOCENTER option is specified) and the sample means are added back when the

forecasts are produced

LetX

t

andY

t

be the observed values of X and Y and letx

t

andy

t

be the values of X

and Y after differencing and subtracting the mean difference The seriesx

t

modeled

by the STATEPSPACE procedure is

x

t

=

x

t

y

t

=

1 B X

t

0 1 4 4 3 1 6

1 B Y

t

0 1 6 4 8 7 1

where B represents the backshift operator

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Chapter 18 Getting Started

After the descriptive statistics PROC STATESPACE prints the Akaike information

criterion (AIC) values for the autoregressive models fit to the series The smallest AIC

value in this case 5517 at lag 2 determines the number of autocovariance matrices

analyzed in the canonical correlation phase

A schematic representation of the autocorrelations is printed next This indicates

which elements of the autocorrelation matrices at different lags are significantlygreater or less than 0

The second page of the STATESPACE printed output is shown in Figure 183

The STATESPACE Procedure

Schematic Representation of Partial Autocorrelations

NameLag 1 2 3 4 5 6 7 8 9 10

x ++ +

y ++

+ is gt 2std error - is lt -2std error is between

Yule-Walker Estimates for Minimum AIC

--------Lag=1------- --------Lag=2-------

x y x y

x 0257438 0202237 0170812 0133554

y 0292177 0469297 -000537 -000048

Figure 183 Partial Autocorrelations and VAR Model

Figure 183 shows a schematic representation of the partial autocorrelations similar

to the autocorrelations shown in Figure 182 The selection of a second order autore-

gressive model by the AIC statistic looks reasonable in this case because the partialautocorrelations for lags greater than 2 are not significant

Next the Yule-Walker estimates for the selected autoregressive model are printed

This output shows the coefficient matrices of the vector autoregressive model at each

lag

Selected State Space Model Form and Preliminary Estimates

After the autoregressive order selection process has determined the number of lags to

consider the canonical correlation analysis phase selects the state vector By default

output for this process is not printed You can use the CANCORR option to print

details of the canonical correlation analysis See the section Canonical Correlation

Analysis later in this chapter for an explanation of this process

Once the state vector is selected the state space model is estimated by approximate

maximum likelihood Information from the canonical correlation analysis and from

the preliminary autoregression is used to form preliminary estimates of the state space

model parameters These preliminary estimates are used as starting values for the

iterative estimation process

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Part 2 General Information

The form of the state vector and the preliminary estimates are printed next as shown

in Figure 184

The STATESPACE Procedure

Selected Statespace Form and Preliminary Estimates

State Vector

x(TT) y(TT) x(T+1T)

Estimate of Transition Matrix

0 0 1

0291536 0468762 -000411

024869 024484 0204257

Input Matrix for Innovation

1 0

0 1

0257438 0202237

Variance Matrix for Innovation

0945196 0100786

0100786 1014703

Figure 184 Preliminary Estimates of State Space Model

Figure 184 first prints the state vector as X[TT] Y[TT] X[T+1T] This notation

indicates that the state vector is

z

t

=

2

4

x

t j t

y

t j t

x

t + 1 j t

3

5

The notationx

t + 1 j t

indicates the conditional expectation or prediction of x

t + 1

based

on the information available at time t andx

t j t

andy

t j t

arex

t

andy

t

respectively

The remainder of Figure 184 shows the preliminary estimates of the transition matrix

F

the input matrixG

and the covariance matrix

e e

Estimated State Space Model

The next page of the STATESPACE output prints the final estimates of the fittedmodel as shown in Figure 185 This output has the same form as in Figure 184 but

shows the maximum likelihood estimates instead of the preliminary estimates

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Chapter 18 Getting Started

The STATESPACE Procedure

Selected Statespace Form and Fitted Model

State Vector

x(TT) y(TT) x(T+1T)

Estimate of Transition Matrix

0 0 1

0297273 047376 -001998

02301 0228425 0256031

Input Matrix for Innovation

1 0

0 1

0257284 0202273

Variance Matrix for Innovation

0945188 01007520100752 1014712

Figure 185 Fitted State Space Model

The estimated state space model shown in Figure 185 is

2

4

x

t + 1 j t + 1

y

t + 1 j t + 1

x

t + 2 j t + 1

3

5

=

2

4

0 0 1

0 2 9 7 0 4 7 4 0 0 2 0

0 2 3 0 0 2 2 8 0 2 5 6

3

5

2

4

x

t

y

t

x

t + 1 j t

3

5

+

2

4

1 0

0 1

0 2 5 7 0 2 0 2

3

5

e

t + 1

n

t + 1

v a r

e

t + 1

n

t + 1

=

0 9 4 5 0 1 0 1

0 1 0 1 1 0 1 5

The next page of the STATESPACE output lists the estimates of the free parameters

in theF

andG

matrices with standard errors and t statistics as shown in Figure 186

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Part 2 General Information

The STATESPACE Procedure

Parameter Estimates

Standard

Parameter Estimate Error t Value

F(21) 0297273 0129995 229

F(22) 0473760 0115688 410F(23) -001998 0313025 -006

F(31) 0230100 0126226 182

F(32) 0228425 0112978 202

F(33) 0256031 0305256 084

G(31) 0257284 0071060 362

G(32) 0202273 0068593 295

Figure 186 Final Parameter Estimates

Convergence Failures

The maximum likelihood estimates are computed by an iterative nonlinear maximiza-

tion algorithm which may not converge If the estimates fail to converge warning

messages are printed in the output

If you encounter convergence problems you should recheck the stationarity of the

data and ensure that the specified differencing orders are correct Attempting to fit

state space models to nonstationary data is a common cause of convergence failure

You can also use the MAXIT= option to increase the number of iterations allowed

or experiment with the convergence tolerance options DETTOL= and PARMTOL=

Forecast Data Set

The following statements print the output data set The WHERE statement excludes

the first 190 observations from the output so that only the forecasts and the last 10

actual observations are printed

proc print data=outid t

where t gt 190

run

The PROC PRINT output is shown in Figure 187

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Part 2 General Information

Figure 188 Plot of Forecasts

Controlling Printed Output

By default the STATESPACE procedure produces a large amount of printed output

The NOPRINT option suppresses all printed output You can suppress the printed

output for the autoregressive model selection process with the PRINTOUT=NONE

option The descriptive statistics and state space model estimation output are still

printed when PRINTOUT=NONE is specified You can produce more detailed output

with the PRINTOUT=LONG option and by specifying the printing control optionsCANCORR COVB and PRINT

Specifying the State Space Model

Instead of allowing the STATESPACE procedure to select the model automatically

you can use FORM and RESTRICT statements to specify a state space model

Specifying the State Vector

Use the FORM statement to control the form of the state vector You can use this

feature to force PROC STATESPACE to estimate and forecast a model different from

the model it would select automatically You can also use this feature to reestimate

the automatically selected model (possibly with restrictions) without repeating thecanonical correlation analysis

The FORM statement specifies the number of lags of each variable to include in

the state vector For example the statement FORM X 3 forces the state vector to

includex

t j t

x

t + 1 j t

andx

t + 2 j t

The following statement specifies the state vector

x

t j t

y

t j t

x

t + 1 j t

which is the same state vector selected in the preceding example

f o r m x 2 y 1

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Chapter 18 Getting Started

You can specify the form for only some of the variables and allow PROC STATES-

PACE to select the form for the other variables If only some of the variables are

specified in the FORM statement canonical correlation analysis is used to determine

the number of lags included in the state vector for the remaining variables not spec-

ified by the FORM statement If the FORM statement includes specifications for all

the variables listed in the VAR statement the state vector is completely defined and

the canonical correlation analysis is not performed

Restricting the F and G matrices

After you know the form of the state vector you can use the RESTRICT statement

to fix some parameters in theF

andG

matrices to specified values One use of this

feature is to remove insignificant parameters by restricting them to 0

In the introductory example shown in the preceding section the F[23] parameter is

not significant (The parameters estimation output shown in Figure 186 gives the t

statistic for F[23] as -006 F[33] and F[31] also have low significance witht 2

)

The following statements reestimate this model with F[23] restricted to 0 The

FORM statement is used to specify the state vector and thus bypass the canonicalcorrelation analysis

proc statespace data=in out=out lead=10

var x(1) y(1)

id t

form x 2 y 1

restrict f(23)=0

run

The final estimates produced by these statements are shown in Figure 189

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Part 2 General Information

The STATESPACE Procedure

Selected Statespace Form and Fitted Model

State Vector

x(TT) y(TT) x(T+1T)

Estimate of Transition Matrix

0 0 1

0290051 0467468 0

0227051 0226139 026436

Input Matrix for Innovation

1 0

0 1

0256826 0202022

Variance Matrix for Innovation

0945175 01006960100696 1014733

The STATESPACE Procedure

Parameter Estimates

Standard

Parameter Estimate Error t Value

F(21) 0290051 0063904 454

F(22) 0467468 0060430 774

F(31) 0227051 0125221 181

F(32) 0226139 0111711 202

F(33) 0264360 0299537 088G(31) 0256826 0070994 362

G(32) 0202022 0068507 295

Figure 189 Results using RESTRICT Statement

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Chapter 18 Syntax

Syntax

The STATESPACE procedure uses the following statements

PROC STATESPACE options

BY variable FORM variable value

ID variable

INITIAL F(rowcolumn)=value

G(rowcolumn)=value

RESTRICT F(rowcolumn)=value

G(rowcolumn)=value

VAR variable (difference difference

)

Functional Summary

The statements and options used by PROC STATESPACE are summarized in the

following table

Description Statement Option

Input Data Set Options

specify the input data set PROC STATESPACE DATA=

prevent subtraction of sample mean PROC STATESPACE NOCENTER

specify the ID variable ID

specify the observed series and differencing VAR

Options for Autoregressive Estimates

specify the maximum order PROC STATESPACE ARMAX=

specify maximum lag for autocovariances PROC STATESPACE LAGMAX=

output only minimum AIC model PROC STATESPACE MINIC

specify the amount of detail printed PROC STATESPACE PRINTOUT=

write preliminary AR models to a data set PROC STATESPACE OUTAR=

Options for Canonical Correlation Analysis

print the sequence of canonical correlations PROC STATESPACE CANCORR

specify upper limit of dimension of state

vector

PROC STATESPACE DIMMAX=

specify the minimum number of lags PROC STATESPACE PASTMIN=

specify the multiplier of the degrees of

freedom

PROC STATESPACE SIGCORR=

Options for State Space Model Estimation

specify starting values INITIAL

print covariance matrix of parameter estimates PROC STATESPACE COVB

specify the convergence criterion PROC STATESPACE DETTOL=

specify the convergence criterion PROC STATESPACE PARMTOL=

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Part 2 General Information

Description Statement Option

print the details of the iterations PROC STATESPACE ITPRINT

specify an upper limit of the number of lags PROC STATESPACE KLAG=

specify maximum number of iterations

allowed

PROC STATESPACE MAXIT=

suppress the final estimation PROC STATESPACE NOEST

write the state space model parameter esti-

mates to an output data set

PROC STATESPACE OUTMODEL=

use conditional least squares for final estimates PROC STATESPACE RESIDEST

specify criterion for testing for singularity PROC STATESPACE SINGULAR=

Options for Forecasting

start forecasting before end of the input data PROC STATESPACE BACK=

specify the time interval between observations PROC STATESPACE INTERVAL=

specify multiple periods in the time series PROC STATESPACE INTPER=specify how many periods to forecast PROC STATESPACE LEAD=

specify the output data set for forecasts PROC STATESPACE OUT=

print forecasts PROC STATESPACE PRINT

Options to Specify the State Space Model

specify the state vector FORM

specify the parameter values RESTRICT

BY Groups

specify BY-group processing BY

Printing

suppresses all printed output NOPRINT

PROC STATESPACE Statement

PROC STATESPACE options

The following options can be specified in the PROC STATESPACE statement

Printing Options NOPRINT

suppresses all printed output

Input Data Options

DATA= SAS-data-set

specifies the name of the SAS data set to be used by the procedure If the DATA=

option is omitted the most recently created SAS data set is used

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Chapter 18 Syntax

LAGMAX= k

specifies the number of lags for which the sample autocovariance matrix is computed

The LAGMAX= option controls the number of lags printed in the schematic repre-

sentation of the autocorrelations

The sample autocovariance matrix of lag i denoted asC

i

is computed as

C

i

=

1

N 1

N

X

t = 1 + i

x

t

x

0

t i

wherex

t

is the differenced and centered data andN

is the number of observations

(If the NOCENTER option is specified 1 is not subtracted fromN

) LAGMAX= k

specifies thatC

0

throughC

k

are computed The default is LAGMAX=10

NOCENTER

prevents subtraction of the sample mean from the input series (after any specified

differencing) before the analysis

Options for Preliminary Autoregressive Models ARMAX= n

specifies the maximum order of the preliminary autoregressive models The AR-

MAX= option controls the autoregressive orders for which information criteria are

printed and controls the number of lags printed in the schematic representation of

partial autocorrelations The default is ARMAX=10 See Preliminary Autoregres-

sive Models later in this chapter for details

MINIC

writes to the OUTAR= data set only the preliminary Yule-Walker estimates for the

VAR model producing the minimum AIC See OUTAR= Data Set later in this chap-

ter for details

OUTAR= SAS-data-set

writes the Yule-Walker estimates of the preliminary autoregressive models to a SAS

data set See OUTAR= Data Set later in this chapter for details

PRINTOUT= SHORT | LONG | NONE

determines the amount of detail printed PRINTOUT=LONG prints the lagged co-

variance matrices the partial autoregressive matrices and estimates of the resid-

ual covariance matrices from the sequence of autoregressive models PRINT-

OUT=NONE suppresses the output for the preliminary autoregressive models The

descriptive statistics and state space model estimation output are still printed when

PRINTOUT=NONE is specified PRINTOUT=SHORT is the default

Canonical Correlation Analysis Options

CANCORR

prints the canonical correlations and information criterion for each candidate state

vector considered See Canonical Correlation Analysis later in this chapter for

details

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Part 2 General Information

DIMMAX= n

specifies the upper limit to the dimension of the state vector The DIMMAX= option

can be used to limit the size of the model selected The default is DIMMAX=10

PASTMIN= n

specifies the minimum number of lags to include in the canonical correlation analy-

sis The default is PASTMIN=0 See Canonical Correlation Analysis later in thischapter for details

SIGCORR= value

specifies the multiplier of the degrees of freedom for the penalty term in the informa-

tion criterion used to select the state space form The default is SIGCORR=2 The

larger the value of the SIGCORR= option the smaller the state vector tends to be

Hence a large value causes a simpler model to be fit See Canonical Correlations

Analysis later in this chapter for details

State Space Model Estimation Options

COVB

prints the inverse of the observed information matrix for the parameter estimatesThis matrix is an estimate of the covariance matrix for the parameter estimates

DETTOL= value

specifies the convergence criterion The DETTOL= and PARMTOL= option values

are used together to test for convergence of the estimation process If during an

iteration the relative change of the parameter estimates is less than the PARMTOL=

value and the relative change of the determinant of the innovation variance matrix

is less than the DETTOL= value then iteration ceases and the current estimates are

accepted The default is DETTOL=1E-5

ITPRINT

prints the iterations during the estimation process

KLAG= n

sets an upper limit for the number of lags of the sample autocovariance matrix used

in computing the approximate likelihood function If the data have a strong moving

average character a larger KLAG= value may be necessary to obtain good estimates

The default is KLAG=15 See Parameter Estimation later in this chapter for details

MAXIT= n

sets an upper limit to the number of iterations in the maximum likelihood or condi-

tional least-squares estimation The default is MAXIT=50

NOEST

suppresses the final maximum likelihood estimation of the selected model

OUTMODEL= SAS-data-set

writes the parameter estimates and their standard errors to a SAS data set See OUT-

MODEL= Data Set later in this chapter for details

PARMTOL= value

specifies the convergence criterion The DETTOL= and PARMTOL= option values

are used together to test for convergence of the estimation process If during an

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Chapter 18 Syntax

iteration the relative change of the parameter estimates is less than the PARMTOL=

value and the relative change of the determinant of the innovation variance matrix

is less than the DETTOL= value then iteration ceases and the current estimates are

accepted The default is PARMTOL=001

RESIDEST

computes the final estimates using conditional least squares on the raw data Thistype of estimation may be more stable than the default maximum likelihood method

but is usually more computationally expensive See Parameter Estimation later in

this chapter for details of the conditional least squares method

SINGULAR= value

specifies the criterion for testing for singularity of a matrix A matrix is declared

singular if a scaled pivot is less than the SINGULAR= value when sweeping the

matrix The default is SINGULAR=1E-7

Forecasting Options

BACK= n

starts forecasting n periods before the end of the input data The BACK= option valuemust not be greater than the number of observations The default is BACK=0

INTERVAL= interval

specifies the time interval between observations The INTERVAL= value is used

in conjunction with the ID variable to check that the input data are in order and

have no missing periods The INTERVAL= option is also used to extrapolate the ID

values past the end of the input data See Chapter 3 ldquoDate Intervals Formats and

Functionsrdquo for details on the INTERVAL= values allowed

INTPER= n

specifies that each input observation corresponds to n time periods For example

the options INTERVAL=MONTH and INTPER=2 specify bimonthly data and are

equivalent to specifying INTERVAL=MONTH2 If the INTERVAL= option is not

specified the INTPER= option controls the increment used to generate ID values for

the forecast observations The default is INTPER=1

LEAD= n

specifies how many forecast observations are produced The forecasts start at the

point set by the BACK= option The default is LEAD=0 which produces no fore-

casts

OUT= SAS-data-set

writes the residuals actual values forecasts and forecast standard errors to a SAS

data set See OUT= Data Set later in this chapter for details

PRINT

prints the forecasts

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Part 2 General Information

BY Statement

BY variable

A BY statement can be used with the STATESPACE procedure to obtain separateanalyses on observations in groups defined by the BY variables

FORM Statement

FORM variable value

The FORM statement specifies the number of times a variable is included in the state

vector Values can be specified for any variable listed in the VAR statement If a

value is specified for each variable in the VAR statement the state vector for the state

space model is entirely specified and automatic selection of the state space model is

not performed

The FORM statement forces the state vectorz

t

to contain a specific variable a given

number of times For example if Y is one of the variables inx

t

then the statement

form y 3

forces the state vector to containY

t

Y

t + 1 j t

andY

t + 2 j t

possibly along with other

variables

The following statements illustrate the use of the FORM statement

proc statespace data=in

var x y

f o r m x 3 y 2

run

These statements fit a state space model with the following state vector

z

t

=

2

6

6

6

6

4

x

t j t

y

t j t

x

t + 1 j t

y

t + 1 j t

x

t + 2 j t

3

7

7

7

7

5

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Chapter 18 Syntax

ID Statement

ID variable

The ID statement specifies a variable that identifies observations in the input data setThe variable specified in the ID statement is included in the OUT= data set The

values of the ID variable are extrapolated for the forecast observations based on the

values of the INTERVAL= and INTPER= options

INITIAL Statement

INITIAL F (rowcolumn)= value G(row column)= value

The INITIAL statement gives initial values to the specified elements of theF

andG

matrices These initial values are used as starting values for the iterative estimation

Parts of theF

andG

matrices represent fixed structural identities If an element

specified is a fixed structural element instead of a free parameter the corresponding

initialization is ignored

The following is an example of an INITIAL statement

initial f(32)=0 g(41)=0 g(51)=0

RESTRICT Statement

RESTRICT F(rowcolumn)= value G(rowcolumn)= value

The RESTRICT statement restricts the specified elements of theF

andG

matrices

to the specified values

To use the restrict statement you need to know the form of the model Either specify

the form of the model with the FORM statement or do a preliminary run perhaps

with the NOEST option to find the form of the model that PROC STATESPACE

selects for the data

The following is an example of a RESTRICT statement

restrict f(32)=0 g(41)=0 g(51)=0

Parts of theF

andG

matrices represent fixed structural identities If a restriction is

specified for an element that is a fixed structural element instead of a free parameter

the restriction is ignored

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Part 2 General Information

VAR Statement

VAR variable (difference difference )

The VAR statement specifies the variables in the input data set to model and fore-cast The VAR statement also specifies differencing of the input variables The VAR

statement is required

Differencing is specified by following the variable name with a list of difference

periods separated by commas See the section Stationarity and Differencing for

more information on differencing of input variables

The order in which variables are listed in the VAR statement controls the order in

which variables are included in the state vector Usually potential inputs should be

listed before potential outputs

For example assuming the input data are monthly the following VAR statement

specifies modeling and forecasting of the one period and seasonal second differenceof X and Y

var x(112) y(112)

In this example the vector time series analyzed is

x

t

=

1 B 1 B

1 2

X

t

x

1 B 1 B

1 2

Y

t

y

where B represents the back shift operator and x and y represent the means of thedifferenced series If the NOCENTER option is specified the mean differences are

not subtracted

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Chapter 18 Details

Details

Missing Values

The STATESPACE procedure does not support missing values The procedure uses

the first contiguous group of observations with no missing values for any of the VARstatement variables Observations at the beginning of the data set with missing values

for any VAR statement variable are not used or included in the output data set

Stationarity and Differencing

The state space model used by the STATESPACE procedure assumes that the time

series are stationary Hence the data should be checked for stationarity One way

to check for stationarity is to plot the series A graph of series over time can show a

time trend or variability changes

You can also check stationarity by using the sample autocorrelation functions dis-

played by the ARIMA procedure The autocorrelation functions of nonstationaryseries tend to decay slowly See Chapter 7 ldquoThe ARIMA Procedurerdquo for more in-

formation

Another alternative is to use the STATIONARITY= option on the IDENTIFY state-

ment in PROC ARIMA to apply Dickey-Fuller tests for unit roots in the time series

See Chapter 7 ldquoThe ARIMA Procedurerdquo for more information on Dickey-Fuller unit

root tests

The most popular way to transform a nonstationary series to stationarity is by dif-

ferencing Differencing of the time series is specified in the VAR statement For

example to take a simple first difference of the series X use this statement

var x(1)

In this example the change in X from one period to the next is analyzed When the se-

ries has a seasonal pattern differencing at a period equal to the length of the seasonal

cycle may be desirable For example suppose the variable X is measured quarterly

and shows a seasonal cycle over the year You can use the following statement to

analyze the series of changes from the same quarter in the previous year

var x(4)

To difference twice add another differencing period to the list For ex-

ample the following statement analyzes the series of second differences

X

t

X

t 1

X

t 1

X

t 2

= X

t

2 X

t 1

+ X

t 2

var x(11)

The following statement analyzes the seasonal second difference series

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Part 2 General Information

var x(14)

The series modeled is the 1-period difference of the 4-period difference

X

t

X

t 4

X

t 1

X

t 5

= X

t

X

t 1

X

t 4

+ X

t 5

Another way to obtain stationary series is to use a regression on time to de-trend the

data If the time series has a deterministic linear trend regressing the series on time

produces residuals that should be stationary The following statements write residuals

of X and Y to the variable RX and RY in the output data set DETREND

data a

set a

t=_n_

run

proc reg data=a

model x y = t

output out=detrend r=rx ry

run

You then use PROC STATESPACE to forecast the de-trended series RX and RY A

disadvantage of this method is that you need to add the trend back to the forecast

series in an additional step A more serious disadvantage of the de-trending method

is that it assumes a deterministic trend In practice most time series appear to have a

stochastic rather than a deterministic trend Differencing is a more flexible and often

more appropriate method

There are several other methods to handle nonstationary time series For more infor-

mation and examples refer to Brockwell and Davis (1991)

Preliminary Autoregressive Models

After computing the sample autocovariance matrices PROC STATESPACE fits a

sequence of vector autoregressive models These preliminary autoregressive models

are used to estimate the autoregressive order of the process and limit the order of the

autocovariances considered in the state vector selection process

Yule-Walker Equations for Forward and Backward Models

Unlike a univariate autoregressive model a multivariate autoregressive model has

different forms depending on whether the present observation is being predicted from

the past observations or from the future observations

Letx

t

be the r -component stationary time series given by the VAR statement after

differencing and subtracting the vector of sample means (If the NOCENTER option

is specified the mean is not subtracted) Let n be the number of observations of x

t

from the input data set

Lete

t

be a vector white noise sequence with mean vector 0 and variance matrix

p

and letn

t

be a vector white noise sequence with mean vector 0 and variance matrix

p

Let p be the order of the vector autoregressive model forx

t

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Chapter 18 Details

The forward autoregressive form based on the past observations is written as follows

x

t

=

p

X

i = 1

p

i

x

t i

+ e

t

The backward autoregressive form based on the future observations is written as fol-

lows

x

t

=

p

X

i = 1

p

i

x

t + i

+ n

t

LettingE

denote the expected value operator the autocovariance sequence for thex

t

series

i

is

i

= E x

t

x

0

t i

The Yule-Walker equations for the autoregressive model that matches the first p ele-

ments of the autocovariance sequence are

2

6

6

4

0

1

p 1

0

1

0

p 2

0

p 1

0

p 2

0

3

7

7

5

2

6

6

4

p

1

p

2

p

p

3

7

7

5

=

2

6

6

4

1

2

p

3

7

7

5

and

2

6

6

6

4

0

0

1

0

p 1

1

0

0

p 2

p 1

p 2

0

3

7

7

7

5

2

6

6

4

p

1

p

2

p

p

3

7

7

5

=

2

6

6

4

0

1

0

2

0

p

3

7

7

5

Here

p

i

are the coefficient matrices for the past observation form of the vector au-toregressive model and

p

i

are the coefficient matrices for the future observation

form More information on the Yule-Walker equations in the multivariate setting can

be found in Whittle (1963) and Ansley and Newbold (1979)

The innovation variance matrices for the two forms can be written as follows

p

=

0

p

X

i = 1

p

i

0

i

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Part 2 General Information

p

=

0

p

X

i = 1

p

i

i

The autoregressive models are fit to the data using the preceding Yule-Walker equa-

tions with

i

replaced by the sample covariance sequenceC

i

The covariance matri-

ces are calculated as

C

i

=

1

N 1

N

X

t = i + 1

x

t

x

0

t i

Let b

p

b

p

b

p

and b

p

represent the Yule-Walker estimates of

p

p

p

and

p

respectively These matrices are written to an output data set when the OUTAR=

option is specified

When the PRINTOUT=LONG option is specified the sequence of matrices b

p

and

the corresponding correlation matrices are printed The sequence of matricesb

p isused to compute Akaike information criteria for selection of the autoregressive order

of the process

Akaike Information Criterion

The Akaike information criterion or AIC is defined as -2( maximum of log likeli-

hood )+2(number of parameters) Since the vector autoregressive models are esti-

mates from the Yule-Walker equations not by maximum likelihood the exact likeli-

hood values are not available for computing the AIC However for the vector autore-

gressive model the maximum of the log likelihood can be approximated as

l n L

n

2

l n j

b

p

j

Thus the AIC for the order p model is computed as

A I C

p

= n l n j

b

p

j + 2 p r

2

You can use the printed AIC array to compute a likelihood ratio test of the autoregres-

sive order The log-likelihood ratio test statistic for testing the order p model against

the order p 1

model is

n l n j

b

p

j + n l n j

b

p 1

j

This quantity is asymptotically distributed as a

2 withr

2 degrees of freedom if the

series is autoregressive of order p 1

It can be computed from the AIC array as

A I C

p 1

A I C

p

+ 2 r

2

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You can evaluate the significance of these test statistics with the PROBCHI function

in a SAS DATA step or with a

2 table

Determining the Autoregressive Order

Although the autoregressive models can be used for prediction their primary value is

to aid in the selection of a suitable portion of the sample covariance matrix for use in

computing canonical correlations If the multivariate time series x

t is of autoregres-sive order p then the vector of past values to lag p is considered to contain essentially

all the information relevant for prediction of future values of the time series

By default PROC STATESPACE selects the order p producing the autoregressive

model with the smallestA I C

p

If the value p for the minimumA I C

p

is less than the

value of the PASTMIN= option then p is set to the PASTMIN= value Alternatively

you can use the ARMAX= and PASTMIN= options to force PROC STATESPACE to

use an order you select

Significance Limits for Partial Autocorrelations

The STATESPACE procedure prints a schematic representation of the partial autocor-

relation matrices indicating which partial autocorrelations are significantly greater orsignificantly less than 0 Figure 1810 shows an example of this table

The STATESPACE Procedure

Schematic Representation of Partial Autocorrelations

NameLag 1 2 3 4 5 6 7 8 9 10

x ++ +

y ++

+ is gt 2std error - is lt -2std error is between

Figure 1810 Significant Partial Autocorrelations

The partial autocorrelations are from the sample partial autoregressive matrices b

p

p

The standard errors used for the significance limits of the partial autocorrelations are

computed from the sequence of matrices

p

and

p

Under the assumption that the observed series arises from an autoregressive process

of order p 1

the pth sample partial autoregressive matrix b

p

p

has an asymptotic

variance matrix 1

n

1

p

p

The significance limits for b

p

p

used in the schematic plot of the sample partial autore-

gressive sequence are derived by replacing

p

and

p

with their sample estimators

to produce the variance estimate as follows

d

V a r

b

p

p

=

1

n r p

b

1

p

b

p

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Canonical Correlation Analysis

Given the order p letp

t

be the vector of current and past values relevant to prediction

of x

t + 1

p

t

= x

0

t

x

0

t 1

x

0

t p

0

Letf

t

be the vector of current and future values

f

t

= x

0

t

x

0

t + 1

x

0

t + p

0

In the canonical correlation analysis consider submatrices of the sample covariance

matrix of p

t

andf

t

This covariance matrixV

has a block Hankel form

V =

2

6

6

6

4

C

0

C

0

1

C

0

2

C

0

p

C

0

1

C

0

2

C

0

3

C

0

p + 1

C

0

p

C

0

p + 1

C

0

p + 2

C

0

2 p

3

7

7

7

5

State Vector Selection Process

The canonical correlation analysis forms a sequence of potential state vectorsz

j

t

Examine a sequencef

j

t

of subvectors of f

t

and form the submatrixV

j consisting

of the rows and columns of V

corresponding to the components of f

j

t

and compute

its canonical correlations

The smallest canonical correlation of V

j is then used in the selection of the com-

ponents of the state vector The selection process is described in the following For

more details about this process refer to Akaike (1976)

In the following discussion the notationx

t + k j t

denotes the wide sense conditional

expectation (best linear predictor) of x

t + k

given allx

s

with s less than or equal to t

In the notationx

i t + 1

the first subscript denotes the ith component of x

t + 1

The initial state vectorz

1

t

is set tox

t

The sequencef

j

t

is initialized by setting

f

1

t

= z

1

0

t

x

1 t + 1 j t

0

= x

0

t

x

1 t + 1 j t

0

That is start by considering whether to addx

1 t + 1 j t

to the initial state vectorz

1

t

The procedure forms the submatrixV

1 corresponding tof

1

t

and computes its canon-

ical correlations Denote the smallest canonical correlation of V

1 as

m i n

If

m i n

is

significantly greater than 0x

1 t + 1 j t

is added to the state vector

If the smallest canonical correlation of V

1 is not significantly greater than 0 then a

linear combination of f

1

t

is uncorrelated with the pastp

t

Assuming that the determi-

nant of C

0

is not 0 (that is no input series is a constant) you can take the coefficient

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of x

1 t + 1 j t

in this linear combination to be 1 Denote the coefficients of z

1

t

in this

linear combination as

This gives the relationship

x

1 t + 1 j t

=

0

x

t

Therefore the current state vector already contains all the past information useful forpredicting

x

1 t + 1

and any greater leads of x

1 t

The variablex

1 t + 1 j t

is not added to

the state vector nor are any termsx

1 t + k j t

considered as possible components of the

state vector The variablex

1

is no longer active for state vector selection

The process described forx

1 t + 1 j t

is repeated for the remaining elements of f

t

The

next candidate for inclusion in the state vector is the next component of f

t

corre-

sponding to an active variable Components of f

t

corresponding to inactive variables

that produced a zero

m i n

in a previous step are skipped

Denote the next candidate asx

l t + k j t

The vectorf

j

t

is formed from the current state

vector andx

l t + k j t

as follows

f

j

t

= z

j

0

t

x

l t + k j t

0

The matrixV

j is formed fromf

j

t

and its canonical correlations are computed The

smallest canonical correlation of V

j is judged to be either greater than or equal to 0

If it is judged to be greater than 0x

l t + k j t

is added to the state vector If it is judged

to be 0 then a linear combination of f

j

t

is uncorrelated with thep

t

and the variable

x

l

is now inactive

The state vector selection process continues until no active variables remain

Testing Significance of Canonical Correlations For each step in the canonical correlation sequence the significance of the small-

est canonical correlation

m i n

is judged by an information criterion from Akaike

(1976) This information criterion is

n l n 1

2

m i n

r p + 1 q + 1

where q is the dimension of f

j

t

at the current step r is the order of the state vector p

is the order of the vector autoregressive process and

is the value of the SIGCORR=

option The default is SIGCORR=2 If this information criterion is less than or equal

to 0

m i n

is taken to be 0 otherwise it is taken to be significantly greater than 0

(Do not confuse this information criterion with the AIC)

Variables inx

t + p j t

are not added in the model even with positive information crite-

rion because of the singularity of V

You can force the consideration of more candi-

date state variables by increasing the size of theV

matrix by specifying a PASTMIN=

option value larger than p

Printing the Canonical Correlations

To print the details of the canonical correlation analysis process specify the CAN-

CORR option in the PROC STATESPACE statement The CANCORR option prints

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the candidate state vectors the canonical correlations and the information criteria for

testing the significance of the smallest canonical correlation

Bartlettrsquos

2 and its degrees of freedom are also printed when the CANCORR option

is specified The formula used for Bartlettrsquos

2 is

2

= n 5 r p + 1 q + 1 l n 1

2

m i n

withr p + 1 q + 1

degrees of freedom

Figure 1811 shows the output of the CANCORR option for the introductory example

shown in the Getting Started section of this chapter

The STATESPACE Procedure

Canonical Correlations Analysis

Information Chi

x(TT) y(TT) x(T+1T) Criterion Square DF

1 1 0237045 3566167 114505 4

Information Chi

x(TT) y(TT) x(T+1T) y(T+1T) Criterion Square DF

1 1 0238244 0056565 -535906 0636134 3

Information Chi

x(TT) y(TT) x(T+1T) x(T+2T) Criterion Square DF

1 1 0237602 0087493 -446312 1525353 3

Figure 1811 Canonical Correlations Analysis

New variables are added to the state vector if the information criteria are positive Inthis example

y

t + 1 j t

andx

t + 2 j t

are not added to the state space vector because the

information criteria for these models are negative

If the information criterion is nearly 0 then you may want to investigate models

that arise if the opposite decision is made regarding

m i n

This investigation can be

accomplished by using a FORM statement to specify part or all of the state vector

Preliminary Estimates of F

When a candidate variablex

l t + k j t

yields a zero

m i n

and is not added to the state

vector a linear combination of f

j

t

is uncorrelated with thep

t

Because of the method

used to construct thef

j

t

sequence the coefficient of x

l t + k j t

inl

can be taken as 1

Denote the coefficients of z

j

t

in this linear combination as l

This gives the relationship

x

l t + k j t

= l

0

z

j

t

The vectorl

is used as a preliminary estimate of the first r columns of the row of the

transition matrixF

corresponding tox

l t + k 1 j t

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Parameter Estimation

The model isz

t + 1

= F z

t

+ G e

t + 1

wheree

t

is a sequence of independent multivari-

ate normal innovations with mean vector 0 and variance

e e

The observed sequence

x

t

composes the first r components of z

t

and thusx

t

= H z

t

where H is ther s

matrix I

r

0

LetE

be ther n

matrix of innovations

E = e

1

e

n

If the number of observations n is reasonably large the log likelihood L can be

approximated up to an additive constant as follows

L =

n

2

l n j

e e

j

1

2

t r a c e

1

e e

E E

0

The elements of

e e

are taken as free parameters and are estimated as follows

S

0

=

1

n

E E

0

Replacing

e e

byS

0

in the likelihood equation the log likelihood up to an additive

constant is

L =

n

2

l n j S

0

j

Letting B be the backshift operator the formal relation betweenx

t

ande

t

is

x

t

= H I B F

1

G e

t

e

t

= H I B F

1

G

1

x

t

=

1

X

i = 0

i

x

t i

LettingC

i

be the ith lagged sample covariance of x

t

and neglecting end effects the

matrixS

0

is

S

0

=

1

X

i j = 0

i

C

i + j

0

j

For the computation of S

0

the infinite sum is truncated at the value of the KLAG=

option The value of the KLAG= option should be large enough that the sequence

i

is approximately 0 beyond that point

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Let

be the vector of free parameters in theF

andG

matrices The derivative of the

log likelihood with respect to the parameter

is

L

=

n

2

t r a c e

S

1

0

S

0

The second derivative is

2

L

0

=

n

2

t r a c e

S

1

0

S

0

0

S

1

0

S

0

t r a c e

S

1

0

2

S

0

0

Near the maximum the first term is unimportant and the second term can be approx-

imated to give the following second derivative approximation

2

L

0

=

n t r a c e

S

1

0

E

E

0

0

The first derivative matrix and this second derivative matrix approximation are com-

puted from the sample covariance matrixC

0

and the truncated sequence

i

The

approximate likelihood function is maximized by a modified Newton-Raphson algo-

rithm employing these derivative matrices

The matrixS

0

is used as the estimate of the innovation covariance matrix

e e

The

negative of the inverse of the second derivative matrix at the maximum is used as an

approximate covariance matrix for the parameter estimates The standard errors of

the parameter estimates printed in the parameter estimates tables are taken from the

diagonal of this covariance matrix The parameter covariance matrix is printed when

the COVB option is specified

If the data are nearly nonstationary a better estimate of

e e

and the other parameters

can sometimes be obtained by specifying the RESIDEST option The RESIDEST

option estimates the parameters using conditional least squares instead of maximum

likelihood

The residuals are computed using the state space equation and the sample mean val-

ues of the variables in the model as start-up values The estimate of S

0

is then com-

puted using the residuals from the ith observation on where i is the maximum number

of times any variable occurs in the state vector A multivariate Gauss-Marquardt al-

gorithm is used to minimizej S

0

j

Refer to Harvey (1981a) for a further description

of this method

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Forecasting

Given estimates of F

G

and

e e

forecasts of x

t

are computed from the conditional

expectation of z

t

In forecasting the parameters F G and

e e

are replaced with the estimates or by val-

ues specified in the RESTRICT statement One-step-ahead forecasting is performedfor the observation

x

t

wheret n b

Heren

is the number of observations and

b is the value of the BACK= option For the observationx

t

wheret n b

m-

step-ahead forecasting is performed form = t n + b

The forecasts are generated

recursively with the initial conditionz

0

= 0

The m-step-ahead forecast of z

t + m

isz

t + m j t

wherez

t + m j t

denotes the conditional

expectation of z

t + m

given the information available at time t The m-step-ahead

forecast of x

t + m

isx

t + m j t

= H z

t + m j t

where the matrixH = I

r

0

Let

i

= F

i

G

Note that the lasts r

elements of z

t

consist of the elements of x

u j t

foru t

The state vector z

t + m

can be represented as

z

t + m

= F

m

z

t

+

m 1

X

i = 0

i

e

t + m i

Sincee

t + i j t

= 0

fori 0

the m-step-ahead forecastz

t + m j t

is

z

t + m j t

= F

m

z

t

= F z

t + m 1 j t

Therefore the m-step-ahead forecast of x

t + m

is

x

t + m j t

= H z

t + m j t

The m-step-ahead forecast error is

z

t + m

z

t + m j t

=

m 1

X

i = 0

i

e

t + m i

The variance of the m-step-ahead forecast error is

V

z m

=

m 1

X

i = 0

i

e e

0

i

LettingV

z 0

= 0

the variance of the m-step-ahead forecast error of z

t + m

V

z m

can

be computed recursively as follows

V

z m

= V

z m 1

+

m 1

e e

0

m 1

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The variance of the m-step-ahead forecast error of x

t + m

is ther r

left upper sub-

matrix of V

z m

that is

V

x m

= H V

z m

H

0

Unless the NOCENTER option is specified the sample mean vector is added to the

forecast When differencing is specified the forecasts xt + m j t

plus the sample mean

vector are integrated back to produce forecasts for the original series

Lety

t

be the original series specified by the VAR statement with some 0 values

appended corresponding to the unobserved past observations Let B be the backshift

operator and let B

be thes s

matrix polynomial in the backshift operator

corresponding to the differencing specified by the VAR statement The off-diagonal

elements of

i

are 0 Note that

0

= I

s

whereI

s

is thes s

identity matrix Then

z

t

= B y

t

This gives the relationship

y

t

=

1

B z

t

=

1

X

i = 0

i

z

t i

where

1

B =

P

1

i = 0

i

B

i and

0

= I

s

The m-step-ahead forecast of y

t + m

is

y

t + m j t

=

m 1

X

i = 0

i

z

t + m i j t

+

1

X

i = m

i

z

t + m i

The m-step-ahead forecast error of y

t + m

is

m 1

X

i = 0

i

z

t + m i

z

t + m i j t

=

m 1

X

i = 0

i

X

u = 0

u

i u

e

t + m i

LettingV

y 0

= 0

the variance of the m-step-ahead forecast error of y

t + m

V

y m

is

V

y m

=

m 1

X

i = 0

i

X

u = 0

u

i u

e e

i

X

u = 0

u

i u

0

= V

y m 1

+

m 1

X

u = 0

u

m 1 u

e e

m 1

X

u = 0

u

m 1 u

0

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Relation of ARMA and State Space Forms

Every state space model has an ARMA representation and conversely every ARMA

model has a state space representation This section discusses this equivalence The

following material is adapted from Akaike (1974) where there is a more complete

discussion Pham-Dinh-Tuan (1978) also contains a discussion of this material

Suppose you are given the following ARMA model

B x

t

= B e

t

or in more detail

x

t

1

x

t 1

p

x

t p

= e

t

+

1

e

t 1

+ +

q

e

t q

(1)

wheree

t

is a sequence of independent multivariate normal random vectors with mean

0 and variance matrix

e e

B is the backshift operator (B x

t

= x

t 1

) B

and

B

are matrix polynomials in B andx

t

is the observed process

If the roots of the determinantial equationj B j = 0

are outside the unit circle in

the complex plane the model can also be written as

x

t

=

1

B B e

t

=

1

X

i = 0

i

e

t i

The

i

matrices are known as the impulse response matrices and can be computed

as

1

B B

You can assume p q

since if this is not initially true you can add more terms

i

that are identically 0 without changing the model

To write this set of equations in a state space form proceed as follows Letx

t + i j t

be

the conditional expectation of x

t + i

givenx

w

forw t

The following relations hold

x

t + i j t

=

1

X

j = i

j

e

t + i j

x

t + i j t + 1

= x

t + i j t

+

i 1

e

t + 1

However from equation (1) you can derive the following relationship

x

t + p j t

=

1

x

t + p 1 j t

+ +

p

x

t

(2)

Hence wheni = p

you can substitute forx

t + p j t

in the right-hand side of equation

(2) and close the system of equations

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This substitution results in the following model in the state space form

z

t + 1

= F z

t

+ G e

t + 1

2

6

6

6

4

x

t + 1

x

t + 2 j t + 1

x

t + p j t + 1

3

7

7

7

5

=

2

6

6

4

0 I 0 0

0 0 I 0

p

p 1

1

3

7

7

5

2

6

6

6

4

x

t

x

t + 1 j t

x

t + p 1 j t

3

7

7

7

5

+

2

6

6

4

I

1

p 1

3

7

7

5

e

t + 1

Note that the state vectorz

t

is composed of conditional expectations of x

t

and the

first r components of z

t

are equal tox

t

The state space form can be cast into an ARMA form by solving the system of dif-

ference equations for the first r components

When converting from an ARMA form to a state space form you can generate a

state vector larger than needed that is the state space model may not be a minimal

representation When going from a state space form to an ARMA form you can have

nontrivial common factors in the autoregressive and moving average operators that

yield an ARMA model larger than necessary

If the state space form used is not a minimal representation some but not all compo-

nents of x

t + i j t

may be linearly dependent This situation corresponds to

p

p 1

being of less than full rank when B

and B

have no common nontrivial

left factors In this casez

t

consists of a subset of the possible components of

x

t + i j t

i = 1 2 p 1

However once a component of x

t + i j t

(for example the

jth one) is linearly dependent on the previous conditional expectations then all sub-

sequent jth components of x

t + k j t

fork i

must also be linearly dependent Note

that in this case equivalent but seemingly different structures can arise if the order of

the components withinx

t

is changed

OUT= Data Set

The forecasts are contained in the output data set specified by the OUT= option on the

PROC STATESPACE statement The OUT= data set contains the following variables

the BY variables

the ID variable

the VAR statement variables These variables contain the actual values from

the input data set

FORi numeric variables containing the forecasts The variable FORi contains

the forecasts for the ith variable in the VAR statement list Forecasts are one-

step-ahead predictions until the end of the data or until the observation specified

by the BACK= option

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RESi numeric variables containing the residual for the forecast of the ith vari-

able in the VAR statement list For forecast observations the actual values are

missing and the RESi variables contain missing values

STDi numeric variables containing the standard deviation for the forecast of

the ith variable in the VAR statement list The values of the STDi variables can

be used to construct univariate confidence limits for the corresponding fore-casts However such confidence limits do not take into account the covariance

of the forecasts

OUTAR= Data Set

The OUTAR= data set contains the estimates of the preliminary autoregressive mod-

els The OUTAR= data set contains the following variables

ORDER a numeric variable containing the order p of the autoregressive model

that the observation represents

AIC a numeric variable containing the value of the information criterionA I C

p

SIGFl numeric variables containing the estimate of the innovation covariance

matrices for the forward autoregressive models The variable SIGFl contains

the lth column of b

p

in the observations with ORDER= p

SIGBl numeric variables containing the estimate of the innovation covariance

matrices for the backward autoregressive models The variable SIGBl contains

the lth column of b

p

in the observations with ORDER= p

FORk ndashl numeric variables containing the estimates of the autoregressive pa-

rameter matrices for the forward models The variable FORk ndashl contains the

lth column of the lag k autoregressive parameter matrix b

p

k

in the observations

with ORDER= p

BACk ndashl numeric variables containing the estimates of the autoregressive pa-

rameter matrices for the backward models The variable BACk ndashl contains the

lth column of the lag k autoregressive parameter matrix b

p

k

in the observations

with ORDER= p

The estimates for the order p autoregressive model can be selected as those observa-

tions with ORDER= p Within these observations the k lth element of

p

i

is given by

the value of the FORindashl variable in the k th observation The k lth element of

p

i

is

given by the value of BACindashl variable in the k th observation The k lth element of

p

is given by SIGFl in the k th observation The k lth element of

p

is given by SIGBlin the k th observation

Table 181 shows an example of the OUTAR= data set with ARMAX=3 andx

t

of

dimension 2 In Table 181 i j

indicate the ijth element of the matrix

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Table 181 Values in the OUTAR= Data Set

Obs ORDER AIC SIGF1 SIGF2 SIGB1 SIGB2 FOR1ndash 1 FOR1ndash 2 FOR2ndash 1 FOR2ndash 2 FOR3ndash 1

1 0 AIC0

0 1 1

0 1 2

0 1 1

0 1 2

2 0 AIC0

0 2 1

0 2 2

0 2 1

0 2 2

3 1 AIC1

1 1 1

1 1 2

1 1 1

1 1 2

1

1

1 1

1

1

1 2

4 1 AIC1

1 2 1

1 1 2

1 2 1

1 1 2

1

1

2 1

1

1

2 2

5 2 AIC2

2 1 1

2 1 2

2 1 1

2 1 2

2

1

1 1

2

1

1 2

2

2

1 1

2

2

1 2

6 2 AIC2

2 2 1

2 1 2

2 2 1

2 1 2

2

1

2 1

2

1

2 2

2

2

2 1

2

2

2 2

7 3 AIC3

3 1 1

3 1 2

3 1 1

3 1 2

3

1

1 1

3

1

1 2

3

2

1 1

3

2

1 2

3

3

1 1

8 3 AIC3

3 2 1

3 1 2

3 2 1

3 1 2

3

1

2 1

3

1

2 2

3

2

2 1

3

2

2 2

3

3

2 1

Obs FOR3ndash 2 BACK1ndash 1 BACK1ndash 2 BACK2ndash 1 BACK2ndash 2 BACK3ndash 1 BACK3ndash 2

1

2

3

1

1

1 1

1

1

1 2

4

1

1

2 1

1

1

2 2

5

2

1

1 1

2

1

1 2

2

2

1 1

2

2

1 2

6

2

1

2 1

2

1

2 2

2

2

2 1

2

2

2 2

7

3

3

1 2

3

1

1 1

3

1

1 2

3

2

1 1

3

2

1 2

3

3

1 1

3

3

1 2

8

3

3

2 2

3

1

2 1

3

1

2 2

3

2

2 1

3

2

2 2

3

3

2 1

3

3

2 2

The estimated autoregressive parameters can be used in the IML procedure to obtain

autoregressive estimates of the spectral density function or forecasts based on the

autoregressive models

OUTMODEL= Data Set

The OUTMODEL= data set contains the estimates of theF

andG

matrices and their

standard errors the names of the components of the state vector and the estimates of

the innovation covariance matrix The variables contained in the OUTMODEL= data

set are as follows

the BY variables

STATEVEC a character variable containing the name of the component of the

state vector corresponding to the observation The STATEVEC variable has

the value STD for standard deviations observations which contain the standarderrors for the estimates given in the preceding observation

Fndash j numeric variables containing the columns of theF

matrix The variable

Fndash j contains the jth column of F

The number of Fndash j variables is equal to

the value of the DIMMAX= option If the model is of smaller dimension the

extraneous variables are set to missing

Gndash j numeric variables containing the columns of theG

matrix The variable

Gndash j contains the jth column of G

The number of Gndash j variables is equal to r

the dimension of x

t

given by the number of variables in the VAR statement

SIGndash j numeric variables containing the columns of the innovation covariance

matrix The variable SIGndash j contains the jth column of

e e

There are r vari-ables SIGndash j

Table 182 shows an example of the OUTMODEL= data set withx

t

= x

t

y

t

0

z

t

= x

t

y

t

x

t + 1 j t

0 and DIMMAX=4 In Table 182F

i j

andG

i j

are the ijth

elements of F

andG

respectively Note that all elements for Fndash4 are missing because

F

is a3 3

matrix

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Chapter 18 Details

Table 182 Value in the OUTMODEL= Data Set

Obs STATEVEC Fndash 1 Fndash 2 Fndash 3 Fndash 4 Gndash 1 Gndash 2 SIGndash 1 SIGndash 2

1 X(TT) 0 0 1 1 0

1 1

1 2

2 STD

3 Y(TT) F2 1

F2 2

F2 3

0 1

2 1

2 2

4 STD std F2 1

std F2 2

std F2 3

5 X(T+1T) F3 1

F3 2

F3 3

G3 1

G3 2

6 STD std F3 1

std F3 2

std F3 3

std G3 1

std G3 2

Printed Output

The printed output produced by the STATESPACE procedure is described in the fol-

lowing

1 descriptive statistics which include the number of observations used the

names of the variables their means and standard deviations (Std) and the dif-

ferencing operations used

2 the Akaike information criteria for the sequence of preliminary autoregressive

models

3 if the PRINTOUT=LONG option is specified the sample autocovariance ma-

trices of the input series at various lags

4 if the PRINTOUT=LONG option is specified the sample autocorrelation ma-

trices of the input series

5 a schematic representation of the autocorrelation matrices showing the signif-

icant autocorrelations

6 if the PRINTOUT=LONG option is specified the partial autoregressive ma-

trices (These are

p

p

as described in Preliminary Autoregressive Modelsearlier in this chapter)

7 a schematic representation of the partial autocorrelation matrices showing the

significant partial autocorrelations

8 the Yule-Walker estimates of the autoregressive parameters for the autoregres-

sive model with the minimum AIC

9 if the PRINTOUT=LONG option is specified the autocovariance matrices of

the residuals of the minimum AIC model This is the sequence of estimated

innovation variance matrices for the solutions of the Yule-Walker equations

10 if the PRINTOUT=LONG option is specified the autocorrelation matrices of the residuals of the minimum AIC model

11 If the CANCORR option is specified the canonical correlations analysis for

each potential state vector considered in the state vector selection process

This includes the potential state vector the canonical correlations the informa-

tion criterion for the smallest canonical correlation Bartlettrsquos

2 statistic (ldquoChi

Squarerdquo) for the smallest canonical correlation and the degrees of freedom of

Bartlettrsquos

2

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Part 2 General Information

12 the components of the chosen state vector

13 the preliminary estimate of the transition matrixF

the input matrixG

and

the variance matrix for the innovations

e e

14 if the ITPRINT option is specified the iteration history of the likelihood max-

imization For each iteration this shows the iteration number the number of step halvings the determinant of the innovation variance matrix the damping

factor Lambda and the values of the parameters

15 the state vector printed again to aid interpretation of the following listing of F

andG

16 the final estimate of the transition matrixF

17 the final estimate of the input matrixG

18 the final estimate of the variance matrix for the innovations

e e

19 a table listing the estimates of the free parameters inF

andG

and their standarderrors and t statistics

20 if the COVB option is specified the covariance matrix of the parameter esti-

mates

21 if the COVB option is specified the correlation matrix of the parameter esti-

mates

22 if the PRINT option is specified the forecasts and their standard errors

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Chapter 18 Details

ODS Table Names

PROC STATESPACE assigns a name to each table it creates You can use these

names to reference the table when using the Output Delivery System (ODS) to select

tables and create output data sets These names are listed in the following table For

more information on ODS see Chapter 6 ldquoUsing the Output Delivery Systemrdquo

Table 183 ODS Tables Produced in PROC STATESPACE

ODS Table Name Description Option

NObs Number of observations default

Summary Simple summary statistics table default

InfoCriterion Information criterion table default

CovLags Covariance Matrices of Input Series PRINTOUT=LONG

CorrLags Correlation Matrices of Input Series PRINTOUT=LONG

PartialAR Partial Autoregressive Matrices PRINTOUT=LONG

YWEstimates Yule-Walker Estimates for Minimum AIC default

CovResiduals Covariance of Residuals PRINTOUT=LONGCorrResiduals Residual Correlations from AR Models PRINTOUT=LONG

StateVector State vector table default

CorrGraph Schematic Representation of Correlations default

TransitionMatrix Transition Matrix default

InputMatrix Input Matrix default

VarInnov Variance Matrix for the Innovation default

CovB Covariance of Parameter Estimates COVB

CorrB Correlation of Parameter Estimates COVB

CanCorr Canonical Correlation Analysis CANCORR

IterHistory Iterative Fitting table ITPRINT

ParameterEstimates Parameter Estimates Table default

Forecasts Forecasts Table PRINT

ConvergenceStatus Convergence Status Table default

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Part 2 General Information

Example

Example 181 Series J from Box and Jenkins

This example analyzes the gas furnace data (series J) from Box and Jenkins (The

data are not shown Refer to Box and Jenkins (1976) for the data)

First a model is selected and fit automatically using the following statements

title1 rsquoGas Furnace Datarsquo

title2 rsquoBox amp Jenkins Series Jrsquo

title3 rsquoAutomatically Selected Modelrsquo

proc statespace data=seriesj cancorr

var x y

run

The results for the automatically selected model are shown in Output 1811

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Chapter 18 Example

Output 1811 Results for Automatically Selected Model

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Number of Observations 296

Standard

Variable Mean Error

x -005683 1072766

y 5350912 3202121

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Information Criterion for Autoregressive Models

Lag=0 Lag=1 Lag=2 Lag=3 Lag=4 Lag=5 Lag=6 Lag=7 Lag=8

6513862 -103357 -163296 -164512 -165152 -164891 -164934 -164315 -163856

Information

Criterion for

Autoregressive

Models

Lag=9 Lag=10

-16348 -163359

Schematic Representation of Correlations

NameLag 0 1 2 3 4 5 6 7 8 9 10

x +- +- +- +- +- +- +- +- +- +- +-

y -+ -+ -+ -+ -+ -+ -+ -+ -+ -+ -+

+ is gt 2std error - is lt -2std error is between

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Part 2 General Information

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Schematic Representation of Partial Autocorrelations

NameLag 1 2 3 4 5 6 7 8 9 10

x + - + -

y -+ -- - + +

+ is gt 2std error - is lt -2std error is between

Yule-Walker Estimates for Minimum AIC

------Lag=1------ ------Lag=2------ ------Lag=3------ ------Lag=4------

x y x y x y x y

x 1925887 -000124 -120166 0004224 0116918 -000867 0104236 0003268

y 0050496 1299793 -002046 -03277 -071182 -025701 0195411 0133417

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Canonical Correlations Analysis

Information Chi

x(TT) y(TT) x(T+1T) Criterion Square DF

1 1 0804883 2929228 3047481 8

Information Chix(TT) y(TT) x(T+1T) y(T+1T) Criterion Square DF

1 1 0906681 0607529 1223358 1347237 7

Information Chi

x(TT) y(TT) x (T+1T) y (T+1T) x (T+2T) Criterion Square DF

1 1 0909434 0610278 0186274 -154701 1034705 6

Information Chi

x(TT) y(TT) x (T+1T) y (T+1T) y (T+2T) Criterion Square DF

1 1 091014 0618937 0206823 0940392 1280924 6

Information Chi

x(TT) y(TT) x(T+1T) y(T+1T) y(T+2T) y(T+3T) Criterion Square DF

1 1 0912963 0628785 0226598 0083258 -794103 2041584 5

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Chapter 18 Example

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Selected Statespace Form and Preliminary Estimates

State Vector

x(TT) y(TT) x(T+1T) y(T+1T) y(T+2T)

Estimate of Transition Matrix

0 0 1 0 0

0 0 0 1 0

-084718 0026794 1711715 -005019 0

0 0 0 0 1

-019785 0334274 -018174 -123557 1787475

Input Matrix for Innovation

1 0

0 11925887 -000124

0050496 1299793

0142421 1361696

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Selected Statespace Form and Preliminary Estimates

Variance Matrix for Innovation

0035274 -000734

-000734 0097569

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Part 2 General Information

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Selected Statespace Form and Fitted Model

State Vector

x(TT) y(TT) x(T+1T) y(T+1T) y(T+2T)

Estimate of Transition Matrix

0 0 1 0 0

0 0 0 1 0

-086192 0030609 1724235 -005483 0

0 0 0 0 1

-034839 0292124 -009435 -109823 1671418

Input Matrix for Innovation

1 0

0 1192442 -000416

0015621 1258495

008058 1353204

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Selected Statespace Form and Fitted Model

Variance Matrix for Innovation

0035579 -000728

-000728 0095577

Parameter Estimates

Standard

Parameter Estimate Error t Value

F(31) -086192 0072961 -1181

F(32) 0030609 0026167 117

F(33) 1724235 0061599 2799

F(34) -005483 0030169 -182

F(51) -034839 0135253 -258

F(52) 0292124 0046299 631

F(53) -009435 0096527 -098F(54) -109823 0109525 -1003

F(55) 1671418 0083737 1996

G(31) 1924420 0058162 3309

G(32) -000416 0035255 -012

G(41) 0015621 0095771 016

G(42) 1258495 0055742 2258

G(51) 0080580 0151622 053

G(52) 1353204 0091388 1481

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Chapter 18 Example

The two series are believed to have a transfer function relation with the gas rate

(variable X) as the input and the CO2

concentration (variable Y) as the output Since

the parameter estimates shown in Output 1811 support this kind of model the model

is reestimated with the feedback parameters restricted to 0 The following statements

fit the transfer function (no feedback) model

title3 rsquoTransfer Function Modelrsquo

proc statespace data=seriesj printout=none

var x y

restrict f(32)=0 f(34)=0

g(32)=0 g(41)=0 g(51)=0

run

The last 2 pages of the output are shown in Output 1812

Output 1812 STATESPACE Output for Transfer Function Model

Gas Furnace Data

Box amp Jenkins Series J

Transfer Function Model

The STATESPACE Procedure

Selected Statespace Form and Fitted Model

State Vector

x(TT) y(TT) x(T+1T) y(T+1T) y(T+2T)

Estimate of Transition Matrix

0 0 1 0 0

0 0 0 1 0

-068882 0 1598717 0 0

0 0 0 0 1

-035944 0284179 -00963 -107313 1650047

Input Matrix for Innovation

1 0

0 1

1923446 0

0 1260856

0 1346332

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Part 2 General Information

Gas Furnace Data

Box amp Jenkins Series J

Transfer Function Model

The STATESPACE Procedure

Selected Statespace Form and Fitted Model

Variance Matrix for Innovation

0036995 -00072

-00072 0095712

Parameter Estimates

Standard

Parameter Estimate Error t Value

F(31) -068882 0050549 -1363

F(33) 1598717 0050924 3139

F(51) -035944 0229044 -157

F(52) 0284179 0096944 293

F(53) -009630 0140876 -068

F(54) -107313 0250385 -429

F(55) 1650047 0188533 875G(31) 1923446 0056328 3415

G(42) 1260856 0056464 2233

G(52) 1346332 0091086 1478

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Chapter 18 References

References

Akaike H (1974) Markovian Representation of Stochastic Processes and Its Ap-

plication to the Analysis of Autoregressive Moving Average Processes Annals

of the Institute of Statistical Mathematics 26 363-387

Akaike H (1976) Canonical Correlations Analysis of Time Series and the Use of

an Information Criterion in Advances and Case Studies in System Identification

eds R Mehra and DG Lainiotis New York Academic Press

Anderson TW (1971) The Statistical Analysis of Time Series New York John

Wiley amp Sons

Ansley CF and Newbold P (1979) Multivariate Partial Autocorrelations Pro-

ceedings of the Business and Economic Statistics Section American Statistical

Association 349-353

Box GEP and Jenkins G (1976) Time Series Analysis Forecasting and Control

San Francisco Holden-Day

Brockwell PJ and Davis RA (1991) Time Series Theory and Methods 2nd Edi-

tion Springer-Verlag

Hannan EJ (1970) Multiple Time Series New York John Wiley amp Sons

Hannan EJ (1976) The Identification and Parameterization of ARMAX and State

Space Forms Econometrica 44 713-722

Harvey AC (1981a) The Econometric Analysis of Time Series New York John

Wiley amp Sons

Harvey AC (1981b) Time Series Models New York John Wiley amp Sons

Jones RH (1974) Identification and Autoregressive Spectrum Estimation IEEE Transactions on Automatic Control AC-19 894-897

Pham-Dinh-Tuan (1978) On the Fitting of Multivariate Processes of the

Autoregressive-Moving Average Type Biometrika 65 99-107

Priestley MB (1980) System Identification Kalman Filtering and Stochastic Con-

trol in Directions in Time Series eds DR Brillinger and GC Tiao Institute

of Mathematical Statistics

Whittle P (1963) On the Fitting of Multivariate Autoregressions and the Approxi-

mate Canonical Factorization of a Spectral Density Matrix Biometrika 50 129-

134

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The correct bibliographic citation for this manual is as follows SAS Institute Inc SAS

ET S U serrsquos Gui de Version 8 Cary NC SAS Institute Inc 1999 1546 pp

SASET S Userrsquos Guide Version 8

Copyright copy 1999 by S AS I ns t it u t e I nc Ca ry N C U S A

ISBN 1ndash58025ndash489ndash6

All r ight s res erved P r int ed in t he U n it ed St a t es of A merica N o pa rt of t his p u blica t ionma y b e reprodu ced s t ored in a ret r ieva l s ys t em or t ra ns mit t ed in a ny form or b y a nymeans electronic mechanical photocopying or otherwise without the prior writtenpermission of the publisher SAS Institute Inc

US Government Restricted Rights Notice Use du plicat ion or disclosure of the

software by the government is subject to restrictions as set forth in FAR 52227ndash19Commercial Computer Softw ar e-Restricted Rights (J une 1987)

SAS Institute Inc SAS Campus Drive Cary North Carolina 27513

1st print ing October 1999

S AS reg a nd a ll ot her SA S I ns t it u t e I nc p rodu ct or s ervice na m es a re regis t ered t ra dema rkst d k f SA S I t it t I i t h U S A d t h t i reg i di t U SA

Page 9: Statespace Model

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Chapter 18 Getting Started

After the descriptive statistics PROC STATESPACE prints the Akaike information

criterion (AIC) values for the autoregressive models fit to the series The smallest AIC

value in this case 5517 at lag 2 determines the number of autocovariance matrices

analyzed in the canonical correlation phase

A schematic representation of the autocorrelations is printed next This indicates

which elements of the autocorrelation matrices at different lags are significantlygreater or less than 0

The second page of the STATESPACE printed output is shown in Figure 183

The STATESPACE Procedure

Schematic Representation of Partial Autocorrelations

NameLag 1 2 3 4 5 6 7 8 9 10

x ++ +

y ++

+ is gt 2std error - is lt -2std error is between

Yule-Walker Estimates for Minimum AIC

--------Lag=1------- --------Lag=2-------

x y x y

x 0257438 0202237 0170812 0133554

y 0292177 0469297 -000537 -000048

Figure 183 Partial Autocorrelations and VAR Model

Figure 183 shows a schematic representation of the partial autocorrelations similar

to the autocorrelations shown in Figure 182 The selection of a second order autore-

gressive model by the AIC statistic looks reasonable in this case because the partialautocorrelations for lags greater than 2 are not significant

Next the Yule-Walker estimates for the selected autoregressive model are printed

This output shows the coefficient matrices of the vector autoregressive model at each

lag

Selected State Space Model Form and Preliminary Estimates

After the autoregressive order selection process has determined the number of lags to

consider the canonical correlation analysis phase selects the state vector By default

output for this process is not printed You can use the CANCORR option to print

details of the canonical correlation analysis See the section Canonical Correlation

Analysis later in this chapter for an explanation of this process

Once the state vector is selected the state space model is estimated by approximate

maximum likelihood Information from the canonical correlation analysis and from

the preliminary autoregression is used to form preliminary estimates of the state space

model parameters These preliminary estimates are used as starting values for the

iterative estimation process

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Part 2 General Information

The form of the state vector and the preliminary estimates are printed next as shown

in Figure 184

The STATESPACE Procedure

Selected Statespace Form and Preliminary Estimates

State Vector

x(TT) y(TT) x(T+1T)

Estimate of Transition Matrix

0 0 1

0291536 0468762 -000411

024869 024484 0204257

Input Matrix for Innovation

1 0

0 1

0257438 0202237

Variance Matrix for Innovation

0945196 0100786

0100786 1014703

Figure 184 Preliminary Estimates of State Space Model

Figure 184 first prints the state vector as X[TT] Y[TT] X[T+1T] This notation

indicates that the state vector is

z

t

=

2

4

x

t j t

y

t j t

x

t + 1 j t

3

5

The notationx

t + 1 j t

indicates the conditional expectation or prediction of x

t + 1

based

on the information available at time t andx

t j t

andy

t j t

arex

t

andy

t

respectively

The remainder of Figure 184 shows the preliminary estimates of the transition matrix

F

the input matrixG

and the covariance matrix

e e

Estimated State Space Model

The next page of the STATESPACE output prints the final estimates of the fittedmodel as shown in Figure 185 This output has the same form as in Figure 184 but

shows the maximum likelihood estimates instead of the preliminary estimates

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Chapter 18 Getting Started

The STATESPACE Procedure

Selected Statespace Form and Fitted Model

State Vector

x(TT) y(TT) x(T+1T)

Estimate of Transition Matrix

0 0 1

0297273 047376 -001998

02301 0228425 0256031

Input Matrix for Innovation

1 0

0 1

0257284 0202273

Variance Matrix for Innovation

0945188 01007520100752 1014712

Figure 185 Fitted State Space Model

The estimated state space model shown in Figure 185 is

2

4

x

t + 1 j t + 1

y

t + 1 j t + 1

x

t + 2 j t + 1

3

5

=

2

4

0 0 1

0 2 9 7 0 4 7 4 0 0 2 0

0 2 3 0 0 2 2 8 0 2 5 6

3

5

2

4

x

t

y

t

x

t + 1 j t

3

5

+

2

4

1 0

0 1

0 2 5 7 0 2 0 2

3

5

e

t + 1

n

t + 1

v a r

e

t + 1

n

t + 1

=

0 9 4 5 0 1 0 1

0 1 0 1 1 0 1 5

The next page of the STATESPACE output lists the estimates of the free parameters

in theF

andG

matrices with standard errors and t statistics as shown in Figure 186

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Part 2 General Information

The STATESPACE Procedure

Parameter Estimates

Standard

Parameter Estimate Error t Value

F(21) 0297273 0129995 229

F(22) 0473760 0115688 410F(23) -001998 0313025 -006

F(31) 0230100 0126226 182

F(32) 0228425 0112978 202

F(33) 0256031 0305256 084

G(31) 0257284 0071060 362

G(32) 0202273 0068593 295

Figure 186 Final Parameter Estimates

Convergence Failures

The maximum likelihood estimates are computed by an iterative nonlinear maximiza-

tion algorithm which may not converge If the estimates fail to converge warning

messages are printed in the output

If you encounter convergence problems you should recheck the stationarity of the

data and ensure that the specified differencing orders are correct Attempting to fit

state space models to nonstationary data is a common cause of convergence failure

You can also use the MAXIT= option to increase the number of iterations allowed

or experiment with the convergence tolerance options DETTOL= and PARMTOL=

Forecast Data Set

The following statements print the output data set The WHERE statement excludes

the first 190 observations from the output so that only the forecasts and the last 10

actual observations are printed

proc print data=outid t

where t gt 190

run

The PROC PRINT output is shown in Figure 187

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Part 2 General Information

Figure 188 Plot of Forecasts

Controlling Printed Output

By default the STATESPACE procedure produces a large amount of printed output

The NOPRINT option suppresses all printed output You can suppress the printed

output for the autoregressive model selection process with the PRINTOUT=NONE

option The descriptive statistics and state space model estimation output are still

printed when PRINTOUT=NONE is specified You can produce more detailed output

with the PRINTOUT=LONG option and by specifying the printing control optionsCANCORR COVB and PRINT

Specifying the State Space Model

Instead of allowing the STATESPACE procedure to select the model automatically

you can use FORM and RESTRICT statements to specify a state space model

Specifying the State Vector

Use the FORM statement to control the form of the state vector You can use this

feature to force PROC STATESPACE to estimate and forecast a model different from

the model it would select automatically You can also use this feature to reestimate

the automatically selected model (possibly with restrictions) without repeating thecanonical correlation analysis

The FORM statement specifies the number of lags of each variable to include in

the state vector For example the statement FORM X 3 forces the state vector to

includex

t j t

x

t + 1 j t

andx

t + 2 j t

The following statement specifies the state vector

x

t j t

y

t j t

x

t + 1 j t

which is the same state vector selected in the preceding example

f o r m x 2 y 1

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Chapter 18 Getting Started

You can specify the form for only some of the variables and allow PROC STATES-

PACE to select the form for the other variables If only some of the variables are

specified in the FORM statement canonical correlation analysis is used to determine

the number of lags included in the state vector for the remaining variables not spec-

ified by the FORM statement If the FORM statement includes specifications for all

the variables listed in the VAR statement the state vector is completely defined and

the canonical correlation analysis is not performed

Restricting the F and G matrices

After you know the form of the state vector you can use the RESTRICT statement

to fix some parameters in theF

andG

matrices to specified values One use of this

feature is to remove insignificant parameters by restricting them to 0

In the introductory example shown in the preceding section the F[23] parameter is

not significant (The parameters estimation output shown in Figure 186 gives the t

statistic for F[23] as -006 F[33] and F[31] also have low significance witht 2

)

The following statements reestimate this model with F[23] restricted to 0 The

FORM statement is used to specify the state vector and thus bypass the canonicalcorrelation analysis

proc statespace data=in out=out lead=10

var x(1) y(1)

id t

form x 2 y 1

restrict f(23)=0

run

The final estimates produced by these statements are shown in Figure 189

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Part 2 General Information

The STATESPACE Procedure

Selected Statespace Form and Fitted Model

State Vector

x(TT) y(TT) x(T+1T)

Estimate of Transition Matrix

0 0 1

0290051 0467468 0

0227051 0226139 026436

Input Matrix for Innovation

1 0

0 1

0256826 0202022

Variance Matrix for Innovation

0945175 01006960100696 1014733

The STATESPACE Procedure

Parameter Estimates

Standard

Parameter Estimate Error t Value

F(21) 0290051 0063904 454

F(22) 0467468 0060430 774

F(31) 0227051 0125221 181

F(32) 0226139 0111711 202

F(33) 0264360 0299537 088G(31) 0256826 0070994 362

G(32) 0202022 0068507 295

Figure 189 Results using RESTRICT Statement

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Chapter 18 Syntax

Syntax

The STATESPACE procedure uses the following statements

PROC STATESPACE options

BY variable FORM variable value

ID variable

INITIAL F(rowcolumn)=value

G(rowcolumn)=value

RESTRICT F(rowcolumn)=value

G(rowcolumn)=value

VAR variable (difference difference

)

Functional Summary

The statements and options used by PROC STATESPACE are summarized in the

following table

Description Statement Option

Input Data Set Options

specify the input data set PROC STATESPACE DATA=

prevent subtraction of sample mean PROC STATESPACE NOCENTER

specify the ID variable ID

specify the observed series and differencing VAR

Options for Autoregressive Estimates

specify the maximum order PROC STATESPACE ARMAX=

specify maximum lag for autocovariances PROC STATESPACE LAGMAX=

output only minimum AIC model PROC STATESPACE MINIC

specify the amount of detail printed PROC STATESPACE PRINTOUT=

write preliminary AR models to a data set PROC STATESPACE OUTAR=

Options for Canonical Correlation Analysis

print the sequence of canonical correlations PROC STATESPACE CANCORR

specify upper limit of dimension of state

vector

PROC STATESPACE DIMMAX=

specify the minimum number of lags PROC STATESPACE PASTMIN=

specify the multiplier of the degrees of

freedom

PROC STATESPACE SIGCORR=

Options for State Space Model Estimation

specify starting values INITIAL

print covariance matrix of parameter estimates PROC STATESPACE COVB

specify the convergence criterion PROC STATESPACE DETTOL=

specify the convergence criterion PROC STATESPACE PARMTOL=

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Part 2 General Information

Description Statement Option

print the details of the iterations PROC STATESPACE ITPRINT

specify an upper limit of the number of lags PROC STATESPACE KLAG=

specify maximum number of iterations

allowed

PROC STATESPACE MAXIT=

suppress the final estimation PROC STATESPACE NOEST

write the state space model parameter esti-

mates to an output data set

PROC STATESPACE OUTMODEL=

use conditional least squares for final estimates PROC STATESPACE RESIDEST

specify criterion for testing for singularity PROC STATESPACE SINGULAR=

Options for Forecasting

start forecasting before end of the input data PROC STATESPACE BACK=

specify the time interval between observations PROC STATESPACE INTERVAL=

specify multiple periods in the time series PROC STATESPACE INTPER=specify how many periods to forecast PROC STATESPACE LEAD=

specify the output data set for forecasts PROC STATESPACE OUT=

print forecasts PROC STATESPACE PRINT

Options to Specify the State Space Model

specify the state vector FORM

specify the parameter values RESTRICT

BY Groups

specify BY-group processing BY

Printing

suppresses all printed output NOPRINT

PROC STATESPACE Statement

PROC STATESPACE options

The following options can be specified in the PROC STATESPACE statement

Printing Options NOPRINT

suppresses all printed output

Input Data Options

DATA= SAS-data-set

specifies the name of the SAS data set to be used by the procedure If the DATA=

option is omitted the most recently created SAS data set is used

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Chapter 18 Syntax

LAGMAX= k

specifies the number of lags for which the sample autocovariance matrix is computed

The LAGMAX= option controls the number of lags printed in the schematic repre-

sentation of the autocorrelations

The sample autocovariance matrix of lag i denoted asC

i

is computed as

C

i

=

1

N 1

N

X

t = 1 + i

x

t

x

0

t i

wherex

t

is the differenced and centered data andN

is the number of observations

(If the NOCENTER option is specified 1 is not subtracted fromN

) LAGMAX= k

specifies thatC

0

throughC

k

are computed The default is LAGMAX=10

NOCENTER

prevents subtraction of the sample mean from the input series (after any specified

differencing) before the analysis

Options for Preliminary Autoregressive Models ARMAX= n

specifies the maximum order of the preliminary autoregressive models The AR-

MAX= option controls the autoregressive orders for which information criteria are

printed and controls the number of lags printed in the schematic representation of

partial autocorrelations The default is ARMAX=10 See Preliminary Autoregres-

sive Models later in this chapter for details

MINIC

writes to the OUTAR= data set only the preliminary Yule-Walker estimates for the

VAR model producing the minimum AIC See OUTAR= Data Set later in this chap-

ter for details

OUTAR= SAS-data-set

writes the Yule-Walker estimates of the preliminary autoregressive models to a SAS

data set See OUTAR= Data Set later in this chapter for details

PRINTOUT= SHORT | LONG | NONE

determines the amount of detail printed PRINTOUT=LONG prints the lagged co-

variance matrices the partial autoregressive matrices and estimates of the resid-

ual covariance matrices from the sequence of autoregressive models PRINT-

OUT=NONE suppresses the output for the preliminary autoregressive models The

descriptive statistics and state space model estimation output are still printed when

PRINTOUT=NONE is specified PRINTOUT=SHORT is the default

Canonical Correlation Analysis Options

CANCORR

prints the canonical correlations and information criterion for each candidate state

vector considered See Canonical Correlation Analysis later in this chapter for

details

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Part 2 General Information

DIMMAX= n

specifies the upper limit to the dimension of the state vector The DIMMAX= option

can be used to limit the size of the model selected The default is DIMMAX=10

PASTMIN= n

specifies the minimum number of lags to include in the canonical correlation analy-

sis The default is PASTMIN=0 See Canonical Correlation Analysis later in thischapter for details

SIGCORR= value

specifies the multiplier of the degrees of freedom for the penalty term in the informa-

tion criterion used to select the state space form The default is SIGCORR=2 The

larger the value of the SIGCORR= option the smaller the state vector tends to be

Hence a large value causes a simpler model to be fit See Canonical Correlations

Analysis later in this chapter for details

State Space Model Estimation Options

COVB

prints the inverse of the observed information matrix for the parameter estimatesThis matrix is an estimate of the covariance matrix for the parameter estimates

DETTOL= value

specifies the convergence criterion The DETTOL= and PARMTOL= option values

are used together to test for convergence of the estimation process If during an

iteration the relative change of the parameter estimates is less than the PARMTOL=

value and the relative change of the determinant of the innovation variance matrix

is less than the DETTOL= value then iteration ceases and the current estimates are

accepted The default is DETTOL=1E-5

ITPRINT

prints the iterations during the estimation process

KLAG= n

sets an upper limit for the number of lags of the sample autocovariance matrix used

in computing the approximate likelihood function If the data have a strong moving

average character a larger KLAG= value may be necessary to obtain good estimates

The default is KLAG=15 See Parameter Estimation later in this chapter for details

MAXIT= n

sets an upper limit to the number of iterations in the maximum likelihood or condi-

tional least-squares estimation The default is MAXIT=50

NOEST

suppresses the final maximum likelihood estimation of the selected model

OUTMODEL= SAS-data-set

writes the parameter estimates and their standard errors to a SAS data set See OUT-

MODEL= Data Set later in this chapter for details

PARMTOL= value

specifies the convergence criterion The DETTOL= and PARMTOL= option values

are used together to test for convergence of the estimation process If during an

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Chapter 18 Syntax

iteration the relative change of the parameter estimates is less than the PARMTOL=

value and the relative change of the determinant of the innovation variance matrix

is less than the DETTOL= value then iteration ceases and the current estimates are

accepted The default is PARMTOL=001

RESIDEST

computes the final estimates using conditional least squares on the raw data Thistype of estimation may be more stable than the default maximum likelihood method

but is usually more computationally expensive See Parameter Estimation later in

this chapter for details of the conditional least squares method

SINGULAR= value

specifies the criterion for testing for singularity of a matrix A matrix is declared

singular if a scaled pivot is less than the SINGULAR= value when sweeping the

matrix The default is SINGULAR=1E-7

Forecasting Options

BACK= n

starts forecasting n periods before the end of the input data The BACK= option valuemust not be greater than the number of observations The default is BACK=0

INTERVAL= interval

specifies the time interval between observations The INTERVAL= value is used

in conjunction with the ID variable to check that the input data are in order and

have no missing periods The INTERVAL= option is also used to extrapolate the ID

values past the end of the input data See Chapter 3 ldquoDate Intervals Formats and

Functionsrdquo for details on the INTERVAL= values allowed

INTPER= n

specifies that each input observation corresponds to n time periods For example

the options INTERVAL=MONTH and INTPER=2 specify bimonthly data and are

equivalent to specifying INTERVAL=MONTH2 If the INTERVAL= option is not

specified the INTPER= option controls the increment used to generate ID values for

the forecast observations The default is INTPER=1

LEAD= n

specifies how many forecast observations are produced The forecasts start at the

point set by the BACK= option The default is LEAD=0 which produces no fore-

casts

OUT= SAS-data-set

writes the residuals actual values forecasts and forecast standard errors to a SAS

data set See OUT= Data Set later in this chapter for details

PRINT

prints the forecasts

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Part 2 General Information

BY Statement

BY variable

A BY statement can be used with the STATESPACE procedure to obtain separateanalyses on observations in groups defined by the BY variables

FORM Statement

FORM variable value

The FORM statement specifies the number of times a variable is included in the state

vector Values can be specified for any variable listed in the VAR statement If a

value is specified for each variable in the VAR statement the state vector for the state

space model is entirely specified and automatic selection of the state space model is

not performed

The FORM statement forces the state vectorz

t

to contain a specific variable a given

number of times For example if Y is one of the variables inx

t

then the statement

form y 3

forces the state vector to containY

t

Y

t + 1 j t

andY

t + 2 j t

possibly along with other

variables

The following statements illustrate the use of the FORM statement

proc statespace data=in

var x y

f o r m x 3 y 2

run

These statements fit a state space model with the following state vector

z

t

=

2

6

6

6

6

4

x

t j t

y

t j t

x

t + 1 j t

y

t + 1 j t

x

t + 2 j t

3

7

7

7

7

5

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Chapter 18 Syntax

ID Statement

ID variable

The ID statement specifies a variable that identifies observations in the input data setThe variable specified in the ID statement is included in the OUT= data set The

values of the ID variable are extrapolated for the forecast observations based on the

values of the INTERVAL= and INTPER= options

INITIAL Statement

INITIAL F (rowcolumn)= value G(row column)= value

The INITIAL statement gives initial values to the specified elements of theF

andG

matrices These initial values are used as starting values for the iterative estimation

Parts of theF

andG

matrices represent fixed structural identities If an element

specified is a fixed structural element instead of a free parameter the corresponding

initialization is ignored

The following is an example of an INITIAL statement

initial f(32)=0 g(41)=0 g(51)=0

RESTRICT Statement

RESTRICT F(rowcolumn)= value G(rowcolumn)= value

The RESTRICT statement restricts the specified elements of theF

andG

matrices

to the specified values

To use the restrict statement you need to know the form of the model Either specify

the form of the model with the FORM statement or do a preliminary run perhaps

with the NOEST option to find the form of the model that PROC STATESPACE

selects for the data

The following is an example of a RESTRICT statement

restrict f(32)=0 g(41)=0 g(51)=0

Parts of theF

andG

matrices represent fixed structural identities If a restriction is

specified for an element that is a fixed structural element instead of a free parameter

the restriction is ignored

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Part 2 General Information

VAR Statement

VAR variable (difference difference )

The VAR statement specifies the variables in the input data set to model and fore-cast The VAR statement also specifies differencing of the input variables The VAR

statement is required

Differencing is specified by following the variable name with a list of difference

periods separated by commas See the section Stationarity and Differencing for

more information on differencing of input variables

The order in which variables are listed in the VAR statement controls the order in

which variables are included in the state vector Usually potential inputs should be

listed before potential outputs

For example assuming the input data are monthly the following VAR statement

specifies modeling and forecasting of the one period and seasonal second differenceof X and Y

var x(112) y(112)

In this example the vector time series analyzed is

x

t

=

1 B 1 B

1 2

X

t

x

1 B 1 B

1 2

Y

t

y

where B represents the back shift operator and x and y represent the means of thedifferenced series If the NOCENTER option is specified the mean differences are

not subtracted

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Chapter 18 Details

Details

Missing Values

The STATESPACE procedure does not support missing values The procedure uses

the first contiguous group of observations with no missing values for any of the VARstatement variables Observations at the beginning of the data set with missing values

for any VAR statement variable are not used or included in the output data set

Stationarity and Differencing

The state space model used by the STATESPACE procedure assumes that the time

series are stationary Hence the data should be checked for stationarity One way

to check for stationarity is to plot the series A graph of series over time can show a

time trend or variability changes

You can also check stationarity by using the sample autocorrelation functions dis-

played by the ARIMA procedure The autocorrelation functions of nonstationaryseries tend to decay slowly See Chapter 7 ldquoThe ARIMA Procedurerdquo for more in-

formation

Another alternative is to use the STATIONARITY= option on the IDENTIFY state-

ment in PROC ARIMA to apply Dickey-Fuller tests for unit roots in the time series

See Chapter 7 ldquoThe ARIMA Procedurerdquo for more information on Dickey-Fuller unit

root tests

The most popular way to transform a nonstationary series to stationarity is by dif-

ferencing Differencing of the time series is specified in the VAR statement For

example to take a simple first difference of the series X use this statement

var x(1)

In this example the change in X from one period to the next is analyzed When the se-

ries has a seasonal pattern differencing at a period equal to the length of the seasonal

cycle may be desirable For example suppose the variable X is measured quarterly

and shows a seasonal cycle over the year You can use the following statement to

analyze the series of changes from the same quarter in the previous year

var x(4)

To difference twice add another differencing period to the list For ex-

ample the following statement analyzes the series of second differences

X

t

X

t 1

X

t 1

X

t 2

= X

t

2 X

t 1

+ X

t 2

var x(11)

The following statement analyzes the seasonal second difference series

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Part 2 General Information

var x(14)

The series modeled is the 1-period difference of the 4-period difference

X

t

X

t 4

X

t 1

X

t 5

= X

t

X

t 1

X

t 4

+ X

t 5

Another way to obtain stationary series is to use a regression on time to de-trend the

data If the time series has a deterministic linear trend regressing the series on time

produces residuals that should be stationary The following statements write residuals

of X and Y to the variable RX and RY in the output data set DETREND

data a

set a

t=_n_

run

proc reg data=a

model x y = t

output out=detrend r=rx ry

run

You then use PROC STATESPACE to forecast the de-trended series RX and RY A

disadvantage of this method is that you need to add the trend back to the forecast

series in an additional step A more serious disadvantage of the de-trending method

is that it assumes a deterministic trend In practice most time series appear to have a

stochastic rather than a deterministic trend Differencing is a more flexible and often

more appropriate method

There are several other methods to handle nonstationary time series For more infor-

mation and examples refer to Brockwell and Davis (1991)

Preliminary Autoregressive Models

After computing the sample autocovariance matrices PROC STATESPACE fits a

sequence of vector autoregressive models These preliminary autoregressive models

are used to estimate the autoregressive order of the process and limit the order of the

autocovariances considered in the state vector selection process

Yule-Walker Equations for Forward and Backward Models

Unlike a univariate autoregressive model a multivariate autoregressive model has

different forms depending on whether the present observation is being predicted from

the past observations or from the future observations

Letx

t

be the r -component stationary time series given by the VAR statement after

differencing and subtracting the vector of sample means (If the NOCENTER option

is specified the mean is not subtracted) Let n be the number of observations of x

t

from the input data set

Lete

t

be a vector white noise sequence with mean vector 0 and variance matrix

p

and letn

t

be a vector white noise sequence with mean vector 0 and variance matrix

p

Let p be the order of the vector autoregressive model forx

t

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Chapter 18 Details

The forward autoregressive form based on the past observations is written as follows

x

t

=

p

X

i = 1

p

i

x

t i

+ e

t

The backward autoregressive form based on the future observations is written as fol-

lows

x

t

=

p

X

i = 1

p

i

x

t + i

+ n

t

LettingE

denote the expected value operator the autocovariance sequence for thex

t

series

i

is

i

= E x

t

x

0

t i

The Yule-Walker equations for the autoregressive model that matches the first p ele-

ments of the autocovariance sequence are

2

6

6

4

0

1

p 1

0

1

0

p 2

0

p 1

0

p 2

0

3

7

7

5

2

6

6

4

p

1

p

2

p

p

3

7

7

5

=

2

6

6

4

1

2

p

3

7

7

5

and

2

6

6

6

4

0

0

1

0

p 1

1

0

0

p 2

p 1

p 2

0

3

7

7

7

5

2

6

6

4

p

1

p

2

p

p

3

7

7

5

=

2

6

6

4

0

1

0

2

0

p

3

7

7

5

Here

p

i

are the coefficient matrices for the past observation form of the vector au-toregressive model and

p

i

are the coefficient matrices for the future observation

form More information on the Yule-Walker equations in the multivariate setting can

be found in Whittle (1963) and Ansley and Newbold (1979)

The innovation variance matrices for the two forms can be written as follows

p

=

0

p

X

i = 1

p

i

0

i

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Part 2 General Information

p

=

0

p

X

i = 1

p

i

i

The autoregressive models are fit to the data using the preceding Yule-Walker equa-

tions with

i

replaced by the sample covariance sequenceC

i

The covariance matri-

ces are calculated as

C

i

=

1

N 1

N

X

t = i + 1

x

t

x

0

t i

Let b

p

b

p

b

p

and b

p

represent the Yule-Walker estimates of

p

p

p

and

p

respectively These matrices are written to an output data set when the OUTAR=

option is specified

When the PRINTOUT=LONG option is specified the sequence of matrices b

p

and

the corresponding correlation matrices are printed The sequence of matricesb

p isused to compute Akaike information criteria for selection of the autoregressive order

of the process

Akaike Information Criterion

The Akaike information criterion or AIC is defined as -2( maximum of log likeli-

hood )+2(number of parameters) Since the vector autoregressive models are esti-

mates from the Yule-Walker equations not by maximum likelihood the exact likeli-

hood values are not available for computing the AIC However for the vector autore-

gressive model the maximum of the log likelihood can be approximated as

l n L

n

2

l n j

b

p

j

Thus the AIC for the order p model is computed as

A I C

p

= n l n j

b

p

j + 2 p r

2

You can use the printed AIC array to compute a likelihood ratio test of the autoregres-

sive order The log-likelihood ratio test statistic for testing the order p model against

the order p 1

model is

n l n j

b

p

j + n l n j

b

p 1

j

This quantity is asymptotically distributed as a

2 withr

2 degrees of freedom if the

series is autoregressive of order p 1

It can be computed from the AIC array as

A I C

p 1

A I C

p

+ 2 r

2

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Chapter 18 Details

You can evaluate the significance of these test statistics with the PROBCHI function

in a SAS DATA step or with a

2 table

Determining the Autoregressive Order

Although the autoregressive models can be used for prediction their primary value is

to aid in the selection of a suitable portion of the sample covariance matrix for use in

computing canonical correlations If the multivariate time series x

t is of autoregres-sive order p then the vector of past values to lag p is considered to contain essentially

all the information relevant for prediction of future values of the time series

By default PROC STATESPACE selects the order p producing the autoregressive

model with the smallestA I C

p

If the value p for the minimumA I C

p

is less than the

value of the PASTMIN= option then p is set to the PASTMIN= value Alternatively

you can use the ARMAX= and PASTMIN= options to force PROC STATESPACE to

use an order you select

Significance Limits for Partial Autocorrelations

The STATESPACE procedure prints a schematic representation of the partial autocor-

relation matrices indicating which partial autocorrelations are significantly greater orsignificantly less than 0 Figure 1810 shows an example of this table

The STATESPACE Procedure

Schematic Representation of Partial Autocorrelations

NameLag 1 2 3 4 5 6 7 8 9 10

x ++ +

y ++

+ is gt 2std error - is lt -2std error is between

Figure 1810 Significant Partial Autocorrelations

The partial autocorrelations are from the sample partial autoregressive matrices b

p

p

The standard errors used for the significance limits of the partial autocorrelations are

computed from the sequence of matrices

p

and

p

Under the assumption that the observed series arises from an autoregressive process

of order p 1

the pth sample partial autoregressive matrix b

p

p

has an asymptotic

variance matrix 1

n

1

p

p

The significance limits for b

p

p

used in the schematic plot of the sample partial autore-

gressive sequence are derived by replacing

p

and

p

with their sample estimators

to produce the variance estimate as follows

d

V a r

b

p

p

=

1

n r p

b

1

p

b

p

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Part 2 General Information

Canonical Correlation Analysis

Given the order p letp

t

be the vector of current and past values relevant to prediction

of x

t + 1

p

t

= x

0

t

x

0

t 1

x

0

t p

0

Letf

t

be the vector of current and future values

f

t

= x

0

t

x

0

t + 1

x

0

t + p

0

In the canonical correlation analysis consider submatrices of the sample covariance

matrix of p

t

andf

t

This covariance matrixV

has a block Hankel form

V =

2

6

6

6

4

C

0

C

0

1

C

0

2

C

0

p

C

0

1

C

0

2

C

0

3

C

0

p + 1

C

0

p

C

0

p + 1

C

0

p + 2

C

0

2 p

3

7

7

7

5

State Vector Selection Process

The canonical correlation analysis forms a sequence of potential state vectorsz

j

t

Examine a sequencef

j

t

of subvectors of f

t

and form the submatrixV

j consisting

of the rows and columns of V

corresponding to the components of f

j

t

and compute

its canonical correlations

The smallest canonical correlation of V

j is then used in the selection of the com-

ponents of the state vector The selection process is described in the following For

more details about this process refer to Akaike (1976)

In the following discussion the notationx

t + k j t

denotes the wide sense conditional

expectation (best linear predictor) of x

t + k

given allx

s

with s less than or equal to t

In the notationx

i t + 1

the first subscript denotes the ith component of x

t + 1

The initial state vectorz

1

t

is set tox

t

The sequencef

j

t

is initialized by setting

f

1

t

= z

1

0

t

x

1 t + 1 j t

0

= x

0

t

x

1 t + 1 j t

0

That is start by considering whether to addx

1 t + 1 j t

to the initial state vectorz

1

t

The procedure forms the submatrixV

1 corresponding tof

1

t

and computes its canon-

ical correlations Denote the smallest canonical correlation of V

1 as

m i n

If

m i n

is

significantly greater than 0x

1 t + 1 j t

is added to the state vector

If the smallest canonical correlation of V

1 is not significantly greater than 0 then a

linear combination of f

1

t

is uncorrelated with the pastp

t

Assuming that the determi-

nant of C

0

is not 0 (that is no input series is a constant) you can take the coefficient

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Chapter 18 Details

of x

1 t + 1 j t

in this linear combination to be 1 Denote the coefficients of z

1

t

in this

linear combination as

This gives the relationship

x

1 t + 1 j t

=

0

x

t

Therefore the current state vector already contains all the past information useful forpredicting

x

1 t + 1

and any greater leads of x

1 t

The variablex

1 t + 1 j t

is not added to

the state vector nor are any termsx

1 t + k j t

considered as possible components of the

state vector The variablex

1

is no longer active for state vector selection

The process described forx

1 t + 1 j t

is repeated for the remaining elements of f

t

The

next candidate for inclusion in the state vector is the next component of f

t

corre-

sponding to an active variable Components of f

t

corresponding to inactive variables

that produced a zero

m i n

in a previous step are skipped

Denote the next candidate asx

l t + k j t

The vectorf

j

t

is formed from the current state

vector andx

l t + k j t

as follows

f

j

t

= z

j

0

t

x

l t + k j t

0

The matrixV

j is formed fromf

j

t

and its canonical correlations are computed The

smallest canonical correlation of V

j is judged to be either greater than or equal to 0

If it is judged to be greater than 0x

l t + k j t

is added to the state vector If it is judged

to be 0 then a linear combination of f

j

t

is uncorrelated with thep

t

and the variable

x

l

is now inactive

The state vector selection process continues until no active variables remain

Testing Significance of Canonical Correlations For each step in the canonical correlation sequence the significance of the small-

est canonical correlation

m i n

is judged by an information criterion from Akaike

(1976) This information criterion is

n l n 1

2

m i n

r p + 1 q + 1

where q is the dimension of f

j

t

at the current step r is the order of the state vector p

is the order of the vector autoregressive process and

is the value of the SIGCORR=

option The default is SIGCORR=2 If this information criterion is less than or equal

to 0

m i n

is taken to be 0 otherwise it is taken to be significantly greater than 0

(Do not confuse this information criterion with the AIC)

Variables inx

t + p j t

are not added in the model even with positive information crite-

rion because of the singularity of V

You can force the consideration of more candi-

date state variables by increasing the size of theV

matrix by specifying a PASTMIN=

option value larger than p

Printing the Canonical Correlations

To print the details of the canonical correlation analysis process specify the CAN-

CORR option in the PROC STATESPACE statement The CANCORR option prints

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the candidate state vectors the canonical correlations and the information criteria for

testing the significance of the smallest canonical correlation

Bartlettrsquos

2 and its degrees of freedom are also printed when the CANCORR option

is specified The formula used for Bartlettrsquos

2 is

2

= n 5 r p + 1 q + 1 l n 1

2

m i n

withr p + 1 q + 1

degrees of freedom

Figure 1811 shows the output of the CANCORR option for the introductory example

shown in the Getting Started section of this chapter

The STATESPACE Procedure

Canonical Correlations Analysis

Information Chi

x(TT) y(TT) x(T+1T) Criterion Square DF

1 1 0237045 3566167 114505 4

Information Chi

x(TT) y(TT) x(T+1T) y(T+1T) Criterion Square DF

1 1 0238244 0056565 -535906 0636134 3

Information Chi

x(TT) y(TT) x(T+1T) x(T+2T) Criterion Square DF

1 1 0237602 0087493 -446312 1525353 3

Figure 1811 Canonical Correlations Analysis

New variables are added to the state vector if the information criteria are positive Inthis example

y

t + 1 j t

andx

t + 2 j t

are not added to the state space vector because the

information criteria for these models are negative

If the information criterion is nearly 0 then you may want to investigate models

that arise if the opposite decision is made regarding

m i n

This investigation can be

accomplished by using a FORM statement to specify part or all of the state vector

Preliminary Estimates of F

When a candidate variablex

l t + k j t

yields a zero

m i n

and is not added to the state

vector a linear combination of f

j

t

is uncorrelated with thep

t

Because of the method

used to construct thef

j

t

sequence the coefficient of x

l t + k j t

inl

can be taken as 1

Denote the coefficients of z

j

t

in this linear combination as l

This gives the relationship

x

l t + k j t

= l

0

z

j

t

The vectorl

is used as a preliminary estimate of the first r columns of the row of the

transition matrixF

corresponding tox

l t + k 1 j t

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Chapter 18 Details

Parameter Estimation

The model isz

t + 1

= F z

t

+ G e

t + 1

wheree

t

is a sequence of independent multivari-

ate normal innovations with mean vector 0 and variance

e e

The observed sequence

x

t

composes the first r components of z

t

and thusx

t

= H z

t

where H is ther s

matrix I

r

0

LetE

be ther n

matrix of innovations

E = e

1

e

n

If the number of observations n is reasonably large the log likelihood L can be

approximated up to an additive constant as follows

L =

n

2

l n j

e e

j

1

2

t r a c e

1

e e

E E

0

The elements of

e e

are taken as free parameters and are estimated as follows

S

0

=

1

n

E E

0

Replacing

e e

byS

0

in the likelihood equation the log likelihood up to an additive

constant is

L =

n

2

l n j S

0

j

Letting B be the backshift operator the formal relation betweenx

t

ande

t

is

x

t

= H I B F

1

G e

t

e

t

= H I B F

1

G

1

x

t

=

1

X

i = 0

i

x

t i

LettingC

i

be the ith lagged sample covariance of x

t

and neglecting end effects the

matrixS

0

is

S

0

=

1

X

i j = 0

i

C

i + j

0

j

For the computation of S

0

the infinite sum is truncated at the value of the KLAG=

option The value of the KLAG= option should be large enough that the sequence

i

is approximately 0 beyond that point

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Let

be the vector of free parameters in theF

andG

matrices The derivative of the

log likelihood with respect to the parameter

is

L

=

n

2

t r a c e

S

1

0

S

0

The second derivative is

2

L

0

=

n

2

t r a c e

S

1

0

S

0

0

S

1

0

S

0

t r a c e

S

1

0

2

S

0

0

Near the maximum the first term is unimportant and the second term can be approx-

imated to give the following second derivative approximation

2

L

0

=

n t r a c e

S

1

0

E

E

0

0

The first derivative matrix and this second derivative matrix approximation are com-

puted from the sample covariance matrixC

0

and the truncated sequence

i

The

approximate likelihood function is maximized by a modified Newton-Raphson algo-

rithm employing these derivative matrices

The matrixS

0

is used as the estimate of the innovation covariance matrix

e e

The

negative of the inverse of the second derivative matrix at the maximum is used as an

approximate covariance matrix for the parameter estimates The standard errors of

the parameter estimates printed in the parameter estimates tables are taken from the

diagonal of this covariance matrix The parameter covariance matrix is printed when

the COVB option is specified

If the data are nearly nonstationary a better estimate of

e e

and the other parameters

can sometimes be obtained by specifying the RESIDEST option The RESIDEST

option estimates the parameters using conditional least squares instead of maximum

likelihood

The residuals are computed using the state space equation and the sample mean val-

ues of the variables in the model as start-up values The estimate of S

0

is then com-

puted using the residuals from the ith observation on where i is the maximum number

of times any variable occurs in the state vector A multivariate Gauss-Marquardt al-

gorithm is used to minimizej S

0

j

Refer to Harvey (1981a) for a further description

of this method

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Forecasting

Given estimates of F

G

and

e e

forecasts of x

t

are computed from the conditional

expectation of z

t

In forecasting the parameters F G and

e e

are replaced with the estimates or by val-

ues specified in the RESTRICT statement One-step-ahead forecasting is performedfor the observation

x

t

wheret n b

Heren

is the number of observations and

b is the value of the BACK= option For the observationx

t

wheret n b

m-

step-ahead forecasting is performed form = t n + b

The forecasts are generated

recursively with the initial conditionz

0

= 0

The m-step-ahead forecast of z

t + m

isz

t + m j t

wherez

t + m j t

denotes the conditional

expectation of z

t + m

given the information available at time t The m-step-ahead

forecast of x

t + m

isx

t + m j t

= H z

t + m j t

where the matrixH = I

r

0

Let

i

= F

i

G

Note that the lasts r

elements of z

t

consist of the elements of x

u j t

foru t

The state vector z

t + m

can be represented as

z

t + m

= F

m

z

t

+

m 1

X

i = 0

i

e

t + m i

Sincee

t + i j t

= 0

fori 0

the m-step-ahead forecastz

t + m j t

is

z

t + m j t

= F

m

z

t

= F z

t + m 1 j t

Therefore the m-step-ahead forecast of x

t + m

is

x

t + m j t

= H z

t + m j t

The m-step-ahead forecast error is

z

t + m

z

t + m j t

=

m 1

X

i = 0

i

e

t + m i

The variance of the m-step-ahead forecast error is

V

z m

=

m 1

X

i = 0

i

e e

0

i

LettingV

z 0

= 0

the variance of the m-step-ahead forecast error of z

t + m

V

z m

can

be computed recursively as follows

V

z m

= V

z m 1

+

m 1

e e

0

m 1

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The variance of the m-step-ahead forecast error of x

t + m

is ther r

left upper sub-

matrix of V

z m

that is

V

x m

= H V

z m

H

0

Unless the NOCENTER option is specified the sample mean vector is added to the

forecast When differencing is specified the forecasts xt + m j t

plus the sample mean

vector are integrated back to produce forecasts for the original series

Lety

t

be the original series specified by the VAR statement with some 0 values

appended corresponding to the unobserved past observations Let B be the backshift

operator and let B

be thes s

matrix polynomial in the backshift operator

corresponding to the differencing specified by the VAR statement The off-diagonal

elements of

i

are 0 Note that

0

= I

s

whereI

s

is thes s

identity matrix Then

z

t

= B y

t

This gives the relationship

y

t

=

1

B z

t

=

1

X

i = 0

i

z

t i

where

1

B =

P

1

i = 0

i

B

i and

0

= I

s

The m-step-ahead forecast of y

t + m

is

y

t + m j t

=

m 1

X

i = 0

i

z

t + m i j t

+

1

X

i = m

i

z

t + m i

The m-step-ahead forecast error of y

t + m

is

m 1

X

i = 0

i

z

t + m i

z

t + m i j t

=

m 1

X

i = 0

i

X

u = 0

u

i u

e

t + m i

LettingV

y 0

= 0

the variance of the m-step-ahead forecast error of y

t + m

V

y m

is

V

y m

=

m 1

X

i = 0

i

X

u = 0

u

i u

e e

i

X

u = 0

u

i u

0

= V

y m 1

+

m 1

X

u = 0

u

m 1 u

e e

m 1

X

u = 0

u

m 1 u

0

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Chapter 18 Details

Relation of ARMA and State Space Forms

Every state space model has an ARMA representation and conversely every ARMA

model has a state space representation This section discusses this equivalence The

following material is adapted from Akaike (1974) where there is a more complete

discussion Pham-Dinh-Tuan (1978) also contains a discussion of this material

Suppose you are given the following ARMA model

B x

t

= B e

t

or in more detail

x

t

1

x

t 1

p

x

t p

= e

t

+

1

e

t 1

+ +

q

e

t q

(1)

wheree

t

is a sequence of independent multivariate normal random vectors with mean

0 and variance matrix

e e

B is the backshift operator (B x

t

= x

t 1

) B

and

B

are matrix polynomials in B andx

t

is the observed process

If the roots of the determinantial equationj B j = 0

are outside the unit circle in

the complex plane the model can also be written as

x

t

=

1

B B e

t

=

1

X

i = 0

i

e

t i

The

i

matrices are known as the impulse response matrices and can be computed

as

1

B B

You can assume p q

since if this is not initially true you can add more terms

i

that are identically 0 without changing the model

To write this set of equations in a state space form proceed as follows Letx

t + i j t

be

the conditional expectation of x

t + i

givenx

w

forw t

The following relations hold

x

t + i j t

=

1

X

j = i

j

e

t + i j

x

t + i j t + 1

= x

t + i j t

+

i 1

e

t + 1

However from equation (1) you can derive the following relationship

x

t + p j t

=

1

x

t + p 1 j t

+ +

p

x

t

(2)

Hence wheni = p

you can substitute forx

t + p j t

in the right-hand side of equation

(2) and close the system of equations

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This substitution results in the following model in the state space form

z

t + 1

= F z

t

+ G e

t + 1

2

6

6

6

4

x

t + 1

x

t + 2 j t + 1

x

t + p j t + 1

3

7

7

7

5

=

2

6

6

4

0 I 0 0

0 0 I 0

p

p 1

1

3

7

7

5

2

6

6

6

4

x

t

x

t + 1 j t

x

t + p 1 j t

3

7

7

7

5

+

2

6

6

4

I

1

p 1

3

7

7

5

e

t + 1

Note that the state vectorz

t

is composed of conditional expectations of x

t

and the

first r components of z

t

are equal tox

t

The state space form can be cast into an ARMA form by solving the system of dif-

ference equations for the first r components

When converting from an ARMA form to a state space form you can generate a

state vector larger than needed that is the state space model may not be a minimal

representation When going from a state space form to an ARMA form you can have

nontrivial common factors in the autoregressive and moving average operators that

yield an ARMA model larger than necessary

If the state space form used is not a minimal representation some but not all compo-

nents of x

t + i j t

may be linearly dependent This situation corresponds to

p

p 1

being of less than full rank when B

and B

have no common nontrivial

left factors In this casez

t

consists of a subset of the possible components of

x

t + i j t

i = 1 2 p 1

However once a component of x

t + i j t

(for example the

jth one) is linearly dependent on the previous conditional expectations then all sub-

sequent jth components of x

t + k j t

fork i

must also be linearly dependent Note

that in this case equivalent but seemingly different structures can arise if the order of

the components withinx

t

is changed

OUT= Data Set

The forecasts are contained in the output data set specified by the OUT= option on the

PROC STATESPACE statement The OUT= data set contains the following variables

the BY variables

the ID variable

the VAR statement variables These variables contain the actual values from

the input data set

FORi numeric variables containing the forecasts The variable FORi contains

the forecasts for the ith variable in the VAR statement list Forecasts are one-

step-ahead predictions until the end of the data or until the observation specified

by the BACK= option

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RESi numeric variables containing the residual for the forecast of the ith vari-

able in the VAR statement list For forecast observations the actual values are

missing and the RESi variables contain missing values

STDi numeric variables containing the standard deviation for the forecast of

the ith variable in the VAR statement list The values of the STDi variables can

be used to construct univariate confidence limits for the corresponding fore-casts However such confidence limits do not take into account the covariance

of the forecasts

OUTAR= Data Set

The OUTAR= data set contains the estimates of the preliminary autoregressive mod-

els The OUTAR= data set contains the following variables

ORDER a numeric variable containing the order p of the autoregressive model

that the observation represents

AIC a numeric variable containing the value of the information criterionA I C

p

SIGFl numeric variables containing the estimate of the innovation covariance

matrices for the forward autoregressive models The variable SIGFl contains

the lth column of b

p

in the observations with ORDER= p

SIGBl numeric variables containing the estimate of the innovation covariance

matrices for the backward autoregressive models The variable SIGBl contains

the lth column of b

p

in the observations with ORDER= p

FORk ndashl numeric variables containing the estimates of the autoregressive pa-

rameter matrices for the forward models The variable FORk ndashl contains the

lth column of the lag k autoregressive parameter matrix b

p

k

in the observations

with ORDER= p

BACk ndashl numeric variables containing the estimates of the autoregressive pa-

rameter matrices for the backward models The variable BACk ndashl contains the

lth column of the lag k autoregressive parameter matrix b

p

k

in the observations

with ORDER= p

The estimates for the order p autoregressive model can be selected as those observa-

tions with ORDER= p Within these observations the k lth element of

p

i

is given by

the value of the FORindashl variable in the k th observation The k lth element of

p

i

is

given by the value of BACindashl variable in the k th observation The k lth element of

p

is given by SIGFl in the k th observation The k lth element of

p

is given by SIGBlin the k th observation

Table 181 shows an example of the OUTAR= data set with ARMAX=3 andx

t

of

dimension 2 In Table 181 i j

indicate the ijth element of the matrix

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Table 181 Values in the OUTAR= Data Set

Obs ORDER AIC SIGF1 SIGF2 SIGB1 SIGB2 FOR1ndash 1 FOR1ndash 2 FOR2ndash 1 FOR2ndash 2 FOR3ndash 1

1 0 AIC0

0 1 1

0 1 2

0 1 1

0 1 2

2 0 AIC0

0 2 1

0 2 2

0 2 1

0 2 2

3 1 AIC1

1 1 1

1 1 2

1 1 1

1 1 2

1

1

1 1

1

1

1 2

4 1 AIC1

1 2 1

1 1 2

1 2 1

1 1 2

1

1

2 1

1

1

2 2

5 2 AIC2

2 1 1

2 1 2

2 1 1

2 1 2

2

1

1 1

2

1

1 2

2

2

1 1

2

2

1 2

6 2 AIC2

2 2 1

2 1 2

2 2 1

2 1 2

2

1

2 1

2

1

2 2

2

2

2 1

2

2

2 2

7 3 AIC3

3 1 1

3 1 2

3 1 1

3 1 2

3

1

1 1

3

1

1 2

3

2

1 1

3

2

1 2

3

3

1 1

8 3 AIC3

3 2 1

3 1 2

3 2 1

3 1 2

3

1

2 1

3

1

2 2

3

2

2 1

3

2

2 2

3

3

2 1

Obs FOR3ndash 2 BACK1ndash 1 BACK1ndash 2 BACK2ndash 1 BACK2ndash 2 BACK3ndash 1 BACK3ndash 2

1

2

3

1

1

1 1

1

1

1 2

4

1

1

2 1

1

1

2 2

5

2

1

1 1

2

1

1 2

2

2

1 1

2

2

1 2

6

2

1

2 1

2

1

2 2

2

2

2 1

2

2

2 2

7

3

3

1 2

3

1

1 1

3

1

1 2

3

2

1 1

3

2

1 2

3

3

1 1

3

3

1 2

8

3

3

2 2

3

1

2 1

3

1

2 2

3

2

2 1

3

2

2 2

3

3

2 1

3

3

2 2

The estimated autoregressive parameters can be used in the IML procedure to obtain

autoregressive estimates of the spectral density function or forecasts based on the

autoregressive models

OUTMODEL= Data Set

The OUTMODEL= data set contains the estimates of theF

andG

matrices and their

standard errors the names of the components of the state vector and the estimates of

the innovation covariance matrix The variables contained in the OUTMODEL= data

set are as follows

the BY variables

STATEVEC a character variable containing the name of the component of the

state vector corresponding to the observation The STATEVEC variable has

the value STD for standard deviations observations which contain the standarderrors for the estimates given in the preceding observation

Fndash j numeric variables containing the columns of theF

matrix The variable

Fndash j contains the jth column of F

The number of Fndash j variables is equal to

the value of the DIMMAX= option If the model is of smaller dimension the

extraneous variables are set to missing

Gndash j numeric variables containing the columns of theG

matrix The variable

Gndash j contains the jth column of G

The number of Gndash j variables is equal to r

the dimension of x

t

given by the number of variables in the VAR statement

SIGndash j numeric variables containing the columns of the innovation covariance

matrix The variable SIGndash j contains the jth column of

e e

There are r vari-ables SIGndash j

Table 182 shows an example of the OUTMODEL= data set withx

t

= x

t

y

t

0

z

t

= x

t

y

t

x

t + 1 j t

0 and DIMMAX=4 In Table 182F

i j

andG

i j

are the ijth

elements of F

andG

respectively Note that all elements for Fndash4 are missing because

F

is a3 3

matrix

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Table 182 Value in the OUTMODEL= Data Set

Obs STATEVEC Fndash 1 Fndash 2 Fndash 3 Fndash 4 Gndash 1 Gndash 2 SIGndash 1 SIGndash 2

1 X(TT) 0 0 1 1 0

1 1

1 2

2 STD

3 Y(TT) F2 1

F2 2

F2 3

0 1

2 1

2 2

4 STD std F2 1

std F2 2

std F2 3

5 X(T+1T) F3 1

F3 2

F3 3

G3 1

G3 2

6 STD std F3 1

std F3 2

std F3 3

std G3 1

std G3 2

Printed Output

The printed output produced by the STATESPACE procedure is described in the fol-

lowing

1 descriptive statistics which include the number of observations used the

names of the variables their means and standard deviations (Std) and the dif-

ferencing operations used

2 the Akaike information criteria for the sequence of preliminary autoregressive

models

3 if the PRINTOUT=LONG option is specified the sample autocovariance ma-

trices of the input series at various lags

4 if the PRINTOUT=LONG option is specified the sample autocorrelation ma-

trices of the input series

5 a schematic representation of the autocorrelation matrices showing the signif-

icant autocorrelations

6 if the PRINTOUT=LONG option is specified the partial autoregressive ma-

trices (These are

p

p

as described in Preliminary Autoregressive Modelsearlier in this chapter)

7 a schematic representation of the partial autocorrelation matrices showing the

significant partial autocorrelations

8 the Yule-Walker estimates of the autoregressive parameters for the autoregres-

sive model with the minimum AIC

9 if the PRINTOUT=LONG option is specified the autocovariance matrices of

the residuals of the minimum AIC model This is the sequence of estimated

innovation variance matrices for the solutions of the Yule-Walker equations

10 if the PRINTOUT=LONG option is specified the autocorrelation matrices of the residuals of the minimum AIC model

11 If the CANCORR option is specified the canonical correlations analysis for

each potential state vector considered in the state vector selection process

This includes the potential state vector the canonical correlations the informa-

tion criterion for the smallest canonical correlation Bartlettrsquos

2 statistic (ldquoChi

Squarerdquo) for the smallest canonical correlation and the degrees of freedom of

Bartlettrsquos

2

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Part 2 General Information

12 the components of the chosen state vector

13 the preliminary estimate of the transition matrixF

the input matrixG

and

the variance matrix for the innovations

e e

14 if the ITPRINT option is specified the iteration history of the likelihood max-

imization For each iteration this shows the iteration number the number of step halvings the determinant of the innovation variance matrix the damping

factor Lambda and the values of the parameters

15 the state vector printed again to aid interpretation of the following listing of F

andG

16 the final estimate of the transition matrixF

17 the final estimate of the input matrixG

18 the final estimate of the variance matrix for the innovations

e e

19 a table listing the estimates of the free parameters inF

andG

and their standarderrors and t statistics

20 if the COVB option is specified the covariance matrix of the parameter esti-

mates

21 if the COVB option is specified the correlation matrix of the parameter esti-

mates

22 if the PRINT option is specified the forecasts and their standard errors

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Chapter 18 Details

ODS Table Names

PROC STATESPACE assigns a name to each table it creates You can use these

names to reference the table when using the Output Delivery System (ODS) to select

tables and create output data sets These names are listed in the following table For

more information on ODS see Chapter 6 ldquoUsing the Output Delivery Systemrdquo

Table 183 ODS Tables Produced in PROC STATESPACE

ODS Table Name Description Option

NObs Number of observations default

Summary Simple summary statistics table default

InfoCriterion Information criterion table default

CovLags Covariance Matrices of Input Series PRINTOUT=LONG

CorrLags Correlation Matrices of Input Series PRINTOUT=LONG

PartialAR Partial Autoregressive Matrices PRINTOUT=LONG

YWEstimates Yule-Walker Estimates for Minimum AIC default

CovResiduals Covariance of Residuals PRINTOUT=LONGCorrResiduals Residual Correlations from AR Models PRINTOUT=LONG

StateVector State vector table default

CorrGraph Schematic Representation of Correlations default

TransitionMatrix Transition Matrix default

InputMatrix Input Matrix default

VarInnov Variance Matrix for the Innovation default

CovB Covariance of Parameter Estimates COVB

CorrB Correlation of Parameter Estimates COVB

CanCorr Canonical Correlation Analysis CANCORR

IterHistory Iterative Fitting table ITPRINT

ParameterEstimates Parameter Estimates Table default

Forecasts Forecasts Table PRINT

ConvergenceStatus Convergence Status Table default

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Part 2 General Information

Example

Example 181 Series J from Box and Jenkins

This example analyzes the gas furnace data (series J) from Box and Jenkins (The

data are not shown Refer to Box and Jenkins (1976) for the data)

First a model is selected and fit automatically using the following statements

title1 rsquoGas Furnace Datarsquo

title2 rsquoBox amp Jenkins Series Jrsquo

title3 rsquoAutomatically Selected Modelrsquo

proc statespace data=seriesj cancorr

var x y

run

The results for the automatically selected model are shown in Output 1811

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Chapter 18 Example

Output 1811 Results for Automatically Selected Model

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Number of Observations 296

Standard

Variable Mean Error

x -005683 1072766

y 5350912 3202121

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Information Criterion for Autoregressive Models

Lag=0 Lag=1 Lag=2 Lag=3 Lag=4 Lag=5 Lag=6 Lag=7 Lag=8

6513862 -103357 -163296 -164512 -165152 -164891 -164934 -164315 -163856

Information

Criterion for

Autoregressive

Models

Lag=9 Lag=10

-16348 -163359

Schematic Representation of Correlations

NameLag 0 1 2 3 4 5 6 7 8 9 10

x +- +- +- +- +- +- +- +- +- +- +-

y -+ -+ -+ -+ -+ -+ -+ -+ -+ -+ -+

+ is gt 2std error - is lt -2std error is between

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Part 2 General Information

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Schematic Representation of Partial Autocorrelations

NameLag 1 2 3 4 5 6 7 8 9 10

x + - + -

y -+ -- - + +

+ is gt 2std error - is lt -2std error is between

Yule-Walker Estimates for Minimum AIC

------Lag=1------ ------Lag=2------ ------Lag=3------ ------Lag=4------

x y x y x y x y

x 1925887 -000124 -120166 0004224 0116918 -000867 0104236 0003268

y 0050496 1299793 -002046 -03277 -071182 -025701 0195411 0133417

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Canonical Correlations Analysis

Information Chi

x(TT) y(TT) x(T+1T) Criterion Square DF

1 1 0804883 2929228 3047481 8

Information Chix(TT) y(TT) x(T+1T) y(T+1T) Criterion Square DF

1 1 0906681 0607529 1223358 1347237 7

Information Chi

x(TT) y(TT) x (T+1T) y (T+1T) x (T+2T) Criterion Square DF

1 1 0909434 0610278 0186274 -154701 1034705 6

Information Chi

x(TT) y(TT) x (T+1T) y (T+1T) y (T+2T) Criterion Square DF

1 1 091014 0618937 0206823 0940392 1280924 6

Information Chi

x(TT) y(TT) x(T+1T) y(T+1T) y(T+2T) y(T+3T) Criterion Square DF

1 1 0912963 0628785 0226598 0083258 -794103 2041584 5

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Chapter 18 Example

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Selected Statespace Form and Preliminary Estimates

State Vector

x(TT) y(TT) x(T+1T) y(T+1T) y(T+2T)

Estimate of Transition Matrix

0 0 1 0 0

0 0 0 1 0

-084718 0026794 1711715 -005019 0

0 0 0 0 1

-019785 0334274 -018174 -123557 1787475

Input Matrix for Innovation

1 0

0 11925887 -000124

0050496 1299793

0142421 1361696

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Selected Statespace Form and Preliminary Estimates

Variance Matrix for Innovation

0035274 -000734

-000734 0097569

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Part 2 General Information

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Selected Statespace Form and Fitted Model

State Vector

x(TT) y(TT) x(T+1T) y(T+1T) y(T+2T)

Estimate of Transition Matrix

0 0 1 0 0

0 0 0 1 0

-086192 0030609 1724235 -005483 0

0 0 0 0 1

-034839 0292124 -009435 -109823 1671418

Input Matrix for Innovation

1 0

0 1192442 -000416

0015621 1258495

008058 1353204

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Selected Statespace Form and Fitted Model

Variance Matrix for Innovation

0035579 -000728

-000728 0095577

Parameter Estimates

Standard

Parameter Estimate Error t Value

F(31) -086192 0072961 -1181

F(32) 0030609 0026167 117

F(33) 1724235 0061599 2799

F(34) -005483 0030169 -182

F(51) -034839 0135253 -258

F(52) 0292124 0046299 631

F(53) -009435 0096527 -098F(54) -109823 0109525 -1003

F(55) 1671418 0083737 1996

G(31) 1924420 0058162 3309

G(32) -000416 0035255 -012

G(41) 0015621 0095771 016

G(42) 1258495 0055742 2258

G(51) 0080580 0151622 053

G(52) 1353204 0091388 1481

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Chapter 18 Example

The two series are believed to have a transfer function relation with the gas rate

(variable X) as the input and the CO2

concentration (variable Y) as the output Since

the parameter estimates shown in Output 1811 support this kind of model the model

is reestimated with the feedback parameters restricted to 0 The following statements

fit the transfer function (no feedback) model

title3 rsquoTransfer Function Modelrsquo

proc statespace data=seriesj printout=none

var x y

restrict f(32)=0 f(34)=0

g(32)=0 g(41)=0 g(51)=0

run

The last 2 pages of the output are shown in Output 1812

Output 1812 STATESPACE Output for Transfer Function Model

Gas Furnace Data

Box amp Jenkins Series J

Transfer Function Model

The STATESPACE Procedure

Selected Statespace Form and Fitted Model

State Vector

x(TT) y(TT) x(T+1T) y(T+1T) y(T+2T)

Estimate of Transition Matrix

0 0 1 0 0

0 0 0 1 0

-068882 0 1598717 0 0

0 0 0 0 1

-035944 0284179 -00963 -107313 1650047

Input Matrix for Innovation

1 0

0 1

1923446 0

0 1260856

0 1346332

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Part 2 General Information

Gas Furnace Data

Box amp Jenkins Series J

Transfer Function Model

The STATESPACE Procedure

Selected Statespace Form and Fitted Model

Variance Matrix for Innovation

0036995 -00072

-00072 0095712

Parameter Estimates

Standard

Parameter Estimate Error t Value

F(31) -068882 0050549 -1363

F(33) 1598717 0050924 3139

F(51) -035944 0229044 -157

F(52) 0284179 0096944 293

F(53) -009630 0140876 -068

F(54) -107313 0250385 -429

F(55) 1650047 0188533 875G(31) 1923446 0056328 3415

G(42) 1260856 0056464 2233

G(52) 1346332 0091086 1478

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Chapter 18 References

References

Akaike H (1974) Markovian Representation of Stochastic Processes and Its Ap-

plication to the Analysis of Autoregressive Moving Average Processes Annals

of the Institute of Statistical Mathematics 26 363-387

Akaike H (1976) Canonical Correlations Analysis of Time Series and the Use of

an Information Criterion in Advances and Case Studies in System Identification

eds R Mehra and DG Lainiotis New York Academic Press

Anderson TW (1971) The Statistical Analysis of Time Series New York John

Wiley amp Sons

Ansley CF and Newbold P (1979) Multivariate Partial Autocorrelations Pro-

ceedings of the Business and Economic Statistics Section American Statistical

Association 349-353

Box GEP and Jenkins G (1976) Time Series Analysis Forecasting and Control

San Francisco Holden-Day

Brockwell PJ and Davis RA (1991) Time Series Theory and Methods 2nd Edi-

tion Springer-Verlag

Hannan EJ (1970) Multiple Time Series New York John Wiley amp Sons

Hannan EJ (1976) The Identification and Parameterization of ARMAX and State

Space Forms Econometrica 44 713-722

Harvey AC (1981a) The Econometric Analysis of Time Series New York John

Wiley amp Sons

Harvey AC (1981b) Time Series Models New York John Wiley amp Sons

Jones RH (1974) Identification and Autoregressive Spectrum Estimation IEEE Transactions on Automatic Control AC-19 894-897

Pham-Dinh-Tuan (1978) On the Fitting of Multivariate Processes of the

Autoregressive-Moving Average Type Biometrika 65 99-107

Priestley MB (1980) System Identification Kalman Filtering and Stochastic Con-

trol in Directions in Time Series eds DR Brillinger and GC Tiao Institute

of Mathematical Statistics

Whittle P (1963) On the Fitting of Multivariate Autoregressions and the Approxi-

mate Canonical Factorization of a Spectral Density Matrix Biometrika 50 129-

134

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The correct bibliographic citation for this manual is as follows SAS Institute Inc SAS

ET S U serrsquos Gui de Version 8 Cary NC SAS Institute Inc 1999 1546 pp

SASET S Userrsquos Guide Version 8

Copyright copy 1999 by S AS I ns t it u t e I nc Ca ry N C U S A

ISBN 1ndash58025ndash489ndash6

All r ight s res erved P r int ed in t he U n it ed St a t es of A merica N o pa rt of t his p u blica t ionma y b e reprodu ced s t ored in a ret r ieva l s ys t em or t ra ns mit t ed in a ny form or b y a nymeans electronic mechanical photocopying or otherwise without the prior writtenpermission of the publisher SAS Institute Inc

US Government Restricted Rights Notice Use du plicat ion or disclosure of the

software by the government is subject to restrictions as set forth in FAR 52227ndash19Commercial Computer Softw ar e-Restricted Rights (J une 1987)

SAS Institute Inc SAS Campus Drive Cary North Carolina 27513

1st print ing October 1999

S AS reg a nd a ll ot her SA S I ns t it u t e I nc p rodu ct or s ervice na m es a re regis t ered t ra dema rkst d k f SA S I t it t I i t h U S A d t h t i reg i di t U SA

Page 10: Statespace Model

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Part 2 General Information

The form of the state vector and the preliminary estimates are printed next as shown

in Figure 184

The STATESPACE Procedure

Selected Statespace Form and Preliminary Estimates

State Vector

x(TT) y(TT) x(T+1T)

Estimate of Transition Matrix

0 0 1

0291536 0468762 -000411

024869 024484 0204257

Input Matrix for Innovation

1 0

0 1

0257438 0202237

Variance Matrix for Innovation

0945196 0100786

0100786 1014703

Figure 184 Preliminary Estimates of State Space Model

Figure 184 first prints the state vector as X[TT] Y[TT] X[T+1T] This notation

indicates that the state vector is

z

t

=

2

4

x

t j t

y

t j t

x

t + 1 j t

3

5

The notationx

t + 1 j t

indicates the conditional expectation or prediction of x

t + 1

based

on the information available at time t andx

t j t

andy

t j t

arex

t

andy

t

respectively

The remainder of Figure 184 shows the preliminary estimates of the transition matrix

F

the input matrixG

and the covariance matrix

e e

Estimated State Space Model

The next page of the STATESPACE output prints the final estimates of the fittedmodel as shown in Figure 185 This output has the same form as in Figure 184 but

shows the maximum likelihood estimates instead of the preliminary estimates

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Chapter 18 Getting Started

The STATESPACE Procedure

Selected Statespace Form and Fitted Model

State Vector

x(TT) y(TT) x(T+1T)

Estimate of Transition Matrix

0 0 1

0297273 047376 -001998

02301 0228425 0256031

Input Matrix for Innovation

1 0

0 1

0257284 0202273

Variance Matrix for Innovation

0945188 01007520100752 1014712

Figure 185 Fitted State Space Model

The estimated state space model shown in Figure 185 is

2

4

x

t + 1 j t + 1

y

t + 1 j t + 1

x

t + 2 j t + 1

3

5

=

2

4

0 0 1

0 2 9 7 0 4 7 4 0 0 2 0

0 2 3 0 0 2 2 8 0 2 5 6

3

5

2

4

x

t

y

t

x

t + 1 j t

3

5

+

2

4

1 0

0 1

0 2 5 7 0 2 0 2

3

5

e

t + 1

n

t + 1

v a r

e

t + 1

n

t + 1

=

0 9 4 5 0 1 0 1

0 1 0 1 1 0 1 5

The next page of the STATESPACE output lists the estimates of the free parameters

in theF

andG

matrices with standard errors and t statistics as shown in Figure 186

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Part 2 General Information

The STATESPACE Procedure

Parameter Estimates

Standard

Parameter Estimate Error t Value

F(21) 0297273 0129995 229

F(22) 0473760 0115688 410F(23) -001998 0313025 -006

F(31) 0230100 0126226 182

F(32) 0228425 0112978 202

F(33) 0256031 0305256 084

G(31) 0257284 0071060 362

G(32) 0202273 0068593 295

Figure 186 Final Parameter Estimates

Convergence Failures

The maximum likelihood estimates are computed by an iterative nonlinear maximiza-

tion algorithm which may not converge If the estimates fail to converge warning

messages are printed in the output

If you encounter convergence problems you should recheck the stationarity of the

data and ensure that the specified differencing orders are correct Attempting to fit

state space models to nonstationary data is a common cause of convergence failure

You can also use the MAXIT= option to increase the number of iterations allowed

or experiment with the convergence tolerance options DETTOL= and PARMTOL=

Forecast Data Set

The following statements print the output data set The WHERE statement excludes

the first 190 observations from the output so that only the forecasts and the last 10

actual observations are printed

proc print data=outid t

where t gt 190

run

The PROC PRINT output is shown in Figure 187

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Part 2 General Information

Figure 188 Plot of Forecasts

Controlling Printed Output

By default the STATESPACE procedure produces a large amount of printed output

The NOPRINT option suppresses all printed output You can suppress the printed

output for the autoregressive model selection process with the PRINTOUT=NONE

option The descriptive statistics and state space model estimation output are still

printed when PRINTOUT=NONE is specified You can produce more detailed output

with the PRINTOUT=LONG option and by specifying the printing control optionsCANCORR COVB and PRINT

Specifying the State Space Model

Instead of allowing the STATESPACE procedure to select the model automatically

you can use FORM and RESTRICT statements to specify a state space model

Specifying the State Vector

Use the FORM statement to control the form of the state vector You can use this

feature to force PROC STATESPACE to estimate and forecast a model different from

the model it would select automatically You can also use this feature to reestimate

the automatically selected model (possibly with restrictions) without repeating thecanonical correlation analysis

The FORM statement specifies the number of lags of each variable to include in

the state vector For example the statement FORM X 3 forces the state vector to

includex

t j t

x

t + 1 j t

andx

t + 2 j t

The following statement specifies the state vector

x

t j t

y

t j t

x

t + 1 j t

which is the same state vector selected in the preceding example

f o r m x 2 y 1

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Chapter 18 Getting Started

You can specify the form for only some of the variables and allow PROC STATES-

PACE to select the form for the other variables If only some of the variables are

specified in the FORM statement canonical correlation analysis is used to determine

the number of lags included in the state vector for the remaining variables not spec-

ified by the FORM statement If the FORM statement includes specifications for all

the variables listed in the VAR statement the state vector is completely defined and

the canonical correlation analysis is not performed

Restricting the F and G matrices

After you know the form of the state vector you can use the RESTRICT statement

to fix some parameters in theF

andG

matrices to specified values One use of this

feature is to remove insignificant parameters by restricting them to 0

In the introductory example shown in the preceding section the F[23] parameter is

not significant (The parameters estimation output shown in Figure 186 gives the t

statistic for F[23] as -006 F[33] and F[31] also have low significance witht 2

)

The following statements reestimate this model with F[23] restricted to 0 The

FORM statement is used to specify the state vector and thus bypass the canonicalcorrelation analysis

proc statespace data=in out=out lead=10

var x(1) y(1)

id t

form x 2 y 1

restrict f(23)=0

run

The final estimates produced by these statements are shown in Figure 189

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Part 2 General Information

The STATESPACE Procedure

Selected Statespace Form and Fitted Model

State Vector

x(TT) y(TT) x(T+1T)

Estimate of Transition Matrix

0 0 1

0290051 0467468 0

0227051 0226139 026436

Input Matrix for Innovation

1 0

0 1

0256826 0202022

Variance Matrix for Innovation

0945175 01006960100696 1014733

The STATESPACE Procedure

Parameter Estimates

Standard

Parameter Estimate Error t Value

F(21) 0290051 0063904 454

F(22) 0467468 0060430 774

F(31) 0227051 0125221 181

F(32) 0226139 0111711 202

F(33) 0264360 0299537 088G(31) 0256826 0070994 362

G(32) 0202022 0068507 295

Figure 189 Results using RESTRICT Statement

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Chapter 18 Syntax

Syntax

The STATESPACE procedure uses the following statements

PROC STATESPACE options

BY variable FORM variable value

ID variable

INITIAL F(rowcolumn)=value

G(rowcolumn)=value

RESTRICT F(rowcolumn)=value

G(rowcolumn)=value

VAR variable (difference difference

)

Functional Summary

The statements and options used by PROC STATESPACE are summarized in the

following table

Description Statement Option

Input Data Set Options

specify the input data set PROC STATESPACE DATA=

prevent subtraction of sample mean PROC STATESPACE NOCENTER

specify the ID variable ID

specify the observed series and differencing VAR

Options for Autoregressive Estimates

specify the maximum order PROC STATESPACE ARMAX=

specify maximum lag for autocovariances PROC STATESPACE LAGMAX=

output only minimum AIC model PROC STATESPACE MINIC

specify the amount of detail printed PROC STATESPACE PRINTOUT=

write preliminary AR models to a data set PROC STATESPACE OUTAR=

Options for Canonical Correlation Analysis

print the sequence of canonical correlations PROC STATESPACE CANCORR

specify upper limit of dimension of state

vector

PROC STATESPACE DIMMAX=

specify the minimum number of lags PROC STATESPACE PASTMIN=

specify the multiplier of the degrees of

freedom

PROC STATESPACE SIGCORR=

Options for State Space Model Estimation

specify starting values INITIAL

print covariance matrix of parameter estimates PROC STATESPACE COVB

specify the convergence criterion PROC STATESPACE DETTOL=

specify the convergence criterion PROC STATESPACE PARMTOL=

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Part 2 General Information

Description Statement Option

print the details of the iterations PROC STATESPACE ITPRINT

specify an upper limit of the number of lags PROC STATESPACE KLAG=

specify maximum number of iterations

allowed

PROC STATESPACE MAXIT=

suppress the final estimation PROC STATESPACE NOEST

write the state space model parameter esti-

mates to an output data set

PROC STATESPACE OUTMODEL=

use conditional least squares for final estimates PROC STATESPACE RESIDEST

specify criterion for testing for singularity PROC STATESPACE SINGULAR=

Options for Forecasting

start forecasting before end of the input data PROC STATESPACE BACK=

specify the time interval between observations PROC STATESPACE INTERVAL=

specify multiple periods in the time series PROC STATESPACE INTPER=specify how many periods to forecast PROC STATESPACE LEAD=

specify the output data set for forecasts PROC STATESPACE OUT=

print forecasts PROC STATESPACE PRINT

Options to Specify the State Space Model

specify the state vector FORM

specify the parameter values RESTRICT

BY Groups

specify BY-group processing BY

Printing

suppresses all printed output NOPRINT

PROC STATESPACE Statement

PROC STATESPACE options

The following options can be specified in the PROC STATESPACE statement

Printing Options NOPRINT

suppresses all printed output

Input Data Options

DATA= SAS-data-set

specifies the name of the SAS data set to be used by the procedure If the DATA=

option is omitted the most recently created SAS data set is used

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Chapter 18 Syntax

LAGMAX= k

specifies the number of lags for which the sample autocovariance matrix is computed

The LAGMAX= option controls the number of lags printed in the schematic repre-

sentation of the autocorrelations

The sample autocovariance matrix of lag i denoted asC

i

is computed as

C

i

=

1

N 1

N

X

t = 1 + i

x

t

x

0

t i

wherex

t

is the differenced and centered data andN

is the number of observations

(If the NOCENTER option is specified 1 is not subtracted fromN

) LAGMAX= k

specifies thatC

0

throughC

k

are computed The default is LAGMAX=10

NOCENTER

prevents subtraction of the sample mean from the input series (after any specified

differencing) before the analysis

Options for Preliminary Autoregressive Models ARMAX= n

specifies the maximum order of the preliminary autoregressive models The AR-

MAX= option controls the autoregressive orders for which information criteria are

printed and controls the number of lags printed in the schematic representation of

partial autocorrelations The default is ARMAX=10 See Preliminary Autoregres-

sive Models later in this chapter for details

MINIC

writes to the OUTAR= data set only the preliminary Yule-Walker estimates for the

VAR model producing the minimum AIC See OUTAR= Data Set later in this chap-

ter for details

OUTAR= SAS-data-set

writes the Yule-Walker estimates of the preliminary autoregressive models to a SAS

data set See OUTAR= Data Set later in this chapter for details

PRINTOUT= SHORT | LONG | NONE

determines the amount of detail printed PRINTOUT=LONG prints the lagged co-

variance matrices the partial autoregressive matrices and estimates of the resid-

ual covariance matrices from the sequence of autoregressive models PRINT-

OUT=NONE suppresses the output for the preliminary autoregressive models The

descriptive statistics and state space model estimation output are still printed when

PRINTOUT=NONE is specified PRINTOUT=SHORT is the default

Canonical Correlation Analysis Options

CANCORR

prints the canonical correlations and information criterion for each candidate state

vector considered See Canonical Correlation Analysis later in this chapter for

details

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DIMMAX= n

specifies the upper limit to the dimension of the state vector The DIMMAX= option

can be used to limit the size of the model selected The default is DIMMAX=10

PASTMIN= n

specifies the minimum number of lags to include in the canonical correlation analy-

sis The default is PASTMIN=0 See Canonical Correlation Analysis later in thischapter for details

SIGCORR= value

specifies the multiplier of the degrees of freedom for the penalty term in the informa-

tion criterion used to select the state space form The default is SIGCORR=2 The

larger the value of the SIGCORR= option the smaller the state vector tends to be

Hence a large value causes a simpler model to be fit See Canonical Correlations

Analysis later in this chapter for details

State Space Model Estimation Options

COVB

prints the inverse of the observed information matrix for the parameter estimatesThis matrix is an estimate of the covariance matrix for the parameter estimates

DETTOL= value

specifies the convergence criterion The DETTOL= and PARMTOL= option values

are used together to test for convergence of the estimation process If during an

iteration the relative change of the parameter estimates is less than the PARMTOL=

value and the relative change of the determinant of the innovation variance matrix

is less than the DETTOL= value then iteration ceases and the current estimates are

accepted The default is DETTOL=1E-5

ITPRINT

prints the iterations during the estimation process

KLAG= n

sets an upper limit for the number of lags of the sample autocovariance matrix used

in computing the approximate likelihood function If the data have a strong moving

average character a larger KLAG= value may be necessary to obtain good estimates

The default is KLAG=15 See Parameter Estimation later in this chapter for details

MAXIT= n

sets an upper limit to the number of iterations in the maximum likelihood or condi-

tional least-squares estimation The default is MAXIT=50

NOEST

suppresses the final maximum likelihood estimation of the selected model

OUTMODEL= SAS-data-set

writes the parameter estimates and their standard errors to a SAS data set See OUT-

MODEL= Data Set later in this chapter for details

PARMTOL= value

specifies the convergence criterion The DETTOL= and PARMTOL= option values

are used together to test for convergence of the estimation process If during an

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Chapter 18 Syntax

iteration the relative change of the parameter estimates is less than the PARMTOL=

value and the relative change of the determinant of the innovation variance matrix

is less than the DETTOL= value then iteration ceases and the current estimates are

accepted The default is PARMTOL=001

RESIDEST

computes the final estimates using conditional least squares on the raw data Thistype of estimation may be more stable than the default maximum likelihood method

but is usually more computationally expensive See Parameter Estimation later in

this chapter for details of the conditional least squares method

SINGULAR= value

specifies the criterion for testing for singularity of a matrix A matrix is declared

singular if a scaled pivot is less than the SINGULAR= value when sweeping the

matrix The default is SINGULAR=1E-7

Forecasting Options

BACK= n

starts forecasting n periods before the end of the input data The BACK= option valuemust not be greater than the number of observations The default is BACK=0

INTERVAL= interval

specifies the time interval between observations The INTERVAL= value is used

in conjunction with the ID variable to check that the input data are in order and

have no missing periods The INTERVAL= option is also used to extrapolate the ID

values past the end of the input data See Chapter 3 ldquoDate Intervals Formats and

Functionsrdquo for details on the INTERVAL= values allowed

INTPER= n

specifies that each input observation corresponds to n time periods For example

the options INTERVAL=MONTH and INTPER=2 specify bimonthly data and are

equivalent to specifying INTERVAL=MONTH2 If the INTERVAL= option is not

specified the INTPER= option controls the increment used to generate ID values for

the forecast observations The default is INTPER=1

LEAD= n

specifies how many forecast observations are produced The forecasts start at the

point set by the BACK= option The default is LEAD=0 which produces no fore-

casts

OUT= SAS-data-set

writes the residuals actual values forecasts and forecast standard errors to a SAS

data set See OUT= Data Set later in this chapter for details

PRINT

prints the forecasts

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Part 2 General Information

BY Statement

BY variable

A BY statement can be used with the STATESPACE procedure to obtain separateanalyses on observations in groups defined by the BY variables

FORM Statement

FORM variable value

The FORM statement specifies the number of times a variable is included in the state

vector Values can be specified for any variable listed in the VAR statement If a

value is specified for each variable in the VAR statement the state vector for the state

space model is entirely specified and automatic selection of the state space model is

not performed

The FORM statement forces the state vectorz

t

to contain a specific variable a given

number of times For example if Y is one of the variables inx

t

then the statement

form y 3

forces the state vector to containY

t

Y

t + 1 j t

andY

t + 2 j t

possibly along with other

variables

The following statements illustrate the use of the FORM statement

proc statespace data=in

var x y

f o r m x 3 y 2

run

These statements fit a state space model with the following state vector

z

t

=

2

6

6

6

6

4

x

t j t

y

t j t

x

t + 1 j t

y

t + 1 j t

x

t + 2 j t

3

7

7

7

7

5

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Chapter 18 Syntax

ID Statement

ID variable

The ID statement specifies a variable that identifies observations in the input data setThe variable specified in the ID statement is included in the OUT= data set The

values of the ID variable are extrapolated for the forecast observations based on the

values of the INTERVAL= and INTPER= options

INITIAL Statement

INITIAL F (rowcolumn)= value G(row column)= value

The INITIAL statement gives initial values to the specified elements of theF

andG

matrices These initial values are used as starting values for the iterative estimation

Parts of theF

andG

matrices represent fixed structural identities If an element

specified is a fixed structural element instead of a free parameter the corresponding

initialization is ignored

The following is an example of an INITIAL statement

initial f(32)=0 g(41)=0 g(51)=0

RESTRICT Statement

RESTRICT F(rowcolumn)= value G(rowcolumn)= value

The RESTRICT statement restricts the specified elements of theF

andG

matrices

to the specified values

To use the restrict statement you need to know the form of the model Either specify

the form of the model with the FORM statement or do a preliminary run perhaps

with the NOEST option to find the form of the model that PROC STATESPACE

selects for the data

The following is an example of a RESTRICT statement

restrict f(32)=0 g(41)=0 g(51)=0

Parts of theF

andG

matrices represent fixed structural identities If a restriction is

specified for an element that is a fixed structural element instead of a free parameter

the restriction is ignored

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Part 2 General Information

VAR Statement

VAR variable (difference difference )

The VAR statement specifies the variables in the input data set to model and fore-cast The VAR statement also specifies differencing of the input variables The VAR

statement is required

Differencing is specified by following the variable name with a list of difference

periods separated by commas See the section Stationarity and Differencing for

more information on differencing of input variables

The order in which variables are listed in the VAR statement controls the order in

which variables are included in the state vector Usually potential inputs should be

listed before potential outputs

For example assuming the input data are monthly the following VAR statement

specifies modeling and forecasting of the one period and seasonal second differenceof X and Y

var x(112) y(112)

In this example the vector time series analyzed is

x

t

=

1 B 1 B

1 2

X

t

x

1 B 1 B

1 2

Y

t

y

where B represents the back shift operator and x and y represent the means of thedifferenced series If the NOCENTER option is specified the mean differences are

not subtracted

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Chapter 18 Details

Details

Missing Values

The STATESPACE procedure does not support missing values The procedure uses

the first contiguous group of observations with no missing values for any of the VARstatement variables Observations at the beginning of the data set with missing values

for any VAR statement variable are not used or included in the output data set

Stationarity and Differencing

The state space model used by the STATESPACE procedure assumes that the time

series are stationary Hence the data should be checked for stationarity One way

to check for stationarity is to plot the series A graph of series over time can show a

time trend or variability changes

You can also check stationarity by using the sample autocorrelation functions dis-

played by the ARIMA procedure The autocorrelation functions of nonstationaryseries tend to decay slowly See Chapter 7 ldquoThe ARIMA Procedurerdquo for more in-

formation

Another alternative is to use the STATIONARITY= option on the IDENTIFY state-

ment in PROC ARIMA to apply Dickey-Fuller tests for unit roots in the time series

See Chapter 7 ldquoThe ARIMA Procedurerdquo for more information on Dickey-Fuller unit

root tests

The most popular way to transform a nonstationary series to stationarity is by dif-

ferencing Differencing of the time series is specified in the VAR statement For

example to take a simple first difference of the series X use this statement

var x(1)

In this example the change in X from one period to the next is analyzed When the se-

ries has a seasonal pattern differencing at a period equal to the length of the seasonal

cycle may be desirable For example suppose the variable X is measured quarterly

and shows a seasonal cycle over the year You can use the following statement to

analyze the series of changes from the same quarter in the previous year

var x(4)

To difference twice add another differencing period to the list For ex-

ample the following statement analyzes the series of second differences

X

t

X

t 1

X

t 1

X

t 2

= X

t

2 X

t 1

+ X

t 2

var x(11)

The following statement analyzes the seasonal second difference series

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Part 2 General Information

var x(14)

The series modeled is the 1-period difference of the 4-period difference

X

t

X

t 4

X

t 1

X

t 5

= X

t

X

t 1

X

t 4

+ X

t 5

Another way to obtain stationary series is to use a regression on time to de-trend the

data If the time series has a deterministic linear trend regressing the series on time

produces residuals that should be stationary The following statements write residuals

of X and Y to the variable RX and RY in the output data set DETREND

data a

set a

t=_n_

run

proc reg data=a

model x y = t

output out=detrend r=rx ry

run

You then use PROC STATESPACE to forecast the de-trended series RX and RY A

disadvantage of this method is that you need to add the trend back to the forecast

series in an additional step A more serious disadvantage of the de-trending method

is that it assumes a deterministic trend In practice most time series appear to have a

stochastic rather than a deterministic trend Differencing is a more flexible and often

more appropriate method

There are several other methods to handle nonstationary time series For more infor-

mation and examples refer to Brockwell and Davis (1991)

Preliminary Autoregressive Models

After computing the sample autocovariance matrices PROC STATESPACE fits a

sequence of vector autoregressive models These preliminary autoregressive models

are used to estimate the autoregressive order of the process and limit the order of the

autocovariances considered in the state vector selection process

Yule-Walker Equations for Forward and Backward Models

Unlike a univariate autoregressive model a multivariate autoregressive model has

different forms depending on whether the present observation is being predicted from

the past observations or from the future observations

Letx

t

be the r -component stationary time series given by the VAR statement after

differencing and subtracting the vector of sample means (If the NOCENTER option

is specified the mean is not subtracted) Let n be the number of observations of x

t

from the input data set

Lete

t

be a vector white noise sequence with mean vector 0 and variance matrix

p

and letn

t

be a vector white noise sequence with mean vector 0 and variance matrix

p

Let p be the order of the vector autoregressive model forx

t

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Chapter 18 Details

The forward autoregressive form based on the past observations is written as follows

x

t

=

p

X

i = 1

p

i

x

t i

+ e

t

The backward autoregressive form based on the future observations is written as fol-

lows

x

t

=

p

X

i = 1

p

i

x

t + i

+ n

t

LettingE

denote the expected value operator the autocovariance sequence for thex

t

series

i

is

i

= E x

t

x

0

t i

The Yule-Walker equations for the autoregressive model that matches the first p ele-

ments of the autocovariance sequence are

2

6

6

4

0

1

p 1

0

1

0

p 2

0

p 1

0

p 2

0

3

7

7

5

2

6

6

4

p

1

p

2

p

p

3

7

7

5

=

2

6

6

4

1

2

p

3

7

7

5

and

2

6

6

6

4

0

0

1

0

p 1

1

0

0

p 2

p 1

p 2

0

3

7

7

7

5

2

6

6

4

p

1

p

2

p

p

3

7

7

5

=

2

6

6

4

0

1

0

2

0

p

3

7

7

5

Here

p

i

are the coefficient matrices for the past observation form of the vector au-toregressive model and

p

i

are the coefficient matrices for the future observation

form More information on the Yule-Walker equations in the multivariate setting can

be found in Whittle (1963) and Ansley and Newbold (1979)

The innovation variance matrices for the two forms can be written as follows

p

=

0

p

X

i = 1

p

i

0

i

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Part 2 General Information

p

=

0

p

X

i = 1

p

i

i

The autoregressive models are fit to the data using the preceding Yule-Walker equa-

tions with

i

replaced by the sample covariance sequenceC

i

The covariance matri-

ces are calculated as

C

i

=

1

N 1

N

X

t = i + 1

x

t

x

0

t i

Let b

p

b

p

b

p

and b

p

represent the Yule-Walker estimates of

p

p

p

and

p

respectively These matrices are written to an output data set when the OUTAR=

option is specified

When the PRINTOUT=LONG option is specified the sequence of matrices b

p

and

the corresponding correlation matrices are printed The sequence of matricesb

p isused to compute Akaike information criteria for selection of the autoregressive order

of the process

Akaike Information Criterion

The Akaike information criterion or AIC is defined as -2( maximum of log likeli-

hood )+2(number of parameters) Since the vector autoregressive models are esti-

mates from the Yule-Walker equations not by maximum likelihood the exact likeli-

hood values are not available for computing the AIC However for the vector autore-

gressive model the maximum of the log likelihood can be approximated as

l n L

n

2

l n j

b

p

j

Thus the AIC for the order p model is computed as

A I C

p

= n l n j

b

p

j + 2 p r

2

You can use the printed AIC array to compute a likelihood ratio test of the autoregres-

sive order The log-likelihood ratio test statistic for testing the order p model against

the order p 1

model is

n l n j

b

p

j + n l n j

b

p 1

j

This quantity is asymptotically distributed as a

2 withr

2 degrees of freedom if the

series is autoregressive of order p 1

It can be computed from the AIC array as

A I C

p 1

A I C

p

+ 2 r

2

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Chapter 18 Details

You can evaluate the significance of these test statistics with the PROBCHI function

in a SAS DATA step or with a

2 table

Determining the Autoregressive Order

Although the autoregressive models can be used for prediction their primary value is

to aid in the selection of a suitable portion of the sample covariance matrix for use in

computing canonical correlations If the multivariate time series x

t is of autoregres-sive order p then the vector of past values to lag p is considered to contain essentially

all the information relevant for prediction of future values of the time series

By default PROC STATESPACE selects the order p producing the autoregressive

model with the smallestA I C

p

If the value p for the minimumA I C

p

is less than the

value of the PASTMIN= option then p is set to the PASTMIN= value Alternatively

you can use the ARMAX= and PASTMIN= options to force PROC STATESPACE to

use an order you select

Significance Limits for Partial Autocorrelations

The STATESPACE procedure prints a schematic representation of the partial autocor-

relation matrices indicating which partial autocorrelations are significantly greater orsignificantly less than 0 Figure 1810 shows an example of this table

The STATESPACE Procedure

Schematic Representation of Partial Autocorrelations

NameLag 1 2 3 4 5 6 7 8 9 10

x ++ +

y ++

+ is gt 2std error - is lt -2std error is between

Figure 1810 Significant Partial Autocorrelations

The partial autocorrelations are from the sample partial autoregressive matrices b

p

p

The standard errors used for the significance limits of the partial autocorrelations are

computed from the sequence of matrices

p

and

p

Under the assumption that the observed series arises from an autoregressive process

of order p 1

the pth sample partial autoregressive matrix b

p

p

has an asymptotic

variance matrix 1

n

1

p

p

The significance limits for b

p

p

used in the schematic plot of the sample partial autore-

gressive sequence are derived by replacing

p

and

p

with their sample estimators

to produce the variance estimate as follows

d

V a r

b

p

p

=

1

n r p

b

1

p

b

p

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Part 2 General Information

Canonical Correlation Analysis

Given the order p letp

t

be the vector of current and past values relevant to prediction

of x

t + 1

p

t

= x

0

t

x

0

t 1

x

0

t p

0

Letf

t

be the vector of current and future values

f

t

= x

0

t

x

0

t + 1

x

0

t + p

0

In the canonical correlation analysis consider submatrices of the sample covariance

matrix of p

t

andf

t

This covariance matrixV

has a block Hankel form

V =

2

6

6

6

4

C

0

C

0

1

C

0

2

C

0

p

C

0

1

C

0

2

C

0

3

C

0

p + 1

C

0

p

C

0

p + 1

C

0

p + 2

C

0

2 p

3

7

7

7

5

State Vector Selection Process

The canonical correlation analysis forms a sequence of potential state vectorsz

j

t

Examine a sequencef

j

t

of subvectors of f

t

and form the submatrixV

j consisting

of the rows and columns of V

corresponding to the components of f

j

t

and compute

its canonical correlations

The smallest canonical correlation of V

j is then used in the selection of the com-

ponents of the state vector The selection process is described in the following For

more details about this process refer to Akaike (1976)

In the following discussion the notationx

t + k j t

denotes the wide sense conditional

expectation (best linear predictor) of x

t + k

given allx

s

with s less than or equal to t

In the notationx

i t + 1

the first subscript denotes the ith component of x

t + 1

The initial state vectorz

1

t

is set tox

t

The sequencef

j

t

is initialized by setting

f

1

t

= z

1

0

t

x

1 t + 1 j t

0

= x

0

t

x

1 t + 1 j t

0

That is start by considering whether to addx

1 t + 1 j t

to the initial state vectorz

1

t

The procedure forms the submatrixV

1 corresponding tof

1

t

and computes its canon-

ical correlations Denote the smallest canonical correlation of V

1 as

m i n

If

m i n

is

significantly greater than 0x

1 t + 1 j t

is added to the state vector

If the smallest canonical correlation of V

1 is not significantly greater than 0 then a

linear combination of f

1

t

is uncorrelated with the pastp

t

Assuming that the determi-

nant of C

0

is not 0 (that is no input series is a constant) you can take the coefficient

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Chapter 18 Details

of x

1 t + 1 j t

in this linear combination to be 1 Denote the coefficients of z

1

t

in this

linear combination as

This gives the relationship

x

1 t + 1 j t

=

0

x

t

Therefore the current state vector already contains all the past information useful forpredicting

x

1 t + 1

and any greater leads of x

1 t

The variablex

1 t + 1 j t

is not added to

the state vector nor are any termsx

1 t + k j t

considered as possible components of the

state vector The variablex

1

is no longer active for state vector selection

The process described forx

1 t + 1 j t

is repeated for the remaining elements of f

t

The

next candidate for inclusion in the state vector is the next component of f

t

corre-

sponding to an active variable Components of f

t

corresponding to inactive variables

that produced a zero

m i n

in a previous step are skipped

Denote the next candidate asx

l t + k j t

The vectorf

j

t

is formed from the current state

vector andx

l t + k j t

as follows

f

j

t

= z

j

0

t

x

l t + k j t

0

The matrixV

j is formed fromf

j

t

and its canonical correlations are computed The

smallest canonical correlation of V

j is judged to be either greater than or equal to 0

If it is judged to be greater than 0x

l t + k j t

is added to the state vector If it is judged

to be 0 then a linear combination of f

j

t

is uncorrelated with thep

t

and the variable

x

l

is now inactive

The state vector selection process continues until no active variables remain

Testing Significance of Canonical Correlations For each step in the canonical correlation sequence the significance of the small-

est canonical correlation

m i n

is judged by an information criterion from Akaike

(1976) This information criterion is

n l n 1

2

m i n

r p + 1 q + 1

where q is the dimension of f

j

t

at the current step r is the order of the state vector p

is the order of the vector autoregressive process and

is the value of the SIGCORR=

option The default is SIGCORR=2 If this information criterion is less than or equal

to 0

m i n

is taken to be 0 otherwise it is taken to be significantly greater than 0

(Do not confuse this information criterion with the AIC)

Variables inx

t + p j t

are not added in the model even with positive information crite-

rion because of the singularity of V

You can force the consideration of more candi-

date state variables by increasing the size of theV

matrix by specifying a PASTMIN=

option value larger than p

Printing the Canonical Correlations

To print the details of the canonical correlation analysis process specify the CAN-

CORR option in the PROC STATESPACE statement The CANCORR option prints

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Part 2 General Information

the candidate state vectors the canonical correlations and the information criteria for

testing the significance of the smallest canonical correlation

Bartlettrsquos

2 and its degrees of freedom are also printed when the CANCORR option

is specified The formula used for Bartlettrsquos

2 is

2

= n 5 r p + 1 q + 1 l n 1

2

m i n

withr p + 1 q + 1

degrees of freedom

Figure 1811 shows the output of the CANCORR option for the introductory example

shown in the Getting Started section of this chapter

The STATESPACE Procedure

Canonical Correlations Analysis

Information Chi

x(TT) y(TT) x(T+1T) Criterion Square DF

1 1 0237045 3566167 114505 4

Information Chi

x(TT) y(TT) x(T+1T) y(T+1T) Criterion Square DF

1 1 0238244 0056565 -535906 0636134 3

Information Chi

x(TT) y(TT) x(T+1T) x(T+2T) Criterion Square DF

1 1 0237602 0087493 -446312 1525353 3

Figure 1811 Canonical Correlations Analysis

New variables are added to the state vector if the information criteria are positive Inthis example

y

t + 1 j t

andx

t + 2 j t

are not added to the state space vector because the

information criteria for these models are negative

If the information criterion is nearly 0 then you may want to investigate models

that arise if the opposite decision is made regarding

m i n

This investigation can be

accomplished by using a FORM statement to specify part or all of the state vector

Preliminary Estimates of F

When a candidate variablex

l t + k j t

yields a zero

m i n

and is not added to the state

vector a linear combination of f

j

t

is uncorrelated with thep

t

Because of the method

used to construct thef

j

t

sequence the coefficient of x

l t + k j t

inl

can be taken as 1

Denote the coefficients of z

j

t

in this linear combination as l

This gives the relationship

x

l t + k j t

= l

0

z

j

t

The vectorl

is used as a preliminary estimate of the first r columns of the row of the

transition matrixF

corresponding tox

l t + k 1 j t

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Chapter 18 Details

Parameter Estimation

The model isz

t + 1

= F z

t

+ G e

t + 1

wheree

t

is a sequence of independent multivari-

ate normal innovations with mean vector 0 and variance

e e

The observed sequence

x

t

composes the first r components of z

t

and thusx

t

= H z

t

where H is ther s

matrix I

r

0

LetE

be ther n

matrix of innovations

E = e

1

e

n

If the number of observations n is reasonably large the log likelihood L can be

approximated up to an additive constant as follows

L =

n

2

l n j

e e

j

1

2

t r a c e

1

e e

E E

0

The elements of

e e

are taken as free parameters and are estimated as follows

S

0

=

1

n

E E

0

Replacing

e e

byS

0

in the likelihood equation the log likelihood up to an additive

constant is

L =

n

2

l n j S

0

j

Letting B be the backshift operator the formal relation betweenx

t

ande

t

is

x

t

= H I B F

1

G e

t

e

t

= H I B F

1

G

1

x

t

=

1

X

i = 0

i

x

t i

LettingC

i

be the ith lagged sample covariance of x

t

and neglecting end effects the

matrixS

0

is

S

0

=

1

X

i j = 0

i

C

i + j

0

j

For the computation of S

0

the infinite sum is truncated at the value of the KLAG=

option The value of the KLAG= option should be large enough that the sequence

i

is approximately 0 beyond that point

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Part 2 General Information

Let

be the vector of free parameters in theF

andG

matrices The derivative of the

log likelihood with respect to the parameter

is

L

=

n

2

t r a c e

S

1

0

S

0

The second derivative is

2

L

0

=

n

2

t r a c e

S

1

0

S

0

0

S

1

0

S

0

t r a c e

S

1

0

2

S

0

0

Near the maximum the first term is unimportant and the second term can be approx-

imated to give the following second derivative approximation

2

L

0

=

n t r a c e

S

1

0

E

E

0

0

The first derivative matrix and this second derivative matrix approximation are com-

puted from the sample covariance matrixC

0

and the truncated sequence

i

The

approximate likelihood function is maximized by a modified Newton-Raphson algo-

rithm employing these derivative matrices

The matrixS

0

is used as the estimate of the innovation covariance matrix

e e

The

negative of the inverse of the second derivative matrix at the maximum is used as an

approximate covariance matrix for the parameter estimates The standard errors of

the parameter estimates printed in the parameter estimates tables are taken from the

diagonal of this covariance matrix The parameter covariance matrix is printed when

the COVB option is specified

If the data are nearly nonstationary a better estimate of

e e

and the other parameters

can sometimes be obtained by specifying the RESIDEST option The RESIDEST

option estimates the parameters using conditional least squares instead of maximum

likelihood

The residuals are computed using the state space equation and the sample mean val-

ues of the variables in the model as start-up values The estimate of S

0

is then com-

puted using the residuals from the ith observation on where i is the maximum number

of times any variable occurs in the state vector A multivariate Gauss-Marquardt al-

gorithm is used to minimizej S

0

j

Refer to Harvey (1981a) for a further description

of this method

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Chapter 18 Details

Forecasting

Given estimates of F

G

and

e e

forecasts of x

t

are computed from the conditional

expectation of z

t

In forecasting the parameters F G and

e e

are replaced with the estimates or by val-

ues specified in the RESTRICT statement One-step-ahead forecasting is performedfor the observation

x

t

wheret n b

Heren

is the number of observations and

b is the value of the BACK= option For the observationx

t

wheret n b

m-

step-ahead forecasting is performed form = t n + b

The forecasts are generated

recursively with the initial conditionz

0

= 0

The m-step-ahead forecast of z

t + m

isz

t + m j t

wherez

t + m j t

denotes the conditional

expectation of z

t + m

given the information available at time t The m-step-ahead

forecast of x

t + m

isx

t + m j t

= H z

t + m j t

where the matrixH = I

r

0

Let

i

= F

i

G

Note that the lasts r

elements of z

t

consist of the elements of x

u j t

foru t

The state vector z

t + m

can be represented as

z

t + m

= F

m

z

t

+

m 1

X

i = 0

i

e

t + m i

Sincee

t + i j t

= 0

fori 0

the m-step-ahead forecastz

t + m j t

is

z

t + m j t

= F

m

z

t

= F z

t + m 1 j t

Therefore the m-step-ahead forecast of x

t + m

is

x

t + m j t

= H z

t + m j t

The m-step-ahead forecast error is

z

t + m

z

t + m j t

=

m 1

X

i = 0

i

e

t + m i

The variance of the m-step-ahead forecast error is

V

z m

=

m 1

X

i = 0

i

e e

0

i

LettingV

z 0

= 0

the variance of the m-step-ahead forecast error of z

t + m

V

z m

can

be computed recursively as follows

V

z m

= V

z m 1

+

m 1

e e

0

m 1

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Part 2 General Information

The variance of the m-step-ahead forecast error of x

t + m

is ther r

left upper sub-

matrix of V

z m

that is

V

x m

= H V

z m

H

0

Unless the NOCENTER option is specified the sample mean vector is added to the

forecast When differencing is specified the forecasts xt + m j t

plus the sample mean

vector are integrated back to produce forecasts for the original series

Lety

t

be the original series specified by the VAR statement with some 0 values

appended corresponding to the unobserved past observations Let B be the backshift

operator and let B

be thes s

matrix polynomial in the backshift operator

corresponding to the differencing specified by the VAR statement The off-diagonal

elements of

i

are 0 Note that

0

= I

s

whereI

s

is thes s

identity matrix Then

z

t

= B y

t

This gives the relationship

y

t

=

1

B z

t

=

1

X

i = 0

i

z

t i

where

1

B =

P

1

i = 0

i

B

i and

0

= I

s

The m-step-ahead forecast of y

t + m

is

y

t + m j t

=

m 1

X

i = 0

i

z

t + m i j t

+

1

X

i = m

i

z

t + m i

The m-step-ahead forecast error of y

t + m

is

m 1

X

i = 0

i

z

t + m i

z

t + m i j t

=

m 1

X

i = 0

i

X

u = 0

u

i u

e

t + m i

LettingV

y 0

= 0

the variance of the m-step-ahead forecast error of y

t + m

V

y m

is

V

y m

=

m 1

X

i = 0

i

X

u = 0

u

i u

e e

i

X

u = 0

u

i u

0

= V

y m 1

+

m 1

X

u = 0

u

m 1 u

e e

m 1

X

u = 0

u

m 1 u

0

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Chapter 18 Details

Relation of ARMA and State Space Forms

Every state space model has an ARMA representation and conversely every ARMA

model has a state space representation This section discusses this equivalence The

following material is adapted from Akaike (1974) where there is a more complete

discussion Pham-Dinh-Tuan (1978) also contains a discussion of this material

Suppose you are given the following ARMA model

B x

t

= B e

t

or in more detail

x

t

1

x

t 1

p

x

t p

= e

t

+

1

e

t 1

+ +

q

e

t q

(1)

wheree

t

is a sequence of independent multivariate normal random vectors with mean

0 and variance matrix

e e

B is the backshift operator (B x

t

= x

t 1

) B

and

B

are matrix polynomials in B andx

t

is the observed process

If the roots of the determinantial equationj B j = 0

are outside the unit circle in

the complex plane the model can also be written as

x

t

=

1

B B e

t

=

1

X

i = 0

i

e

t i

The

i

matrices are known as the impulse response matrices and can be computed

as

1

B B

You can assume p q

since if this is not initially true you can add more terms

i

that are identically 0 without changing the model

To write this set of equations in a state space form proceed as follows Letx

t + i j t

be

the conditional expectation of x

t + i

givenx

w

forw t

The following relations hold

x

t + i j t

=

1

X

j = i

j

e

t + i j

x

t + i j t + 1

= x

t + i j t

+

i 1

e

t + 1

However from equation (1) you can derive the following relationship

x

t + p j t

=

1

x

t + p 1 j t

+ +

p

x

t

(2)

Hence wheni = p

you can substitute forx

t + p j t

in the right-hand side of equation

(2) and close the system of equations

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Part 2 General Information

This substitution results in the following model in the state space form

z

t + 1

= F z

t

+ G e

t + 1

2

6

6

6

4

x

t + 1

x

t + 2 j t + 1

x

t + p j t + 1

3

7

7

7

5

=

2

6

6

4

0 I 0 0

0 0 I 0

p

p 1

1

3

7

7

5

2

6

6

6

4

x

t

x

t + 1 j t

x

t + p 1 j t

3

7

7

7

5

+

2

6

6

4

I

1

p 1

3

7

7

5

e

t + 1

Note that the state vectorz

t

is composed of conditional expectations of x

t

and the

first r components of z

t

are equal tox

t

The state space form can be cast into an ARMA form by solving the system of dif-

ference equations for the first r components

When converting from an ARMA form to a state space form you can generate a

state vector larger than needed that is the state space model may not be a minimal

representation When going from a state space form to an ARMA form you can have

nontrivial common factors in the autoregressive and moving average operators that

yield an ARMA model larger than necessary

If the state space form used is not a minimal representation some but not all compo-

nents of x

t + i j t

may be linearly dependent This situation corresponds to

p

p 1

being of less than full rank when B

and B

have no common nontrivial

left factors In this casez

t

consists of a subset of the possible components of

x

t + i j t

i = 1 2 p 1

However once a component of x

t + i j t

(for example the

jth one) is linearly dependent on the previous conditional expectations then all sub-

sequent jth components of x

t + k j t

fork i

must also be linearly dependent Note

that in this case equivalent but seemingly different structures can arise if the order of

the components withinx

t

is changed

OUT= Data Set

The forecasts are contained in the output data set specified by the OUT= option on the

PROC STATESPACE statement The OUT= data set contains the following variables

the BY variables

the ID variable

the VAR statement variables These variables contain the actual values from

the input data set

FORi numeric variables containing the forecasts The variable FORi contains

the forecasts for the ith variable in the VAR statement list Forecasts are one-

step-ahead predictions until the end of the data or until the observation specified

by the BACK= option

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Chapter 18 Details

RESi numeric variables containing the residual for the forecast of the ith vari-

able in the VAR statement list For forecast observations the actual values are

missing and the RESi variables contain missing values

STDi numeric variables containing the standard deviation for the forecast of

the ith variable in the VAR statement list The values of the STDi variables can

be used to construct univariate confidence limits for the corresponding fore-casts However such confidence limits do not take into account the covariance

of the forecasts

OUTAR= Data Set

The OUTAR= data set contains the estimates of the preliminary autoregressive mod-

els The OUTAR= data set contains the following variables

ORDER a numeric variable containing the order p of the autoregressive model

that the observation represents

AIC a numeric variable containing the value of the information criterionA I C

p

SIGFl numeric variables containing the estimate of the innovation covariance

matrices for the forward autoregressive models The variable SIGFl contains

the lth column of b

p

in the observations with ORDER= p

SIGBl numeric variables containing the estimate of the innovation covariance

matrices for the backward autoregressive models The variable SIGBl contains

the lth column of b

p

in the observations with ORDER= p

FORk ndashl numeric variables containing the estimates of the autoregressive pa-

rameter matrices for the forward models The variable FORk ndashl contains the

lth column of the lag k autoregressive parameter matrix b

p

k

in the observations

with ORDER= p

BACk ndashl numeric variables containing the estimates of the autoregressive pa-

rameter matrices for the backward models The variable BACk ndashl contains the

lth column of the lag k autoregressive parameter matrix b

p

k

in the observations

with ORDER= p

The estimates for the order p autoregressive model can be selected as those observa-

tions with ORDER= p Within these observations the k lth element of

p

i

is given by

the value of the FORindashl variable in the k th observation The k lth element of

p

i

is

given by the value of BACindashl variable in the k th observation The k lth element of

p

is given by SIGFl in the k th observation The k lth element of

p

is given by SIGBlin the k th observation

Table 181 shows an example of the OUTAR= data set with ARMAX=3 andx

t

of

dimension 2 In Table 181 i j

indicate the ijth element of the matrix

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Part 2 General Information

Table 181 Values in the OUTAR= Data Set

Obs ORDER AIC SIGF1 SIGF2 SIGB1 SIGB2 FOR1ndash 1 FOR1ndash 2 FOR2ndash 1 FOR2ndash 2 FOR3ndash 1

1 0 AIC0

0 1 1

0 1 2

0 1 1

0 1 2

2 0 AIC0

0 2 1

0 2 2

0 2 1

0 2 2

3 1 AIC1

1 1 1

1 1 2

1 1 1

1 1 2

1

1

1 1

1

1

1 2

4 1 AIC1

1 2 1

1 1 2

1 2 1

1 1 2

1

1

2 1

1

1

2 2

5 2 AIC2

2 1 1

2 1 2

2 1 1

2 1 2

2

1

1 1

2

1

1 2

2

2

1 1

2

2

1 2

6 2 AIC2

2 2 1

2 1 2

2 2 1

2 1 2

2

1

2 1

2

1

2 2

2

2

2 1

2

2

2 2

7 3 AIC3

3 1 1

3 1 2

3 1 1

3 1 2

3

1

1 1

3

1

1 2

3

2

1 1

3

2

1 2

3

3

1 1

8 3 AIC3

3 2 1

3 1 2

3 2 1

3 1 2

3

1

2 1

3

1

2 2

3

2

2 1

3

2

2 2

3

3

2 1

Obs FOR3ndash 2 BACK1ndash 1 BACK1ndash 2 BACK2ndash 1 BACK2ndash 2 BACK3ndash 1 BACK3ndash 2

1

2

3

1

1

1 1

1

1

1 2

4

1

1

2 1

1

1

2 2

5

2

1

1 1

2

1

1 2

2

2

1 1

2

2

1 2

6

2

1

2 1

2

1

2 2

2

2

2 1

2

2

2 2

7

3

3

1 2

3

1

1 1

3

1

1 2

3

2

1 1

3

2

1 2

3

3

1 1

3

3

1 2

8

3

3

2 2

3

1

2 1

3

1

2 2

3

2

2 1

3

2

2 2

3

3

2 1

3

3

2 2

The estimated autoregressive parameters can be used in the IML procedure to obtain

autoregressive estimates of the spectral density function or forecasts based on the

autoregressive models

OUTMODEL= Data Set

The OUTMODEL= data set contains the estimates of theF

andG

matrices and their

standard errors the names of the components of the state vector and the estimates of

the innovation covariance matrix The variables contained in the OUTMODEL= data

set are as follows

the BY variables

STATEVEC a character variable containing the name of the component of the

state vector corresponding to the observation The STATEVEC variable has

the value STD for standard deviations observations which contain the standarderrors for the estimates given in the preceding observation

Fndash j numeric variables containing the columns of theF

matrix The variable

Fndash j contains the jth column of F

The number of Fndash j variables is equal to

the value of the DIMMAX= option If the model is of smaller dimension the

extraneous variables are set to missing

Gndash j numeric variables containing the columns of theG

matrix The variable

Gndash j contains the jth column of G

The number of Gndash j variables is equal to r

the dimension of x

t

given by the number of variables in the VAR statement

SIGndash j numeric variables containing the columns of the innovation covariance

matrix The variable SIGndash j contains the jth column of

e e

There are r vari-ables SIGndash j

Table 182 shows an example of the OUTMODEL= data set withx

t

= x

t

y

t

0

z

t

= x

t

y

t

x

t + 1 j t

0 and DIMMAX=4 In Table 182F

i j

andG

i j

are the ijth

elements of F

andG

respectively Note that all elements for Fndash4 are missing because

F

is a3 3

matrix

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Chapter 18 Details

Table 182 Value in the OUTMODEL= Data Set

Obs STATEVEC Fndash 1 Fndash 2 Fndash 3 Fndash 4 Gndash 1 Gndash 2 SIGndash 1 SIGndash 2

1 X(TT) 0 0 1 1 0

1 1

1 2

2 STD

3 Y(TT) F2 1

F2 2

F2 3

0 1

2 1

2 2

4 STD std F2 1

std F2 2

std F2 3

5 X(T+1T) F3 1

F3 2

F3 3

G3 1

G3 2

6 STD std F3 1

std F3 2

std F3 3

std G3 1

std G3 2

Printed Output

The printed output produced by the STATESPACE procedure is described in the fol-

lowing

1 descriptive statistics which include the number of observations used the

names of the variables their means and standard deviations (Std) and the dif-

ferencing operations used

2 the Akaike information criteria for the sequence of preliminary autoregressive

models

3 if the PRINTOUT=LONG option is specified the sample autocovariance ma-

trices of the input series at various lags

4 if the PRINTOUT=LONG option is specified the sample autocorrelation ma-

trices of the input series

5 a schematic representation of the autocorrelation matrices showing the signif-

icant autocorrelations

6 if the PRINTOUT=LONG option is specified the partial autoregressive ma-

trices (These are

p

p

as described in Preliminary Autoregressive Modelsearlier in this chapter)

7 a schematic representation of the partial autocorrelation matrices showing the

significant partial autocorrelations

8 the Yule-Walker estimates of the autoregressive parameters for the autoregres-

sive model with the minimum AIC

9 if the PRINTOUT=LONG option is specified the autocovariance matrices of

the residuals of the minimum AIC model This is the sequence of estimated

innovation variance matrices for the solutions of the Yule-Walker equations

10 if the PRINTOUT=LONG option is specified the autocorrelation matrices of the residuals of the minimum AIC model

11 If the CANCORR option is specified the canonical correlations analysis for

each potential state vector considered in the state vector selection process

This includes the potential state vector the canonical correlations the informa-

tion criterion for the smallest canonical correlation Bartlettrsquos

2 statistic (ldquoChi

Squarerdquo) for the smallest canonical correlation and the degrees of freedom of

Bartlettrsquos

2

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Part 2 General Information

12 the components of the chosen state vector

13 the preliminary estimate of the transition matrixF

the input matrixG

and

the variance matrix for the innovations

e e

14 if the ITPRINT option is specified the iteration history of the likelihood max-

imization For each iteration this shows the iteration number the number of step halvings the determinant of the innovation variance matrix the damping

factor Lambda and the values of the parameters

15 the state vector printed again to aid interpretation of the following listing of F

andG

16 the final estimate of the transition matrixF

17 the final estimate of the input matrixG

18 the final estimate of the variance matrix for the innovations

e e

19 a table listing the estimates of the free parameters inF

andG

and their standarderrors and t statistics

20 if the COVB option is specified the covariance matrix of the parameter esti-

mates

21 if the COVB option is specified the correlation matrix of the parameter esti-

mates

22 if the PRINT option is specified the forecasts and their standard errors

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Chapter 18 Details

ODS Table Names

PROC STATESPACE assigns a name to each table it creates You can use these

names to reference the table when using the Output Delivery System (ODS) to select

tables and create output data sets These names are listed in the following table For

more information on ODS see Chapter 6 ldquoUsing the Output Delivery Systemrdquo

Table 183 ODS Tables Produced in PROC STATESPACE

ODS Table Name Description Option

NObs Number of observations default

Summary Simple summary statistics table default

InfoCriterion Information criterion table default

CovLags Covariance Matrices of Input Series PRINTOUT=LONG

CorrLags Correlation Matrices of Input Series PRINTOUT=LONG

PartialAR Partial Autoregressive Matrices PRINTOUT=LONG

YWEstimates Yule-Walker Estimates for Minimum AIC default

CovResiduals Covariance of Residuals PRINTOUT=LONGCorrResiduals Residual Correlations from AR Models PRINTOUT=LONG

StateVector State vector table default

CorrGraph Schematic Representation of Correlations default

TransitionMatrix Transition Matrix default

InputMatrix Input Matrix default

VarInnov Variance Matrix for the Innovation default

CovB Covariance of Parameter Estimates COVB

CorrB Correlation of Parameter Estimates COVB

CanCorr Canonical Correlation Analysis CANCORR

IterHistory Iterative Fitting table ITPRINT

ParameterEstimates Parameter Estimates Table default

Forecasts Forecasts Table PRINT

ConvergenceStatus Convergence Status Table default

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Part 2 General Information

Example

Example 181 Series J from Box and Jenkins

This example analyzes the gas furnace data (series J) from Box and Jenkins (The

data are not shown Refer to Box and Jenkins (1976) for the data)

First a model is selected and fit automatically using the following statements

title1 rsquoGas Furnace Datarsquo

title2 rsquoBox amp Jenkins Series Jrsquo

title3 rsquoAutomatically Selected Modelrsquo

proc statespace data=seriesj cancorr

var x y

run

The results for the automatically selected model are shown in Output 1811

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Chapter 18 Example

Output 1811 Results for Automatically Selected Model

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Number of Observations 296

Standard

Variable Mean Error

x -005683 1072766

y 5350912 3202121

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Information Criterion for Autoregressive Models

Lag=0 Lag=1 Lag=2 Lag=3 Lag=4 Lag=5 Lag=6 Lag=7 Lag=8

6513862 -103357 -163296 -164512 -165152 -164891 -164934 -164315 -163856

Information

Criterion for

Autoregressive

Models

Lag=9 Lag=10

-16348 -163359

Schematic Representation of Correlations

NameLag 0 1 2 3 4 5 6 7 8 9 10

x +- +- +- +- +- +- +- +- +- +- +-

y -+ -+ -+ -+ -+ -+ -+ -+ -+ -+ -+

+ is gt 2std error - is lt -2std error is between

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Part 2 General Information

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Schematic Representation of Partial Autocorrelations

NameLag 1 2 3 4 5 6 7 8 9 10

x + - + -

y -+ -- - + +

+ is gt 2std error - is lt -2std error is between

Yule-Walker Estimates for Minimum AIC

------Lag=1------ ------Lag=2------ ------Lag=3------ ------Lag=4------

x y x y x y x y

x 1925887 -000124 -120166 0004224 0116918 -000867 0104236 0003268

y 0050496 1299793 -002046 -03277 -071182 -025701 0195411 0133417

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Canonical Correlations Analysis

Information Chi

x(TT) y(TT) x(T+1T) Criterion Square DF

1 1 0804883 2929228 3047481 8

Information Chix(TT) y(TT) x(T+1T) y(T+1T) Criterion Square DF

1 1 0906681 0607529 1223358 1347237 7

Information Chi

x(TT) y(TT) x (T+1T) y (T+1T) x (T+2T) Criterion Square DF

1 1 0909434 0610278 0186274 -154701 1034705 6

Information Chi

x(TT) y(TT) x (T+1T) y (T+1T) y (T+2T) Criterion Square DF

1 1 091014 0618937 0206823 0940392 1280924 6

Information Chi

x(TT) y(TT) x(T+1T) y(T+1T) y(T+2T) y(T+3T) Criterion Square DF

1 1 0912963 0628785 0226598 0083258 -794103 2041584 5

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Chapter 18 Example

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Selected Statespace Form and Preliminary Estimates

State Vector

x(TT) y(TT) x(T+1T) y(T+1T) y(T+2T)

Estimate of Transition Matrix

0 0 1 0 0

0 0 0 1 0

-084718 0026794 1711715 -005019 0

0 0 0 0 1

-019785 0334274 -018174 -123557 1787475

Input Matrix for Innovation

1 0

0 11925887 -000124

0050496 1299793

0142421 1361696

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Selected Statespace Form and Preliminary Estimates

Variance Matrix for Innovation

0035274 -000734

-000734 0097569

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Part 2 General Information

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Selected Statespace Form and Fitted Model

State Vector

x(TT) y(TT) x(T+1T) y(T+1T) y(T+2T)

Estimate of Transition Matrix

0 0 1 0 0

0 0 0 1 0

-086192 0030609 1724235 -005483 0

0 0 0 0 1

-034839 0292124 -009435 -109823 1671418

Input Matrix for Innovation

1 0

0 1192442 -000416

0015621 1258495

008058 1353204

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Selected Statespace Form and Fitted Model

Variance Matrix for Innovation

0035579 -000728

-000728 0095577

Parameter Estimates

Standard

Parameter Estimate Error t Value

F(31) -086192 0072961 -1181

F(32) 0030609 0026167 117

F(33) 1724235 0061599 2799

F(34) -005483 0030169 -182

F(51) -034839 0135253 -258

F(52) 0292124 0046299 631

F(53) -009435 0096527 -098F(54) -109823 0109525 -1003

F(55) 1671418 0083737 1996

G(31) 1924420 0058162 3309

G(32) -000416 0035255 -012

G(41) 0015621 0095771 016

G(42) 1258495 0055742 2258

G(51) 0080580 0151622 053

G(52) 1353204 0091388 1481

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Chapter 18 Example

The two series are believed to have a transfer function relation with the gas rate

(variable X) as the input and the CO2

concentration (variable Y) as the output Since

the parameter estimates shown in Output 1811 support this kind of model the model

is reestimated with the feedback parameters restricted to 0 The following statements

fit the transfer function (no feedback) model

title3 rsquoTransfer Function Modelrsquo

proc statespace data=seriesj printout=none

var x y

restrict f(32)=0 f(34)=0

g(32)=0 g(41)=0 g(51)=0

run

The last 2 pages of the output are shown in Output 1812

Output 1812 STATESPACE Output for Transfer Function Model

Gas Furnace Data

Box amp Jenkins Series J

Transfer Function Model

The STATESPACE Procedure

Selected Statespace Form and Fitted Model

State Vector

x(TT) y(TT) x(T+1T) y(T+1T) y(T+2T)

Estimate of Transition Matrix

0 0 1 0 0

0 0 0 1 0

-068882 0 1598717 0 0

0 0 0 0 1

-035944 0284179 -00963 -107313 1650047

Input Matrix for Innovation

1 0

0 1

1923446 0

0 1260856

0 1346332

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Part 2 General Information

Gas Furnace Data

Box amp Jenkins Series J

Transfer Function Model

The STATESPACE Procedure

Selected Statespace Form and Fitted Model

Variance Matrix for Innovation

0036995 -00072

-00072 0095712

Parameter Estimates

Standard

Parameter Estimate Error t Value

F(31) -068882 0050549 -1363

F(33) 1598717 0050924 3139

F(51) -035944 0229044 -157

F(52) 0284179 0096944 293

F(53) -009630 0140876 -068

F(54) -107313 0250385 -429

F(55) 1650047 0188533 875G(31) 1923446 0056328 3415

G(42) 1260856 0056464 2233

G(52) 1346332 0091086 1478

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Chapter 18 References

References

Akaike H (1974) Markovian Representation of Stochastic Processes and Its Ap-

plication to the Analysis of Autoregressive Moving Average Processes Annals

of the Institute of Statistical Mathematics 26 363-387

Akaike H (1976) Canonical Correlations Analysis of Time Series and the Use of

an Information Criterion in Advances and Case Studies in System Identification

eds R Mehra and DG Lainiotis New York Academic Press

Anderson TW (1971) The Statistical Analysis of Time Series New York John

Wiley amp Sons

Ansley CF and Newbold P (1979) Multivariate Partial Autocorrelations Pro-

ceedings of the Business and Economic Statistics Section American Statistical

Association 349-353

Box GEP and Jenkins G (1976) Time Series Analysis Forecasting and Control

San Francisco Holden-Day

Brockwell PJ and Davis RA (1991) Time Series Theory and Methods 2nd Edi-

tion Springer-Verlag

Hannan EJ (1970) Multiple Time Series New York John Wiley amp Sons

Hannan EJ (1976) The Identification and Parameterization of ARMAX and State

Space Forms Econometrica 44 713-722

Harvey AC (1981a) The Econometric Analysis of Time Series New York John

Wiley amp Sons

Harvey AC (1981b) Time Series Models New York John Wiley amp Sons

Jones RH (1974) Identification and Autoregressive Spectrum Estimation IEEE Transactions on Automatic Control AC-19 894-897

Pham-Dinh-Tuan (1978) On the Fitting of Multivariate Processes of the

Autoregressive-Moving Average Type Biometrika 65 99-107

Priestley MB (1980) System Identification Kalman Filtering and Stochastic Con-

trol in Directions in Time Series eds DR Brillinger and GC Tiao Institute

of Mathematical Statistics

Whittle P (1963) On the Fitting of Multivariate Autoregressions and the Approxi-

mate Canonical Factorization of a Spectral Density Matrix Biometrika 50 129-

134

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The correct bibliographic citation for this manual is as follows SAS Institute Inc SAS

ET S U serrsquos Gui de Version 8 Cary NC SAS Institute Inc 1999 1546 pp

SASET S Userrsquos Guide Version 8

Copyright copy 1999 by S AS I ns t it u t e I nc Ca ry N C U S A

ISBN 1ndash58025ndash489ndash6

All r ight s res erved P r int ed in t he U n it ed St a t es of A merica N o pa rt of t his p u blica t ionma y b e reprodu ced s t ored in a ret r ieva l s ys t em or t ra ns mit t ed in a ny form or b y a nymeans electronic mechanical photocopying or otherwise without the prior writtenpermission of the publisher SAS Institute Inc

US Government Restricted Rights Notice Use du plicat ion or disclosure of the

software by the government is subject to restrictions as set forth in FAR 52227ndash19Commercial Computer Softw ar e-Restricted Rights (J une 1987)

SAS Institute Inc SAS Campus Drive Cary North Carolina 27513

1st print ing October 1999

S AS reg a nd a ll ot her SA S I ns t it u t e I nc p rodu ct or s ervice na m es a re regis t ered t ra dema rkst d k f SA S I t it t I i t h U S A d t h t i reg i di t U SA

Page 11: Statespace Model

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Chapter 18 Getting Started

The STATESPACE Procedure

Selected Statespace Form and Fitted Model

State Vector

x(TT) y(TT) x(T+1T)

Estimate of Transition Matrix

0 0 1

0297273 047376 -001998

02301 0228425 0256031

Input Matrix for Innovation

1 0

0 1

0257284 0202273

Variance Matrix for Innovation

0945188 01007520100752 1014712

Figure 185 Fitted State Space Model

The estimated state space model shown in Figure 185 is

2

4

x

t + 1 j t + 1

y

t + 1 j t + 1

x

t + 2 j t + 1

3

5

=

2

4

0 0 1

0 2 9 7 0 4 7 4 0 0 2 0

0 2 3 0 0 2 2 8 0 2 5 6

3

5

2

4

x

t

y

t

x

t + 1 j t

3

5

+

2

4

1 0

0 1

0 2 5 7 0 2 0 2

3

5

e

t + 1

n

t + 1

v a r

e

t + 1

n

t + 1

=

0 9 4 5 0 1 0 1

0 1 0 1 1 0 1 5

The next page of the STATESPACE output lists the estimates of the free parameters

in theF

andG

matrices with standard errors and t statistics as shown in Figure 186

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Part 2 General Information

The STATESPACE Procedure

Parameter Estimates

Standard

Parameter Estimate Error t Value

F(21) 0297273 0129995 229

F(22) 0473760 0115688 410F(23) -001998 0313025 -006

F(31) 0230100 0126226 182

F(32) 0228425 0112978 202

F(33) 0256031 0305256 084

G(31) 0257284 0071060 362

G(32) 0202273 0068593 295

Figure 186 Final Parameter Estimates

Convergence Failures

The maximum likelihood estimates are computed by an iterative nonlinear maximiza-

tion algorithm which may not converge If the estimates fail to converge warning

messages are printed in the output

If you encounter convergence problems you should recheck the stationarity of the

data and ensure that the specified differencing orders are correct Attempting to fit

state space models to nonstationary data is a common cause of convergence failure

You can also use the MAXIT= option to increase the number of iterations allowed

or experiment with the convergence tolerance options DETTOL= and PARMTOL=

Forecast Data Set

The following statements print the output data set The WHERE statement excludes

the first 190 observations from the output so that only the forecasts and the last 10

actual observations are printed

proc print data=outid t

where t gt 190

run

The PROC PRINT output is shown in Figure 187

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Part 2 General Information

Figure 188 Plot of Forecasts

Controlling Printed Output

By default the STATESPACE procedure produces a large amount of printed output

The NOPRINT option suppresses all printed output You can suppress the printed

output for the autoregressive model selection process with the PRINTOUT=NONE

option The descriptive statistics and state space model estimation output are still

printed when PRINTOUT=NONE is specified You can produce more detailed output

with the PRINTOUT=LONG option and by specifying the printing control optionsCANCORR COVB and PRINT

Specifying the State Space Model

Instead of allowing the STATESPACE procedure to select the model automatically

you can use FORM and RESTRICT statements to specify a state space model

Specifying the State Vector

Use the FORM statement to control the form of the state vector You can use this

feature to force PROC STATESPACE to estimate and forecast a model different from

the model it would select automatically You can also use this feature to reestimate

the automatically selected model (possibly with restrictions) without repeating thecanonical correlation analysis

The FORM statement specifies the number of lags of each variable to include in

the state vector For example the statement FORM X 3 forces the state vector to

includex

t j t

x

t + 1 j t

andx

t + 2 j t

The following statement specifies the state vector

x

t j t

y

t j t

x

t + 1 j t

which is the same state vector selected in the preceding example

f o r m x 2 y 1

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Chapter 18 Getting Started

You can specify the form for only some of the variables and allow PROC STATES-

PACE to select the form for the other variables If only some of the variables are

specified in the FORM statement canonical correlation analysis is used to determine

the number of lags included in the state vector for the remaining variables not spec-

ified by the FORM statement If the FORM statement includes specifications for all

the variables listed in the VAR statement the state vector is completely defined and

the canonical correlation analysis is not performed

Restricting the F and G matrices

After you know the form of the state vector you can use the RESTRICT statement

to fix some parameters in theF

andG

matrices to specified values One use of this

feature is to remove insignificant parameters by restricting them to 0

In the introductory example shown in the preceding section the F[23] parameter is

not significant (The parameters estimation output shown in Figure 186 gives the t

statistic for F[23] as -006 F[33] and F[31] also have low significance witht 2

)

The following statements reestimate this model with F[23] restricted to 0 The

FORM statement is used to specify the state vector and thus bypass the canonicalcorrelation analysis

proc statespace data=in out=out lead=10

var x(1) y(1)

id t

form x 2 y 1

restrict f(23)=0

run

The final estimates produced by these statements are shown in Figure 189

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Part 2 General Information

The STATESPACE Procedure

Selected Statespace Form and Fitted Model

State Vector

x(TT) y(TT) x(T+1T)

Estimate of Transition Matrix

0 0 1

0290051 0467468 0

0227051 0226139 026436

Input Matrix for Innovation

1 0

0 1

0256826 0202022

Variance Matrix for Innovation

0945175 01006960100696 1014733

The STATESPACE Procedure

Parameter Estimates

Standard

Parameter Estimate Error t Value

F(21) 0290051 0063904 454

F(22) 0467468 0060430 774

F(31) 0227051 0125221 181

F(32) 0226139 0111711 202

F(33) 0264360 0299537 088G(31) 0256826 0070994 362

G(32) 0202022 0068507 295

Figure 189 Results using RESTRICT Statement

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Chapter 18 Syntax

Syntax

The STATESPACE procedure uses the following statements

PROC STATESPACE options

BY variable FORM variable value

ID variable

INITIAL F(rowcolumn)=value

G(rowcolumn)=value

RESTRICT F(rowcolumn)=value

G(rowcolumn)=value

VAR variable (difference difference

)

Functional Summary

The statements and options used by PROC STATESPACE are summarized in the

following table

Description Statement Option

Input Data Set Options

specify the input data set PROC STATESPACE DATA=

prevent subtraction of sample mean PROC STATESPACE NOCENTER

specify the ID variable ID

specify the observed series and differencing VAR

Options for Autoregressive Estimates

specify the maximum order PROC STATESPACE ARMAX=

specify maximum lag for autocovariances PROC STATESPACE LAGMAX=

output only minimum AIC model PROC STATESPACE MINIC

specify the amount of detail printed PROC STATESPACE PRINTOUT=

write preliminary AR models to a data set PROC STATESPACE OUTAR=

Options for Canonical Correlation Analysis

print the sequence of canonical correlations PROC STATESPACE CANCORR

specify upper limit of dimension of state

vector

PROC STATESPACE DIMMAX=

specify the minimum number of lags PROC STATESPACE PASTMIN=

specify the multiplier of the degrees of

freedom

PROC STATESPACE SIGCORR=

Options for State Space Model Estimation

specify starting values INITIAL

print covariance matrix of parameter estimates PROC STATESPACE COVB

specify the convergence criterion PROC STATESPACE DETTOL=

specify the convergence criterion PROC STATESPACE PARMTOL=

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Part 2 General Information

Description Statement Option

print the details of the iterations PROC STATESPACE ITPRINT

specify an upper limit of the number of lags PROC STATESPACE KLAG=

specify maximum number of iterations

allowed

PROC STATESPACE MAXIT=

suppress the final estimation PROC STATESPACE NOEST

write the state space model parameter esti-

mates to an output data set

PROC STATESPACE OUTMODEL=

use conditional least squares for final estimates PROC STATESPACE RESIDEST

specify criterion for testing for singularity PROC STATESPACE SINGULAR=

Options for Forecasting

start forecasting before end of the input data PROC STATESPACE BACK=

specify the time interval between observations PROC STATESPACE INTERVAL=

specify multiple periods in the time series PROC STATESPACE INTPER=specify how many periods to forecast PROC STATESPACE LEAD=

specify the output data set for forecasts PROC STATESPACE OUT=

print forecasts PROC STATESPACE PRINT

Options to Specify the State Space Model

specify the state vector FORM

specify the parameter values RESTRICT

BY Groups

specify BY-group processing BY

Printing

suppresses all printed output NOPRINT

PROC STATESPACE Statement

PROC STATESPACE options

The following options can be specified in the PROC STATESPACE statement

Printing Options NOPRINT

suppresses all printed output

Input Data Options

DATA= SAS-data-set

specifies the name of the SAS data set to be used by the procedure If the DATA=

option is omitted the most recently created SAS data set is used

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Chapter 18 Syntax

LAGMAX= k

specifies the number of lags for which the sample autocovariance matrix is computed

The LAGMAX= option controls the number of lags printed in the schematic repre-

sentation of the autocorrelations

The sample autocovariance matrix of lag i denoted asC

i

is computed as

C

i

=

1

N 1

N

X

t = 1 + i

x

t

x

0

t i

wherex

t

is the differenced and centered data andN

is the number of observations

(If the NOCENTER option is specified 1 is not subtracted fromN

) LAGMAX= k

specifies thatC

0

throughC

k

are computed The default is LAGMAX=10

NOCENTER

prevents subtraction of the sample mean from the input series (after any specified

differencing) before the analysis

Options for Preliminary Autoregressive Models ARMAX= n

specifies the maximum order of the preliminary autoregressive models The AR-

MAX= option controls the autoregressive orders for which information criteria are

printed and controls the number of lags printed in the schematic representation of

partial autocorrelations The default is ARMAX=10 See Preliminary Autoregres-

sive Models later in this chapter for details

MINIC

writes to the OUTAR= data set only the preliminary Yule-Walker estimates for the

VAR model producing the minimum AIC See OUTAR= Data Set later in this chap-

ter for details

OUTAR= SAS-data-set

writes the Yule-Walker estimates of the preliminary autoregressive models to a SAS

data set See OUTAR= Data Set later in this chapter for details

PRINTOUT= SHORT | LONG | NONE

determines the amount of detail printed PRINTOUT=LONG prints the lagged co-

variance matrices the partial autoregressive matrices and estimates of the resid-

ual covariance matrices from the sequence of autoregressive models PRINT-

OUT=NONE suppresses the output for the preliminary autoregressive models The

descriptive statistics and state space model estimation output are still printed when

PRINTOUT=NONE is specified PRINTOUT=SHORT is the default

Canonical Correlation Analysis Options

CANCORR

prints the canonical correlations and information criterion for each candidate state

vector considered See Canonical Correlation Analysis later in this chapter for

details

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Part 2 General Information

DIMMAX= n

specifies the upper limit to the dimension of the state vector The DIMMAX= option

can be used to limit the size of the model selected The default is DIMMAX=10

PASTMIN= n

specifies the minimum number of lags to include in the canonical correlation analy-

sis The default is PASTMIN=0 See Canonical Correlation Analysis later in thischapter for details

SIGCORR= value

specifies the multiplier of the degrees of freedom for the penalty term in the informa-

tion criterion used to select the state space form The default is SIGCORR=2 The

larger the value of the SIGCORR= option the smaller the state vector tends to be

Hence a large value causes a simpler model to be fit See Canonical Correlations

Analysis later in this chapter for details

State Space Model Estimation Options

COVB

prints the inverse of the observed information matrix for the parameter estimatesThis matrix is an estimate of the covariance matrix for the parameter estimates

DETTOL= value

specifies the convergence criterion The DETTOL= and PARMTOL= option values

are used together to test for convergence of the estimation process If during an

iteration the relative change of the parameter estimates is less than the PARMTOL=

value and the relative change of the determinant of the innovation variance matrix

is less than the DETTOL= value then iteration ceases and the current estimates are

accepted The default is DETTOL=1E-5

ITPRINT

prints the iterations during the estimation process

KLAG= n

sets an upper limit for the number of lags of the sample autocovariance matrix used

in computing the approximate likelihood function If the data have a strong moving

average character a larger KLAG= value may be necessary to obtain good estimates

The default is KLAG=15 See Parameter Estimation later in this chapter for details

MAXIT= n

sets an upper limit to the number of iterations in the maximum likelihood or condi-

tional least-squares estimation The default is MAXIT=50

NOEST

suppresses the final maximum likelihood estimation of the selected model

OUTMODEL= SAS-data-set

writes the parameter estimates and their standard errors to a SAS data set See OUT-

MODEL= Data Set later in this chapter for details

PARMTOL= value

specifies the convergence criterion The DETTOL= and PARMTOL= option values

are used together to test for convergence of the estimation process If during an

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Chapter 18 Syntax

iteration the relative change of the parameter estimates is less than the PARMTOL=

value and the relative change of the determinant of the innovation variance matrix

is less than the DETTOL= value then iteration ceases and the current estimates are

accepted The default is PARMTOL=001

RESIDEST

computes the final estimates using conditional least squares on the raw data Thistype of estimation may be more stable than the default maximum likelihood method

but is usually more computationally expensive See Parameter Estimation later in

this chapter for details of the conditional least squares method

SINGULAR= value

specifies the criterion for testing for singularity of a matrix A matrix is declared

singular if a scaled pivot is less than the SINGULAR= value when sweeping the

matrix The default is SINGULAR=1E-7

Forecasting Options

BACK= n

starts forecasting n periods before the end of the input data The BACK= option valuemust not be greater than the number of observations The default is BACK=0

INTERVAL= interval

specifies the time interval between observations The INTERVAL= value is used

in conjunction with the ID variable to check that the input data are in order and

have no missing periods The INTERVAL= option is also used to extrapolate the ID

values past the end of the input data See Chapter 3 ldquoDate Intervals Formats and

Functionsrdquo for details on the INTERVAL= values allowed

INTPER= n

specifies that each input observation corresponds to n time periods For example

the options INTERVAL=MONTH and INTPER=2 specify bimonthly data and are

equivalent to specifying INTERVAL=MONTH2 If the INTERVAL= option is not

specified the INTPER= option controls the increment used to generate ID values for

the forecast observations The default is INTPER=1

LEAD= n

specifies how many forecast observations are produced The forecasts start at the

point set by the BACK= option The default is LEAD=0 which produces no fore-

casts

OUT= SAS-data-set

writes the residuals actual values forecasts and forecast standard errors to a SAS

data set See OUT= Data Set later in this chapter for details

PRINT

prints the forecasts

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Part 2 General Information

BY Statement

BY variable

A BY statement can be used with the STATESPACE procedure to obtain separateanalyses on observations in groups defined by the BY variables

FORM Statement

FORM variable value

The FORM statement specifies the number of times a variable is included in the state

vector Values can be specified for any variable listed in the VAR statement If a

value is specified for each variable in the VAR statement the state vector for the state

space model is entirely specified and automatic selection of the state space model is

not performed

The FORM statement forces the state vectorz

t

to contain a specific variable a given

number of times For example if Y is one of the variables inx

t

then the statement

form y 3

forces the state vector to containY

t

Y

t + 1 j t

andY

t + 2 j t

possibly along with other

variables

The following statements illustrate the use of the FORM statement

proc statespace data=in

var x y

f o r m x 3 y 2

run

These statements fit a state space model with the following state vector

z

t

=

2

6

6

6

6

4

x

t j t

y

t j t

x

t + 1 j t

y

t + 1 j t

x

t + 2 j t

3

7

7

7

7

5

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Chapter 18 Syntax

ID Statement

ID variable

The ID statement specifies a variable that identifies observations in the input data setThe variable specified in the ID statement is included in the OUT= data set The

values of the ID variable are extrapolated for the forecast observations based on the

values of the INTERVAL= and INTPER= options

INITIAL Statement

INITIAL F (rowcolumn)= value G(row column)= value

The INITIAL statement gives initial values to the specified elements of theF

andG

matrices These initial values are used as starting values for the iterative estimation

Parts of theF

andG

matrices represent fixed structural identities If an element

specified is a fixed structural element instead of a free parameter the corresponding

initialization is ignored

The following is an example of an INITIAL statement

initial f(32)=0 g(41)=0 g(51)=0

RESTRICT Statement

RESTRICT F(rowcolumn)= value G(rowcolumn)= value

The RESTRICT statement restricts the specified elements of theF

andG

matrices

to the specified values

To use the restrict statement you need to know the form of the model Either specify

the form of the model with the FORM statement or do a preliminary run perhaps

with the NOEST option to find the form of the model that PROC STATESPACE

selects for the data

The following is an example of a RESTRICT statement

restrict f(32)=0 g(41)=0 g(51)=0

Parts of theF

andG

matrices represent fixed structural identities If a restriction is

specified for an element that is a fixed structural element instead of a free parameter

the restriction is ignored

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Part 2 General Information

VAR Statement

VAR variable (difference difference )

The VAR statement specifies the variables in the input data set to model and fore-cast The VAR statement also specifies differencing of the input variables The VAR

statement is required

Differencing is specified by following the variable name with a list of difference

periods separated by commas See the section Stationarity and Differencing for

more information on differencing of input variables

The order in which variables are listed in the VAR statement controls the order in

which variables are included in the state vector Usually potential inputs should be

listed before potential outputs

For example assuming the input data are monthly the following VAR statement

specifies modeling and forecasting of the one period and seasonal second differenceof X and Y

var x(112) y(112)

In this example the vector time series analyzed is

x

t

=

1 B 1 B

1 2

X

t

x

1 B 1 B

1 2

Y

t

y

where B represents the back shift operator and x and y represent the means of thedifferenced series If the NOCENTER option is specified the mean differences are

not subtracted

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Chapter 18 Details

Details

Missing Values

The STATESPACE procedure does not support missing values The procedure uses

the first contiguous group of observations with no missing values for any of the VARstatement variables Observations at the beginning of the data set with missing values

for any VAR statement variable are not used or included in the output data set

Stationarity and Differencing

The state space model used by the STATESPACE procedure assumes that the time

series are stationary Hence the data should be checked for stationarity One way

to check for stationarity is to plot the series A graph of series over time can show a

time trend or variability changes

You can also check stationarity by using the sample autocorrelation functions dis-

played by the ARIMA procedure The autocorrelation functions of nonstationaryseries tend to decay slowly See Chapter 7 ldquoThe ARIMA Procedurerdquo for more in-

formation

Another alternative is to use the STATIONARITY= option on the IDENTIFY state-

ment in PROC ARIMA to apply Dickey-Fuller tests for unit roots in the time series

See Chapter 7 ldquoThe ARIMA Procedurerdquo for more information on Dickey-Fuller unit

root tests

The most popular way to transform a nonstationary series to stationarity is by dif-

ferencing Differencing of the time series is specified in the VAR statement For

example to take a simple first difference of the series X use this statement

var x(1)

In this example the change in X from one period to the next is analyzed When the se-

ries has a seasonal pattern differencing at a period equal to the length of the seasonal

cycle may be desirable For example suppose the variable X is measured quarterly

and shows a seasonal cycle over the year You can use the following statement to

analyze the series of changes from the same quarter in the previous year

var x(4)

To difference twice add another differencing period to the list For ex-

ample the following statement analyzes the series of second differences

X

t

X

t 1

X

t 1

X

t 2

= X

t

2 X

t 1

+ X

t 2

var x(11)

The following statement analyzes the seasonal second difference series

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Part 2 General Information

var x(14)

The series modeled is the 1-period difference of the 4-period difference

X

t

X

t 4

X

t 1

X

t 5

= X

t

X

t 1

X

t 4

+ X

t 5

Another way to obtain stationary series is to use a regression on time to de-trend the

data If the time series has a deterministic linear trend regressing the series on time

produces residuals that should be stationary The following statements write residuals

of X and Y to the variable RX and RY in the output data set DETREND

data a

set a

t=_n_

run

proc reg data=a

model x y = t

output out=detrend r=rx ry

run

You then use PROC STATESPACE to forecast the de-trended series RX and RY A

disadvantage of this method is that you need to add the trend back to the forecast

series in an additional step A more serious disadvantage of the de-trending method

is that it assumes a deterministic trend In practice most time series appear to have a

stochastic rather than a deterministic trend Differencing is a more flexible and often

more appropriate method

There are several other methods to handle nonstationary time series For more infor-

mation and examples refer to Brockwell and Davis (1991)

Preliminary Autoregressive Models

After computing the sample autocovariance matrices PROC STATESPACE fits a

sequence of vector autoregressive models These preliminary autoregressive models

are used to estimate the autoregressive order of the process and limit the order of the

autocovariances considered in the state vector selection process

Yule-Walker Equations for Forward and Backward Models

Unlike a univariate autoregressive model a multivariate autoregressive model has

different forms depending on whether the present observation is being predicted from

the past observations or from the future observations

Letx

t

be the r -component stationary time series given by the VAR statement after

differencing and subtracting the vector of sample means (If the NOCENTER option

is specified the mean is not subtracted) Let n be the number of observations of x

t

from the input data set

Lete

t

be a vector white noise sequence with mean vector 0 and variance matrix

p

and letn

t

be a vector white noise sequence with mean vector 0 and variance matrix

p

Let p be the order of the vector autoregressive model forx

t

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Chapter 18 Details

The forward autoregressive form based on the past observations is written as follows

x

t

=

p

X

i = 1

p

i

x

t i

+ e

t

The backward autoregressive form based on the future observations is written as fol-

lows

x

t

=

p

X

i = 1

p

i

x

t + i

+ n

t

LettingE

denote the expected value operator the autocovariance sequence for thex

t

series

i

is

i

= E x

t

x

0

t i

The Yule-Walker equations for the autoregressive model that matches the first p ele-

ments of the autocovariance sequence are

2

6

6

4

0

1

p 1

0

1

0

p 2

0

p 1

0

p 2

0

3

7

7

5

2

6

6

4

p

1

p

2

p

p

3

7

7

5

=

2

6

6

4

1

2

p

3

7

7

5

and

2

6

6

6

4

0

0

1

0

p 1

1

0

0

p 2

p 1

p 2

0

3

7

7

7

5

2

6

6

4

p

1

p

2

p

p

3

7

7

5

=

2

6

6

4

0

1

0

2

0

p

3

7

7

5

Here

p

i

are the coefficient matrices for the past observation form of the vector au-toregressive model and

p

i

are the coefficient matrices for the future observation

form More information on the Yule-Walker equations in the multivariate setting can

be found in Whittle (1963) and Ansley and Newbold (1979)

The innovation variance matrices for the two forms can be written as follows

p

=

0

p

X

i = 1

p

i

0

i

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Part 2 General Information

p

=

0

p

X

i = 1

p

i

i

The autoregressive models are fit to the data using the preceding Yule-Walker equa-

tions with

i

replaced by the sample covariance sequenceC

i

The covariance matri-

ces are calculated as

C

i

=

1

N 1

N

X

t = i + 1

x

t

x

0

t i

Let b

p

b

p

b

p

and b

p

represent the Yule-Walker estimates of

p

p

p

and

p

respectively These matrices are written to an output data set when the OUTAR=

option is specified

When the PRINTOUT=LONG option is specified the sequence of matrices b

p

and

the corresponding correlation matrices are printed The sequence of matricesb

p isused to compute Akaike information criteria for selection of the autoregressive order

of the process

Akaike Information Criterion

The Akaike information criterion or AIC is defined as -2( maximum of log likeli-

hood )+2(number of parameters) Since the vector autoregressive models are esti-

mates from the Yule-Walker equations not by maximum likelihood the exact likeli-

hood values are not available for computing the AIC However for the vector autore-

gressive model the maximum of the log likelihood can be approximated as

l n L

n

2

l n j

b

p

j

Thus the AIC for the order p model is computed as

A I C

p

= n l n j

b

p

j + 2 p r

2

You can use the printed AIC array to compute a likelihood ratio test of the autoregres-

sive order The log-likelihood ratio test statistic for testing the order p model against

the order p 1

model is

n l n j

b

p

j + n l n j

b

p 1

j

This quantity is asymptotically distributed as a

2 withr

2 degrees of freedom if the

series is autoregressive of order p 1

It can be computed from the AIC array as

A I C

p 1

A I C

p

+ 2 r

2

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Chapter 18 Details

You can evaluate the significance of these test statistics with the PROBCHI function

in a SAS DATA step or with a

2 table

Determining the Autoregressive Order

Although the autoregressive models can be used for prediction their primary value is

to aid in the selection of a suitable portion of the sample covariance matrix for use in

computing canonical correlations If the multivariate time series x

t is of autoregres-sive order p then the vector of past values to lag p is considered to contain essentially

all the information relevant for prediction of future values of the time series

By default PROC STATESPACE selects the order p producing the autoregressive

model with the smallestA I C

p

If the value p for the minimumA I C

p

is less than the

value of the PASTMIN= option then p is set to the PASTMIN= value Alternatively

you can use the ARMAX= and PASTMIN= options to force PROC STATESPACE to

use an order you select

Significance Limits for Partial Autocorrelations

The STATESPACE procedure prints a schematic representation of the partial autocor-

relation matrices indicating which partial autocorrelations are significantly greater orsignificantly less than 0 Figure 1810 shows an example of this table

The STATESPACE Procedure

Schematic Representation of Partial Autocorrelations

NameLag 1 2 3 4 5 6 7 8 9 10

x ++ +

y ++

+ is gt 2std error - is lt -2std error is between

Figure 1810 Significant Partial Autocorrelations

The partial autocorrelations are from the sample partial autoregressive matrices b

p

p

The standard errors used for the significance limits of the partial autocorrelations are

computed from the sequence of matrices

p

and

p

Under the assumption that the observed series arises from an autoregressive process

of order p 1

the pth sample partial autoregressive matrix b

p

p

has an asymptotic

variance matrix 1

n

1

p

p

The significance limits for b

p

p

used in the schematic plot of the sample partial autore-

gressive sequence are derived by replacing

p

and

p

with their sample estimators

to produce the variance estimate as follows

d

V a r

b

p

p

=

1

n r p

b

1

p

b

p

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Part 2 General Information

Canonical Correlation Analysis

Given the order p letp

t

be the vector of current and past values relevant to prediction

of x

t + 1

p

t

= x

0

t

x

0

t 1

x

0

t p

0

Letf

t

be the vector of current and future values

f

t

= x

0

t

x

0

t + 1

x

0

t + p

0

In the canonical correlation analysis consider submatrices of the sample covariance

matrix of p

t

andf

t

This covariance matrixV

has a block Hankel form

V =

2

6

6

6

4

C

0

C

0

1

C

0

2

C

0

p

C

0

1

C

0

2

C

0

3

C

0

p + 1

C

0

p

C

0

p + 1

C

0

p + 2

C

0

2 p

3

7

7

7

5

State Vector Selection Process

The canonical correlation analysis forms a sequence of potential state vectorsz

j

t

Examine a sequencef

j

t

of subvectors of f

t

and form the submatrixV

j consisting

of the rows and columns of V

corresponding to the components of f

j

t

and compute

its canonical correlations

The smallest canonical correlation of V

j is then used in the selection of the com-

ponents of the state vector The selection process is described in the following For

more details about this process refer to Akaike (1976)

In the following discussion the notationx

t + k j t

denotes the wide sense conditional

expectation (best linear predictor) of x

t + k

given allx

s

with s less than or equal to t

In the notationx

i t + 1

the first subscript denotes the ith component of x

t + 1

The initial state vectorz

1

t

is set tox

t

The sequencef

j

t

is initialized by setting

f

1

t

= z

1

0

t

x

1 t + 1 j t

0

= x

0

t

x

1 t + 1 j t

0

That is start by considering whether to addx

1 t + 1 j t

to the initial state vectorz

1

t

The procedure forms the submatrixV

1 corresponding tof

1

t

and computes its canon-

ical correlations Denote the smallest canonical correlation of V

1 as

m i n

If

m i n

is

significantly greater than 0x

1 t + 1 j t

is added to the state vector

If the smallest canonical correlation of V

1 is not significantly greater than 0 then a

linear combination of f

1

t

is uncorrelated with the pastp

t

Assuming that the determi-

nant of C

0

is not 0 (that is no input series is a constant) you can take the coefficient

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Chapter 18 Details

of x

1 t + 1 j t

in this linear combination to be 1 Denote the coefficients of z

1

t

in this

linear combination as

This gives the relationship

x

1 t + 1 j t

=

0

x

t

Therefore the current state vector already contains all the past information useful forpredicting

x

1 t + 1

and any greater leads of x

1 t

The variablex

1 t + 1 j t

is not added to

the state vector nor are any termsx

1 t + k j t

considered as possible components of the

state vector The variablex

1

is no longer active for state vector selection

The process described forx

1 t + 1 j t

is repeated for the remaining elements of f

t

The

next candidate for inclusion in the state vector is the next component of f

t

corre-

sponding to an active variable Components of f

t

corresponding to inactive variables

that produced a zero

m i n

in a previous step are skipped

Denote the next candidate asx

l t + k j t

The vectorf

j

t

is formed from the current state

vector andx

l t + k j t

as follows

f

j

t

= z

j

0

t

x

l t + k j t

0

The matrixV

j is formed fromf

j

t

and its canonical correlations are computed The

smallest canonical correlation of V

j is judged to be either greater than or equal to 0

If it is judged to be greater than 0x

l t + k j t

is added to the state vector If it is judged

to be 0 then a linear combination of f

j

t

is uncorrelated with thep

t

and the variable

x

l

is now inactive

The state vector selection process continues until no active variables remain

Testing Significance of Canonical Correlations For each step in the canonical correlation sequence the significance of the small-

est canonical correlation

m i n

is judged by an information criterion from Akaike

(1976) This information criterion is

n l n 1

2

m i n

r p + 1 q + 1

where q is the dimension of f

j

t

at the current step r is the order of the state vector p

is the order of the vector autoregressive process and

is the value of the SIGCORR=

option The default is SIGCORR=2 If this information criterion is less than or equal

to 0

m i n

is taken to be 0 otherwise it is taken to be significantly greater than 0

(Do not confuse this information criterion with the AIC)

Variables inx

t + p j t

are not added in the model even with positive information crite-

rion because of the singularity of V

You can force the consideration of more candi-

date state variables by increasing the size of theV

matrix by specifying a PASTMIN=

option value larger than p

Printing the Canonical Correlations

To print the details of the canonical correlation analysis process specify the CAN-

CORR option in the PROC STATESPACE statement The CANCORR option prints

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Part 2 General Information

the candidate state vectors the canonical correlations and the information criteria for

testing the significance of the smallest canonical correlation

Bartlettrsquos

2 and its degrees of freedom are also printed when the CANCORR option

is specified The formula used for Bartlettrsquos

2 is

2

= n 5 r p + 1 q + 1 l n 1

2

m i n

withr p + 1 q + 1

degrees of freedom

Figure 1811 shows the output of the CANCORR option for the introductory example

shown in the Getting Started section of this chapter

The STATESPACE Procedure

Canonical Correlations Analysis

Information Chi

x(TT) y(TT) x(T+1T) Criterion Square DF

1 1 0237045 3566167 114505 4

Information Chi

x(TT) y(TT) x(T+1T) y(T+1T) Criterion Square DF

1 1 0238244 0056565 -535906 0636134 3

Information Chi

x(TT) y(TT) x(T+1T) x(T+2T) Criterion Square DF

1 1 0237602 0087493 -446312 1525353 3

Figure 1811 Canonical Correlations Analysis

New variables are added to the state vector if the information criteria are positive Inthis example

y

t + 1 j t

andx

t + 2 j t

are not added to the state space vector because the

information criteria for these models are negative

If the information criterion is nearly 0 then you may want to investigate models

that arise if the opposite decision is made regarding

m i n

This investigation can be

accomplished by using a FORM statement to specify part or all of the state vector

Preliminary Estimates of F

When a candidate variablex

l t + k j t

yields a zero

m i n

and is not added to the state

vector a linear combination of f

j

t

is uncorrelated with thep

t

Because of the method

used to construct thef

j

t

sequence the coefficient of x

l t + k j t

inl

can be taken as 1

Denote the coefficients of z

j

t

in this linear combination as l

This gives the relationship

x

l t + k j t

= l

0

z

j

t

The vectorl

is used as a preliminary estimate of the first r columns of the row of the

transition matrixF

corresponding tox

l t + k 1 j t

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Chapter 18 Details

Parameter Estimation

The model isz

t + 1

= F z

t

+ G e

t + 1

wheree

t

is a sequence of independent multivari-

ate normal innovations with mean vector 0 and variance

e e

The observed sequence

x

t

composes the first r components of z

t

and thusx

t

= H z

t

where H is ther s

matrix I

r

0

LetE

be ther n

matrix of innovations

E = e

1

e

n

If the number of observations n is reasonably large the log likelihood L can be

approximated up to an additive constant as follows

L =

n

2

l n j

e e

j

1

2

t r a c e

1

e e

E E

0

The elements of

e e

are taken as free parameters and are estimated as follows

S

0

=

1

n

E E

0

Replacing

e e

byS

0

in the likelihood equation the log likelihood up to an additive

constant is

L =

n

2

l n j S

0

j

Letting B be the backshift operator the formal relation betweenx

t

ande

t

is

x

t

= H I B F

1

G e

t

e

t

= H I B F

1

G

1

x

t

=

1

X

i = 0

i

x

t i

LettingC

i

be the ith lagged sample covariance of x

t

and neglecting end effects the

matrixS

0

is

S

0

=

1

X

i j = 0

i

C

i + j

0

j

For the computation of S

0

the infinite sum is truncated at the value of the KLAG=

option The value of the KLAG= option should be large enough that the sequence

i

is approximately 0 beyond that point

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Let

be the vector of free parameters in theF

andG

matrices The derivative of the

log likelihood with respect to the parameter

is

L

=

n

2

t r a c e

S

1

0

S

0

The second derivative is

2

L

0

=

n

2

t r a c e

S

1

0

S

0

0

S

1

0

S

0

t r a c e

S

1

0

2

S

0

0

Near the maximum the first term is unimportant and the second term can be approx-

imated to give the following second derivative approximation

2

L

0

=

n t r a c e

S

1

0

E

E

0

0

The first derivative matrix and this second derivative matrix approximation are com-

puted from the sample covariance matrixC

0

and the truncated sequence

i

The

approximate likelihood function is maximized by a modified Newton-Raphson algo-

rithm employing these derivative matrices

The matrixS

0

is used as the estimate of the innovation covariance matrix

e e

The

negative of the inverse of the second derivative matrix at the maximum is used as an

approximate covariance matrix for the parameter estimates The standard errors of

the parameter estimates printed in the parameter estimates tables are taken from the

diagonal of this covariance matrix The parameter covariance matrix is printed when

the COVB option is specified

If the data are nearly nonstationary a better estimate of

e e

and the other parameters

can sometimes be obtained by specifying the RESIDEST option The RESIDEST

option estimates the parameters using conditional least squares instead of maximum

likelihood

The residuals are computed using the state space equation and the sample mean val-

ues of the variables in the model as start-up values The estimate of S

0

is then com-

puted using the residuals from the ith observation on where i is the maximum number

of times any variable occurs in the state vector A multivariate Gauss-Marquardt al-

gorithm is used to minimizej S

0

j

Refer to Harvey (1981a) for a further description

of this method

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Chapter 18 Details

Forecasting

Given estimates of F

G

and

e e

forecasts of x

t

are computed from the conditional

expectation of z

t

In forecasting the parameters F G and

e e

are replaced with the estimates or by val-

ues specified in the RESTRICT statement One-step-ahead forecasting is performedfor the observation

x

t

wheret n b

Heren

is the number of observations and

b is the value of the BACK= option For the observationx

t

wheret n b

m-

step-ahead forecasting is performed form = t n + b

The forecasts are generated

recursively with the initial conditionz

0

= 0

The m-step-ahead forecast of z

t + m

isz

t + m j t

wherez

t + m j t

denotes the conditional

expectation of z

t + m

given the information available at time t The m-step-ahead

forecast of x

t + m

isx

t + m j t

= H z

t + m j t

where the matrixH = I

r

0

Let

i

= F

i

G

Note that the lasts r

elements of z

t

consist of the elements of x

u j t

foru t

The state vector z

t + m

can be represented as

z

t + m

= F

m

z

t

+

m 1

X

i = 0

i

e

t + m i

Sincee

t + i j t

= 0

fori 0

the m-step-ahead forecastz

t + m j t

is

z

t + m j t

= F

m

z

t

= F z

t + m 1 j t

Therefore the m-step-ahead forecast of x

t + m

is

x

t + m j t

= H z

t + m j t

The m-step-ahead forecast error is

z

t + m

z

t + m j t

=

m 1

X

i = 0

i

e

t + m i

The variance of the m-step-ahead forecast error is

V

z m

=

m 1

X

i = 0

i

e e

0

i

LettingV

z 0

= 0

the variance of the m-step-ahead forecast error of z

t + m

V

z m

can

be computed recursively as follows

V

z m

= V

z m 1

+

m 1

e e

0

m 1

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The variance of the m-step-ahead forecast error of x

t + m

is ther r

left upper sub-

matrix of V

z m

that is

V

x m

= H V

z m

H

0

Unless the NOCENTER option is specified the sample mean vector is added to the

forecast When differencing is specified the forecasts xt + m j t

plus the sample mean

vector are integrated back to produce forecasts for the original series

Lety

t

be the original series specified by the VAR statement with some 0 values

appended corresponding to the unobserved past observations Let B be the backshift

operator and let B

be thes s

matrix polynomial in the backshift operator

corresponding to the differencing specified by the VAR statement The off-diagonal

elements of

i

are 0 Note that

0

= I

s

whereI

s

is thes s

identity matrix Then

z

t

= B y

t

This gives the relationship

y

t

=

1

B z

t

=

1

X

i = 0

i

z

t i

where

1

B =

P

1

i = 0

i

B

i and

0

= I

s

The m-step-ahead forecast of y

t + m

is

y

t + m j t

=

m 1

X

i = 0

i

z

t + m i j t

+

1

X

i = m

i

z

t + m i

The m-step-ahead forecast error of y

t + m

is

m 1

X

i = 0

i

z

t + m i

z

t + m i j t

=

m 1

X

i = 0

i

X

u = 0

u

i u

e

t + m i

LettingV

y 0

= 0

the variance of the m-step-ahead forecast error of y

t + m

V

y m

is

V

y m

=

m 1

X

i = 0

i

X

u = 0

u

i u

e e

i

X

u = 0

u

i u

0

= V

y m 1

+

m 1

X

u = 0

u

m 1 u

e e

m 1

X

u = 0

u

m 1 u

0

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Chapter 18 Details

Relation of ARMA and State Space Forms

Every state space model has an ARMA representation and conversely every ARMA

model has a state space representation This section discusses this equivalence The

following material is adapted from Akaike (1974) where there is a more complete

discussion Pham-Dinh-Tuan (1978) also contains a discussion of this material

Suppose you are given the following ARMA model

B x

t

= B e

t

or in more detail

x

t

1

x

t 1

p

x

t p

= e

t

+

1

e

t 1

+ +

q

e

t q

(1)

wheree

t

is a sequence of independent multivariate normal random vectors with mean

0 and variance matrix

e e

B is the backshift operator (B x

t

= x

t 1

) B

and

B

are matrix polynomials in B andx

t

is the observed process

If the roots of the determinantial equationj B j = 0

are outside the unit circle in

the complex plane the model can also be written as

x

t

=

1

B B e

t

=

1

X

i = 0

i

e

t i

The

i

matrices are known as the impulse response matrices and can be computed

as

1

B B

You can assume p q

since if this is not initially true you can add more terms

i

that are identically 0 without changing the model

To write this set of equations in a state space form proceed as follows Letx

t + i j t

be

the conditional expectation of x

t + i

givenx

w

forw t

The following relations hold

x

t + i j t

=

1

X

j = i

j

e

t + i j

x

t + i j t + 1

= x

t + i j t

+

i 1

e

t + 1

However from equation (1) you can derive the following relationship

x

t + p j t

=

1

x

t + p 1 j t

+ +

p

x

t

(2)

Hence wheni = p

you can substitute forx

t + p j t

in the right-hand side of equation

(2) and close the system of equations

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Part 2 General Information

This substitution results in the following model in the state space form

z

t + 1

= F z

t

+ G e

t + 1

2

6

6

6

4

x

t + 1

x

t + 2 j t + 1

x

t + p j t + 1

3

7

7

7

5

=

2

6

6

4

0 I 0 0

0 0 I 0

p

p 1

1

3

7

7

5

2

6

6

6

4

x

t

x

t + 1 j t

x

t + p 1 j t

3

7

7

7

5

+

2

6

6

4

I

1

p 1

3

7

7

5

e

t + 1

Note that the state vectorz

t

is composed of conditional expectations of x

t

and the

first r components of z

t

are equal tox

t

The state space form can be cast into an ARMA form by solving the system of dif-

ference equations for the first r components

When converting from an ARMA form to a state space form you can generate a

state vector larger than needed that is the state space model may not be a minimal

representation When going from a state space form to an ARMA form you can have

nontrivial common factors in the autoregressive and moving average operators that

yield an ARMA model larger than necessary

If the state space form used is not a minimal representation some but not all compo-

nents of x

t + i j t

may be linearly dependent This situation corresponds to

p

p 1

being of less than full rank when B

and B

have no common nontrivial

left factors In this casez

t

consists of a subset of the possible components of

x

t + i j t

i = 1 2 p 1

However once a component of x

t + i j t

(for example the

jth one) is linearly dependent on the previous conditional expectations then all sub-

sequent jth components of x

t + k j t

fork i

must also be linearly dependent Note

that in this case equivalent but seemingly different structures can arise if the order of

the components withinx

t

is changed

OUT= Data Set

The forecasts are contained in the output data set specified by the OUT= option on the

PROC STATESPACE statement The OUT= data set contains the following variables

the BY variables

the ID variable

the VAR statement variables These variables contain the actual values from

the input data set

FORi numeric variables containing the forecasts The variable FORi contains

the forecasts for the ith variable in the VAR statement list Forecasts are one-

step-ahead predictions until the end of the data or until the observation specified

by the BACK= option

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Chapter 18 Details

RESi numeric variables containing the residual for the forecast of the ith vari-

able in the VAR statement list For forecast observations the actual values are

missing and the RESi variables contain missing values

STDi numeric variables containing the standard deviation for the forecast of

the ith variable in the VAR statement list The values of the STDi variables can

be used to construct univariate confidence limits for the corresponding fore-casts However such confidence limits do not take into account the covariance

of the forecasts

OUTAR= Data Set

The OUTAR= data set contains the estimates of the preliminary autoregressive mod-

els The OUTAR= data set contains the following variables

ORDER a numeric variable containing the order p of the autoregressive model

that the observation represents

AIC a numeric variable containing the value of the information criterionA I C

p

SIGFl numeric variables containing the estimate of the innovation covariance

matrices for the forward autoregressive models The variable SIGFl contains

the lth column of b

p

in the observations with ORDER= p

SIGBl numeric variables containing the estimate of the innovation covariance

matrices for the backward autoregressive models The variable SIGBl contains

the lth column of b

p

in the observations with ORDER= p

FORk ndashl numeric variables containing the estimates of the autoregressive pa-

rameter matrices for the forward models The variable FORk ndashl contains the

lth column of the lag k autoregressive parameter matrix b

p

k

in the observations

with ORDER= p

BACk ndashl numeric variables containing the estimates of the autoregressive pa-

rameter matrices for the backward models The variable BACk ndashl contains the

lth column of the lag k autoregressive parameter matrix b

p

k

in the observations

with ORDER= p

The estimates for the order p autoregressive model can be selected as those observa-

tions with ORDER= p Within these observations the k lth element of

p

i

is given by

the value of the FORindashl variable in the k th observation The k lth element of

p

i

is

given by the value of BACindashl variable in the k th observation The k lth element of

p

is given by SIGFl in the k th observation The k lth element of

p

is given by SIGBlin the k th observation

Table 181 shows an example of the OUTAR= data set with ARMAX=3 andx

t

of

dimension 2 In Table 181 i j

indicate the ijth element of the matrix

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Part 2 General Information

Table 181 Values in the OUTAR= Data Set

Obs ORDER AIC SIGF1 SIGF2 SIGB1 SIGB2 FOR1ndash 1 FOR1ndash 2 FOR2ndash 1 FOR2ndash 2 FOR3ndash 1

1 0 AIC0

0 1 1

0 1 2

0 1 1

0 1 2

2 0 AIC0

0 2 1

0 2 2

0 2 1

0 2 2

3 1 AIC1

1 1 1

1 1 2

1 1 1

1 1 2

1

1

1 1

1

1

1 2

4 1 AIC1

1 2 1

1 1 2

1 2 1

1 1 2

1

1

2 1

1

1

2 2

5 2 AIC2

2 1 1

2 1 2

2 1 1

2 1 2

2

1

1 1

2

1

1 2

2

2

1 1

2

2

1 2

6 2 AIC2

2 2 1

2 1 2

2 2 1

2 1 2

2

1

2 1

2

1

2 2

2

2

2 1

2

2

2 2

7 3 AIC3

3 1 1

3 1 2

3 1 1

3 1 2

3

1

1 1

3

1

1 2

3

2

1 1

3

2

1 2

3

3

1 1

8 3 AIC3

3 2 1

3 1 2

3 2 1

3 1 2

3

1

2 1

3

1

2 2

3

2

2 1

3

2

2 2

3

3

2 1

Obs FOR3ndash 2 BACK1ndash 1 BACK1ndash 2 BACK2ndash 1 BACK2ndash 2 BACK3ndash 1 BACK3ndash 2

1

2

3

1

1

1 1

1

1

1 2

4

1

1

2 1

1

1

2 2

5

2

1

1 1

2

1

1 2

2

2

1 1

2

2

1 2

6

2

1

2 1

2

1

2 2

2

2

2 1

2

2

2 2

7

3

3

1 2

3

1

1 1

3

1

1 2

3

2

1 1

3

2

1 2

3

3

1 1

3

3

1 2

8

3

3

2 2

3

1

2 1

3

1

2 2

3

2

2 1

3

2

2 2

3

3

2 1

3

3

2 2

The estimated autoregressive parameters can be used in the IML procedure to obtain

autoregressive estimates of the spectral density function or forecasts based on the

autoregressive models

OUTMODEL= Data Set

The OUTMODEL= data set contains the estimates of theF

andG

matrices and their

standard errors the names of the components of the state vector and the estimates of

the innovation covariance matrix The variables contained in the OUTMODEL= data

set are as follows

the BY variables

STATEVEC a character variable containing the name of the component of the

state vector corresponding to the observation The STATEVEC variable has

the value STD for standard deviations observations which contain the standarderrors for the estimates given in the preceding observation

Fndash j numeric variables containing the columns of theF

matrix The variable

Fndash j contains the jth column of F

The number of Fndash j variables is equal to

the value of the DIMMAX= option If the model is of smaller dimension the

extraneous variables are set to missing

Gndash j numeric variables containing the columns of theG

matrix The variable

Gndash j contains the jth column of G

The number of Gndash j variables is equal to r

the dimension of x

t

given by the number of variables in the VAR statement

SIGndash j numeric variables containing the columns of the innovation covariance

matrix The variable SIGndash j contains the jth column of

e e

There are r vari-ables SIGndash j

Table 182 shows an example of the OUTMODEL= data set withx

t

= x

t

y

t

0

z

t

= x

t

y

t

x

t + 1 j t

0 and DIMMAX=4 In Table 182F

i j

andG

i j

are the ijth

elements of F

andG

respectively Note that all elements for Fndash4 are missing because

F

is a3 3

matrix

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Chapter 18 Details

Table 182 Value in the OUTMODEL= Data Set

Obs STATEVEC Fndash 1 Fndash 2 Fndash 3 Fndash 4 Gndash 1 Gndash 2 SIGndash 1 SIGndash 2

1 X(TT) 0 0 1 1 0

1 1

1 2

2 STD

3 Y(TT) F2 1

F2 2

F2 3

0 1

2 1

2 2

4 STD std F2 1

std F2 2

std F2 3

5 X(T+1T) F3 1

F3 2

F3 3

G3 1

G3 2

6 STD std F3 1

std F3 2

std F3 3

std G3 1

std G3 2

Printed Output

The printed output produced by the STATESPACE procedure is described in the fol-

lowing

1 descriptive statistics which include the number of observations used the

names of the variables their means and standard deviations (Std) and the dif-

ferencing operations used

2 the Akaike information criteria for the sequence of preliminary autoregressive

models

3 if the PRINTOUT=LONG option is specified the sample autocovariance ma-

trices of the input series at various lags

4 if the PRINTOUT=LONG option is specified the sample autocorrelation ma-

trices of the input series

5 a schematic representation of the autocorrelation matrices showing the signif-

icant autocorrelations

6 if the PRINTOUT=LONG option is specified the partial autoregressive ma-

trices (These are

p

p

as described in Preliminary Autoregressive Modelsearlier in this chapter)

7 a schematic representation of the partial autocorrelation matrices showing the

significant partial autocorrelations

8 the Yule-Walker estimates of the autoregressive parameters for the autoregres-

sive model with the minimum AIC

9 if the PRINTOUT=LONG option is specified the autocovariance matrices of

the residuals of the minimum AIC model This is the sequence of estimated

innovation variance matrices for the solutions of the Yule-Walker equations

10 if the PRINTOUT=LONG option is specified the autocorrelation matrices of the residuals of the minimum AIC model

11 If the CANCORR option is specified the canonical correlations analysis for

each potential state vector considered in the state vector selection process

This includes the potential state vector the canonical correlations the informa-

tion criterion for the smallest canonical correlation Bartlettrsquos

2 statistic (ldquoChi

Squarerdquo) for the smallest canonical correlation and the degrees of freedom of

Bartlettrsquos

2

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Part 2 General Information

12 the components of the chosen state vector

13 the preliminary estimate of the transition matrixF

the input matrixG

and

the variance matrix for the innovations

e e

14 if the ITPRINT option is specified the iteration history of the likelihood max-

imization For each iteration this shows the iteration number the number of step halvings the determinant of the innovation variance matrix the damping

factor Lambda and the values of the parameters

15 the state vector printed again to aid interpretation of the following listing of F

andG

16 the final estimate of the transition matrixF

17 the final estimate of the input matrixG

18 the final estimate of the variance matrix for the innovations

e e

19 a table listing the estimates of the free parameters inF

andG

and their standarderrors and t statistics

20 if the COVB option is specified the covariance matrix of the parameter esti-

mates

21 if the COVB option is specified the correlation matrix of the parameter esti-

mates

22 if the PRINT option is specified the forecasts and their standard errors

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Chapter 18 Details

ODS Table Names

PROC STATESPACE assigns a name to each table it creates You can use these

names to reference the table when using the Output Delivery System (ODS) to select

tables and create output data sets These names are listed in the following table For

more information on ODS see Chapter 6 ldquoUsing the Output Delivery Systemrdquo

Table 183 ODS Tables Produced in PROC STATESPACE

ODS Table Name Description Option

NObs Number of observations default

Summary Simple summary statistics table default

InfoCriterion Information criterion table default

CovLags Covariance Matrices of Input Series PRINTOUT=LONG

CorrLags Correlation Matrices of Input Series PRINTOUT=LONG

PartialAR Partial Autoregressive Matrices PRINTOUT=LONG

YWEstimates Yule-Walker Estimates for Minimum AIC default

CovResiduals Covariance of Residuals PRINTOUT=LONGCorrResiduals Residual Correlations from AR Models PRINTOUT=LONG

StateVector State vector table default

CorrGraph Schematic Representation of Correlations default

TransitionMatrix Transition Matrix default

InputMatrix Input Matrix default

VarInnov Variance Matrix for the Innovation default

CovB Covariance of Parameter Estimates COVB

CorrB Correlation of Parameter Estimates COVB

CanCorr Canonical Correlation Analysis CANCORR

IterHistory Iterative Fitting table ITPRINT

ParameterEstimates Parameter Estimates Table default

Forecasts Forecasts Table PRINT

ConvergenceStatus Convergence Status Table default

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Part 2 General Information

Example

Example 181 Series J from Box and Jenkins

This example analyzes the gas furnace data (series J) from Box and Jenkins (The

data are not shown Refer to Box and Jenkins (1976) for the data)

First a model is selected and fit automatically using the following statements

title1 rsquoGas Furnace Datarsquo

title2 rsquoBox amp Jenkins Series Jrsquo

title3 rsquoAutomatically Selected Modelrsquo

proc statespace data=seriesj cancorr

var x y

run

The results for the automatically selected model are shown in Output 1811

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Chapter 18 Example

Output 1811 Results for Automatically Selected Model

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Number of Observations 296

Standard

Variable Mean Error

x -005683 1072766

y 5350912 3202121

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Information Criterion for Autoregressive Models

Lag=0 Lag=1 Lag=2 Lag=3 Lag=4 Lag=5 Lag=6 Lag=7 Lag=8

6513862 -103357 -163296 -164512 -165152 -164891 -164934 -164315 -163856

Information

Criterion for

Autoregressive

Models

Lag=9 Lag=10

-16348 -163359

Schematic Representation of Correlations

NameLag 0 1 2 3 4 5 6 7 8 9 10

x +- +- +- +- +- +- +- +- +- +- +-

y -+ -+ -+ -+ -+ -+ -+ -+ -+ -+ -+

+ is gt 2std error - is lt -2std error is between

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Part 2 General Information

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Schematic Representation of Partial Autocorrelations

NameLag 1 2 3 4 5 6 7 8 9 10

x + - + -

y -+ -- - + +

+ is gt 2std error - is lt -2std error is between

Yule-Walker Estimates for Minimum AIC

------Lag=1------ ------Lag=2------ ------Lag=3------ ------Lag=4------

x y x y x y x y

x 1925887 -000124 -120166 0004224 0116918 -000867 0104236 0003268

y 0050496 1299793 -002046 -03277 -071182 -025701 0195411 0133417

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Canonical Correlations Analysis

Information Chi

x(TT) y(TT) x(T+1T) Criterion Square DF

1 1 0804883 2929228 3047481 8

Information Chix(TT) y(TT) x(T+1T) y(T+1T) Criterion Square DF

1 1 0906681 0607529 1223358 1347237 7

Information Chi

x(TT) y(TT) x (T+1T) y (T+1T) x (T+2T) Criterion Square DF

1 1 0909434 0610278 0186274 -154701 1034705 6

Information Chi

x(TT) y(TT) x (T+1T) y (T+1T) y (T+2T) Criterion Square DF

1 1 091014 0618937 0206823 0940392 1280924 6

Information Chi

x(TT) y(TT) x(T+1T) y(T+1T) y(T+2T) y(T+3T) Criterion Square DF

1 1 0912963 0628785 0226598 0083258 -794103 2041584 5

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Chapter 18 Example

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Selected Statespace Form and Preliminary Estimates

State Vector

x(TT) y(TT) x(T+1T) y(T+1T) y(T+2T)

Estimate of Transition Matrix

0 0 1 0 0

0 0 0 1 0

-084718 0026794 1711715 -005019 0

0 0 0 0 1

-019785 0334274 -018174 -123557 1787475

Input Matrix for Innovation

1 0

0 11925887 -000124

0050496 1299793

0142421 1361696

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Selected Statespace Form and Preliminary Estimates

Variance Matrix for Innovation

0035274 -000734

-000734 0097569

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Part 2 General Information

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Selected Statespace Form and Fitted Model

State Vector

x(TT) y(TT) x(T+1T) y(T+1T) y(T+2T)

Estimate of Transition Matrix

0 0 1 0 0

0 0 0 1 0

-086192 0030609 1724235 -005483 0

0 0 0 0 1

-034839 0292124 -009435 -109823 1671418

Input Matrix for Innovation

1 0

0 1192442 -000416

0015621 1258495

008058 1353204

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Selected Statespace Form and Fitted Model

Variance Matrix for Innovation

0035579 -000728

-000728 0095577

Parameter Estimates

Standard

Parameter Estimate Error t Value

F(31) -086192 0072961 -1181

F(32) 0030609 0026167 117

F(33) 1724235 0061599 2799

F(34) -005483 0030169 -182

F(51) -034839 0135253 -258

F(52) 0292124 0046299 631

F(53) -009435 0096527 -098F(54) -109823 0109525 -1003

F(55) 1671418 0083737 1996

G(31) 1924420 0058162 3309

G(32) -000416 0035255 -012

G(41) 0015621 0095771 016

G(42) 1258495 0055742 2258

G(51) 0080580 0151622 053

G(52) 1353204 0091388 1481

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Chapter 18 Example

The two series are believed to have a transfer function relation with the gas rate

(variable X) as the input and the CO2

concentration (variable Y) as the output Since

the parameter estimates shown in Output 1811 support this kind of model the model

is reestimated with the feedback parameters restricted to 0 The following statements

fit the transfer function (no feedback) model

title3 rsquoTransfer Function Modelrsquo

proc statespace data=seriesj printout=none

var x y

restrict f(32)=0 f(34)=0

g(32)=0 g(41)=0 g(51)=0

run

The last 2 pages of the output are shown in Output 1812

Output 1812 STATESPACE Output for Transfer Function Model

Gas Furnace Data

Box amp Jenkins Series J

Transfer Function Model

The STATESPACE Procedure

Selected Statespace Form and Fitted Model

State Vector

x(TT) y(TT) x(T+1T) y(T+1T) y(T+2T)

Estimate of Transition Matrix

0 0 1 0 0

0 0 0 1 0

-068882 0 1598717 0 0

0 0 0 0 1

-035944 0284179 -00963 -107313 1650047

Input Matrix for Innovation

1 0

0 1

1923446 0

0 1260856

0 1346332

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Part 2 General Information

Gas Furnace Data

Box amp Jenkins Series J

Transfer Function Model

The STATESPACE Procedure

Selected Statespace Form and Fitted Model

Variance Matrix for Innovation

0036995 -00072

-00072 0095712

Parameter Estimates

Standard

Parameter Estimate Error t Value

F(31) -068882 0050549 -1363

F(33) 1598717 0050924 3139

F(51) -035944 0229044 -157

F(52) 0284179 0096944 293

F(53) -009630 0140876 -068

F(54) -107313 0250385 -429

F(55) 1650047 0188533 875G(31) 1923446 0056328 3415

G(42) 1260856 0056464 2233

G(52) 1346332 0091086 1478

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Chapter 18 References

References

Akaike H (1974) Markovian Representation of Stochastic Processes and Its Ap-

plication to the Analysis of Autoregressive Moving Average Processes Annals

of the Institute of Statistical Mathematics 26 363-387

Akaike H (1976) Canonical Correlations Analysis of Time Series and the Use of

an Information Criterion in Advances and Case Studies in System Identification

eds R Mehra and DG Lainiotis New York Academic Press

Anderson TW (1971) The Statistical Analysis of Time Series New York John

Wiley amp Sons

Ansley CF and Newbold P (1979) Multivariate Partial Autocorrelations Pro-

ceedings of the Business and Economic Statistics Section American Statistical

Association 349-353

Box GEP and Jenkins G (1976) Time Series Analysis Forecasting and Control

San Francisco Holden-Day

Brockwell PJ and Davis RA (1991) Time Series Theory and Methods 2nd Edi-

tion Springer-Verlag

Hannan EJ (1970) Multiple Time Series New York John Wiley amp Sons

Hannan EJ (1976) The Identification and Parameterization of ARMAX and State

Space Forms Econometrica 44 713-722

Harvey AC (1981a) The Econometric Analysis of Time Series New York John

Wiley amp Sons

Harvey AC (1981b) Time Series Models New York John Wiley amp Sons

Jones RH (1974) Identification and Autoregressive Spectrum Estimation IEEE Transactions on Automatic Control AC-19 894-897

Pham-Dinh-Tuan (1978) On the Fitting of Multivariate Processes of the

Autoregressive-Moving Average Type Biometrika 65 99-107

Priestley MB (1980) System Identification Kalman Filtering and Stochastic Con-

trol in Directions in Time Series eds DR Brillinger and GC Tiao Institute

of Mathematical Statistics

Whittle P (1963) On the Fitting of Multivariate Autoregressions and the Approxi-

mate Canonical Factorization of a Spectral Density Matrix Biometrika 50 129-

134

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The correct bibliographic citation for this manual is as follows SAS Institute Inc SAS

ET S U serrsquos Gui de Version 8 Cary NC SAS Institute Inc 1999 1546 pp

SASET S Userrsquos Guide Version 8

Copyright copy 1999 by S AS I ns t it u t e I nc Ca ry N C U S A

ISBN 1ndash58025ndash489ndash6

All r ight s res erved P r int ed in t he U n it ed St a t es of A merica N o pa rt of t his p u blica t ionma y b e reprodu ced s t ored in a ret r ieva l s ys t em or t ra ns mit t ed in a ny form or b y a nymeans electronic mechanical photocopying or otherwise without the prior writtenpermission of the publisher SAS Institute Inc

US Government Restricted Rights Notice Use du plicat ion or disclosure of the

software by the government is subject to restrictions as set forth in FAR 52227ndash19Commercial Computer Softw ar e-Restricted Rights (J une 1987)

SAS Institute Inc SAS Campus Drive Cary North Carolina 27513

1st print ing October 1999

S AS reg a nd a ll ot her SA S I ns t it u t e I nc p rodu ct or s ervice na m es a re regis t ered t ra dema rkst d k f SA S I t it t I i t h U S A d t h t i reg i di t U SA

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Part 2 General Information

The STATESPACE Procedure

Parameter Estimates

Standard

Parameter Estimate Error t Value

F(21) 0297273 0129995 229

F(22) 0473760 0115688 410F(23) -001998 0313025 -006

F(31) 0230100 0126226 182

F(32) 0228425 0112978 202

F(33) 0256031 0305256 084

G(31) 0257284 0071060 362

G(32) 0202273 0068593 295

Figure 186 Final Parameter Estimates

Convergence Failures

The maximum likelihood estimates are computed by an iterative nonlinear maximiza-

tion algorithm which may not converge If the estimates fail to converge warning

messages are printed in the output

If you encounter convergence problems you should recheck the stationarity of the

data and ensure that the specified differencing orders are correct Attempting to fit

state space models to nonstationary data is a common cause of convergence failure

You can also use the MAXIT= option to increase the number of iterations allowed

or experiment with the convergence tolerance options DETTOL= and PARMTOL=

Forecast Data Set

The following statements print the output data set The WHERE statement excludes

the first 190 observations from the output so that only the forecasts and the last 10

actual observations are printed

proc print data=outid t

where t gt 190

run

The PROC PRINT output is shown in Figure 187

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Part 2 General Information

Figure 188 Plot of Forecasts

Controlling Printed Output

By default the STATESPACE procedure produces a large amount of printed output

The NOPRINT option suppresses all printed output You can suppress the printed

output for the autoregressive model selection process with the PRINTOUT=NONE

option The descriptive statistics and state space model estimation output are still

printed when PRINTOUT=NONE is specified You can produce more detailed output

with the PRINTOUT=LONG option and by specifying the printing control optionsCANCORR COVB and PRINT

Specifying the State Space Model

Instead of allowing the STATESPACE procedure to select the model automatically

you can use FORM and RESTRICT statements to specify a state space model

Specifying the State Vector

Use the FORM statement to control the form of the state vector You can use this

feature to force PROC STATESPACE to estimate and forecast a model different from

the model it would select automatically You can also use this feature to reestimate

the automatically selected model (possibly with restrictions) without repeating thecanonical correlation analysis

The FORM statement specifies the number of lags of each variable to include in

the state vector For example the statement FORM X 3 forces the state vector to

includex

t j t

x

t + 1 j t

andx

t + 2 j t

The following statement specifies the state vector

x

t j t

y

t j t

x

t + 1 j t

which is the same state vector selected in the preceding example

f o r m x 2 y 1

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Chapter 18 Getting Started

You can specify the form for only some of the variables and allow PROC STATES-

PACE to select the form for the other variables If only some of the variables are

specified in the FORM statement canonical correlation analysis is used to determine

the number of lags included in the state vector for the remaining variables not spec-

ified by the FORM statement If the FORM statement includes specifications for all

the variables listed in the VAR statement the state vector is completely defined and

the canonical correlation analysis is not performed

Restricting the F and G matrices

After you know the form of the state vector you can use the RESTRICT statement

to fix some parameters in theF

andG

matrices to specified values One use of this

feature is to remove insignificant parameters by restricting them to 0

In the introductory example shown in the preceding section the F[23] parameter is

not significant (The parameters estimation output shown in Figure 186 gives the t

statistic for F[23] as -006 F[33] and F[31] also have low significance witht 2

)

The following statements reestimate this model with F[23] restricted to 0 The

FORM statement is used to specify the state vector and thus bypass the canonicalcorrelation analysis

proc statespace data=in out=out lead=10

var x(1) y(1)

id t

form x 2 y 1

restrict f(23)=0

run

The final estimates produced by these statements are shown in Figure 189

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Part 2 General Information

The STATESPACE Procedure

Selected Statespace Form and Fitted Model

State Vector

x(TT) y(TT) x(T+1T)

Estimate of Transition Matrix

0 0 1

0290051 0467468 0

0227051 0226139 026436

Input Matrix for Innovation

1 0

0 1

0256826 0202022

Variance Matrix for Innovation

0945175 01006960100696 1014733

The STATESPACE Procedure

Parameter Estimates

Standard

Parameter Estimate Error t Value

F(21) 0290051 0063904 454

F(22) 0467468 0060430 774

F(31) 0227051 0125221 181

F(32) 0226139 0111711 202

F(33) 0264360 0299537 088G(31) 0256826 0070994 362

G(32) 0202022 0068507 295

Figure 189 Results using RESTRICT Statement

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Chapter 18 Syntax

Syntax

The STATESPACE procedure uses the following statements

PROC STATESPACE options

BY variable FORM variable value

ID variable

INITIAL F(rowcolumn)=value

G(rowcolumn)=value

RESTRICT F(rowcolumn)=value

G(rowcolumn)=value

VAR variable (difference difference

)

Functional Summary

The statements and options used by PROC STATESPACE are summarized in the

following table

Description Statement Option

Input Data Set Options

specify the input data set PROC STATESPACE DATA=

prevent subtraction of sample mean PROC STATESPACE NOCENTER

specify the ID variable ID

specify the observed series and differencing VAR

Options for Autoregressive Estimates

specify the maximum order PROC STATESPACE ARMAX=

specify maximum lag for autocovariances PROC STATESPACE LAGMAX=

output only minimum AIC model PROC STATESPACE MINIC

specify the amount of detail printed PROC STATESPACE PRINTOUT=

write preliminary AR models to a data set PROC STATESPACE OUTAR=

Options for Canonical Correlation Analysis

print the sequence of canonical correlations PROC STATESPACE CANCORR

specify upper limit of dimension of state

vector

PROC STATESPACE DIMMAX=

specify the minimum number of lags PROC STATESPACE PASTMIN=

specify the multiplier of the degrees of

freedom

PROC STATESPACE SIGCORR=

Options for State Space Model Estimation

specify starting values INITIAL

print covariance matrix of parameter estimates PROC STATESPACE COVB

specify the convergence criterion PROC STATESPACE DETTOL=

specify the convergence criterion PROC STATESPACE PARMTOL=

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Part 2 General Information

Description Statement Option

print the details of the iterations PROC STATESPACE ITPRINT

specify an upper limit of the number of lags PROC STATESPACE KLAG=

specify maximum number of iterations

allowed

PROC STATESPACE MAXIT=

suppress the final estimation PROC STATESPACE NOEST

write the state space model parameter esti-

mates to an output data set

PROC STATESPACE OUTMODEL=

use conditional least squares for final estimates PROC STATESPACE RESIDEST

specify criterion for testing for singularity PROC STATESPACE SINGULAR=

Options for Forecasting

start forecasting before end of the input data PROC STATESPACE BACK=

specify the time interval between observations PROC STATESPACE INTERVAL=

specify multiple periods in the time series PROC STATESPACE INTPER=specify how many periods to forecast PROC STATESPACE LEAD=

specify the output data set for forecasts PROC STATESPACE OUT=

print forecasts PROC STATESPACE PRINT

Options to Specify the State Space Model

specify the state vector FORM

specify the parameter values RESTRICT

BY Groups

specify BY-group processing BY

Printing

suppresses all printed output NOPRINT

PROC STATESPACE Statement

PROC STATESPACE options

The following options can be specified in the PROC STATESPACE statement

Printing Options NOPRINT

suppresses all printed output

Input Data Options

DATA= SAS-data-set

specifies the name of the SAS data set to be used by the procedure If the DATA=

option is omitted the most recently created SAS data set is used

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Chapter 18 Syntax

LAGMAX= k

specifies the number of lags for which the sample autocovariance matrix is computed

The LAGMAX= option controls the number of lags printed in the schematic repre-

sentation of the autocorrelations

The sample autocovariance matrix of lag i denoted asC

i

is computed as

C

i

=

1

N 1

N

X

t = 1 + i

x

t

x

0

t i

wherex

t

is the differenced and centered data andN

is the number of observations

(If the NOCENTER option is specified 1 is not subtracted fromN

) LAGMAX= k

specifies thatC

0

throughC

k

are computed The default is LAGMAX=10

NOCENTER

prevents subtraction of the sample mean from the input series (after any specified

differencing) before the analysis

Options for Preliminary Autoregressive Models ARMAX= n

specifies the maximum order of the preliminary autoregressive models The AR-

MAX= option controls the autoregressive orders for which information criteria are

printed and controls the number of lags printed in the schematic representation of

partial autocorrelations The default is ARMAX=10 See Preliminary Autoregres-

sive Models later in this chapter for details

MINIC

writes to the OUTAR= data set only the preliminary Yule-Walker estimates for the

VAR model producing the minimum AIC See OUTAR= Data Set later in this chap-

ter for details

OUTAR= SAS-data-set

writes the Yule-Walker estimates of the preliminary autoregressive models to a SAS

data set See OUTAR= Data Set later in this chapter for details

PRINTOUT= SHORT | LONG | NONE

determines the amount of detail printed PRINTOUT=LONG prints the lagged co-

variance matrices the partial autoregressive matrices and estimates of the resid-

ual covariance matrices from the sequence of autoregressive models PRINT-

OUT=NONE suppresses the output for the preliminary autoregressive models The

descriptive statistics and state space model estimation output are still printed when

PRINTOUT=NONE is specified PRINTOUT=SHORT is the default

Canonical Correlation Analysis Options

CANCORR

prints the canonical correlations and information criterion for each candidate state

vector considered See Canonical Correlation Analysis later in this chapter for

details

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Part 2 General Information

DIMMAX= n

specifies the upper limit to the dimension of the state vector The DIMMAX= option

can be used to limit the size of the model selected The default is DIMMAX=10

PASTMIN= n

specifies the minimum number of lags to include in the canonical correlation analy-

sis The default is PASTMIN=0 See Canonical Correlation Analysis later in thischapter for details

SIGCORR= value

specifies the multiplier of the degrees of freedom for the penalty term in the informa-

tion criterion used to select the state space form The default is SIGCORR=2 The

larger the value of the SIGCORR= option the smaller the state vector tends to be

Hence a large value causes a simpler model to be fit See Canonical Correlations

Analysis later in this chapter for details

State Space Model Estimation Options

COVB

prints the inverse of the observed information matrix for the parameter estimatesThis matrix is an estimate of the covariance matrix for the parameter estimates

DETTOL= value

specifies the convergence criterion The DETTOL= and PARMTOL= option values

are used together to test for convergence of the estimation process If during an

iteration the relative change of the parameter estimates is less than the PARMTOL=

value and the relative change of the determinant of the innovation variance matrix

is less than the DETTOL= value then iteration ceases and the current estimates are

accepted The default is DETTOL=1E-5

ITPRINT

prints the iterations during the estimation process

KLAG= n

sets an upper limit for the number of lags of the sample autocovariance matrix used

in computing the approximate likelihood function If the data have a strong moving

average character a larger KLAG= value may be necessary to obtain good estimates

The default is KLAG=15 See Parameter Estimation later in this chapter for details

MAXIT= n

sets an upper limit to the number of iterations in the maximum likelihood or condi-

tional least-squares estimation The default is MAXIT=50

NOEST

suppresses the final maximum likelihood estimation of the selected model

OUTMODEL= SAS-data-set

writes the parameter estimates and their standard errors to a SAS data set See OUT-

MODEL= Data Set later in this chapter for details

PARMTOL= value

specifies the convergence criterion The DETTOL= and PARMTOL= option values

are used together to test for convergence of the estimation process If during an

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Chapter 18 Syntax

iteration the relative change of the parameter estimates is less than the PARMTOL=

value and the relative change of the determinant of the innovation variance matrix

is less than the DETTOL= value then iteration ceases and the current estimates are

accepted The default is PARMTOL=001

RESIDEST

computes the final estimates using conditional least squares on the raw data Thistype of estimation may be more stable than the default maximum likelihood method

but is usually more computationally expensive See Parameter Estimation later in

this chapter for details of the conditional least squares method

SINGULAR= value

specifies the criterion for testing for singularity of a matrix A matrix is declared

singular if a scaled pivot is less than the SINGULAR= value when sweeping the

matrix The default is SINGULAR=1E-7

Forecasting Options

BACK= n

starts forecasting n periods before the end of the input data The BACK= option valuemust not be greater than the number of observations The default is BACK=0

INTERVAL= interval

specifies the time interval between observations The INTERVAL= value is used

in conjunction with the ID variable to check that the input data are in order and

have no missing periods The INTERVAL= option is also used to extrapolate the ID

values past the end of the input data See Chapter 3 ldquoDate Intervals Formats and

Functionsrdquo for details on the INTERVAL= values allowed

INTPER= n

specifies that each input observation corresponds to n time periods For example

the options INTERVAL=MONTH and INTPER=2 specify bimonthly data and are

equivalent to specifying INTERVAL=MONTH2 If the INTERVAL= option is not

specified the INTPER= option controls the increment used to generate ID values for

the forecast observations The default is INTPER=1

LEAD= n

specifies how many forecast observations are produced The forecasts start at the

point set by the BACK= option The default is LEAD=0 which produces no fore-

casts

OUT= SAS-data-set

writes the residuals actual values forecasts and forecast standard errors to a SAS

data set See OUT= Data Set later in this chapter for details

PRINT

prints the forecasts

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Part 2 General Information

BY Statement

BY variable

A BY statement can be used with the STATESPACE procedure to obtain separateanalyses on observations in groups defined by the BY variables

FORM Statement

FORM variable value

The FORM statement specifies the number of times a variable is included in the state

vector Values can be specified for any variable listed in the VAR statement If a

value is specified for each variable in the VAR statement the state vector for the state

space model is entirely specified and automatic selection of the state space model is

not performed

The FORM statement forces the state vectorz

t

to contain a specific variable a given

number of times For example if Y is one of the variables inx

t

then the statement

form y 3

forces the state vector to containY

t

Y

t + 1 j t

andY

t + 2 j t

possibly along with other

variables

The following statements illustrate the use of the FORM statement

proc statespace data=in

var x y

f o r m x 3 y 2

run

These statements fit a state space model with the following state vector

z

t

=

2

6

6

6

6

4

x

t j t

y

t j t

x

t + 1 j t

y

t + 1 j t

x

t + 2 j t

3

7

7

7

7

5

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Chapter 18 Syntax

ID Statement

ID variable

The ID statement specifies a variable that identifies observations in the input data setThe variable specified in the ID statement is included in the OUT= data set The

values of the ID variable are extrapolated for the forecast observations based on the

values of the INTERVAL= and INTPER= options

INITIAL Statement

INITIAL F (rowcolumn)= value G(row column)= value

The INITIAL statement gives initial values to the specified elements of theF

andG

matrices These initial values are used as starting values for the iterative estimation

Parts of theF

andG

matrices represent fixed structural identities If an element

specified is a fixed structural element instead of a free parameter the corresponding

initialization is ignored

The following is an example of an INITIAL statement

initial f(32)=0 g(41)=0 g(51)=0

RESTRICT Statement

RESTRICT F(rowcolumn)= value G(rowcolumn)= value

The RESTRICT statement restricts the specified elements of theF

andG

matrices

to the specified values

To use the restrict statement you need to know the form of the model Either specify

the form of the model with the FORM statement or do a preliminary run perhaps

with the NOEST option to find the form of the model that PROC STATESPACE

selects for the data

The following is an example of a RESTRICT statement

restrict f(32)=0 g(41)=0 g(51)=0

Parts of theF

andG

matrices represent fixed structural identities If a restriction is

specified for an element that is a fixed structural element instead of a free parameter

the restriction is ignored

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Part 2 General Information

VAR Statement

VAR variable (difference difference )

The VAR statement specifies the variables in the input data set to model and fore-cast The VAR statement also specifies differencing of the input variables The VAR

statement is required

Differencing is specified by following the variable name with a list of difference

periods separated by commas See the section Stationarity and Differencing for

more information on differencing of input variables

The order in which variables are listed in the VAR statement controls the order in

which variables are included in the state vector Usually potential inputs should be

listed before potential outputs

For example assuming the input data are monthly the following VAR statement

specifies modeling and forecasting of the one period and seasonal second differenceof X and Y

var x(112) y(112)

In this example the vector time series analyzed is

x

t

=

1 B 1 B

1 2

X

t

x

1 B 1 B

1 2

Y

t

y

where B represents the back shift operator and x and y represent the means of thedifferenced series If the NOCENTER option is specified the mean differences are

not subtracted

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Chapter 18 Details

Details

Missing Values

The STATESPACE procedure does not support missing values The procedure uses

the first contiguous group of observations with no missing values for any of the VARstatement variables Observations at the beginning of the data set with missing values

for any VAR statement variable are not used or included in the output data set

Stationarity and Differencing

The state space model used by the STATESPACE procedure assumes that the time

series are stationary Hence the data should be checked for stationarity One way

to check for stationarity is to plot the series A graph of series over time can show a

time trend or variability changes

You can also check stationarity by using the sample autocorrelation functions dis-

played by the ARIMA procedure The autocorrelation functions of nonstationaryseries tend to decay slowly See Chapter 7 ldquoThe ARIMA Procedurerdquo for more in-

formation

Another alternative is to use the STATIONARITY= option on the IDENTIFY state-

ment in PROC ARIMA to apply Dickey-Fuller tests for unit roots in the time series

See Chapter 7 ldquoThe ARIMA Procedurerdquo for more information on Dickey-Fuller unit

root tests

The most popular way to transform a nonstationary series to stationarity is by dif-

ferencing Differencing of the time series is specified in the VAR statement For

example to take a simple first difference of the series X use this statement

var x(1)

In this example the change in X from one period to the next is analyzed When the se-

ries has a seasonal pattern differencing at a period equal to the length of the seasonal

cycle may be desirable For example suppose the variable X is measured quarterly

and shows a seasonal cycle over the year You can use the following statement to

analyze the series of changes from the same quarter in the previous year

var x(4)

To difference twice add another differencing period to the list For ex-

ample the following statement analyzes the series of second differences

X

t

X

t 1

X

t 1

X

t 2

= X

t

2 X

t 1

+ X

t 2

var x(11)

The following statement analyzes the seasonal second difference series

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Part 2 General Information

var x(14)

The series modeled is the 1-period difference of the 4-period difference

X

t

X

t 4

X

t 1

X

t 5

= X

t

X

t 1

X

t 4

+ X

t 5

Another way to obtain stationary series is to use a regression on time to de-trend the

data If the time series has a deterministic linear trend regressing the series on time

produces residuals that should be stationary The following statements write residuals

of X and Y to the variable RX and RY in the output data set DETREND

data a

set a

t=_n_

run

proc reg data=a

model x y = t

output out=detrend r=rx ry

run

You then use PROC STATESPACE to forecast the de-trended series RX and RY A

disadvantage of this method is that you need to add the trend back to the forecast

series in an additional step A more serious disadvantage of the de-trending method

is that it assumes a deterministic trend In practice most time series appear to have a

stochastic rather than a deterministic trend Differencing is a more flexible and often

more appropriate method

There are several other methods to handle nonstationary time series For more infor-

mation and examples refer to Brockwell and Davis (1991)

Preliminary Autoregressive Models

After computing the sample autocovariance matrices PROC STATESPACE fits a

sequence of vector autoregressive models These preliminary autoregressive models

are used to estimate the autoregressive order of the process and limit the order of the

autocovariances considered in the state vector selection process

Yule-Walker Equations for Forward and Backward Models

Unlike a univariate autoregressive model a multivariate autoregressive model has

different forms depending on whether the present observation is being predicted from

the past observations or from the future observations

Letx

t

be the r -component stationary time series given by the VAR statement after

differencing and subtracting the vector of sample means (If the NOCENTER option

is specified the mean is not subtracted) Let n be the number of observations of x

t

from the input data set

Lete

t

be a vector white noise sequence with mean vector 0 and variance matrix

p

and letn

t

be a vector white noise sequence with mean vector 0 and variance matrix

p

Let p be the order of the vector autoregressive model forx

t

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Chapter 18 Details

The forward autoregressive form based on the past observations is written as follows

x

t

=

p

X

i = 1

p

i

x

t i

+ e

t

The backward autoregressive form based on the future observations is written as fol-

lows

x

t

=

p

X

i = 1

p

i

x

t + i

+ n

t

LettingE

denote the expected value operator the autocovariance sequence for thex

t

series

i

is

i

= E x

t

x

0

t i

The Yule-Walker equations for the autoregressive model that matches the first p ele-

ments of the autocovariance sequence are

2

6

6

4

0

1

p 1

0

1

0

p 2

0

p 1

0

p 2

0

3

7

7

5

2

6

6

4

p

1

p

2

p

p

3

7

7

5

=

2

6

6

4

1

2

p

3

7

7

5

and

2

6

6

6

4

0

0

1

0

p 1

1

0

0

p 2

p 1

p 2

0

3

7

7

7

5

2

6

6

4

p

1

p

2

p

p

3

7

7

5

=

2

6

6

4

0

1

0

2

0

p

3

7

7

5

Here

p

i

are the coefficient matrices for the past observation form of the vector au-toregressive model and

p

i

are the coefficient matrices for the future observation

form More information on the Yule-Walker equations in the multivariate setting can

be found in Whittle (1963) and Ansley and Newbold (1979)

The innovation variance matrices for the two forms can be written as follows

p

=

0

p

X

i = 1

p

i

0

i

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Part 2 General Information

p

=

0

p

X

i = 1

p

i

i

The autoregressive models are fit to the data using the preceding Yule-Walker equa-

tions with

i

replaced by the sample covariance sequenceC

i

The covariance matri-

ces are calculated as

C

i

=

1

N 1

N

X

t = i + 1

x

t

x

0

t i

Let b

p

b

p

b

p

and b

p

represent the Yule-Walker estimates of

p

p

p

and

p

respectively These matrices are written to an output data set when the OUTAR=

option is specified

When the PRINTOUT=LONG option is specified the sequence of matrices b

p

and

the corresponding correlation matrices are printed The sequence of matricesb

p isused to compute Akaike information criteria for selection of the autoregressive order

of the process

Akaike Information Criterion

The Akaike information criterion or AIC is defined as -2( maximum of log likeli-

hood )+2(number of parameters) Since the vector autoregressive models are esti-

mates from the Yule-Walker equations not by maximum likelihood the exact likeli-

hood values are not available for computing the AIC However for the vector autore-

gressive model the maximum of the log likelihood can be approximated as

l n L

n

2

l n j

b

p

j

Thus the AIC for the order p model is computed as

A I C

p

= n l n j

b

p

j + 2 p r

2

You can use the printed AIC array to compute a likelihood ratio test of the autoregres-

sive order The log-likelihood ratio test statistic for testing the order p model against

the order p 1

model is

n l n j

b

p

j + n l n j

b

p 1

j

This quantity is asymptotically distributed as a

2 withr

2 degrees of freedom if the

series is autoregressive of order p 1

It can be computed from the AIC array as

A I C

p 1

A I C

p

+ 2 r

2

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Chapter 18 Details

You can evaluate the significance of these test statistics with the PROBCHI function

in a SAS DATA step or with a

2 table

Determining the Autoregressive Order

Although the autoregressive models can be used for prediction their primary value is

to aid in the selection of a suitable portion of the sample covariance matrix for use in

computing canonical correlations If the multivariate time series x

t is of autoregres-sive order p then the vector of past values to lag p is considered to contain essentially

all the information relevant for prediction of future values of the time series

By default PROC STATESPACE selects the order p producing the autoregressive

model with the smallestA I C

p

If the value p for the minimumA I C

p

is less than the

value of the PASTMIN= option then p is set to the PASTMIN= value Alternatively

you can use the ARMAX= and PASTMIN= options to force PROC STATESPACE to

use an order you select

Significance Limits for Partial Autocorrelations

The STATESPACE procedure prints a schematic representation of the partial autocor-

relation matrices indicating which partial autocorrelations are significantly greater orsignificantly less than 0 Figure 1810 shows an example of this table

The STATESPACE Procedure

Schematic Representation of Partial Autocorrelations

NameLag 1 2 3 4 5 6 7 8 9 10

x ++ +

y ++

+ is gt 2std error - is lt -2std error is between

Figure 1810 Significant Partial Autocorrelations

The partial autocorrelations are from the sample partial autoregressive matrices b

p

p

The standard errors used for the significance limits of the partial autocorrelations are

computed from the sequence of matrices

p

and

p

Under the assumption that the observed series arises from an autoregressive process

of order p 1

the pth sample partial autoregressive matrix b

p

p

has an asymptotic

variance matrix 1

n

1

p

p

The significance limits for b

p

p

used in the schematic plot of the sample partial autore-

gressive sequence are derived by replacing

p

and

p

with their sample estimators

to produce the variance estimate as follows

d

V a r

b

p

p

=

1

n r p

b

1

p

b

p

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Canonical Correlation Analysis

Given the order p letp

t

be the vector of current and past values relevant to prediction

of x

t + 1

p

t

= x

0

t

x

0

t 1

x

0

t p

0

Letf

t

be the vector of current and future values

f

t

= x

0

t

x

0

t + 1

x

0

t + p

0

In the canonical correlation analysis consider submatrices of the sample covariance

matrix of p

t

andf

t

This covariance matrixV

has a block Hankel form

V =

2

6

6

6

4

C

0

C

0

1

C

0

2

C

0

p

C

0

1

C

0

2

C

0

3

C

0

p + 1

C

0

p

C

0

p + 1

C

0

p + 2

C

0

2 p

3

7

7

7

5

State Vector Selection Process

The canonical correlation analysis forms a sequence of potential state vectorsz

j

t

Examine a sequencef

j

t

of subvectors of f

t

and form the submatrixV

j consisting

of the rows and columns of V

corresponding to the components of f

j

t

and compute

its canonical correlations

The smallest canonical correlation of V

j is then used in the selection of the com-

ponents of the state vector The selection process is described in the following For

more details about this process refer to Akaike (1976)

In the following discussion the notationx

t + k j t

denotes the wide sense conditional

expectation (best linear predictor) of x

t + k

given allx

s

with s less than or equal to t

In the notationx

i t + 1

the first subscript denotes the ith component of x

t + 1

The initial state vectorz

1

t

is set tox

t

The sequencef

j

t

is initialized by setting

f

1

t

= z

1

0

t

x

1 t + 1 j t

0

= x

0

t

x

1 t + 1 j t

0

That is start by considering whether to addx

1 t + 1 j t

to the initial state vectorz

1

t

The procedure forms the submatrixV

1 corresponding tof

1

t

and computes its canon-

ical correlations Denote the smallest canonical correlation of V

1 as

m i n

If

m i n

is

significantly greater than 0x

1 t + 1 j t

is added to the state vector

If the smallest canonical correlation of V

1 is not significantly greater than 0 then a

linear combination of f

1

t

is uncorrelated with the pastp

t

Assuming that the determi-

nant of C

0

is not 0 (that is no input series is a constant) you can take the coefficient

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Chapter 18 Details

of x

1 t + 1 j t

in this linear combination to be 1 Denote the coefficients of z

1

t

in this

linear combination as

This gives the relationship

x

1 t + 1 j t

=

0

x

t

Therefore the current state vector already contains all the past information useful forpredicting

x

1 t + 1

and any greater leads of x

1 t

The variablex

1 t + 1 j t

is not added to

the state vector nor are any termsx

1 t + k j t

considered as possible components of the

state vector The variablex

1

is no longer active for state vector selection

The process described forx

1 t + 1 j t

is repeated for the remaining elements of f

t

The

next candidate for inclusion in the state vector is the next component of f

t

corre-

sponding to an active variable Components of f

t

corresponding to inactive variables

that produced a zero

m i n

in a previous step are skipped

Denote the next candidate asx

l t + k j t

The vectorf

j

t

is formed from the current state

vector andx

l t + k j t

as follows

f

j

t

= z

j

0

t

x

l t + k j t

0

The matrixV

j is formed fromf

j

t

and its canonical correlations are computed The

smallest canonical correlation of V

j is judged to be either greater than or equal to 0

If it is judged to be greater than 0x

l t + k j t

is added to the state vector If it is judged

to be 0 then a linear combination of f

j

t

is uncorrelated with thep

t

and the variable

x

l

is now inactive

The state vector selection process continues until no active variables remain

Testing Significance of Canonical Correlations For each step in the canonical correlation sequence the significance of the small-

est canonical correlation

m i n

is judged by an information criterion from Akaike

(1976) This information criterion is

n l n 1

2

m i n

r p + 1 q + 1

where q is the dimension of f

j

t

at the current step r is the order of the state vector p

is the order of the vector autoregressive process and

is the value of the SIGCORR=

option The default is SIGCORR=2 If this information criterion is less than or equal

to 0

m i n

is taken to be 0 otherwise it is taken to be significantly greater than 0

(Do not confuse this information criterion with the AIC)

Variables inx

t + p j t

are not added in the model even with positive information crite-

rion because of the singularity of V

You can force the consideration of more candi-

date state variables by increasing the size of theV

matrix by specifying a PASTMIN=

option value larger than p

Printing the Canonical Correlations

To print the details of the canonical correlation analysis process specify the CAN-

CORR option in the PROC STATESPACE statement The CANCORR option prints

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the candidate state vectors the canonical correlations and the information criteria for

testing the significance of the smallest canonical correlation

Bartlettrsquos

2 and its degrees of freedom are also printed when the CANCORR option

is specified The formula used for Bartlettrsquos

2 is

2

= n 5 r p + 1 q + 1 l n 1

2

m i n

withr p + 1 q + 1

degrees of freedom

Figure 1811 shows the output of the CANCORR option for the introductory example

shown in the Getting Started section of this chapter

The STATESPACE Procedure

Canonical Correlations Analysis

Information Chi

x(TT) y(TT) x(T+1T) Criterion Square DF

1 1 0237045 3566167 114505 4

Information Chi

x(TT) y(TT) x(T+1T) y(T+1T) Criterion Square DF

1 1 0238244 0056565 -535906 0636134 3

Information Chi

x(TT) y(TT) x(T+1T) x(T+2T) Criterion Square DF

1 1 0237602 0087493 -446312 1525353 3

Figure 1811 Canonical Correlations Analysis

New variables are added to the state vector if the information criteria are positive Inthis example

y

t + 1 j t

andx

t + 2 j t

are not added to the state space vector because the

information criteria for these models are negative

If the information criterion is nearly 0 then you may want to investigate models

that arise if the opposite decision is made regarding

m i n

This investigation can be

accomplished by using a FORM statement to specify part or all of the state vector

Preliminary Estimates of F

When a candidate variablex

l t + k j t

yields a zero

m i n

and is not added to the state

vector a linear combination of f

j

t

is uncorrelated with thep

t

Because of the method

used to construct thef

j

t

sequence the coefficient of x

l t + k j t

inl

can be taken as 1

Denote the coefficients of z

j

t

in this linear combination as l

This gives the relationship

x

l t + k j t

= l

0

z

j

t

The vectorl

is used as a preliminary estimate of the first r columns of the row of the

transition matrixF

corresponding tox

l t + k 1 j t

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Chapter 18 Details

Parameter Estimation

The model isz

t + 1

= F z

t

+ G e

t + 1

wheree

t

is a sequence of independent multivari-

ate normal innovations with mean vector 0 and variance

e e

The observed sequence

x

t

composes the first r components of z

t

and thusx

t

= H z

t

where H is ther s

matrix I

r

0

LetE

be ther n

matrix of innovations

E = e

1

e

n

If the number of observations n is reasonably large the log likelihood L can be

approximated up to an additive constant as follows

L =

n

2

l n j

e e

j

1

2

t r a c e

1

e e

E E

0

The elements of

e e

are taken as free parameters and are estimated as follows

S

0

=

1

n

E E

0

Replacing

e e

byS

0

in the likelihood equation the log likelihood up to an additive

constant is

L =

n

2

l n j S

0

j

Letting B be the backshift operator the formal relation betweenx

t

ande

t

is

x

t

= H I B F

1

G e

t

e

t

= H I B F

1

G

1

x

t

=

1

X

i = 0

i

x

t i

LettingC

i

be the ith lagged sample covariance of x

t

and neglecting end effects the

matrixS

0

is

S

0

=

1

X

i j = 0

i

C

i + j

0

j

For the computation of S

0

the infinite sum is truncated at the value of the KLAG=

option The value of the KLAG= option should be large enough that the sequence

i

is approximately 0 beyond that point

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Let

be the vector of free parameters in theF

andG

matrices The derivative of the

log likelihood with respect to the parameter

is

L

=

n

2

t r a c e

S

1

0

S

0

The second derivative is

2

L

0

=

n

2

t r a c e

S

1

0

S

0

0

S

1

0

S

0

t r a c e

S

1

0

2

S

0

0

Near the maximum the first term is unimportant and the second term can be approx-

imated to give the following second derivative approximation

2

L

0

=

n t r a c e

S

1

0

E

E

0

0

The first derivative matrix and this second derivative matrix approximation are com-

puted from the sample covariance matrixC

0

and the truncated sequence

i

The

approximate likelihood function is maximized by a modified Newton-Raphson algo-

rithm employing these derivative matrices

The matrixS

0

is used as the estimate of the innovation covariance matrix

e e

The

negative of the inverse of the second derivative matrix at the maximum is used as an

approximate covariance matrix for the parameter estimates The standard errors of

the parameter estimates printed in the parameter estimates tables are taken from the

diagonal of this covariance matrix The parameter covariance matrix is printed when

the COVB option is specified

If the data are nearly nonstationary a better estimate of

e e

and the other parameters

can sometimes be obtained by specifying the RESIDEST option The RESIDEST

option estimates the parameters using conditional least squares instead of maximum

likelihood

The residuals are computed using the state space equation and the sample mean val-

ues of the variables in the model as start-up values The estimate of S

0

is then com-

puted using the residuals from the ith observation on where i is the maximum number

of times any variable occurs in the state vector A multivariate Gauss-Marquardt al-

gorithm is used to minimizej S

0

j

Refer to Harvey (1981a) for a further description

of this method

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Forecasting

Given estimates of F

G

and

e e

forecasts of x

t

are computed from the conditional

expectation of z

t

In forecasting the parameters F G and

e e

are replaced with the estimates or by val-

ues specified in the RESTRICT statement One-step-ahead forecasting is performedfor the observation

x

t

wheret n b

Heren

is the number of observations and

b is the value of the BACK= option For the observationx

t

wheret n b

m-

step-ahead forecasting is performed form = t n + b

The forecasts are generated

recursively with the initial conditionz

0

= 0

The m-step-ahead forecast of z

t + m

isz

t + m j t

wherez

t + m j t

denotes the conditional

expectation of z

t + m

given the information available at time t The m-step-ahead

forecast of x

t + m

isx

t + m j t

= H z

t + m j t

where the matrixH = I

r

0

Let

i

= F

i

G

Note that the lasts r

elements of z

t

consist of the elements of x

u j t

foru t

The state vector z

t + m

can be represented as

z

t + m

= F

m

z

t

+

m 1

X

i = 0

i

e

t + m i

Sincee

t + i j t

= 0

fori 0

the m-step-ahead forecastz

t + m j t

is

z

t + m j t

= F

m

z

t

= F z

t + m 1 j t

Therefore the m-step-ahead forecast of x

t + m

is

x

t + m j t

= H z

t + m j t

The m-step-ahead forecast error is

z

t + m

z

t + m j t

=

m 1

X

i = 0

i

e

t + m i

The variance of the m-step-ahead forecast error is

V

z m

=

m 1

X

i = 0

i

e e

0

i

LettingV

z 0

= 0

the variance of the m-step-ahead forecast error of z

t + m

V

z m

can

be computed recursively as follows

V

z m

= V

z m 1

+

m 1

e e

0

m 1

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The variance of the m-step-ahead forecast error of x

t + m

is ther r

left upper sub-

matrix of V

z m

that is

V

x m

= H V

z m

H

0

Unless the NOCENTER option is specified the sample mean vector is added to the

forecast When differencing is specified the forecasts xt + m j t

plus the sample mean

vector are integrated back to produce forecasts for the original series

Lety

t

be the original series specified by the VAR statement with some 0 values

appended corresponding to the unobserved past observations Let B be the backshift

operator and let B

be thes s

matrix polynomial in the backshift operator

corresponding to the differencing specified by the VAR statement The off-diagonal

elements of

i

are 0 Note that

0

= I

s

whereI

s

is thes s

identity matrix Then

z

t

= B y

t

This gives the relationship

y

t

=

1

B z

t

=

1

X

i = 0

i

z

t i

where

1

B =

P

1

i = 0

i

B

i and

0

= I

s

The m-step-ahead forecast of y

t + m

is

y

t + m j t

=

m 1

X

i = 0

i

z

t + m i j t

+

1

X

i = m

i

z

t + m i

The m-step-ahead forecast error of y

t + m

is

m 1

X

i = 0

i

z

t + m i

z

t + m i j t

=

m 1

X

i = 0

i

X

u = 0

u

i u

e

t + m i

LettingV

y 0

= 0

the variance of the m-step-ahead forecast error of y

t + m

V

y m

is

V

y m

=

m 1

X

i = 0

i

X

u = 0

u

i u

e e

i

X

u = 0

u

i u

0

= V

y m 1

+

m 1

X

u = 0

u

m 1 u

e e

m 1

X

u = 0

u

m 1 u

0

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Chapter 18 Details

Relation of ARMA and State Space Forms

Every state space model has an ARMA representation and conversely every ARMA

model has a state space representation This section discusses this equivalence The

following material is adapted from Akaike (1974) where there is a more complete

discussion Pham-Dinh-Tuan (1978) also contains a discussion of this material

Suppose you are given the following ARMA model

B x

t

= B e

t

or in more detail

x

t

1

x

t 1

p

x

t p

= e

t

+

1

e

t 1

+ +

q

e

t q

(1)

wheree

t

is a sequence of independent multivariate normal random vectors with mean

0 and variance matrix

e e

B is the backshift operator (B x

t

= x

t 1

) B

and

B

are matrix polynomials in B andx

t

is the observed process

If the roots of the determinantial equationj B j = 0

are outside the unit circle in

the complex plane the model can also be written as

x

t

=

1

B B e

t

=

1

X

i = 0

i

e

t i

The

i

matrices are known as the impulse response matrices and can be computed

as

1

B B

You can assume p q

since if this is not initially true you can add more terms

i

that are identically 0 without changing the model

To write this set of equations in a state space form proceed as follows Letx

t + i j t

be

the conditional expectation of x

t + i

givenx

w

forw t

The following relations hold

x

t + i j t

=

1

X

j = i

j

e

t + i j

x

t + i j t + 1

= x

t + i j t

+

i 1

e

t + 1

However from equation (1) you can derive the following relationship

x

t + p j t

=

1

x

t + p 1 j t

+ +

p

x

t

(2)

Hence wheni = p

you can substitute forx

t + p j t

in the right-hand side of equation

(2) and close the system of equations

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This substitution results in the following model in the state space form

z

t + 1

= F z

t

+ G e

t + 1

2

6

6

6

4

x

t + 1

x

t + 2 j t + 1

x

t + p j t + 1

3

7

7

7

5

=

2

6

6

4

0 I 0 0

0 0 I 0

p

p 1

1

3

7

7

5

2

6

6

6

4

x

t

x

t + 1 j t

x

t + p 1 j t

3

7

7

7

5

+

2

6

6

4

I

1

p 1

3

7

7

5

e

t + 1

Note that the state vectorz

t

is composed of conditional expectations of x

t

and the

first r components of z

t

are equal tox

t

The state space form can be cast into an ARMA form by solving the system of dif-

ference equations for the first r components

When converting from an ARMA form to a state space form you can generate a

state vector larger than needed that is the state space model may not be a minimal

representation When going from a state space form to an ARMA form you can have

nontrivial common factors in the autoregressive and moving average operators that

yield an ARMA model larger than necessary

If the state space form used is not a minimal representation some but not all compo-

nents of x

t + i j t

may be linearly dependent This situation corresponds to

p

p 1

being of less than full rank when B

and B

have no common nontrivial

left factors In this casez

t

consists of a subset of the possible components of

x

t + i j t

i = 1 2 p 1

However once a component of x

t + i j t

(for example the

jth one) is linearly dependent on the previous conditional expectations then all sub-

sequent jth components of x

t + k j t

fork i

must also be linearly dependent Note

that in this case equivalent but seemingly different structures can arise if the order of

the components withinx

t

is changed

OUT= Data Set

The forecasts are contained in the output data set specified by the OUT= option on the

PROC STATESPACE statement The OUT= data set contains the following variables

the BY variables

the ID variable

the VAR statement variables These variables contain the actual values from

the input data set

FORi numeric variables containing the forecasts The variable FORi contains

the forecasts for the ith variable in the VAR statement list Forecasts are one-

step-ahead predictions until the end of the data or until the observation specified

by the BACK= option

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RESi numeric variables containing the residual for the forecast of the ith vari-

able in the VAR statement list For forecast observations the actual values are

missing and the RESi variables contain missing values

STDi numeric variables containing the standard deviation for the forecast of

the ith variable in the VAR statement list The values of the STDi variables can

be used to construct univariate confidence limits for the corresponding fore-casts However such confidence limits do not take into account the covariance

of the forecasts

OUTAR= Data Set

The OUTAR= data set contains the estimates of the preliminary autoregressive mod-

els The OUTAR= data set contains the following variables

ORDER a numeric variable containing the order p of the autoregressive model

that the observation represents

AIC a numeric variable containing the value of the information criterionA I C

p

SIGFl numeric variables containing the estimate of the innovation covariance

matrices for the forward autoregressive models The variable SIGFl contains

the lth column of b

p

in the observations with ORDER= p

SIGBl numeric variables containing the estimate of the innovation covariance

matrices for the backward autoregressive models The variable SIGBl contains

the lth column of b

p

in the observations with ORDER= p

FORk ndashl numeric variables containing the estimates of the autoregressive pa-

rameter matrices for the forward models The variable FORk ndashl contains the

lth column of the lag k autoregressive parameter matrix b

p

k

in the observations

with ORDER= p

BACk ndashl numeric variables containing the estimates of the autoregressive pa-

rameter matrices for the backward models The variable BACk ndashl contains the

lth column of the lag k autoregressive parameter matrix b

p

k

in the observations

with ORDER= p

The estimates for the order p autoregressive model can be selected as those observa-

tions with ORDER= p Within these observations the k lth element of

p

i

is given by

the value of the FORindashl variable in the k th observation The k lth element of

p

i

is

given by the value of BACindashl variable in the k th observation The k lth element of

p

is given by SIGFl in the k th observation The k lth element of

p

is given by SIGBlin the k th observation

Table 181 shows an example of the OUTAR= data set with ARMAX=3 andx

t

of

dimension 2 In Table 181 i j

indicate the ijth element of the matrix

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Table 181 Values in the OUTAR= Data Set

Obs ORDER AIC SIGF1 SIGF2 SIGB1 SIGB2 FOR1ndash 1 FOR1ndash 2 FOR2ndash 1 FOR2ndash 2 FOR3ndash 1

1 0 AIC0

0 1 1

0 1 2

0 1 1

0 1 2

2 0 AIC0

0 2 1

0 2 2

0 2 1

0 2 2

3 1 AIC1

1 1 1

1 1 2

1 1 1

1 1 2

1

1

1 1

1

1

1 2

4 1 AIC1

1 2 1

1 1 2

1 2 1

1 1 2

1

1

2 1

1

1

2 2

5 2 AIC2

2 1 1

2 1 2

2 1 1

2 1 2

2

1

1 1

2

1

1 2

2

2

1 1

2

2

1 2

6 2 AIC2

2 2 1

2 1 2

2 2 1

2 1 2

2

1

2 1

2

1

2 2

2

2

2 1

2

2

2 2

7 3 AIC3

3 1 1

3 1 2

3 1 1

3 1 2

3

1

1 1

3

1

1 2

3

2

1 1

3

2

1 2

3

3

1 1

8 3 AIC3

3 2 1

3 1 2

3 2 1

3 1 2

3

1

2 1

3

1

2 2

3

2

2 1

3

2

2 2

3

3

2 1

Obs FOR3ndash 2 BACK1ndash 1 BACK1ndash 2 BACK2ndash 1 BACK2ndash 2 BACK3ndash 1 BACK3ndash 2

1

2

3

1

1

1 1

1

1

1 2

4

1

1

2 1

1

1

2 2

5

2

1

1 1

2

1

1 2

2

2

1 1

2

2

1 2

6

2

1

2 1

2

1

2 2

2

2

2 1

2

2

2 2

7

3

3

1 2

3

1

1 1

3

1

1 2

3

2

1 1

3

2

1 2

3

3

1 1

3

3

1 2

8

3

3

2 2

3

1

2 1

3

1

2 2

3

2

2 1

3

2

2 2

3

3

2 1

3

3

2 2

The estimated autoregressive parameters can be used in the IML procedure to obtain

autoregressive estimates of the spectral density function or forecasts based on the

autoregressive models

OUTMODEL= Data Set

The OUTMODEL= data set contains the estimates of theF

andG

matrices and their

standard errors the names of the components of the state vector and the estimates of

the innovation covariance matrix The variables contained in the OUTMODEL= data

set are as follows

the BY variables

STATEVEC a character variable containing the name of the component of the

state vector corresponding to the observation The STATEVEC variable has

the value STD for standard deviations observations which contain the standarderrors for the estimates given in the preceding observation

Fndash j numeric variables containing the columns of theF

matrix The variable

Fndash j contains the jth column of F

The number of Fndash j variables is equal to

the value of the DIMMAX= option If the model is of smaller dimension the

extraneous variables are set to missing

Gndash j numeric variables containing the columns of theG

matrix The variable

Gndash j contains the jth column of G

The number of Gndash j variables is equal to r

the dimension of x

t

given by the number of variables in the VAR statement

SIGndash j numeric variables containing the columns of the innovation covariance

matrix The variable SIGndash j contains the jth column of

e e

There are r vari-ables SIGndash j

Table 182 shows an example of the OUTMODEL= data set withx

t

= x

t

y

t

0

z

t

= x

t

y

t

x

t + 1 j t

0 and DIMMAX=4 In Table 182F

i j

andG

i j

are the ijth

elements of F

andG

respectively Note that all elements for Fndash4 are missing because

F

is a3 3

matrix

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Chapter 18 Details

Table 182 Value in the OUTMODEL= Data Set

Obs STATEVEC Fndash 1 Fndash 2 Fndash 3 Fndash 4 Gndash 1 Gndash 2 SIGndash 1 SIGndash 2

1 X(TT) 0 0 1 1 0

1 1

1 2

2 STD

3 Y(TT) F2 1

F2 2

F2 3

0 1

2 1

2 2

4 STD std F2 1

std F2 2

std F2 3

5 X(T+1T) F3 1

F3 2

F3 3

G3 1

G3 2

6 STD std F3 1

std F3 2

std F3 3

std G3 1

std G3 2

Printed Output

The printed output produced by the STATESPACE procedure is described in the fol-

lowing

1 descriptive statistics which include the number of observations used the

names of the variables their means and standard deviations (Std) and the dif-

ferencing operations used

2 the Akaike information criteria for the sequence of preliminary autoregressive

models

3 if the PRINTOUT=LONG option is specified the sample autocovariance ma-

trices of the input series at various lags

4 if the PRINTOUT=LONG option is specified the sample autocorrelation ma-

trices of the input series

5 a schematic representation of the autocorrelation matrices showing the signif-

icant autocorrelations

6 if the PRINTOUT=LONG option is specified the partial autoregressive ma-

trices (These are

p

p

as described in Preliminary Autoregressive Modelsearlier in this chapter)

7 a schematic representation of the partial autocorrelation matrices showing the

significant partial autocorrelations

8 the Yule-Walker estimates of the autoregressive parameters for the autoregres-

sive model with the minimum AIC

9 if the PRINTOUT=LONG option is specified the autocovariance matrices of

the residuals of the minimum AIC model This is the sequence of estimated

innovation variance matrices for the solutions of the Yule-Walker equations

10 if the PRINTOUT=LONG option is specified the autocorrelation matrices of the residuals of the minimum AIC model

11 If the CANCORR option is specified the canonical correlations analysis for

each potential state vector considered in the state vector selection process

This includes the potential state vector the canonical correlations the informa-

tion criterion for the smallest canonical correlation Bartlettrsquos

2 statistic (ldquoChi

Squarerdquo) for the smallest canonical correlation and the degrees of freedom of

Bartlettrsquos

2

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Part 2 General Information

12 the components of the chosen state vector

13 the preliminary estimate of the transition matrixF

the input matrixG

and

the variance matrix for the innovations

e e

14 if the ITPRINT option is specified the iteration history of the likelihood max-

imization For each iteration this shows the iteration number the number of step halvings the determinant of the innovation variance matrix the damping

factor Lambda and the values of the parameters

15 the state vector printed again to aid interpretation of the following listing of F

andG

16 the final estimate of the transition matrixF

17 the final estimate of the input matrixG

18 the final estimate of the variance matrix for the innovations

e e

19 a table listing the estimates of the free parameters inF

andG

and their standarderrors and t statistics

20 if the COVB option is specified the covariance matrix of the parameter esti-

mates

21 if the COVB option is specified the correlation matrix of the parameter esti-

mates

22 if the PRINT option is specified the forecasts and their standard errors

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Chapter 18 Details

ODS Table Names

PROC STATESPACE assigns a name to each table it creates You can use these

names to reference the table when using the Output Delivery System (ODS) to select

tables and create output data sets These names are listed in the following table For

more information on ODS see Chapter 6 ldquoUsing the Output Delivery Systemrdquo

Table 183 ODS Tables Produced in PROC STATESPACE

ODS Table Name Description Option

NObs Number of observations default

Summary Simple summary statistics table default

InfoCriterion Information criterion table default

CovLags Covariance Matrices of Input Series PRINTOUT=LONG

CorrLags Correlation Matrices of Input Series PRINTOUT=LONG

PartialAR Partial Autoregressive Matrices PRINTOUT=LONG

YWEstimates Yule-Walker Estimates for Minimum AIC default

CovResiduals Covariance of Residuals PRINTOUT=LONGCorrResiduals Residual Correlations from AR Models PRINTOUT=LONG

StateVector State vector table default

CorrGraph Schematic Representation of Correlations default

TransitionMatrix Transition Matrix default

InputMatrix Input Matrix default

VarInnov Variance Matrix for the Innovation default

CovB Covariance of Parameter Estimates COVB

CorrB Correlation of Parameter Estimates COVB

CanCorr Canonical Correlation Analysis CANCORR

IterHistory Iterative Fitting table ITPRINT

ParameterEstimates Parameter Estimates Table default

Forecasts Forecasts Table PRINT

ConvergenceStatus Convergence Status Table default

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Part 2 General Information

Example

Example 181 Series J from Box and Jenkins

This example analyzes the gas furnace data (series J) from Box and Jenkins (The

data are not shown Refer to Box and Jenkins (1976) for the data)

First a model is selected and fit automatically using the following statements

title1 rsquoGas Furnace Datarsquo

title2 rsquoBox amp Jenkins Series Jrsquo

title3 rsquoAutomatically Selected Modelrsquo

proc statespace data=seriesj cancorr

var x y

run

The results for the automatically selected model are shown in Output 1811

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Chapter 18 Example

Output 1811 Results for Automatically Selected Model

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Number of Observations 296

Standard

Variable Mean Error

x -005683 1072766

y 5350912 3202121

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Information Criterion for Autoregressive Models

Lag=0 Lag=1 Lag=2 Lag=3 Lag=4 Lag=5 Lag=6 Lag=7 Lag=8

6513862 -103357 -163296 -164512 -165152 -164891 -164934 -164315 -163856

Information

Criterion for

Autoregressive

Models

Lag=9 Lag=10

-16348 -163359

Schematic Representation of Correlations

NameLag 0 1 2 3 4 5 6 7 8 9 10

x +- +- +- +- +- +- +- +- +- +- +-

y -+ -+ -+ -+ -+ -+ -+ -+ -+ -+ -+

+ is gt 2std error - is lt -2std error is between

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Part 2 General Information

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Schematic Representation of Partial Autocorrelations

NameLag 1 2 3 4 5 6 7 8 9 10

x + - + -

y -+ -- - + +

+ is gt 2std error - is lt -2std error is between

Yule-Walker Estimates for Minimum AIC

------Lag=1------ ------Lag=2------ ------Lag=3------ ------Lag=4------

x y x y x y x y

x 1925887 -000124 -120166 0004224 0116918 -000867 0104236 0003268

y 0050496 1299793 -002046 -03277 -071182 -025701 0195411 0133417

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Canonical Correlations Analysis

Information Chi

x(TT) y(TT) x(T+1T) Criterion Square DF

1 1 0804883 2929228 3047481 8

Information Chix(TT) y(TT) x(T+1T) y(T+1T) Criterion Square DF

1 1 0906681 0607529 1223358 1347237 7

Information Chi

x(TT) y(TT) x (T+1T) y (T+1T) x (T+2T) Criterion Square DF

1 1 0909434 0610278 0186274 -154701 1034705 6

Information Chi

x(TT) y(TT) x (T+1T) y (T+1T) y (T+2T) Criterion Square DF

1 1 091014 0618937 0206823 0940392 1280924 6

Information Chi

x(TT) y(TT) x(T+1T) y(T+1T) y(T+2T) y(T+3T) Criterion Square DF

1 1 0912963 0628785 0226598 0083258 -794103 2041584 5

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Chapter 18 Example

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Selected Statespace Form and Preliminary Estimates

State Vector

x(TT) y(TT) x(T+1T) y(T+1T) y(T+2T)

Estimate of Transition Matrix

0 0 1 0 0

0 0 0 1 0

-084718 0026794 1711715 -005019 0

0 0 0 0 1

-019785 0334274 -018174 -123557 1787475

Input Matrix for Innovation

1 0

0 11925887 -000124

0050496 1299793

0142421 1361696

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Selected Statespace Form and Preliminary Estimates

Variance Matrix for Innovation

0035274 -000734

-000734 0097569

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Part 2 General Information

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Selected Statespace Form and Fitted Model

State Vector

x(TT) y(TT) x(T+1T) y(T+1T) y(T+2T)

Estimate of Transition Matrix

0 0 1 0 0

0 0 0 1 0

-086192 0030609 1724235 -005483 0

0 0 0 0 1

-034839 0292124 -009435 -109823 1671418

Input Matrix for Innovation

1 0

0 1192442 -000416

0015621 1258495

008058 1353204

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Selected Statespace Form and Fitted Model

Variance Matrix for Innovation

0035579 -000728

-000728 0095577

Parameter Estimates

Standard

Parameter Estimate Error t Value

F(31) -086192 0072961 -1181

F(32) 0030609 0026167 117

F(33) 1724235 0061599 2799

F(34) -005483 0030169 -182

F(51) -034839 0135253 -258

F(52) 0292124 0046299 631

F(53) -009435 0096527 -098F(54) -109823 0109525 -1003

F(55) 1671418 0083737 1996

G(31) 1924420 0058162 3309

G(32) -000416 0035255 -012

G(41) 0015621 0095771 016

G(42) 1258495 0055742 2258

G(51) 0080580 0151622 053

G(52) 1353204 0091388 1481

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Chapter 18 Example

The two series are believed to have a transfer function relation with the gas rate

(variable X) as the input and the CO2

concentration (variable Y) as the output Since

the parameter estimates shown in Output 1811 support this kind of model the model

is reestimated with the feedback parameters restricted to 0 The following statements

fit the transfer function (no feedback) model

title3 rsquoTransfer Function Modelrsquo

proc statespace data=seriesj printout=none

var x y

restrict f(32)=0 f(34)=0

g(32)=0 g(41)=0 g(51)=0

run

The last 2 pages of the output are shown in Output 1812

Output 1812 STATESPACE Output for Transfer Function Model

Gas Furnace Data

Box amp Jenkins Series J

Transfer Function Model

The STATESPACE Procedure

Selected Statespace Form and Fitted Model

State Vector

x(TT) y(TT) x(T+1T) y(T+1T) y(T+2T)

Estimate of Transition Matrix

0 0 1 0 0

0 0 0 1 0

-068882 0 1598717 0 0

0 0 0 0 1

-035944 0284179 -00963 -107313 1650047

Input Matrix for Innovation

1 0

0 1

1923446 0

0 1260856

0 1346332

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Part 2 General Information

Gas Furnace Data

Box amp Jenkins Series J

Transfer Function Model

The STATESPACE Procedure

Selected Statespace Form and Fitted Model

Variance Matrix for Innovation

0036995 -00072

-00072 0095712

Parameter Estimates

Standard

Parameter Estimate Error t Value

F(31) -068882 0050549 -1363

F(33) 1598717 0050924 3139

F(51) -035944 0229044 -157

F(52) 0284179 0096944 293

F(53) -009630 0140876 -068

F(54) -107313 0250385 -429

F(55) 1650047 0188533 875G(31) 1923446 0056328 3415

G(42) 1260856 0056464 2233

G(52) 1346332 0091086 1478

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Chapter 18 References

References

Akaike H (1974) Markovian Representation of Stochastic Processes and Its Ap-

plication to the Analysis of Autoregressive Moving Average Processes Annals

of the Institute of Statistical Mathematics 26 363-387

Akaike H (1976) Canonical Correlations Analysis of Time Series and the Use of

an Information Criterion in Advances and Case Studies in System Identification

eds R Mehra and DG Lainiotis New York Academic Press

Anderson TW (1971) The Statistical Analysis of Time Series New York John

Wiley amp Sons

Ansley CF and Newbold P (1979) Multivariate Partial Autocorrelations Pro-

ceedings of the Business and Economic Statistics Section American Statistical

Association 349-353

Box GEP and Jenkins G (1976) Time Series Analysis Forecasting and Control

San Francisco Holden-Day

Brockwell PJ and Davis RA (1991) Time Series Theory and Methods 2nd Edi-

tion Springer-Verlag

Hannan EJ (1970) Multiple Time Series New York John Wiley amp Sons

Hannan EJ (1976) The Identification and Parameterization of ARMAX and State

Space Forms Econometrica 44 713-722

Harvey AC (1981a) The Econometric Analysis of Time Series New York John

Wiley amp Sons

Harvey AC (1981b) Time Series Models New York John Wiley amp Sons

Jones RH (1974) Identification and Autoregressive Spectrum Estimation IEEE Transactions on Automatic Control AC-19 894-897

Pham-Dinh-Tuan (1978) On the Fitting of Multivariate Processes of the

Autoregressive-Moving Average Type Biometrika 65 99-107

Priestley MB (1980) System Identification Kalman Filtering and Stochastic Con-

trol in Directions in Time Series eds DR Brillinger and GC Tiao Institute

of Mathematical Statistics

Whittle P (1963) On the Fitting of Multivariate Autoregressions and the Approxi-

mate Canonical Factorization of a Spectral Density Matrix Biometrika 50 129-

134

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The correct bibliographic citation for this manual is as follows SAS Institute Inc SAS

ET S U serrsquos Gui de Version 8 Cary NC SAS Institute Inc 1999 1546 pp

SASET S Userrsquos Guide Version 8

Copyright copy 1999 by S AS I ns t it u t e I nc Ca ry N C U S A

ISBN 1ndash58025ndash489ndash6

All r ight s res erved P r int ed in t he U n it ed St a t es of A merica N o pa rt of t his p u blica t ionma y b e reprodu ced s t ored in a ret r ieva l s ys t em or t ra ns mit t ed in a ny form or b y a nymeans electronic mechanical photocopying or otherwise without the prior writtenpermission of the publisher SAS Institute Inc

US Government Restricted Rights Notice Use du plicat ion or disclosure of the

software by the government is subject to restrictions as set forth in FAR 52227ndash19Commercial Computer Softw ar e-Restricted Rights (J une 1987)

SAS Institute Inc SAS Campus Drive Cary North Carolina 27513

1st print ing October 1999

S AS reg a nd a ll ot her SA S I ns t it u t e I nc p rodu ct or s ervice na m es a re regis t ered t ra dema rkst d k f SA S I t it t I i t h U S A d t h t i reg i di t U SA

Page 13: Statespace Model

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Part 2 General Information

Figure 188 Plot of Forecasts

Controlling Printed Output

By default the STATESPACE procedure produces a large amount of printed output

The NOPRINT option suppresses all printed output You can suppress the printed

output for the autoregressive model selection process with the PRINTOUT=NONE

option The descriptive statistics and state space model estimation output are still

printed when PRINTOUT=NONE is specified You can produce more detailed output

with the PRINTOUT=LONG option and by specifying the printing control optionsCANCORR COVB and PRINT

Specifying the State Space Model

Instead of allowing the STATESPACE procedure to select the model automatically

you can use FORM and RESTRICT statements to specify a state space model

Specifying the State Vector

Use the FORM statement to control the form of the state vector You can use this

feature to force PROC STATESPACE to estimate and forecast a model different from

the model it would select automatically You can also use this feature to reestimate

the automatically selected model (possibly with restrictions) without repeating thecanonical correlation analysis

The FORM statement specifies the number of lags of each variable to include in

the state vector For example the statement FORM X 3 forces the state vector to

includex

t j t

x

t + 1 j t

andx

t + 2 j t

The following statement specifies the state vector

x

t j t

y

t j t

x

t + 1 j t

which is the same state vector selected in the preceding example

f o r m x 2 y 1

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Chapter 18 Getting Started

You can specify the form for only some of the variables and allow PROC STATES-

PACE to select the form for the other variables If only some of the variables are

specified in the FORM statement canonical correlation analysis is used to determine

the number of lags included in the state vector for the remaining variables not spec-

ified by the FORM statement If the FORM statement includes specifications for all

the variables listed in the VAR statement the state vector is completely defined and

the canonical correlation analysis is not performed

Restricting the F and G matrices

After you know the form of the state vector you can use the RESTRICT statement

to fix some parameters in theF

andG

matrices to specified values One use of this

feature is to remove insignificant parameters by restricting them to 0

In the introductory example shown in the preceding section the F[23] parameter is

not significant (The parameters estimation output shown in Figure 186 gives the t

statistic for F[23] as -006 F[33] and F[31] also have low significance witht 2

)

The following statements reestimate this model with F[23] restricted to 0 The

FORM statement is used to specify the state vector and thus bypass the canonicalcorrelation analysis

proc statespace data=in out=out lead=10

var x(1) y(1)

id t

form x 2 y 1

restrict f(23)=0

run

The final estimates produced by these statements are shown in Figure 189

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Part 2 General Information

The STATESPACE Procedure

Selected Statespace Form and Fitted Model

State Vector

x(TT) y(TT) x(T+1T)

Estimate of Transition Matrix

0 0 1

0290051 0467468 0

0227051 0226139 026436

Input Matrix for Innovation

1 0

0 1

0256826 0202022

Variance Matrix for Innovation

0945175 01006960100696 1014733

The STATESPACE Procedure

Parameter Estimates

Standard

Parameter Estimate Error t Value

F(21) 0290051 0063904 454

F(22) 0467468 0060430 774

F(31) 0227051 0125221 181

F(32) 0226139 0111711 202

F(33) 0264360 0299537 088G(31) 0256826 0070994 362

G(32) 0202022 0068507 295

Figure 189 Results using RESTRICT Statement

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Chapter 18 Syntax

Syntax

The STATESPACE procedure uses the following statements

PROC STATESPACE options

BY variable FORM variable value

ID variable

INITIAL F(rowcolumn)=value

G(rowcolumn)=value

RESTRICT F(rowcolumn)=value

G(rowcolumn)=value

VAR variable (difference difference

)

Functional Summary

The statements and options used by PROC STATESPACE are summarized in the

following table

Description Statement Option

Input Data Set Options

specify the input data set PROC STATESPACE DATA=

prevent subtraction of sample mean PROC STATESPACE NOCENTER

specify the ID variable ID

specify the observed series and differencing VAR

Options for Autoregressive Estimates

specify the maximum order PROC STATESPACE ARMAX=

specify maximum lag for autocovariances PROC STATESPACE LAGMAX=

output only minimum AIC model PROC STATESPACE MINIC

specify the amount of detail printed PROC STATESPACE PRINTOUT=

write preliminary AR models to a data set PROC STATESPACE OUTAR=

Options for Canonical Correlation Analysis

print the sequence of canonical correlations PROC STATESPACE CANCORR

specify upper limit of dimension of state

vector

PROC STATESPACE DIMMAX=

specify the minimum number of lags PROC STATESPACE PASTMIN=

specify the multiplier of the degrees of

freedom

PROC STATESPACE SIGCORR=

Options for State Space Model Estimation

specify starting values INITIAL

print covariance matrix of parameter estimates PROC STATESPACE COVB

specify the convergence criterion PROC STATESPACE DETTOL=

specify the convergence criterion PROC STATESPACE PARMTOL=

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Part 2 General Information

Description Statement Option

print the details of the iterations PROC STATESPACE ITPRINT

specify an upper limit of the number of lags PROC STATESPACE KLAG=

specify maximum number of iterations

allowed

PROC STATESPACE MAXIT=

suppress the final estimation PROC STATESPACE NOEST

write the state space model parameter esti-

mates to an output data set

PROC STATESPACE OUTMODEL=

use conditional least squares for final estimates PROC STATESPACE RESIDEST

specify criterion for testing for singularity PROC STATESPACE SINGULAR=

Options for Forecasting

start forecasting before end of the input data PROC STATESPACE BACK=

specify the time interval between observations PROC STATESPACE INTERVAL=

specify multiple periods in the time series PROC STATESPACE INTPER=specify how many periods to forecast PROC STATESPACE LEAD=

specify the output data set for forecasts PROC STATESPACE OUT=

print forecasts PROC STATESPACE PRINT

Options to Specify the State Space Model

specify the state vector FORM

specify the parameter values RESTRICT

BY Groups

specify BY-group processing BY

Printing

suppresses all printed output NOPRINT

PROC STATESPACE Statement

PROC STATESPACE options

The following options can be specified in the PROC STATESPACE statement

Printing Options NOPRINT

suppresses all printed output

Input Data Options

DATA= SAS-data-set

specifies the name of the SAS data set to be used by the procedure If the DATA=

option is omitted the most recently created SAS data set is used

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Chapter 18 Syntax

LAGMAX= k

specifies the number of lags for which the sample autocovariance matrix is computed

The LAGMAX= option controls the number of lags printed in the schematic repre-

sentation of the autocorrelations

The sample autocovariance matrix of lag i denoted asC

i

is computed as

C

i

=

1

N 1

N

X

t = 1 + i

x

t

x

0

t i

wherex

t

is the differenced and centered data andN

is the number of observations

(If the NOCENTER option is specified 1 is not subtracted fromN

) LAGMAX= k

specifies thatC

0

throughC

k

are computed The default is LAGMAX=10

NOCENTER

prevents subtraction of the sample mean from the input series (after any specified

differencing) before the analysis

Options for Preliminary Autoregressive Models ARMAX= n

specifies the maximum order of the preliminary autoregressive models The AR-

MAX= option controls the autoregressive orders for which information criteria are

printed and controls the number of lags printed in the schematic representation of

partial autocorrelations The default is ARMAX=10 See Preliminary Autoregres-

sive Models later in this chapter for details

MINIC

writes to the OUTAR= data set only the preliminary Yule-Walker estimates for the

VAR model producing the minimum AIC See OUTAR= Data Set later in this chap-

ter for details

OUTAR= SAS-data-set

writes the Yule-Walker estimates of the preliminary autoregressive models to a SAS

data set See OUTAR= Data Set later in this chapter for details

PRINTOUT= SHORT | LONG | NONE

determines the amount of detail printed PRINTOUT=LONG prints the lagged co-

variance matrices the partial autoregressive matrices and estimates of the resid-

ual covariance matrices from the sequence of autoregressive models PRINT-

OUT=NONE suppresses the output for the preliminary autoregressive models The

descriptive statistics and state space model estimation output are still printed when

PRINTOUT=NONE is specified PRINTOUT=SHORT is the default

Canonical Correlation Analysis Options

CANCORR

prints the canonical correlations and information criterion for each candidate state

vector considered See Canonical Correlation Analysis later in this chapter for

details

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Part 2 General Information

DIMMAX= n

specifies the upper limit to the dimension of the state vector The DIMMAX= option

can be used to limit the size of the model selected The default is DIMMAX=10

PASTMIN= n

specifies the minimum number of lags to include in the canonical correlation analy-

sis The default is PASTMIN=0 See Canonical Correlation Analysis later in thischapter for details

SIGCORR= value

specifies the multiplier of the degrees of freedom for the penalty term in the informa-

tion criterion used to select the state space form The default is SIGCORR=2 The

larger the value of the SIGCORR= option the smaller the state vector tends to be

Hence a large value causes a simpler model to be fit See Canonical Correlations

Analysis later in this chapter for details

State Space Model Estimation Options

COVB

prints the inverse of the observed information matrix for the parameter estimatesThis matrix is an estimate of the covariance matrix for the parameter estimates

DETTOL= value

specifies the convergence criterion The DETTOL= and PARMTOL= option values

are used together to test for convergence of the estimation process If during an

iteration the relative change of the parameter estimates is less than the PARMTOL=

value and the relative change of the determinant of the innovation variance matrix

is less than the DETTOL= value then iteration ceases and the current estimates are

accepted The default is DETTOL=1E-5

ITPRINT

prints the iterations during the estimation process

KLAG= n

sets an upper limit for the number of lags of the sample autocovariance matrix used

in computing the approximate likelihood function If the data have a strong moving

average character a larger KLAG= value may be necessary to obtain good estimates

The default is KLAG=15 See Parameter Estimation later in this chapter for details

MAXIT= n

sets an upper limit to the number of iterations in the maximum likelihood or condi-

tional least-squares estimation The default is MAXIT=50

NOEST

suppresses the final maximum likelihood estimation of the selected model

OUTMODEL= SAS-data-set

writes the parameter estimates and their standard errors to a SAS data set See OUT-

MODEL= Data Set later in this chapter for details

PARMTOL= value

specifies the convergence criterion The DETTOL= and PARMTOL= option values

are used together to test for convergence of the estimation process If during an

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Chapter 18 Syntax

iteration the relative change of the parameter estimates is less than the PARMTOL=

value and the relative change of the determinant of the innovation variance matrix

is less than the DETTOL= value then iteration ceases and the current estimates are

accepted The default is PARMTOL=001

RESIDEST

computes the final estimates using conditional least squares on the raw data Thistype of estimation may be more stable than the default maximum likelihood method

but is usually more computationally expensive See Parameter Estimation later in

this chapter for details of the conditional least squares method

SINGULAR= value

specifies the criterion for testing for singularity of a matrix A matrix is declared

singular if a scaled pivot is less than the SINGULAR= value when sweeping the

matrix The default is SINGULAR=1E-7

Forecasting Options

BACK= n

starts forecasting n periods before the end of the input data The BACK= option valuemust not be greater than the number of observations The default is BACK=0

INTERVAL= interval

specifies the time interval between observations The INTERVAL= value is used

in conjunction with the ID variable to check that the input data are in order and

have no missing periods The INTERVAL= option is also used to extrapolate the ID

values past the end of the input data See Chapter 3 ldquoDate Intervals Formats and

Functionsrdquo for details on the INTERVAL= values allowed

INTPER= n

specifies that each input observation corresponds to n time periods For example

the options INTERVAL=MONTH and INTPER=2 specify bimonthly data and are

equivalent to specifying INTERVAL=MONTH2 If the INTERVAL= option is not

specified the INTPER= option controls the increment used to generate ID values for

the forecast observations The default is INTPER=1

LEAD= n

specifies how many forecast observations are produced The forecasts start at the

point set by the BACK= option The default is LEAD=0 which produces no fore-

casts

OUT= SAS-data-set

writes the residuals actual values forecasts and forecast standard errors to a SAS

data set See OUT= Data Set later in this chapter for details

PRINT

prints the forecasts

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Part 2 General Information

BY Statement

BY variable

A BY statement can be used with the STATESPACE procedure to obtain separateanalyses on observations in groups defined by the BY variables

FORM Statement

FORM variable value

The FORM statement specifies the number of times a variable is included in the state

vector Values can be specified for any variable listed in the VAR statement If a

value is specified for each variable in the VAR statement the state vector for the state

space model is entirely specified and automatic selection of the state space model is

not performed

The FORM statement forces the state vectorz

t

to contain a specific variable a given

number of times For example if Y is one of the variables inx

t

then the statement

form y 3

forces the state vector to containY

t

Y

t + 1 j t

andY

t + 2 j t

possibly along with other

variables

The following statements illustrate the use of the FORM statement

proc statespace data=in

var x y

f o r m x 3 y 2

run

These statements fit a state space model with the following state vector

z

t

=

2

6

6

6

6

4

x

t j t

y

t j t

x

t + 1 j t

y

t + 1 j t

x

t + 2 j t

3

7

7

7

7

5

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Chapter 18 Syntax

ID Statement

ID variable

The ID statement specifies a variable that identifies observations in the input data setThe variable specified in the ID statement is included in the OUT= data set The

values of the ID variable are extrapolated for the forecast observations based on the

values of the INTERVAL= and INTPER= options

INITIAL Statement

INITIAL F (rowcolumn)= value G(row column)= value

The INITIAL statement gives initial values to the specified elements of theF

andG

matrices These initial values are used as starting values for the iterative estimation

Parts of theF

andG

matrices represent fixed structural identities If an element

specified is a fixed structural element instead of a free parameter the corresponding

initialization is ignored

The following is an example of an INITIAL statement

initial f(32)=0 g(41)=0 g(51)=0

RESTRICT Statement

RESTRICT F(rowcolumn)= value G(rowcolumn)= value

The RESTRICT statement restricts the specified elements of theF

andG

matrices

to the specified values

To use the restrict statement you need to know the form of the model Either specify

the form of the model with the FORM statement or do a preliminary run perhaps

with the NOEST option to find the form of the model that PROC STATESPACE

selects for the data

The following is an example of a RESTRICT statement

restrict f(32)=0 g(41)=0 g(51)=0

Parts of theF

andG

matrices represent fixed structural identities If a restriction is

specified for an element that is a fixed structural element instead of a free parameter

the restriction is ignored

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Part 2 General Information

VAR Statement

VAR variable (difference difference )

The VAR statement specifies the variables in the input data set to model and fore-cast The VAR statement also specifies differencing of the input variables The VAR

statement is required

Differencing is specified by following the variable name with a list of difference

periods separated by commas See the section Stationarity and Differencing for

more information on differencing of input variables

The order in which variables are listed in the VAR statement controls the order in

which variables are included in the state vector Usually potential inputs should be

listed before potential outputs

For example assuming the input data are monthly the following VAR statement

specifies modeling and forecasting of the one period and seasonal second differenceof X and Y

var x(112) y(112)

In this example the vector time series analyzed is

x

t

=

1 B 1 B

1 2

X

t

x

1 B 1 B

1 2

Y

t

y

where B represents the back shift operator and x and y represent the means of thedifferenced series If the NOCENTER option is specified the mean differences are

not subtracted

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Chapter 18 Details

Details

Missing Values

The STATESPACE procedure does not support missing values The procedure uses

the first contiguous group of observations with no missing values for any of the VARstatement variables Observations at the beginning of the data set with missing values

for any VAR statement variable are not used or included in the output data set

Stationarity and Differencing

The state space model used by the STATESPACE procedure assumes that the time

series are stationary Hence the data should be checked for stationarity One way

to check for stationarity is to plot the series A graph of series over time can show a

time trend or variability changes

You can also check stationarity by using the sample autocorrelation functions dis-

played by the ARIMA procedure The autocorrelation functions of nonstationaryseries tend to decay slowly See Chapter 7 ldquoThe ARIMA Procedurerdquo for more in-

formation

Another alternative is to use the STATIONARITY= option on the IDENTIFY state-

ment in PROC ARIMA to apply Dickey-Fuller tests for unit roots in the time series

See Chapter 7 ldquoThe ARIMA Procedurerdquo for more information on Dickey-Fuller unit

root tests

The most popular way to transform a nonstationary series to stationarity is by dif-

ferencing Differencing of the time series is specified in the VAR statement For

example to take a simple first difference of the series X use this statement

var x(1)

In this example the change in X from one period to the next is analyzed When the se-

ries has a seasonal pattern differencing at a period equal to the length of the seasonal

cycle may be desirable For example suppose the variable X is measured quarterly

and shows a seasonal cycle over the year You can use the following statement to

analyze the series of changes from the same quarter in the previous year

var x(4)

To difference twice add another differencing period to the list For ex-

ample the following statement analyzes the series of second differences

X

t

X

t 1

X

t 1

X

t 2

= X

t

2 X

t 1

+ X

t 2

var x(11)

The following statement analyzes the seasonal second difference series

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Part 2 General Information

var x(14)

The series modeled is the 1-period difference of the 4-period difference

X

t

X

t 4

X

t 1

X

t 5

= X

t

X

t 1

X

t 4

+ X

t 5

Another way to obtain stationary series is to use a regression on time to de-trend the

data If the time series has a deterministic linear trend regressing the series on time

produces residuals that should be stationary The following statements write residuals

of X and Y to the variable RX and RY in the output data set DETREND

data a

set a

t=_n_

run

proc reg data=a

model x y = t

output out=detrend r=rx ry

run

You then use PROC STATESPACE to forecast the de-trended series RX and RY A

disadvantage of this method is that you need to add the trend back to the forecast

series in an additional step A more serious disadvantage of the de-trending method

is that it assumes a deterministic trend In practice most time series appear to have a

stochastic rather than a deterministic trend Differencing is a more flexible and often

more appropriate method

There are several other methods to handle nonstationary time series For more infor-

mation and examples refer to Brockwell and Davis (1991)

Preliminary Autoregressive Models

After computing the sample autocovariance matrices PROC STATESPACE fits a

sequence of vector autoregressive models These preliminary autoregressive models

are used to estimate the autoregressive order of the process and limit the order of the

autocovariances considered in the state vector selection process

Yule-Walker Equations for Forward and Backward Models

Unlike a univariate autoregressive model a multivariate autoregressive model has

different forms depending on whether the present observation is being predicted from

the past observations or from the future observations

Letx

t

be the r -component stationary time series given by the VAR statement after

differencing and subtracting the vector of sample means (If the NOCENTER option

is specified the mean is not subtracted) Let n be the number of observations of x

t

from the input data set

Lete

t

be a vector white noise sequence with mean vector 0 and variance matrix

p

and letn

t

be a vector white noise sequence with mean vector 0 and variance matrix

p

Let p be the order of the vector autoregressive model forx

t

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Chapter 18 Details

The forward autoregressive form based on the past observations is written as follows

x

t

=

p

X

i = 1

p

i

x

t i

+ e

t

The backward autoregressive form based on the future observations is written as fol-

lows

x

t

=

p

X

i = 1

p

i

x

t + i

+ n

t

LettingE

denote the expected value operator the autocovariance sequence for thex

t

series

i

is

i

= E x

t

x

0

t i

The Yule-Walker equations for the autoregressive model that matches the first p ele-

ments of the autocovariance sequence are

2

6

6

4

0

1

p 1

0

1

0

p 2

0

p 1

0

p 2

0

3

7

7

5

2

6

6

4

p

1

p

2

p

p

3

7

7

5

=

2

6

6

4

1

2

p

3

7

7

5

and

2

6

6

6

4

0

0

1

0

p 1

1

0

0

p 2

p 1

p 2

0

3

7

7

7

5

2

6

6

4

p

1

p

2

p

p

3

7

7

5

=

2

6

6

4

0

1

0

2

0

p

3

7

7

5

Here

p

i

are the coefficient matrices for the past observation form of the vector au-toregressive model and

p

i

are the coefficient matrices for the future observation

form More information on the Yule-Walker equations in the multivariate setting can

be found in Whittle (1963) and Ansley and Newbold (1979)

The innovation variance matrices for the two forms can be written as follows

p

=

0

p

X

i = 1

p

i

0

i

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Part 2 General Information

p

=

0

p

X

i = 1

p

i

i

The autoregressive models are fit to the data using the preceding Yule-Walker equa-

tions with

i

replaced by the sample covariance sequenceC

i

The covariance matri-

ces are calculated as

C

i

=

1

N 1

N

X

t = i + 1

x

t

x

0

t i

Let b

p

b

p

b

p

and b

p

represent the Yule-Walker estimates of

p

p

p

and

p

respectively These matrices are written to an output data set when the OUTAR=

option is specified

When the PRINTOUT=LONG option is specified the sequence of matrices b

p

and

the corresponding correlation matrices are printed The sequence of matricesb

p isused to compute Akaike information criteria for selection of the autoregressive order

of the process

Akaike Information Criterion

The Akaike information criterion or AIC is defined as -2( maximum of log likeli-

hood )+2(number of parameters) Since the vector autoregressive models are esti-

mates from the Yule-Walker equations not by maximum likelihood the exact likeli-

hood values are not available for computing the AIC However for the vector autore-

gressive model the maximum of the log likelihood can be approximated as

l n L

n

2

l n j

b

p

j

Thus the AIC for the order p model is computed as

A I C

p

= n l n j

b

p

j + 2 p r

2

You can use the printed AIC array to compute a likelihood ratio test of the autoregres-

sive order The log-likelihood ratio test statistic for testing the order p model against

the order p 1

model is

n l n j

b

p

j + n l n j

b

p 1

j

This quantity is asymptotically distributed as a

2 withr

2 degrees of freedom if the

series is autoregressive of order p 1

It can be computed from the AIC array as

A I C

p 1

A I C

p

+ 2 r

2

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Chapter 18 Details

You can evaluate the significance of these test statistics with the PROBCHI function

in a SAS DATA step or with a

2 table

Determining the Autoregressive Order

Although the autoregressive models can be used for prediction their primary value is

to aid in the selection of a suitable portion of the sample covariance matrix for use in

computing canonical correlations If the multivariate time series x

t is of autoregres-sive order p then the vector of past values to lag p is considered to contain essentially

all the information relevant for prediction of future values of the time series

By default PROC STATESPACE selects the order p producing the autoregressive

model with the smallestA I C

p

If the value p for the minimumA I C

p

is less than the

value of the PASTMIN= option then p is set to the PASTMIN= value Alternatively

you can use the ARMAX= and PASTMIN= options to force PROC STATESPACE to

use an order you select

Significance Limits for Partial Autocorrelations

The STATESPACE procedure prints a schematic representation of the partial autocor-

relation matrices indicating which partial autocorrelations are significantly greater orsignificantly less than 0 Figure 1810 shows an example of this table

The STATESPACE Procedure

Schematic Representation of Partial Autocorrelations

NameLag 1 2 3 4 5 6 7 8 9 10

x ++ +

y ++

+ is gt 2std error - is lt -2std error is between

Figure 1810 Significant Partial Autocorrelations

The partial autocorrelations are from the sample partial autoregressive matrices b

p

p

The standard errors used for the significance limits of the partial autocorrelations are

computed from the sequence of matrices

p

and

p

Under the assumption that the observed series arises from an autoregressive process

of order p 1

the pth sample partial autoregressive matrix b

p

p

has an asymptotic

variance matrix 1

n

1

p

p

The significance limits for b

p

p

used in the schematic plot of the sample partial autore-

gressive sequence are derived by replacing

p

and

p

with their sample estimators

to produce the variance estimate as follows

d

V a r

b

p

p

=

1

n r p

b

1

p

b

p

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Part 2 General Information

Canonical Correlation Analysis

Given the order p letp

t

be the vector of current and past values relevant to prediction

of x

t + 1

p

t

= x

0

t

x

0

t 1

x

0

t p

0

Letf

t

be the vector of current and future values

f

t

= x

0

t

x

0

t + 1

x

0

t + p

0

In the canonical correlation analysis consider submatrices of the sample covariance

matrix of p

t

andf

t

This covariance matrixV

has a block Hankel form

V =

2

6

6

6

4

C

0

C

0

1

C

0

2

C

0

p

C

0

1

C

0

2

C

0

3

C

0

p + 1

C

0

p

C

0

p + 1

C

0

p + 2

C

0

2 p

3

7

7

7

5

State Vector Selection Process

The canonical correlation analysis forms a sequence of potential state vectorsz

j

t

Examine a sequencef

j

t

of subvectors of f

t

and form the submatrixV

j consisting

of the rows and columns of V

corresponding to the components of f

j

t

and compute

its canonical correlations

The smallest canonical correlation of V

j is then used in the selection of the com-

ponents of the state vector The selection process is described in the following For

more details about this process refer to Akaike (1976)

In the following discussion the notationx

t + k j t

denotes the wide sense conditional

expectation (best linear predictor) of x

t + k

given allx

s

with s less than or equal to t

In the notationx

i t + 1

the first subscript denotes the ith component of x

t + 1

The initial state vectorz

1

t

is set tox

t

The sequencef

j

t

is initialized by setting

f

1

t

= z

1

0

t

x

1 t + 1 j t

0

= x

0

t

x

1 t + 1 j t

0

That is start by considering whether to addx

1 t + 1 j t

to the initial state vectorz

1

t

The procedure forms the submatrixV

1 corresponding tof

1

t

and computes its canon-

ical correlations Denote the smallest canonical correlation of V

1 as

m i n

If

m i n

is

significantly greater than 0x

1 t + 1 j t

is added to the state vector

If the smallest canonical correlation of V

1 is not significantly greater than 0 then a

linear combination of f

1

t

is uncorrelated with the pastp

t

Assuming that the determi-

nant of C

0

is not 0 (that is no input series is a constant) you can take the coefficient

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Chapter 18 Details

of x

1 t + 1 j t

in this linear combination to be 1 Denote the coefficients of z

1

t

in this

linear combination as

This gives the relationship

x

1 t + 1 j t

=

0

x

t

Therefore the current state vector already contains all the past information useful forpredicting

x

1 t + 1

and any greater leads of x

1 t

The variablex

1 t + 1 j t

is not added to

the state vector nor are any termsx

1 t + k j t

considered as possible components of the

state vector The variablex

1

is no longer active for state vector selection

The process described forx

1 t + 1 j t

is repeated for the remaining elements of f

t

The

next candidate for inclusion in the state vector is the next component of f

t

corre-

sponding to an active variable Components of f

t

corresponding to inactive variables

that produced a zero

m i n

in a previous step are skipped

Denote the next candidate asx

l t + k j t

The vectorf

j

t

is formed from the current state

vector andx

l t + k j t

as follows

f

j

t

= z

j

0

t

x

l t + k j t

0

The matrixV

j is formed fromf

j

t

and its canonical correlations are computed The

smallest canonical correlation of V

j is judged to be either greater than or equal to 0

If it is judged to be greater than 0x

l t + k j t

is added to the state vector If it is judged

to be 0 then a linear combination of f

j

t

is uncorrelated with thep

t

and the variable

x

l

is now inactive

The state vector selection process continues until no active variables remain

Testing Significance of Canonical Correlations For each step in the canonical correlation sequence the significance of the small-

est canonical correlation

m i n

is judged by an information criterion from Akaike

(1976) This information criterion is

n l n 1

2

m i n

r p + 1 q + 1

where q is the dimension of f

j

t

at the current step r is the order of the state vector p

is the order of the vector autoregressive process and

is the value of the SIGCORR=

option The default is SIGCORR=2 If this information criterion is less than or equal

to 0

m i n

is taken to be 0 otherwise it is taken to be significantly greater than 0

(Do not confuse this information criterion with the AIC)

Variables inx

t + p j t

are not added in the model even with positive information crite-

rion because of the singularity of V

You can force the consideration of more candi-

date state variables by increasing the size of theV

matrix by specifying a PASTMIN=

option value larger than p

Printing the Canonical Correlations

To print the details of the canonical correlation analysis process specify the CAN-

CORR option in the PROC STATESPACE statement The CANCORR option prints

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Part 2 General Information

the candidate state vectors the canonical correlations and the information criteria for

testing the significance of the smallest canonical correlation

Bartlettrsquos

2 and its degrees of freedom are also printed when the CANCORR option

is specified The formula used for Bartlettrsquos

2 is

2

= n 5 r p + 1 q + 1 l n 1

2

m i n

withr p + 1 q + 1

degrees of freedom

Figure 1811 shows the output of the CANCORR option for the introductory example

shown in the Getting Started section of this chapter

The STATESPACE Procedure

Canonical Correlations Analysis

Information Chi

x(TT) y(TT) x(T+1T) Criterion Square DF

1 1 0237045 3566167 114505 4

Information Chi

x(TT) y(TT) x(T+1T) y(T+1T) Criterion Square DF

1 1 0238244 0056565 -535906 0636134 3

Information Chi

x(TT) y(TT) x(T+1T) x(T+2T) Criterion Square DF

1 1 0237602 0087493 -446312 1525353 3

Figure 1811 Canonical Correlations Analysis

New variables are added to the state vector if the information criteria are positive Inthis example

y

t + 1 j t

andx

t + 2 j t

are not added to the state space vector because the

information criteria for these models are negative

If the information criterion is nearly 0 then you may want to investigate models

that arise if the opposite decision is made regarding

m i n

This investigation can be

accomplished by using a FORM statement to specify part or all of the state vector

Preliminary Estimates of F

When a candidate variablex

l t + k j t

yields a zero

m i n

and is not added to the state

vector a linear combination of f

j

t

is uncorrelated with thep

t

Because of the method

used to construct thef

j

t

sequence the coefficient of x

l t + k j t

inl

can be taken as 1

Denote the coefficients of z

j

t

in this linear combination as l

This gives the relationship

x

l t + k j t

= l

0

z

j

t

The vectorl

is used as a preliminary estimate of the first r columns of the row of the

transition matrixF

corresponding tox

l t + k 1 j t

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Chapter 18 Details

Parameter Estimation

The model isz

t + 1

= F z

t

+ G e

t + 1

wheree

t

is a sequence of independent multivari-

ate normal innovations with mean vector 0 and variance

e e

The observed sequence

x

t

composes the first r components of z

t

and thusx

t

= H z

t

where H is ther s

matrix I

r

0

LetE

be ther n

matrix of innovations

E = e

1

e

n

If the number of observations n is reasonably large the log likelihood L can be

approximated up to an additive constant as follows

L =

n

2

l n j

e e

j

1

2

t r a c e

1

e e

E E

0

The elements of

e e

are taken as free parameters and are estimated as follows

S

0

=

1

n

E E

0

Replacing

e e

byS

0

in the likelihood equation the log likelihood up to an additive

constant is

L =

n

2

l n j S

0

j

Letting B be the backshift operator the formal relation betweenx

t

ande

t

is

x

t

= H I B F

1

G e

t

e

t

= H I B F

1

G

1

x

t

=

1

X

i = 0

i

x

t i

LettingC

i

be the ith lagged sample covariance of x

t

and neglecting end effects the

matrixS

0

is

S

0

=

1

X

i j = 0

i

C

i + j

0

j

For the computation of S

0

the infinite sum is truncated at the value of the KLAG=

option The value of the KLAG= option should be large enough that the sequence

i

is approximately 0 beyond that point

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Part 2 General Information

Let

be the vector of free parameters in theF

andG

matrices The derivative of the

log likelihood with respect to the parameter

is

L

=

n

2

t r a c e

S

1

0

S

0

The second derivative is

2

L

0

=

n

2

t r a c e

S

1

0

S

0

0

S

1

0

S

0

t r a c e

S

1

0

2

S

0

0

Near the maximum the first term is unimportant and the second term can be approx-

imated to give the following second derivative approximation

2

L

0

=

n t r a c e

S

1

0

E

E

0

0

The first derivative matrix and this second derivative matrix approximation are com-

puted from the sample covariance matrixC

0

and the truncated sequence

i

The

approximate likelihood function is maximized by a modified Newton-Raphson algo-

rithm employing these derivative matrices

The matrixS

0

is used as the estimate of the innovation covariance matrix

e e

The

negative of the inverse of the second derivative matrix at the maximum is used as an

approximate covariance matrix for the parameter estimates The standard errors of

the parameter estimates printed in the parameter estimates tables are taken from the

diagonal of this covariance matrix The parameter covariance matrix is printed when

the COVB option is specified

If the data are nearly nonstationary a better estimate of

e e

and the other parameters

can sometimes be obtained by specifying the RESIDEST option The RESIDEST

option estimates the parameters using conditional least squares instead of maximum

likelihood

The residuals are computed using the state space equation and the sample mean val-

ues of the variables in the model as start-up values The estimate of S

0

is then com-

puted using the residuals from the ith observation on where i is the maximum number

of times any variable occurs in the state vector A multivariate Gauss-Marquardt al-

gorithm is used to minimizej S

0

j

Refer to Harvey (1981a) for a further description

of this method

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Chapter 18 Details

Forecasting

Given estimates of F

G

and

e e

forecasts of x

t

are computed from the conditional

expectation of z

t

In forecasting the parameters F G and

e e

are replaced with the estimates or by val-

ues specified in the RESTRICT statement One-step-ahead forecasting is performedfor the observation

x

t

wheret n b

Heren

is the number of observations and

b is the value of the BACK= option For the observationx

t

wheret n b

m-

step-ahead forecasting is performed form = t n + b

The forecasts are generated

recursively with the initial conditionz

0

= 0

The m-step-ahead forecast of z

t + m

isz

t + m j t

wherez

t + m j t

denotes the conditional

expectation of z

t + m

given the information available at time t The m-step-ahead

forecast of x

t + m

isx

t + m j t

= H z

t + m j t

where the matrixH = I

r

0

Let

i

= F

i

G

Note that the lasts r

elements of z

t

consist of the elements of x

u j t

foru t

The state vector z

t + m

can be represented as

z

t + m

= F

m

z

t

+

m 1

X

i = 0

i

e

t + m i

Sincee

t + i j t

= 0

fori 0

the m-step-ahead forecastz

t + m j t

is

z

t + m j t

= F

m

z

t

= F z

t + m 1 j t

Therefore the m-step-ahead forecast of x

t + m

is

x

t + m j t

= H z

t + m j t

The m-step-ahead forecast error is

z

t + m

z

t + m j t

=

m 1

X

i = 0

i

e

t + m i

The variance of the m-step-ahead forecast error is

V

z m

=

m 1

X

i = 0

i

e e

0

i

LettingV

z 0

= 0

the variance of the m-step-ahead forecast error of z

t + m

V

z m

can

be computed recursively as follows

V

z m

= V

z m 1

+

m 1

e e

0

m 1

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Part 2 General Information

The variance of the m-step-ahead forecast error of x

t + m

is ther r

left upper sub-

matrix of V

z m

that is

V

x m

= H V

z m

H

0

Unless the NOCENTER option is specified the sample mean vector is added to the

forecast When differencing is specified the forecasts xt + m j t

plus the sample mean

vector are integrated back to produce forecasts for the original series

Lety

t

be the original series specified by the VAR statement with some 0 values

appended corresponding to the unobserved past observations Let B be the backshift

operator and let B

be thes s

matrix polynomial in the backshift operator

corresponding to the differencing specified by the VAR statement The off-diagonal

elements of

i

are 0 Note that

0

= I

s

whereI

s

is thes s

identity matrix Then

z

t

= B y

t

This gives the relationship

y

t

=

1

B z

t

=

1

X

i = 0

i

z

t i

where

1

B =

P

1

i = 0

i

B

i and

0

= I

s

The m-step-ahead forecast of y

t + m

is

y

t + m j t

=

m 1

X

i = 0

i

z

t + m i j t

+

1

X

i = m

i

z

t + m i

The m-step-ahead forecast error of y

t + m

is

m 1

X

i = 0

i

z

t + m i

z

t + m i j t

=

m 1

X

i = 0

i

X

u = 0

u

i u

e

t + m i

LettingV

y 0

= 0

the variance of the m-step-ahead forecast error of y

t + m

V

y m

is

V

y m

=

m 1

X

i = 0

i

X

u = 0

u

i u

e e

i

X

u = 0

u

i u

0

= V

y m 1

+

m 1

X

u = 0

u

m 1 u

e e

m 1

X

u = 0

u

m 1 u

0

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Chapter 18 Details

Relation of ARMA and State Space Forms

Every state space model has an ARMA representation and conversely every ARMA

model has a state space representation This section discusses this equivalence The

following material is adapted from Akaike (1974) where there is a more complete

discussion Pham-Dinh-Tuan (1978) also contains a discussion of this material

Suppose you are given the following ARMA model

B x

t

= B e

t

or in more detail

x

t

1

x

t 1

p

x

t p

= e

t

+

1

e

t 1

+ +

q

e

t q

(1)

wheree

t

is a sequence of independent multivariate normal random vectors with mean

0 and variance matrix

e e

B is the backshift operator (B x

t

= x

t 1

) B

and

B

are matrix polynomials in B andx

t

is the observed process

If the roots of the determinantial equationj B j = 0

are outside the unit circle in

the complex plane the model can also be written as

x

t

=

1

B B e

t

=

1

X

i = 0

i

e

t i

The

i

matrices are known as the impulse response matrices and can be computed

as

1

B B

You can assume p q

since if this is not initially true you can add more terms

i

that are identically 0 without changing the model

To write this set of equations in a state space form proceed as follows Letx

t + i j t

be

the conditional expectation of x

t + i

givenx

w

forw t

The following relations hold

x

t + i j t

=

1

X

j = i

j

e

t + i j

x

t + i j t + 1

= x

t + i j t

+

i 1

e

t + 1

However from equation (1) you can derive the following relationship

x

t + p j t

=

1

x

t + p 1 j t

+ +

p

x

t

(2)

Hence wheni = p

you can substitute forx

t + p j t

in the right-hand side of equation

(2) and close the system of equations

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Part 2 General Information

This substitution results in the following model in the state space form

z

t + 1

= F z

t

+ G e

t + 1

2

6

6

6

4

x

t + 1

x

t + 2 j t + 1

x

t + p j t + 1

3

7

7

7

5

=

2

6

6

4

0 I 0 0

0 0 I 0

p

p 1

1

3

7

7

5

2

6

6

6

4

x

t

x

t + 1 j t

x

t + p 1 j t

3

7

7

7

5

+

2

6

6

4

I

1

p 1

3

7

7

5

e

t + 1

Note that the state vectorz

t

is composed of conditional expectations of x

t

and the

first r components of z

t

are equal tox

t

The state space form can be cast into an ARMA form by solving the system of dif-

ference equations for the first r components

When converting from an ARMA form to a state space form you can generate a

state vector larger than needed that is the state space model may not be a minimal

representation When going from a state space form to an ARMA form you can have

nontrivial common factors in the autoregressive and moving average operators that

yield an ARMA model larger than necessary

If the state space form used is not a minimal representation some but not all compo-

nents of x

t + i j t

may be linearly dependent This situation corresponds to

p

p 1

being of less than full rank when B

and B

have no common nontrivial

left factors In this casez

t

consists of a subset of the possible components of

x

t + i j t

i = 1 2 p 1

However once a component of x

t + i j t

(for example the

jth one) is linearly dependent on the previous conditional expectations then all sub-

sequent jth components of x

t + k j t

fork i

must also be linearly dependent Note

that in this case equivalent but seemingly different structures can arise if the order of

the components withinx

t

is changed

OUT= Data Set

The forecasts are contained in the output data set specified by the OUT= option on the

PROC STATESPACE statement The OUT= data set contains the following variables

the BY variables

the ID variable

the VAR statement variables These variables contain the actual values from

the input data set

FORi numeric variables containing the forecasts The variable FORi contains

the forecasts for the ith variable in the VAR statement list Forecasts are one-

step-ahead predictions until the end of the data or until the observation specified

by the BACK= option

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Chapter 18 Details

RESi numeric variables containing the residual for the forecast of the ith vari-

able in the VAR statement list For forecast observations the actual values are

missing and the RESi variables contain missing values

STDi numeric variables containing the standard deviation for the forecast of

the ith variable in the VAR statement list The values of the STDi variables can

be used to construct univariate confidence limits for the corresponding fore-casts However such confidence limits do not take into account the covariance

of the forecasts

OUTAR= Data Set

The OUTAR= data set contains the estimates of the preliminary autoregressive mod-

els The OUTAR= data set contains the following variables

ORDER a numeric variable containing the order p of the autoregressive model

that the observation represents

AIC a numeric variable containing the value of the information criterionA I C

p

SIGFl numeric variables containing the estimate of the innovation covariance

matrices for the forward autoregressive models The variable SIGFl contains

the lth column of b

p

in the observations with ORDER= p

SIGBl numeric variables containing the estimate of the innovation covariance

matrices for the backward autoregressive models The variable SIGBl contains

the lth column of b

p

in the observations with ORDER= p

FORk ndashl numeric variables containing the estimates of the autoregressive pa-

rameter matrices for the forward models The variable FORk ndashl contains the

lth column of the lag k autoregressive parameter matrix b

p

k

in the observations

with ORDER= p

BACk ndashl numeric variables containing the estimates of the autoregressive pa-

rameter matrices for the backward models The variable BACk ndashl contains the

lth column of the lag k autoregressive parameter matrix b

p

k

in the observations

with ORDER= p

The estimates for the order p autoregressive model can be selected as those observa-

tions with ORDER= p Within these observations the k lth element of

p

i

is given by

the value of the FORindashl variable in the k th observation The k lth element of

p

i

is

given by the value of BACindashl variable in the k th observation The k lth element of

p

is given by SIGFl in the k th observation The k lth element of

p

is given by SIGBlin the k th observation

Table 181 shows an example of the OUTAR= data set with ARMAX=3 andx

t

of

dimension 2 In Table 181 i j

indicate the ijth element of the matrix

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Part 2 General Information

Table 181 Values in the OUTAR= Data Set

Obs ORDER AIC SIGF1 SIGF2 SIGB1 SIGB2 FOR1ndash 1 FOR1ndash 2 FOR2ndash 1 FOR2ndash 2 FOR3ndash 1

1 0 AIC0

0 1 1

0 1 2

0 1 1

0 1 2

2 0 AIC0

0 2 1

0 2 2

0 2 1

0 2 2

3 1 AIC1

1 1 1

1 1 2

1 1 1

1 1 2

1

1

1 1

1

1

1 2

4 1 AIC1

1 2 1

1 1 2

1 2 1

1 1 2

1

1

2 1

1

1

2 2

5 2 AIC2

2 1 1

2 1 2

2 1 1

2 1 2

2

1

1 1

2

1

1 2

2

2

1 1

2

2

1 2

6 2 AIC2

2 2 1

2 1 2

2 2 1

2 1 2

2

1

2 1

2

1

2 2

2

2

2 1

2

2

2 2

7 3 AIC3

3 1 1

3 1 2

3 1 1

3 1 2

3

1

1 1

3

1

1 2

3

2

1 1

3

2

1 2

3

3

1 1

8 3 AIC3

3 2 1

3 1 2

3 2 1

3 1 2

3

1

2 1

3

1

2 2

3

2

2 1

3

2

2 2

3

3

2 1

Obs FOR3ndash 2 BACK1ndash 1 BACK1ndash 2 BACK2ndash 1 BACK2ndash 2 BACK3ndash 1 BACK3ndash 2

1

2

3

1

1

1 1

1

1

1 2

4

1

1

2 1

1

1

2 2

5

2

1

1 1

2

1

1 2

2

2

1 1

2

2

1 2

6

2

1

2 1

2

1

2 2

2

2

2 1

2

2

2 2

7

3

3

1 2

3

1

1 1

3

1

1 2

3

2

1 1

3

2

1 2

3

3

1 1

3

3

1 2

8

3

3

2 2

3

1

2 1

3

1

2 2

3

2

2 1

3

2

2 2

3

3

2 1

3

3

2 2

The estimated autoregressive parameters can be used in the IML procedure to obtain

autoregressive estimates of the spectral density function or forecasts based on the

autoregressive models

OUTMODEL= Data Set

The OUTMODEL= data set contains the estimates of theF

andG

matrices and their

standard errors the names of the components of the state vector and the estimates of

the innovation covariance matrix The variables contained in the OUTMODEL= data

set are as follows

the BY variables

STATEVEC a character variable containing the name of the component of the

state vector corresponding to the observation The STATEVEC variable has

the value STD for standard deviations observations which contain the standarderrors for the estimates given in the preceding observation

Fndash j numeric variables containing the columns of theF

matrix The variable

Fndash j contains the jth column of F

The number of Fndash j variables is equal to

the value of the DIMMAX= option If the model is of smaller dimension the

extraneous variables are set to missing

Gndash j numeric variables containing the columns of theG

matrix The variable

Gndash j contains the jth column of G

The number of Gndash j variables is equal to r

the dimension of x

t

given by the number of variables in the VAR statement

SIGndash j numeric variables containing the columns of the innovation covariance

matrix The variable SIGndash j contains the jth column of

e e

There are r vari-ables SIGndash j

Table 182 shows an example of the OUTMODEL= data set withx

t

= x

t

y

t

0

z

t

= x

t

y

t

x

t + 1 j t

0 and DIMMAX=4 In Table 182F

i j

andG

i j

are the ijth

elements of F

andG

respectively Note that all elements for Fndash4 are missing because

F

is a3 3

matrix

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Chapter 18 Details

Table 182 Value in the OUTMODEL= Data Set

Obs STATEVEC Fndash 1 Fndash 2 Fndash 3 Fndash 4 Gndash 1 Gndash 2 SIGndash 1 SIGndash 2

1 X(TT) 0 0 1 1 0

1 1

1 2

2 STD

3 Y(TT) F2 1

F2 2

F2 3

0 1

2 1

2 2

4 STD std F2 1

std F2 2

std F2 3

5 X(T+1T) F3 1

F3 2

F3 3

G3 1

G3 2

6 STD std F3 1

std F3 2

std F3 3

std G3 1

std G3 2

Printed Output

The printed output produced by the STATESPACE procedure is described in the fol-

lowing

1 descriptive statistics which include the number of observations used the

names of the variables their means and standard deviations (Std) and the dif-

ferencing operations used

2 the Akaike information criteria for the sequence of preliminary autoregressive

models

3 if the PRINTOUT=LONG option is specified the sample autocovariance ma-

trices of the input series at various lags

4 if the PRINTOUT=LONG option is specified the sample autocorrelation ma-

trices of the input series

5 a schematic representation of the autocorrelation matrices showing the signif-

icant autocorrelations

6 if the PRINTOUT=LONG option is specified the partial autoregressive ma-

trices (These are

p

p

as described in Preliminary Autoregressive Modelsearlier in this chapter)

7 a schematic representation of the partial autocorrelation matrices showing the

significant partial autocorrelations

8 the Yule-Walker estimates of the autoregressive parameters for the autoregres-

sive model with the minimum AIC

9 if the PRINTOUT=LONG option is specified the autocovariance matrices of

the residuals of the minimum AIC model This is the sequence of estimated

innovation variance matrices for the solutions of the Yule-Walker equations

10 if the PRINTOUT=LONG option is specified the autocorrelation matrices of the residuals of the minimum AIC model

11 If the CANCORR option is specified the canonical correlations analysis for

each potential state vector considered in the state vector selection process

This includes the potential state vector the canonical correlations the informa-

tion criterion for the smallest canonical correlation Bartlettrsquos

2 statistic (ldquoChi

Squarerdquo) for the smallest canonical correlation and the degrees of freedom of

Bartlettrsquos

2

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Part 2 General Information

12 the components of the chosen state vector

13 the preliminary estimate of the transition matrixF

the input matrixG

and

the variance matrix for the innovations

e e

14 if the ITPRINT option is specified the iteration history of the likelihood max-

imization For each iteration this shows the iteration number the number of step halvings the determinant of the innovation variance matrix the damping

factor Lambda and the values of the parameters

15 the state vector printed again to aid interpretation of the following listing of F

andG

16 the final estimate of the transition matrixF

17 the final estimate of the input matrixG

18 the final estimate of the variance matrix for the innovations

e e

19 a table listing the estimates of the free parameters inF

andG

and their standarderrors and t statistics

20 if the COVB option is specified the covariance matrix of the parameter esti-

mates

21 if the COVB option is specified the correlation matrix of the parameter esti-

mates

22 if the PRINT option is specified the forecasts and their standard errors

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Chapter 18 Details

ODS Table Names

PROC STATESPACE assigns a name to each table it creates You can use these

names to reference the table when using the Output Delivery System (ODS) to select

tables and create output data sets These names are listed in the following table For

more information on ODS see Chapter 6 ldquoUsing the Output Delivery Systemrdquo

Table 183 ODS Tables Produced in PROC STATESPACE

ODS Table Name Description Option

NObs Number of observations default

Summary Simple summary statistics table default

InfoCriterion Information criterion table default

CovLags Covariance Matrices of Input Series PRINTOUT=LONG

CorrLags Correlation Matrices of Input Series PRINTOUT=LONG

PartialAR Partial Autoregressive Matrices PRINTOUT=LONG

YWEstimates Yule-Walker Estimates for Minimum AIC default

CovResiduals Covariance of Residuals PRINTOUT=LONGCorrResiduals Residual Correlations from AR Models PRINTOUT=LONG

StateVector State vector table default

CorrGraph Schematic Representation of Correlations default

TransitionMatrix Transition Matrix default

InputMatrix Input Matrix default

VarInnov Variance Matrix for the Innovation default

CovB Covariance of Parameter Estimates COVB

CorrB Correlation of Parameter Estimates COVB

CanCorr Canonical Correlation Analysis CANCORR

IterHistory Iterative Fitting table ITPRINT

ParameterEstimates Parameter Estimates Table default

Forecasts Forecasts Table PRINT

ConvergenceStatus Convergence Status Table default

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Part 2 General Information

Example

Example 181 Series J from Box and Jenkins

This example analyzes the gas furnace data (series J) from Box and Jenkins (The

data are not shown Refer to Box and Jenkins (1976) for the data)

First a model is selected and fit automatically using the following statements

title1 rsquoGas Furnace Datarsquo

title2 rsquoBox amp Jenkins Series Jrsquo

title3 rsquoAutomatically Selected Modelrsquo

proc statespace data=seriesj cancorr

var x y

run

The results for the automatically selected model are shown in Output 1811

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Chapter 18 Example

Output 1811 Results for Automatically Selected Model

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Number of Observations 296

Standard

Variable Mean Error

x -005683 1072766

y 5350912 3202121

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Information Criterion for Autoregressive Models

Lag=0 Lag=1 Lag=2 Lag=3 Lag=4 Lag=5 Lag=6 Lag=7 Lag=8

6513862 -103357 -163296 -164512 -165152 -164891 -164934 -164315 -163856

Information

Criterion for

Autoregressive

Models

Lag=9 Lag=10

-16348 -163359

Schematic Representation of Correlations

NameLag 0 1 2 3 4 5 6 7 8 9 10

x +- +- +- +- +- +- +- +- +- +- +-

y -+ -+ -+ -+ -+ -+ -+ -+ -+ -+ -+

+ is gt 2std error - is lt -2std error is between

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Part 2 General Information

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Schematic Representation of Partial Autocorrelations

NameLag 1 2 3 4 5 6 7 8 9 10

x + - + -

y -+ -- - + +

+ is gt 2std error - is lt -2std error is between

Yule-Walker Estimates for Minimum AIC

------Lag=1------ ------Lag=2------ ------Lag=3------ ------Lag=4------

x y x y x y x y

x 1925887 -000124 -120166 0004224 0116918 -000867 0104236 0003268

y 0050496 1299793 -002046 -03277 -071182 -025701 0195411 0133417

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Canonical Correlations Analysis

Information Chi

x(TT) y(TT) x(T+1T) Criterion Square DF

1 1 0804883 2929228 3047481 8

Information Chix(TT) y(TT) x(T+1T) y(T+1T) Criterion Square DF

1 1 0906681 0607529 1223358 1347237 7

Information Chi

x(TT) y(TT) x (T+1T) y (T+1T) x (T+2T) Criterion Square DF

1 1 0909434 0610278 0186274 -154701 1034705 6

Information Chi

x(TT) y(TT) x (T+1T) y (T+1T) y (T+2T) Criterion Square DF

1 1 091014 0618937 0206823 0940392 1280924 6

Information Chi

x(TT) y(TT) x(T+1T) y(T+1T) y(T+2T) y(T+3T) Criterion Square DF

1 1 0912963 0628785 0226598 0083258 -794103 2041584 5

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Chapter 18 Example

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Selected Statespace Form and Preliminary Estimates

State Vector

x(TT) y(TT) x(T+1T) y(T+1T) y(T+2T)

Estimate of Transition Matrix

0 0 1 0 0

0 0 0 1 0

-084718 0026794 1711715 -005019 0

0 0 0 0 1

-019785 0334274 -018174 -123557 1787475

Input Matrix for Innovation

1 0

0 11925887 -000124

0050496 1299793

0142421 1361696

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Selected Statespace Form and Preliminary Estimates

Variance Matrix for Innovation

0035274 -000734

-000734 0097569

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Part 2 General Information

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Selected Statespace Form and Fitted Model

State Vector

x(TT) y(TT) x(T+1T) y(T+1T) y(T+2T)

Estimate of Transition Matrix

0 0 1 0 0

0 0 0 1 0

-086192 0030609 1724235 -005483 0

0 0 0 0 1

-034839 0292124 -009435 -109823 1671418

Input Matrix for Innovation

1 0

0 1192442 -000416

0015621 1258495

008058 1353204

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Selected Statespace Form and Fitted Model

Variance Matrix for Innovation

0035579 -000728

-000728 0095577

Parameter Estimates

Standard

Parameter Estimate Error t Value

F(31) -086192 0072961 -1181

F(32) 0030609 0026167 117

F(33) 1724235 0061599 2799

F(34) -005483 0030169 -182

F(51) -034839 0135253 -258

F(52) 0292124 0046299 631

F(53) -009435 0096527 -098F(54) -109823 0109525 -1003

F(55) 1671418 0083737 1996

G(31) 1924420 0058162 3309

G(32) -000416 0035255 -012

G(41) 0015621 0095771 016

G(42) 1258495 0055742 2258

G(51) 0080580 0151622 053

G(52) 1353204 0091388 1481

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Chapter 18 Example

The two series are believed to have a transfer function relation with the gas rate

(variable X) as the input and the CO2

concentration (variable Y) as the output Since

the parameter estimates shown in Output 1811 support this kind of model the model

is reestimated with the feedback parameters restricted to 0 The following statements

fit the transfer function (no feedback) model

title3 rsquoTransfer Function Modelrsquo

proc statespace data=seriesj printout=none

var x y

restrict f(32)=0 f(34)=0

g(32)=0 g(41)=0 g(51)=0

run

The last 2 pages of the output are shown in Output 1812

Output 1812 STATESPACE Output for Transfer Function Model

Gas Furnace Data

Box amp Jenkins Series J

Transfer Function Model

The STATESPACE Procedure

Selected Statespace Form and Fitted Model

State Vector

x(TT) y(TT) x(T+1T) y(T+1T) y(T+2T)

Estimate of Transition Matrix

0 0 1 0 0

0 0 0 1 0

-068882 0 1598717 0 0

0 0 0 0 1

-035944 0284179 -00963 -107313 1650047

Input Matrix for Innovation

1 0

0 1

1923446 0

0 1260856

0 1346332

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Part 2 General Information

Gas Furnace Data

Box amp Jenkins Series J

Transfer Function Model

The STATESPACE Procedure

Selected Statespace Form and Fitted Model

Variance Matrix for Innovation

0036995 -00072

-00072 0095712

Parameter Estimates

Standard

Parameter Estimate Error t Value

F(31) -068882 0050549 -1363

F(33) 1598717 0050924 3139

F(51) -035944 0229044 -157

F(52) 0284179 0096944 293

F(53) -009630 0140876 -068

F(54) -107313 0250385 -429

F(55) 1650047 0188533 875G(31) 1923446 0056328 3415

G(42) 1260856 0056464 2233

G(52) 1346332 0091086 1478

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Chapter 18 References

References

Akaike H (1974) Markovian Representation of Stochastic Processes and Its Ap-

plication to the Analysis of Autoregressive Moving Average Processes Annals

of the Institute of Statistical Mathematics 26 363-387

Akaike H (1976) Canonical Correlations Analysis of Time Series and the Use of

an Information Criterion in Advances and Case Studies in System Identification

eds R Mehra and DG Lainiotis New York Academic Press

Anderson TW (1971) The Statistical Analysis of Time Series New York John

Wiley amp Sons

Ansley CF and Newbold P (1979) Multivariate Partial Autocorrelations Pro-

ceedings of the Business and Economic Statistics Section American Statistical

Association 349-353

Box GEP and Jenkins G (1976) Time Series Analysis Forecasting and Control

San Francisco Holden-Day

Brockwell PJ and Davis RA (1991) Time Series Theory and Methods 2nd Edi-

tion Springer-Verlag

Hannan EJ (1970) Multiple Time Series New York John Wiley amp Sons

Hannan EJ (1976) The Identification and Parameterization of ARMAX and State

Space Forms Econometrica 44 713-722

Harvey AC (1981a) The Econometric Analysis of Time Series New York John

Wiley amp Sons

Harvey AC (1981b) Time Series Models New York John Wiley amp Sons

Jones RH (1974) Identification and Autoregressive Spectrum Estimation IEEE Transactions on Automatic Control AC-19 894-897

Pham-Dinh-Tuan (1978) On the Fitting of Multivariate Processes of the

Autoregressive-Moving Average Type Biometrika 65 99-107

Priestley MB (1980) System Identification Kalman Filtering and Stochastic Con-

trol in Directions in Time Series eds DR Brillinger and GC Tiao Institute

of Mathematical Statistics

Whittle P (1963) On the Fitting of Multivariate Autoregressions and the Approxi-

mate Canonical Factorization of a Spectral Density Matrix Biometrika 50 129-

134

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The correct bibliographic citation for this manual is as follows SAS Institute Inc SAS

ET S U serrsquos Gui de Version 8 Cary NC SAS Institute Inc 1999 1546 pp

SASET S Userrsquos Guide Version 8

Copyright copy 1999 by S AS I ns t it u t e I nc Ca ry N C U S A

ISBN 1ndash58025ndash489ndash6

All r ight s res erved P r int ed in t he U n it ed St a t es of A merica N o pa rt of t his p u blica t ionma y b e reprodu ced s t ored in a ret r ieva l s ys t em or t ra ns mit t ed in a ny form or b y a nymeans electronic mechanical photocopying or otherwise without the prior writtenpermission of the publisher SAS Institute Inc

US Government Restricted Rights Notice Use du plicat ion or disclosure of the

software by the government is subject to restrictions as set forth in FAR 52227ndash19Commercial Computer Softw ar e-Restricted Rights (J une 1987)

SAS Institute Inc SAS Campus Drive Cary North Carolina 27513

1st print ing October 1999

S AS reg a nd a ll ot her SA S I ns t it u t e I nc p rodu ct or s ervice na m es a re regis t ered t ra dema rkst d k f SA S I t it t I i t h U S A d t h t i reg i di t U SA

Page 14: Statespace Model

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Part 2 General Information

Figure 188 Plot of Forecasts

Controlling Printed Output

By default the STATESPACE procedure produces a large amount of printed output

The NOPRINT option suppresses all printed output You can suppress the printed

output for the autoregressive model selection process with the PRINTOUT=NONE

option The descriptive statistics and state space model estimation output are still

printed when PRINTOUT=NONE is specified You can produce more detailed output

with the PRINTOUT=LONG option and by specifying the printing control optionsCANCORR COVB and PRINT

Specifying the State Space Model

Instead of allowing the STATESPACE procedure to select the model automatically

you can use FORM and RESTRICT statements to specify a state space model

Specifying the State Vector

Use the FORM statement to control the form of the state vector You can use this

feature to force PROC STATESPACE to estimate and forecast a model different from

the model it would select automatically You can also use this feature to reestimate

the automatically selected model (possibly with restrictions) without repeating thecanonical correlation analysis

The FORM statement specifies the number of lags of each variable to include in

the state vector For example the statement FORM X 3 forces the state vector to

includex

t j t

x

t + 1 j t

andx

t + 2 j t

The following statement specifies the state vector

x

t j t

y

t j t

x

t + 1 j t

which is the same state vector selected in the preceding example

f o r m x 2 y 1

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Chapter 18 Getting Started

You can specify the form for only some of the variables and allow PROC STATES-

PACE to select the form for the other variables If only some of the variables are

specified in the FORM statement canonical correlation analysis is used to determine

the number of lags included in the state vector for the remaining variables not spec-

ified by the FORM statement If the FORM statement includes specifications for all

the variables listed in the VAR statement the state vector is completely defined and

the canonical correlation analysis is not performed

Restricting the F and G matrices

After you know the form of the state vector you can use the RESTRICT statement

to fix some parameters in theF

andG

matrices to specified values One use of this

feature is to remove insignificant parameters by restricting them to 0

In the introductory example shown in the preceding section the F[23] parameter is

not significant (The parameters estimation output shown in Figure 186 gives the t

statistic for F[23] as -006 F[33] and F[31] also have low significance witht 2

)

The following statements reestimate this model with F[23] restricted to 0 The

FORM statement is used to specify the state vector and thus bypass the canonicalcorrelation analysis

proc statespace data=in out=out lead=10

var x(1) y(1)

id t

form x 2 y 1

restrict f(23)=0

run

The final estimates produced by these statements are shown in Figure 189

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Part 2 General Information

The STATESPACE Procedure

Selected Statespace Form and Fitted Model

State Vector

x(TT) y(TT) x(T+1T)

Estimate of Transition Matrix

0 0 1

0290051 0467468 0

0227051 0226139 026436

Input Matrix for Innovation

1 0

0 1

0256826 0202022

Variance Matrix for Innovation

0945175 01006960100696 1014733

The STATESPACE Procedure

Parameter Estimates

Standard

Parameter Estimate Error t Value

F(21) 0290051 0063904 454

F(22) 0467468 0060430 774

F(31) 0227051 0125221 181

F(32) 0226139 0111711 202

F(33) 0264360 0299537 088G(31) 0256826 0070994 362

G(32) 0202022 0068507 295

Figure 189 Results using RESTRICT Statement

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Chapter 18 Syntax

Syntax

The STATESPACE procedure uses the following statements

PROC STATESPACE options

BY variable FORM variable value

ID variable

INITIAL F(rowcolumn)=value

G(rowcolumn)=value

RESTRICT F(rowcolumn)=value

G(rowcolumn)=value

VAR variable (difference difference

)

Functional Summary

The statements and options used by PROC STATESPACE are summarized in the

following table

Description Statement Option

Input Data Set Options

specify the input data set PROC STATESPACE DATA=

prevent subtraction of sample mean PROC STATESPACE NOCENTER

specify the ID variable ID

specify the observed series and differencing VAR

Options for Autoregressive Estimates

specify the maximum order PROC STATESPACE ARMAX=

specify maximum lag for autocovariances PROC STATESPACE LAGMAX=

output only minimum AIC model PROC STATESPACE MINIC

specify the amount of detail printed PROC STATESPACE PRINTOUT=

write preliminary AR models to a data set PROC STATESPACE OUTAR=

Options for Canonical Correlation Analysis

print the sequence of canonical correlations PROC STATESPACE CANCORR

specify upper limit of dimension of state

vector

PROC STATESPACE DIMMAX=

specify the minimum number of lags PROC STATESPACE PASTMIN=

specify the multiplier of the degrees of

freedom

PROC STATESPACE SIGCORR=

Options for State Space Model Estimation

specify starting values INITIAL

print covariance matrix of parameter estimates PROC STATESPACE COVB

specify the convergence criterion PROC STATESPACE DETTOL=

specify the convergence criterion PROC STATESPACE PARMTOL=

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Part 2 General Information

Description Statement Option

print the details of the iterations PROC STATESPACE ITPRINT

specify an upper limit of the number of lags PROC STATESPACE KLAG=

specify maximum number of iterations

allowed

PROC STATESPACE MAXIT=

suppress the final estimation PROC STATESPACE NOEST

write the state space model parameter esti-

mates to an output data set

PROC STATESPACE OUTMODEL=

use conditional least squares for final estimates PROC STATESPACE RESIDEST

specify criterion for testing for singularity PROC STATESPACE SINGULAR=

Options for Forecasting

start forecasting before end of the input data PROC STATESPACE BACK=

specify the time interval between observations PROC STATESPACE INTERVAL=

specify multiple periods in the time series PROC STATESPACE INTPER=specify how many periods to forecast PROC STATESPACE LEAD=

specify the output data set for forecasts PROC STATESPACE OUT=

print forecasts PROC STATESPACE PRINT

Options to Specify the State Space Model

specify the state vector FORM

specify the parameter values RESTRICT

BY Groups

specify BY-group processing BY

Printing

suppresses all printed output NOPRINT

PROC STATESPACE Statement

PROC STATESPACE options

The following options can be specified in the PROC STATESPACE statement

Printing Options NOPRINT

suppresses all printed output

Input Data Options

DATA= SAS-data-set

specifies the name of the SAS data set to be used by the procedure If the DATA=

option is omitted the most recently created SAS data set is used

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Chapter 18 Syntax

LAGMAX= k

specifies the number of lags for which the sample autocovariance matrix is computed

The LAGMAX= option controls the number of lags printed in the schematic repre-

sentation of the autocorrelations

The sample autocovariance matrix of lag i denoted asC

i

is computed as

C

i

=

1

N 1

N

X

t = 1 + i

x

t

x

0

t i

wherex

t

is the differenced and centered data andN

is the number of observations

(If the NOCENTER option is specified 1 is not subtracted fromN

) LAGMAX= k

specifies thatC

0

throughC

k

are computed The default is LAGMAX=10

NOCENTER

prevents subtraction of the sample mean from the input series (after any specified

differencing) before the analysis

Options for Preliminary Autoregressive Models ARMAX= n

specifies the maximum order of the preliminary autoregressive models The AR-

MAX= option controls the autoregressive orders for which information criteria are

printed and controls the number of lags printed in the schematic representation of

partial autocorrelations The default is ARMAX=10 See Preliminary Autoregres-

sive Models later in this chapter for details

MINIC

writes to the OUTAR= data set only the preliminary Yule-Walker estimates for the

VAR model producing the minimum AIC See OUTAR= Data Set later in this chap-

ter for details

OUTAR= SAS-data-set

writes the Yule-Walker estimates of the preliminary autoregressive models to a SAS

data set See OUTAR= Data Set later in this chapter for details

PRINTOUT= SHORT | LONG | NONE

determines the amount of detail printed PRINTOUT=LONG prints the lagged co-

variance matrices the partial autoregressive matrices and estimates of the resid-

ual covariance matrices from the sequence of autoregressive models PRINT-

OUT=NONE suppresses the output for the preliminary autoregressive models The

descriptive statistics and state space model estimation output are still printed when

PRINTOUT=NONE is specified PRINTOUT=SHORT is the default

Canonical Correlation Analysis Options

CANCORR

prints the canonical correlations and information criterion for each candidate state

vector considered See Canonical Correlation Analysis later in this chapter for

details

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Part 2 General Information

DIMMAX= n

specifies the upper limit to the dimension of the state vector The DIMMAX= option

can be used to limit the size of the model selected The default is DIMMAX=10

PASTMIN= n

specifies the minimum number of lags to include in the canonical correlation analy-

sis The default is PASTMIN=0 See Canonical Correlation Analysis later in thischapter for details

SIGCORR= value

specifies the multiplier of the degrees of freedom for the penalty term in the informa-

tion criterion used to select the state space form The default is SIGCORR=2 The

larger the value of the SIGCORR= option the smaller the state vector tends to be

Hence a large value causes a simpler model to be fit See Canonical Correlations

Analysis later in this chapter for details

State Space Model Estimation Options

COVB

prints the inverse of the observed information matrix for the parameter estimatesThis matrix is an estimate of the covariance matrix for the parameter estimates

DETTOL= value

specifies the convergence criterion The DETTOL= and PARMTOL= option values

are used together to test for convergence of the estimation process If during an

iteration the relative change of the parameter estimates is less than the PARMTOL=

value and the relative change of the determinant of the innovation variance matrix

is less than the DETTOL= value then iteration ceases and the current estimates are

accepted The default is DETTOL=1E-5

ITPRINT

prints the iterations during the estimation process

KLAG= n

sets an upper limit for the number of lags of the sample autocovariance matrix used

in computing the approximate likelihood function If the data have a strong moving

average character a larger KLAG= value may be necessary to obtain good estimates

The default is KLAG=15 See Parameter Estimation later in this chapter for details

MAXIT= n

sets an upper limit to the number of iterations in the maximum likelihood or condi-

tional least-squares estimation The default is MAXIT=50

NOEST

suppresses the final maximum likelihood estimation of the selected model

OUTMODEL= SAS-data-set

writes the parameter estimates and their standard errors to a SAS data set See OUT-

MODEL= Data Set later in this chapter for details

PARMTOL= value

specifies the convergence criterion The DETTOL= and PARMTOL= option values

are used together to test for convergence of the estimation process If during an

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Chapter 18 Syntax

iteration the relative change of the parameter estimates is less than the PARMTOL=

value and the relative change of the determinant of the innovation variance matrix

is less than the DETTOL= value then iteration ceases and the current estimates are

accepted The default is PARMTOL=001

RESIDEST

computes the final estimates using conditional least squares on the raw data Thistype of estimation may be more stable than the default maximum likelihood method

but is usually more computationally expensive See Parameter Estimation later in

this chapter for details of the conditional least squares method

SINGULAR= value

specifies the criterion for testing for singularity of a matrix A matrix is declared

singular if a scaled pivot is less than the SINGULAR= value when sweeping the

matrix The default is SINGULAR=1E-7

Forecasting Options

BACK= n

starts forecasting n periods before the end of the input data The BACK= option valuemust not be greater than the number of observations The default is BACK=0

INTERVAL= interval

specifies the time interval between observations The INTERVAL= value is used

in conjunction with the ID variable to check that the input data are in order and

have no missing periods The INTERVAL= option is also used to extrapolate the ID

values past the end of the input data See Chapter 3 ldquoDate Intervals Formats and

Functionsrdquo for details on the INTERVAL= values allowed

INTPER= n

specifies that each input observation corresponds to n time periods For example

the options INTERVAL=MONTH and INTPER=2 specify bimonthly data and are

equivalent to specifying INTERVAL=MONTH2 If the INTERVAL= option is not

specified the INTPER= option controls the increment used to generate ID values for

the forecast observations The default is INTPER=1

LEAD= n

specifies how many forecast observations are produced The forecasts start at the

point set by the BACK= option The default is LEAD=0 which produces no fore-

casts

OUT= SAS-data-set

writes the residuals actual values forecasts and forecast standard errors to a SAS

data set See OUT= Data Set later in this chapter for details

PRINT

prints the forecasts

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Part 2 General Information

BY Statement

BY variable

A BY statement can be used with the STATESPACE procedure to obtain separateanalyses on observations in groups defined by the BY variables

FORM Statement

FORM variable value

The FORM statement specifies the number of times a variable is included in the state

vector Values can be specified for any variable listed in the VAR statement If a

value is specified for each variable in the VAR statement the state vector for the state

space model is entirely specified and automatic selection of the state space model is

not performed

The FORM statement forces the state vectorz

t

to contain a specific variable a given

number of times For example if Y is one of the variables inx

t

then the statement

form y 3

forces the state vector to containY

t

Y

t + 1 j t

andY

t + 2 j t

possibly along with other

variables

The following statements illustrate the use of the FORM statement

proc statespace data=in

var x y

f o r m x 3 y 2

run

These statements fit a state space model with the following state vector

z

t

=

2

6

6

6

6

4

x

t j t

y

t j t

x

t + 1 j t

y

t + 1 j t

x

t + 2 j t

3

7

7

7

7

5

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Chapter 18 Syntax

ID Statement

ID variable

The ID statement specifies a variable that identifies observations in the input data setThe variable specified in the ID statement is included in the OUT= data set The

values of the ID variable are extrapolated for the forecast observations based on the

values of the INTERVAL= and INTPER= options

INITIAL Statement

INITIAL F (rowcolumn)= value G(row column)= value

The INITIAL statement gives initial values to the specified elements of theF

andG

matrices These initial values are used as starting values for the iterative estimation

Parts of theF

andG

matrices represent fixed structural identities If an element

specified is a fixed structural element instead of a free parameter the corresponding

initialization is ignored

The following is an example of an INITIAL statement

initial f(32)=0 g(41)=0 g(51)=0

RESTRICT Statement

RESTRICT F(rowcolumn)= value G(rowcolumn)= value

The RESTRICT statement restricts the specified elements of theF

andG

matrices

to the specified values

To use the restrict statement you need to know the form of the model Either specify

the form of the model with the FORM statement or do a preliminary run perhaps

with the NOEST option to find the form of the model that PROC STATESPACE

selects for the data

The following is an example of a RESTRICT statement

restrict f(32)=0 g(41)=0 g(51)=0

Parts of theF

andG

matrices represent fixed structural identities If a restriction is

specified for an element that is a fixed structural element instead of a free parameter

the restriction is ignored

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Part 2 General Information

VAR Statement

VAR variable (difference difference )

The VAR statement specifies the variables in the input data set to model and fore-cast The VAR statement also specifies differencing of the input variables The VAR

statement is required

Differencing is specified by following the variable name with a list of difference

periods separated by commas See the section Stationarity and Differencing for

more information on differencing of input variables

The order in which variables are listed in the VAR statement controls the order in

which variables are included in the state vector Usually potential inputs should be

listed before potential outputs

For example assuming the input data are monthly the following VAR statement

specifies modeling and forecasting of the one period and seasonal second differenceof X and Y

var x(112) y(112)

In this example the vector time series analyzed is

x

t

=

1 B 1 B

1 2

X

t

x

1 B 1 B

1 2

Y

t

y

where B represents the back shift operator and x and y represent the means of thedifferenced series If the NOCENTER option is specified the mean differences are

not subtracted

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Chapter 18 Details

Details

Missing Values

The STATESPACE procedure does not support missing values The procedure uses

the first contiguous group of observations with no missing values for any of the VARstatement variables Observations at the beginning of the data set with missing values

for any VAR statement variable are not used or included in the output data set

Stationarity and Differencing

The state space model used by the STATESPACE procedure assumes that the time

series are stationary Hence the data should be checked for stationarity One way

to check for stationarity is to plot the series A graph of series over time can show a

time trend or variability changes

You can also check stationarity by using the sample autocorrelation functions dis-

played by the ARIMA procedure The autocorrelation functions of nonstationaryseries tend to decay slowly See Chapter 7 ldquoThe ARIMA Procedurerdquo for more in-

formation

Another alternative is to use the STATIONARITY= option on the IDENTIFY state-

ment in PROC ARIMA to apply Dickey-Fuller tests for unit roots in the time series

See Chapter 7 ldquoThe ARIMA Procedurerdquo for more information on Dickey-Fuller unit

root tests

The most popular way to transform a nonstationary series to stationarity is by dif-

ferencing Differencing of the time series is specified in the VAR statement For

example to take a simple first difference of the series X use this statement

var x(1)

In this example the change in X from one period to the next is analyzed When the se-

ries has a seasonal pattern differencing at a period equal to the length of the seasonal

cycle may be desirable For example suppose the variable X is measured quarterly

and shows a seasonal cycle over the year You can use the following statement to

analyze the series of changes from the same quarter in the previous year

var x(4)

To difference twice add another differencing period to the list For ex-

ample the following statement analyzes the series of second differences

X

t

X

t 1

X

t 1

X

t 2

= X

t

2 X

t 1

+ X

t 2

var x(11)

The following statement analyzes the seasonal second difference series

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Part 2 General Information

var x(14)

The series modeled is the 1-period difference of the 4-period difference

X

t

X

t 4

X

t 1

X

t 5

= X

t

X

t 1

X

t 4

+ X

t 5

Another way to obtain stationary series is to use a regression on time to de-trend the

data If the time series has a deterministic linear trend regressing the series on time

produces residuals that should be stationary The following statements write residuals

of X and Y to the variable RX and RY in the output data set DETREND

data a

set a

t=_n_

run

proc reg data=a

model x y = t

output out=detrend r=rx ry

run

You then use PROC STATESPACE to forecast the de-trended series RX and RY A

disadvantage of this method is that you need to add the trend back to the forecast

series in an additional step A more serious disadvantage of the de-trending method

is that it assumes a deterministic trend In practice most time series appear to have a

stochastic rather than a deterministic trend Differencing is a more flexible and often

more appropriate method

There are several other methods to handle nonstationary time series For more infor-

mation and examples refer to Brockwell and Davis (1991)

Preliminary Autoregressive Models

After computing the sample autocovariance matrices PROC STATESPACE fits a

sequence of vector autoregressive models These preliminary autoregressive models

are used to estimate the autoregressive order of the process and limit the order of the

autocovariances considered in the state vector selection process

Yule-Walker Equations for Forward and Backward Models

Unlike a univariate autoregressive model a multivariate autoregressive model has

different forms depending on whether the present observation is being predicted from

the past observations or from the future observations

Letx

t

be the r -component stationary time series given by the VAR statement after

differencing and subtracting the vector of sample means (If the NOCENTER option

is specified the mean is not subtracted) Let n be the number of observations of x

t

from the input data set

Lete

t

be a vector white noise sequence with mean vector 0 and variance matrix

p

and letn

t

be a vector white noise sequence with mean vector 0 and variance matrix

p

Let p be the order of the vector autoregressive model forx

t

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The forward autoregressive form based on the past observations is written as follows

x

t

=

p

X

i = 1

p

i

x

t i

+ e

t

The backward autoregressive form based on the future observations is written as fol-

lows

x

t

=

p

X

i = 1

p

i

x

t + i

+ n

t

LettingE

denote the expected value operator the autocovariance sequence for thex

t

series

i

is

i

= E x

t

x

0

t i

The Yule-Walker equations for the autoregressive model that matches the first p ele-

ments of the autocovariance sequence are

2

6

6

4

0

1

p 1

0

1

0

p 2

0

p 1

0

p 2

0

3

7

7

5

2

6

6

4

p

1

p

2

p

p

3

7

7

5

=

2

6

6

4

1

2

p

3

7

7

5

and

2

6

6

6

4

0

0

1

0

p 1

1

0

0

p 2

p 1

p 2

0

3

7

7

7

5

2

6

6

4

p

1

p

2

p

p

3

7

7

5

=

2

6

6

4

0

1

0

2

0

p

3

7

7

5

Here

p

i

are the coefficient matrices for the past observation form of the vector au-toregressive model and

p

i

are the coefficient matrices for the future observation

form More information on the Yule-Walker equations in the multivariate setting can

be found in Whittle (1963) and Ansley and Newbold (1979)

The innovation variance matrices for the two forms can be written as follows

p

=

0

p

X

i = 1

p

i

0

i

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p

=

0

p

X

i = 1

p

i

i

The autoregressive models are fit to the data using the preceding Yule-Walker equa-

tions with

i

replaced by the sample covariance sequenceC

i

The covariance matri-

ces are calculated as

C

i

=

1

N 1

N

X

t = i + 1

x

t

x

0

t i

Let b

p

b

p

b

p

and b

p

represent the Yule-Walker estimates of

p

p

p

and

p

respectively These matrices are written to an output data set when the OUTAR=

option is specified

When the PRINTOUT=LONG option is specified the sequence of matrices b

p

and

the corresponding correlation matrices are printed The sequence of matricesb

p isused to compute Akaike information criteria for selection of the autoregressive order

of the process

Akaike Information Criterion

The Akaike information criterion or AIC is defined as -2( maximum of log likeli-

hood )+2(number of parameters) Since the vector autoregressive models are esti-

mates from the Yule-Walker equations not by maximum likelihood the exact likeli-

hood values are not available for computing the AIC However for the vector autore-

gressive model the maximum of the log likelihood can be approximated as

l n L

n

2

l n j

b

p

j

Thus the AIC for the order p model is computed as

A I C

p

= n l n j

b

p

j + 2 p r

2

You can use the printed AIC array to compute a likelihood ratio test of the autoregres-

sive order The log-likelihood ratio test statistic for testing the order p model against

the order p 1

model is

n l n j

b

p

j + n l n j

b

p 1

j

This quantity is asymptotically distributed as a

2 withr

2 degrees of freedom if the

series is autoregressive of order p 1

It can be computed from the AIC array as

A I C

p 1

A I C

p

+ 2 r

2

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You can evaluate the significance of these test statistics with the PROBCHI function

in a SAS DATA step or with a

2 table

Determining the Autoregressive Order

Although the autoregressive models can be used for prediction their primary value is

to aid in the selection of a suitable portion of the sample covariance matrix for use in

computing canonical correlations If the multivariate time series x

t is of autoregres-sive order p then the vector of past values to lag p is considered to contain essentially

all the information relevant for prediction of future values of the time series

By default PROC STATESPACE selects the order p producing the autoregressive

model with the smallestA I C

p

If the value p for the minimumA I C

p

is less than the

value of the PASTMIN= option then p is set to the PASTMIN= value Alternatively

you can use the ARMAX= and PASTMIN= options to force PROC STATESPACE to

use an order you select

Significance Limits for Partial Autocorrelations

The STATESPACE procedure prints a schematic representation of the partial autocor-

relation matrices indicating which partial autocorrelations are significantly greater orsignificantly less than 0 Figure 1810 shows an example of this table

The STATESPACE Procedure

Schematic Representation of Partial Autocorrelations

NameLag 1 2 3 4 5 6 7 8 9 10

x ++ +

y ++

+ is gt 2std error - is lt -2std error is between

Figure 1810 Significant Partial Autocorrelations

The partial autocorrelations are from the sample partial autoregressive matrices b

p

p

The standard errors used for the significance limits of the partial autocorrelations are

computed from the sequence of matrices

p

and

p

Under the assumption that the observed series arises from an autoregressive process

of order p 1

the pth sample partial autoregressive matrix b

p

p

has an asymptotic

variance matrix 1

n

1

p

p

The significance limits for b

p

p

used in the schematic plot of the sample partial autore-

gressive sequence are derived by replacing

p

and

p

with their sample estimators

to produce the variance estimate as follows

d

V a r

b

p

p

=

1

n r p

b

1

p

b

p

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Canonical Correlation Analysis

Given the order p letp

t

be the vector of current and past values relevant to prediction

of x

t + 1

p

t

= x

0

t

x

0

t 1

x

0

t p

0

Letf

t

be the vector of current and future values

f

t

= x

0

t

x

0

t + 1

x

0

t + p

0

In the canonical correlation analysis consider submatrices of the sample covariance

matrix of p

t

andf

t

This covariance matrixV

has a block Hankel form

V =

2

6

6

6

4

C

0

C

0

1

C

0

2

C

0

p

C

0

1

C

0

2

C

0

3

C

0

p + 1

C

0

p

C

0

p + 1

C

0

p + 2

C

0

2 p

3

7

7

7

5

State Vector Selection Process

The canonical correlation analysis forms a sequence of potential state vectorsz

j

t

Examine a sequencef

j

t

of subvectors of f

t

and form the submatrixV

j consisting

of the rows and columns of V

corresponding to the components of f

j

t

and compute

its canonical correlations

The smallest canonical correlation of V

j is then used in the selection of the com-

ponents of the state vector The selection process is described in the following For

more details about this process refer to Akaike (1976)

In the following discussion the notationx

t + k j t

denotes the wide sense conditional

expectation (best linear predictor) of x

t + k

given allx

s

with s less than or equal to t

In the notationx

i t + 1

the first subscript denotes the ith component of x

t + 1

The initial state vectorz

1

t

is set tox

t

The sequencef

j

t

is initialized by setting

f

1

t

= z

1

0

t

x

1 t + 1 j t

0

= x

0

t

x

1 t + 1 j t

0

That is start by considering whether to addx

1 t + 1 j t

to the initial state vectorz

1

t

The procedure forms the submatrixV

1 corresponding tof

1

t

and computes its canon-

ical correlations Denote the smallest canonical correlation of V

1 as

m i n

If

m i n

is

significantly greater than 0x

1 t + 1 j t

is added to the state vector

If the smallest canonical correlation of V

1 is not significantly greater than 0 then a

linear combination of f

1

t

is uncorrelated with the pastp

t

Assuming that the determi-

nant of C

0

is not 0 (that is no input series is a constant) you can take the coefficient

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Chapter 18 Details

of x

1 t + 1 j t

in this linear combination to be 1 Denote the coefficients of z

1

t

in this

linear combination as

This gives the relationship

x

1 t + 1 j t

=

0

x

t

Therefore the current state vector already contains all the past information useful forpredicting

x

1 t + 1

and any greater leads of x

1 t

The variablex

1 t + 1 j t

is not added to

the state vector nor are any termsx

1 t + k j t

considered as possible components of the

state vector The variablex

1

is no longer active for state vector selection

The process described forx

1 t + 1 j t

is repeated for the remaining elements of f

t

The

next candidate for inclusion in the state vector is the next component of f

t

corre-

sponding to an active variable Components of f

t

corresponding to inactive variables

that produced a zero

m i n

in a previous step are skipped

Denote the next candidate asx

l t + k j t

The vectorf

j

t

is formed from the current state

vector andx

l t + k j t

as follows

f

j

t

= z

j

0

t

x

l t + k j t

0

The matrixV

j is formed fromf

j

t

and its canonical correlations are computed The

smallest canonical correlation of V

j is judged to be either greater than or equal to 0

If it is judged to be greater than 0x

l t + k j t

is added to the state vector If it is judged

to be 0 then a linear combination of f

j

t

is uncorrelated with thep

t

and the variable

x

l

is now inactive

The state vector selection process continues until no active variables remain

Testing Significance of Canonical Correlations For each step in the canonical correlation sequence the significance of the small-

est canonical correlation

m i n

is judged by an information criterion from Akaike

(1976) This information criterion is

n l n 1

2

m i n

r p + 1 q + 1

where q is the dimension of f

j

t

at the current step r is the order of the state vector p

is the order of the vector autoregressive process and

is the value of the SIGCORR=

option The default is SIGCORR=2 If this information criterion is less than or equal

to 0

m i n

is taken to be 0 otherwise it is taken to be significantly greater than 0

(Do not confuse this information criterion with the AIC)

Variables inx

t + p j t

are not added in the model even with positive information crite-

rion because of the singularity of V

You can force the consideration of more candi-

date state variables by increasing the size of theV

matrix by specifying a PASTMIN=

option value larger than p

Printing the Canonical Correlations

To print the details of the canonical correlation analysis process specify the CAN-

CORR option in the PROC STATESPACE statement The CANCORR option prints

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the candidate state vectors the canonical correlations and the information criteria for

testing the significance of the smallest canonical correlation

Bartlettrsquos

2 and its degrees of freedom are also printed when the CANCORR option

is specified The formula used for Bartlettrsquos

2 is

2

= n 5 r p + 1 q + 1 l n 1

2

m i n

withr p + 1 q + 1

degrees of freedom

Figure 1811 shows the output of the CANCORR option for the introductory example

shown in the Getting Started section of this chapter

The STATESPACE Procedure

Canonical Correlations Analysis

Information Chi

x(TT) y(TT) x(T+1T) Criterion Square DF

1 1 0237045 3566167 114505 4

Information Chi

x(TT) y(TT) x(T+1T) y(T+1T) Criterion Square DF

1 1 0238244 0056565 -535906 0636134 3

Information Chi

x(TT) y(TT) x(T+1T) x(T+2T) Criterion Square DF

1 1 0237602 0087493 -446312 1525353 3

Figure 1811 Canonical Correlations Analysis

New variables are added to the state vector if the information criteria are positive Inthis example

y

t + 1 j t

andx

t + 2 j t

are not added to the state space vector because the

information criteria for these models are negative

If the information criterion is nearly 0 then you may want to investigate models

that arise if the opposite decision is made regarding

m i n

This investigation can be

accomplished by using a FORM statement to specify part or all of the state vector

Preliminary Estimates of F

When a candidate variablex

l t + k j t

yields a zero

m i n

and is not added to the state

vector a linear combination of f

j

t

is uncorrelated with thep

t

Because of the method

used to construct thef

j

t

sequence the coefficient of x

l t + k j t

inl

can be taken as 1

Denote the coefficients of z

j

t

in this linear combination as l

This gives the relationship

x

l t + k j t

= l

0

z

j

t

The vectorl

is used as a preliminary estimate of the first r columns of the row of the

transition matrixF

corresponding tox

l t + k 1 j t

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Parameter Estimation

The model isz

t + 1

= F z

t

+ G e

t + 1

wheree

t

is a sequence of independent multivari-

ate normal innovations with mean vector 0 and variance

e e

The observed sequence

x

t

composes the first r components of z

t

and thusx

t

= H z

t

where H is ther s

matrix I

r

0

LetE

be ther n

matrix of innovations

E = e

1

e

n

If the number of observations n is reasonably large the log likelihood L can be

approximated up to an additive constant as follows

L =

n

2

l n j

e e

j

1

2

t r a c e

1

e e

E E

0

The elements of

e e

are taken as free parameters and are estimated as follows

S

0

=

1

n

E E

0

Replacing

e e

byS

0

in the likelihood equation the log likelihood up to an additive

constant is

L =

n

2

l n j S

0

j

Letting B be the backshift operator the formal relation betweenx

t

ande

t

is

x

t

= H I B F

1

G e

t

e

t

= H I B F

1

G

1

x

t

=

1

X

i = 0

i

x

t i

LettingC

i

be the ith lagged sample covariance of x

t

and neglecting end effects the

matrixS

0

is

S

0

=

1

X

i j = 0

i

C

i + j

0

j

For the computation of S

0

the infinite sum is truncated at the value of the KLAG=

option The value of the KLAG= option should be large enough that the sequence

i

is approximately 0 beyond that point

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Let

be the vector of free parameters in theF

andG

matrices The derivative of the

log likelihood with respect to the parameter

is

L

=

n

2

t r a c e

S

1

0

S

0

The second derivative is

2

L

0

=

n

2

t r a c e

S

1

0

S

0

0

S

1

0

S

0

t r a c e

S

1

0

2

S

0

0

Near the maximum the first term is unimportant and the second term can be approx-

imated to give the following second derivative approximation

2

L

0

=

n t r a c e

S

1

0

E

E

0

0

The first derivative matrix and this second derivative matrix approximation are com-

puted from the sample covariance matrixC

0

and the truncated sequence

i

The

approximate likelihood function is maximized by a modified Newton-Raphson algo-

rithm employing these derivative matrices

The matrixS

0

is used as the estimate of the innovation covariance matrix

e e

The

negative of the inverse of the second derivative matrix at the maximum is used as an

approximate covariance matrix for the parameter estimates The standard errors of

the parameter estimates printed in the parameter estimates tables are taken from the

diagonal of this covariance matrix The parameter covariance matrix is printed when

the COVB option is specified

If the data are nearly nonstationary a better estimate of

e e

and the other parameters

can sometimes be obtained by specifying the RESIDEST option The RESIDEST

option estimates the parameters using conditional least squares instead of maximum

likelihood

The residuals are computed using the state space equation and the sample mean val-

ues of the variables in the model as start-up values The estimate of S

0

is then com-

puted using the residuals from the ith observation on where i is the maximum number

of times any variable occurs in the state vector A multivariate Gauss-Marquardt al-

gorithm is used to minimizej S

0

j

Refer to Harvey (1981a) for a further description

of this method

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Forecasting

Given estimates of F

G

and

e e

forecasts of x

t

are computed from the conditional

expectation of z

t

In forecasting the parameters F G and

e e

are replaced with the estimates or by val-

ues specified in the RESTRICT statement One-step-ahead forecasting is performedfor the observation

x

t

wheret n b

Heren

is the number of observations and

b is the value of the BACK= option For the observationx

t

wheret n b

m-

step-ahead forecasting is performed form = t n + b

The forecasts are generated

recursively with the initial conditionz

0

= 0

The m-step-ahead forecast of z

t + m

isz

t + m j t

wherez

t + m j t

denotes the conditional

expectation of z

t + m

given the information available at time t The m-step-ahead

forecast of x

t + m

isx

t + m j t

= H z

t + m j t

where the matrixH = I

r

0

Let

i

= F

i

G

Note that the lasts r

elements of z

t

consist of the elements of x

u j t

foru t

The state vector z

t + m

can be represented as

z

t + m

= F

m

z

t

+

m 1

X

i = 0

i

e

t + m i

Sincee

t + i j t

= 0

fori 0

the m-step-ahead forecastz

t + m j t

is

z

t + m j t

= F

m

z

t

= F z

t + m 1 j t

Therefore the m-step-ahead forecast of x

t + m

is

x

t + m j t

= H z

t + m j t

The m-step-ahead forecast error is

z

t + m

z

t + m j t

=

m 1

X

i = 0

i

e

t + m i

The variance of the m-step-ahead forecast error is

V

z m

=

m 1

X

i = 0

i

e e

0

i

LettingV

z 0

= 0

the variance of the m-step-ahead forecast error of z

t + m

V

z m

can

be computed recursively as follows

V

z m

= V

z m 1

+

m 1

e e

0

m 1

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The variance of the m-step-ahead forecast error of x

t + m

is ther r

left upper sub-

matrix of V

z m

that is

V

x m

= H V

z m

H

0

Unless the NOCENTER option is specified the sample mean vector is added to the

forecast When differencing is specified the forecasts xt + m j t

plus the sample mean

vector are integrated back to produce forecasts for the original series

Lety

t

be the original series specified by the VAR statement with some 0 values

appended corresponding to the unobserved past observations Let B be the backshift

operator and let B

be thes s

matrix polynomial in the backshift operator

corresponding to the differencing specified by the VAR statement The off-diagonal

elements of

i

are 0 Note that

0

= I

s

whereI

s

is thes s

identity matrix Then

z

t

= B y

t

This gives the relationship

y

t

=

1

B z

t

=

1

X

i = 0

i

z

t i

where

1

B =

P

1

i = 0

i

B

i and

0

= I

s

The m-step-ahead forecast of y

t + m

is

y

t + m j t

=

m 1

X

i = 0

i

z

t + m i j t

+

1

X

i = m

i

z

t + m i

The m-step-ahead forecast error of y

t + m

is

m 1

X

i = 0

i

z

t + m i

z

t + m i j t

=

m 1

X

i = 0

i

X

u = 0

u

i u

e

t + m i

LettingV

y 0

= 0

the variance of the m-step-ahead forecast error of y

t + m

V

y m

is

V

y m

=

m 1

X

i = 0

i

X

u = 0

u

i u

e e

i

X

u = 0

u

i u

0

= V

y m 1

+

m 1

X

u = 0

u

m 1 u

e e

m 1

X

u = 0

u

m 1 u

0

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Relation of ARMA and State Space Forms

Every state space model has an ARMA representation and conversely every ARMA

model has a state space representation This section discusses this equivalence The

following material is adapted from Akaike (1974) where there is a more complete

discussion Pham-Dinh-Tuan (1978) also contains a discussion of this material

Suppose you are given the following ARMA model

B x

t

= B e

t

or in more detail

x

t

1

x

t 1

p

x

t p

= e

t

+

1

e

t 1

+ +

q

e

t q

(1)

wheree

t

is a sequence of independent multivariate normal random vectors with mean

0 and variance matrix

e e

B is the backshift operator (B x

t

= x

t 1

) B

and

B

are matrix polynomials in B andx

t

is the observed process

If the roots of the determinantial equationj B j = 0

are outside the unit circle in

the complex plane the model can also be written as

x

t

=

1

B B e

t

=

1

X

i = 0

i

e

t i

The

i

matrices are known as the impulse response matrices and can be computed

as

1

B B

You can assume p q

since if this is not initially true you can add more terms

i

that are identically 0 without changing the model

To write this set of equations in a state space form proceed as follows Letx

t + i j t

be

the conditional expectation of x

t + i

givenx

w

forw t

The following relations hold

x

t + i j t

=

1

X

j = i

j

e

t + i j

x

t + i j t + 1

= x

t + i j t

+

i 1

e

t + 1

However from equation (1) you can derive the following relationship

x

t + p j t

=

1

x

t + p 1 j t

+ +

p

x

t

(2)

Hence wheni = p

you can substitute forx

t + p j t

in the right-hand side of equation

(2) and close the system of equations

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This substitution results in the following model in the state space form

z

t + 1

= F z

t

+ G e

t + 1

2

6

6

6

4

x

t + 1

x

t + 2 j t + 1

x

t + p j t + 1

3

7

7

7

5

=

2

6

6

4

0 I 0 0

0 0 I 0

p

p 1

1

3

7

7

5

2

6

6

6

4

x

t

x

t + 1 j t

x

t + p 1 j t

3

7

7

7

5

+

2

6

6

4

I

1

p 1

3

7

7

5

e

t + 1

Note that the state vectorz

t

is composed of conditional expectations of x

t

and the

first r components of z

t

are equal tox

t

The state space form can be cast into an ARMA form by solving the system of dif-

ference equations for the first r components

When converting from an ARMA form to a state space form you can generate a

state vector larger than needed that is the state space model may not be a minimal

representation When going from a state space form to an ARMA form you can have

nontrivial common factors in the autoregressive and moving average operators that

yield an ARMA model larger than necessary

If the state space form used is not a minimal representation some but not all compo-

nents of x

t + i j t

may be linearly dependent This situation corresponds to

p

p 1

being of less than full rank when B

and B

have no common nontrivial

left factors In this casez

t

consists of a subset of the possible components of

x

t + i j t

i = 1 2 p 1

However once a component of x

t + i j t

(for example the

jth one) is linearly dependent on the previous conditional expectations then all sub-

sequent jth components of x

t + k j t

fork i

must also be linearly dependent Note

that in this case equivalent but seemingly different structures can arise if the order of

the components withinx

t

is changed

OUT= Data Set

The forecasts are contained in the output data set specified by the OUT= option on the

PROC STATESPACE statement The OUT= data set contains the following variables

the BY variables

the ID variable

the VAR statement variables These variables contain the actual values from

the input data set

FORi numeric variables containing the forecasts The variable FORi contains

the forecasts for the ith variable in the VAR statement list Forecasts are one-

step-ahead predictions until the end of the data or until the observation specified

by the BACK= option

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Chapter 18 Details

RESi numeric variables containing the residual for the forecast of the ith vari-

able in the VAR statement list For forecast observations the actual values are

missing and the RESi variables contain missing values

STDi numeric variables containing the standard deviation for the forecast of

the ith variable in the VAR statement list The values of the STDi variables can

be used to construct univariate confidence limits for the corresponding fore-casts However such confidence limits do not take into account the covariance

of the forecasts

OUTAR= Data Set

The OUTAR= data set contains the estimates of the preliminary autoregressive mod-

els The OUTAR= data set contains the following variables

ORDER a numeric variable containing the order p of the autoregressive model

that the observation represents

AIC a numeric variable containing the value of the information criterionA I C

p

SIGFl numeric variables containing the estimate of the innovation covariance

matrices for the forward autoregressive models The variable SIGFl contains

the lth column of b

p

in the observations with ORDER= p

SIGBl numeric variables containing the estimate of the innovation covariance

matrices for the backward autoregressive models The variable SIGBl contains

the lth column of b

p

in the observations with ORDER= p

FORk ndashl numeric variables containing the estimates of the autoregressive pa-

rameter matrices for the forward models The variable FORk ndashl contains the

lth column of the lag k autoregressive parameter matrix b

p

k

in the observations

with ORDER= p

BACk ndashl numeric variables containing the estimates of the autoregressive pa-

rameter matrices for the backward models The variable BACk ndashl contains the

lth column of the lag k autoregressive parameter matrix b

p

k

in the observations

with ORDER= p

The estimates for the order p autoregressive model can be selected as those observa-

tions with ORDER= p Within these observations the k lth element of

p

i

is given by

the value of the FORindashl variable in the k th observation The k lth element of

p

i

is

given by the value of BACindashl variable in the k th observation The k lth element of

p

is given by SIGFl in the k th observation The k lth element of

p

is given by SIGBlin the k th observation

Table 181 shows an example of the OUTAR= data set with ARMAX=3 andx

t

of

dimension 2 In Table 181 i j

indicate the ijth element of the matrix

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Part 2 General Information

Table 181 Values in the OUTAR= Data Set

Obs ORDER AIC SIGF1 SIGF2 SIGB1 SIGB2 FOR1ndash 1 FOR1ndash 2 FOR2ndash 1 FOR2ndash 2 FOR3ndash 1

1 0 AIC0

0 1 1

0 1 2

0 1 1

0 1 2

2 0 AIC0

0 2 1

0 2 2

0 2 1

0 2 2

3 1 AIC1

1 1 1

1 1 2

1 1 1

1 1 2

1

1

1 1

1

1

1 2

4 1 AIC1

1 2 1

1 1 2

1 2 1

1 1 2

1

1

2 1

1

1

2 2

5 2 AIC2

2 1 1

2 1 2

2 1 1

2 1 2

2

1

1 1

2

1

1 2

2

2

1 1

2

2

1 2

6 2 AIC2

2 2 1

2 1 2

2 2 1

2 1 2

2

1

2 1

2

1

2 2

2

2

2 1

2

2

2 2

7 3 AIC3

3 1 1

3 1 2

3 1 1

3 1 2

3

1

1 1

3

1

1 2

3

2

1 1

3

2

1 2

3

3

1 1

8 3 AIC3

3 2 1

3 1 2

3 2 1

3 1 2

3

1

2 1

3

1

2 2

3

2

2 1

3

2

2 2

3

3

2 1

Obs FOR3ndash 2 BACK1ndash 1 BACK1ndash 2 BACK2ndash 1 BACK2ndash 2 BACK3ndash 1 BACK3ndash 2

1

2

3

1

1

1 1

1

1

1 2

4

1

1

2 1

1

1

2 2

5

2

1

1 1

2

1

1 2

2

2

1 1

2

2

1 2

6

2

1

2 1

2

1

2 2

2

2

2 1

2

2

2 2

7

3

3

1 2

3

1

1 1

3

1

1 2

3

2

1 1

3

2

1 2

3

3

1 1

3

3

1 2

8

3

3

2 2

3

1

2 1

3

1

2 2

3

2

2 1

3

2

2 2

3

3

2 1

3

3

2 2

The estimated autoregressive parameters can be used in the IML procedure to obtain

autoregressive estimates of the spectral density function or forecasts based on the

autoregressive models

OUTMODEL= Data Set

The OUTMODEL= data set contains the estimates of theF

andG

matrices and their

standard errors the names of the components of the state vector and the estimates of

the innovation covariance matrix The variables contained in the OUTMODEL= data

set are as follows

the BY variables

STATEVEC a character variable containing the name of the component of the

state vector corresponding to the observation The STATEVEC variable has

the value STD for standard deviations observations which contain the standarderrors for the estimates given in the preceding observation

Fndash j numeric variables containing the columns of theF

matrix The variable

Fndash j contains the jth column of F

The number of Fndash j variables is equal to

the value of the DIMMAX= option If the model is of smaller dimension the

extraneous variables are set to missing

Gndash j numeric variables containing the columns of theG

matrix The variable

Gndash j contains the jth column of G

The number of Gndash j variables is equal to r

the dimension of x

t

given by the number of variables in the VAR statement

SIGndash j numeric variables containing the columns of the innovation covariance

matrix The variable SIGndash j contains the jth column of

e e

There are r vari-ables SIGndash j

Table 182 shows an example of the OUTMODEL= data set withx

t

= x

t

y

t

0

z

t

= x

t

y

t

x

t + 1 j t

0 and DIMMAX=4 In Table 182F

i j

andG

i j

are the ijth

elements of F

andG

respectively Note that all elements for Fndash4 are missing because

F

is a3 3

matrix

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Chapter 18 Details

Table 182 Value in the OUTMODEL= Data Set

Obs STATEVEC Fndash 1 Fndash 2 Fndash 3 Fndash 4 Gndash 1 Gndash 2 SIGndash 1 SIGndash 2

1 X(TT) 0 0 1 1 0

1 1

1 2

2 STD

3 Y(TT) F2 1

F2 2

F2 3

0 1

2 1

2 2

4 STD std F2 1

std F2 2

std F2 3

5 X(T+1T) F3 1

F3 2

F3 3

G3 1

G3 2

6 STD std F3 1

std F3 2

std F3 3

std G3 1

std G3 2

Printed Output

The printed output produced by the STATESPACE procedure is described in the fol-

lowing

1 descriptive statistics which include the number of observations used the

names of the variables their means and standard deviations (Std) and the dif-

ferencing operations used

2 the Akaike information criteria for the sequence of preliminary autoregressive

models

3 if the PRINTOUT=LONG option is specified the sample autocovariance ma-

trices of the input series at various lags

4 if the PRINTOUT=LONG option is specified the sample autocorrelation ma-

trices of the input series

5 a schematic representation of the autocorrelation matrices showing the signif-

icant autocorrelations

6 if the PRINTOUT=LONG option is specified the partial autoregressive ma-

trices (These are

p

p

as described in Preliminary Autoregressive Modelsearlier in this chapter)

7 a schematic representation of the partial autocorrelation matrices showing the

significant partial autocorrelations

8 the Yule-Walker estimates of the autoregressive parameters for the autoregres-

sive model with the minimum AIC

9 if the PRINTOUT=LONG option is specified the autocovariance matrices of

the residuals of the minimum AIC model This is the sequence of estimated

innovation variance matrices for the solutions of the Yule-Walker equations

10 if the PRINTOUT=LONG option is specified the autocorrelation matrices of the residuals of the minimum AIC model

11 If the CANCORR option is specified the canonical correlations analysis for

each potential state vector considered in the state vector selection process

This includes the potential state vector the canonical correlations the informa-

tion criterion for the smallest canonical correlation Bartlettrsquos

2 statistic (ldquoChi

Squarerdquo) for the smallest canonical correlation and the degrees of freedom of

Bartlettrsquos

2

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Part 2 General Information

12 the components of the chosen state vector

13 the preliminary estimate of the transition matrixF

the input matrixG

and

the variance matrix for the innovations

e e

14 if the ITPRINT option is specified the iteration history of the likelihood max-

imization For each iteration this shows the iteration number the number of step halvings the determinant of the innovation variance matrix the damping

factor Lambda and the values of the parameters

15 the state vector printed again to aid interpretation of the following listing of F

andG

16 the final estimate of the transition matrixF

17 the final estimate of the input matrixG

18 the final estimate of the variance matrix for the innovations

e e

19 a table listing the estimates of the free parameters inF

andG

and their standarderrors and t statistics

20 if the COVB option is specified the covariance matrix of the parameter esti-

mates

21 if the COVB option is specified the correlation matrix of the parameter esti-

mates

22 if the PRINT option is specified the forecasts and their standard errors

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Chapter 18 Details

ODS Table Names

PROC STATESPACE assigns a name to each table it creates You can use these

names to reference the table when using the Output Delivery System (ODS) to select

tables and create output data sets These names are listed in the following table For

more information on ODS see Chapter 6 ldquoUsing the Output Delivery Systemrdquo

Table 183 ODS Tables Produced in PROC STATESPACE

ODS Table Name Description Option

NObs Number of observations default

Summary Simple summary statistics table default

InfoCriterion Information criterion table default

CovLags Covariance Matrices of Input Series PRINTOUT=LONG

CorrLags Correlation Matrices of Input Series PRINTOUT=LONG

PartialAR Partial Autoregressive Matrices PRINTOUT=LONG

YWEstimates Yule-Walker Estimates for Minimum AIC default

CovResiduals Covariance of Residuals PRINTOUT=LONGCorrResiduals Residual Correlations from AR Models PRINTOUT=LONG

StateVector State vector table default

CorrGraph Schematic Representation of Correlations default

TransitionMatrix Transition Matrix default

InputMatrix Input Matrix default

VarInnov Variance Matrix for the Innovation default

CovB Covariance of Parameter Estimates COVB

CorrB Correlation of Parameter Estimates COVB

CanCorr Canonical Correlation Analysis CANCORR

IterHistory Iterative Fitting table ITPRINT

ParameterEstimates Parameter Estimates Table default

Forecasts Forecasts Table PRINT

ConvergenceStatus Convergence Status Table default

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Part 2 General Information

Example

Example 181 Series J from Box and Jenkins

This example analyzes the gas furnace data (series J) from Box and Jenkins (The

data are not shown Refer to Box and Jenkins (1976) for the data)

First a model is selected and fit automatically using the following statements

title1 rsquoGas Furnace Datarsquo

title2 rsquoBox amp Jenkins Series Jrsquo

title3 rsquoAutomatically Selected Modelrsquo

proc statespace data=seriesj cancorr

var x y

run

The results for the automatically selected model are shown in Output 1811

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Chapter 18 Example

Output 1811 Results for Automatically Selected Model

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Number of Observations 296

Standard

Variable Mean Error

x -005683 1072766

y 5350912 3202121

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Information Criterion for Autoregressive Models

Lag=0 Lag=1 Lag=2 Lag=3 Lag=4 Lag=5 Lag=6 Lag=7 Lag=8

6513862 -103357 -163296 -164512 -165152 -164891 -164934 -164315 -163856

Information

Criterion for

Autoregressive

Models

Lag=9 Lag=10

-16348 -163359

Schematic Representation of Correlations

NameLag 0 1 2 3 4 5 6 7 8 9 10

x +- +- +- +- +- +- +- +- +- +- +-

y -+ -+ -+ -+ -+ -+ -+ -+ -+ -+ -+

+ is gt 2std error - is lt -2std error is between

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Part 2 General Information

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Schematic Representation of Partial Autocorrelations

NameLag 1 2 3 4 5 6 7 8 9 10

x + - + -

y -+ -- - + +

+ is gt 2std error - is lt -2std error is between

Yule-Walker Estimates for Minimum AIC

------Lag=1------ ------Lag=2------ ------Lag=3------ ------Lag=4------

x y x y x y x y

x 1925887 -000124 -120166 0004224 0116918 -000867 0104236 0003268

y 0050496 1299793 -002046 -03277 -071182 -025701 0195411 0133417

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Canonical Correlations Analysis

Information Chi

x(TT) y(TT) x(T+1T) Criterion Square DF

1 1 0804883 2929228 3047481 8

Information Chix(TT) y(TT) x(T+1T) y(T+1T) Criterion Square DF

1 1 0906681 0607529 1223358 1347237 7

Information Chi

x(TT) y(TT) x (T+1T) y (T+1T) x (T+2T) Criterion Square DF

1 1 0909434 0610278 0186274 -154701 1034705 6

Information Chi

x(TT) y(TT) x (T+1T) y (T+1T) y (T+2T) Criterion Square DF

1 1 091014 0618937 0206823 0940392 1280924 6

Information Chi

x(TT) y(TT) x(T+1T) y(T+1T) y(T+2T) y(T+3T) Criterion Square DF

1 1 0912963 0628785 0226598 0083258 -794103 2041584 5

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Chapter 18 Example

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Selected Statespace Form and Preliminary Estimates

State Vector

x(TT) y(TT) x(T+1T) y(T+1T) y(T+2T)

Estimate of Transition Matrix

0 0 1 0 0

0 0 0 1 0

-084718 0026794 1711715 -005019 0

0 0 0 0 1

-019785 0334274 -018174 -123557 1787475

Input Matrix for Innovation

1 0

0 11925887 -000124

0050496 1299793

0142421 1361696

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Selected Statespace Form and Preliminary Estimates

Variance Matrix for Innovation

0035274 -000734

-000734 0097569

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Part 2 General Information

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Selected Statespace Form and Fitted Model

State Vector

x(TT) y(TT) x(T+1T) y(T+1T) y(T+2T)

Estimate of Transition Matrix

0 0 1 0 0

0 0 0 1 0

-086192 0030609 1724235 -005483 0

0 0 0 0 1

-034839 0292124 -009435 -109823 1671418

Input Matrix for Innovation

1 0

0 1192442 -000416

0015621 1258495

008058 1353204

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Selected Statespace Form and Fitted Model

Variance Matrix for Innovation

0035579 -000728

-000728 0095577

Parameter Estimates

Standard

Parameter Estimate Error t Value

F(31) -086192 0072961 -1181

F(32) 0030609 0026167 117

F(33) 1724235 0061599 2799

F(34) -005483 0030169 -182

F(51) -034839 0135253 -258

F(52) 0292124 0046299 631

F(53) -009435 0096527 -098F(54) -109823 0109525 -1003

F(55) 1671418 0083737 1996

G(31) 1924420 0058162 3309

G(32) -000416 0035255 -012

G(41) 0015621 0095771 016

G(42) 1258495 0055742 2258

G(51) 0080580 0151622 053

G(52) 1353204 0091388 1481

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Chapter 18 Example

The two series are believed to have a transfer function relation with the gas rate

(variable X) as the input and the CO2

concentration (variable Y) as the output Since

the parameter estimates shown in Output 1811 support this kind of model the model

is reestimated with the feedback parameters restricted to 0 The following statements

fit the transfer function (no feedback) model

title3 rsquoTransfer Function Modelrsquo

proc statespace data=seriesj printout=none

var x y

restrict f(32)=0 f(34)=0

g(32)=0 g(41)=0 g(51)=0

run

The last 2 pages of the output are shown in Output 1812

Output 1812 STATESPACE Output for Transfer Function Model

Gas Furnace Data

Box amp Jenkins Series J

Transfer Function Model

The STATESPACE Procedure

Selected Statespace Form and Fitted Model

State Vector

x(TT) y(TT) x(T+1T) y(T+1T) y(T+2T)

Estimate of Transition Matrix

0 0 1 0 0

0 0 0 1 0

-068882 0 1598717 0 0

0 0 0 0 1

-035944 0284179 -00963 -107313 1650047

Input Matrix for Innovation

1 0

0 1

1923446 0

0 1260856

0 1346332

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Part 2 General Information

Gas Furnace Data

Box amp Jenkins Series J

Transfer Function Model

The STATESPACE Procedure

Selected Statespace Form and Fitted Model

Variance Matrix for Innovation

0036995 -00072

-00072 0095712

Parameter Estimates

Standard

Parameter Estimate Error t Value

F(31) -068882 0050549 -1363

F(33) 1598717 0050924 3139

F(51) -035944 0229044 -157

F(52) 0284179 0096944 293

F(53) -009630 0140876 -068

F(54) -107313 0250385 -429

F(55) 1650047 0188533 875G(31) 1923446 0056328 3415

G(42) 1260856 0056464 2233

G(52) 1346332 0091086 1478

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Chapter 18 References

References

Akaike H (1974) Markovian Representation of Stochastic Processes and Its Ap-

plication to the Analysis of Autoregressive Moving Average Processes Annals

of the Institute of Statistical Mathematics 26 363-387

Akaike H (1976) Canonical Correlations Analysis of Time Series and the Use of

an Information Criterion in Advances and Case Studies in System Identification

eds R Mehra and DG Lainiotis New York Academic Press

Anderson TW (1971) The Statistical Analysis of Time Series New York John

Wiley amp Sons

Ansley CF and Newbold P (1979) Multivariate Partial Autocorrelations Pro-

ceedings of the Business and Economic Statistics Section American Statistical

Association 349-353

Box GEP and Jenkins G (1976) Time Series Analysis Forecasting and Control

San Francisco Holden-Day

Brockwell PJ and Davis RA (1991) Time Series Theory and Methods 2nd Edi-

tion Springer-Verlag

Hannan EJ (1970) Multiple Time Series New York John Wiley amp Sons

Hannan EJ (1976) The Identification and Parameterization of ARMAX and State

Space Forms Econometrica 44 713-722

Harvey AC (1981a) The Econometric Analysis of Time Series New York John

Wiley amp Sons

Harvey AC (1981b) Time Series Models New York John Wiley amp Sons

Jones RH (1974) Identification and Autoregressive Spectrum Estimation IEEE Transactions on Automatic Control AC-19 894-897

Pham-Dinh-Tuan (1978) On the Fitting of Multivariate Processes of the

Autoregressive-Moving Average Type Biometrika 65 99-107

Priestley MB (1980) System Identification Kalman Filtering and Stochastic Con-

trol in Directions in Time Series eds DR Brillinger and GC Tiao Institute

of Mathematical Statistics

Whittle P (1963) On the Fitting of Multivariate Autoregressions and the Approxi-

mate Canonical Factorization of a Spectral Density Matrix Biometrika 50 129-

134

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The correct bibliographic citation for this manual is as follows SAS Institute Inc SAS

ET S U serrsquos Gui de Version 8 Cary NC SAS Institute Inc 1999 1546 pp

SASET S Userrsquos Guide Version 8

Copyright copy 1999 by S AS I ns t it u t e I nc Ca ry N C U S A

ISBN 1ndash58025ndash489ndash6

All r ight s res erved P r int ed in t he U n it ed St a t es of A merica N o pa rt of t his p u blica t ionma y b e reprodu ced s t ored in a ret r ieva l s ys t em or t ra ns mit t ed in a ny form or b y a nymeans electronic mechanical photocopying or otherwise without the prior writtenpermission of the publisher SAS Institute Inc

US Government Restricted Rights Notice Use du plicat ion or disclosure of the

software by the government is subject to restrictions as set forth in FAR 52227ndash19Commercial Computer Softw ar e-Restricted Rights (J une 1987)

SAS Institute Inc SAS Campus Drive Cary North Carolina 27513

1st print ing October 1999

S AS reg a nd a ll ot her SA S I ns t it u t e I nc p rodu ct or s ervice na m es a re regis t ered t ra dema rkst d k f SA S I t it t I i t h U S A d t h t i reg i di t U SA

Page 15: Statespace Model

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Chapter 18 Getting Started

You can specify the form for only some of the variables and allow PROC STATES-

PACE to select the form for the other variables If only some of the variables are

specified in the FORM statement canonical correlation analysis is used to determine

the number of lags included in the state vector for the remaining variables not spec-

ified by the FORM statement If the FORM statement includes specifications for all

the variables listed in the VAR statement the state vector is completely defined and

the canonical correlation analysis is not performed

Restricting the F and G matrices

After you know the form of the state vector you can use the RESTRICT statement

to fix some parameters in theF

andG

matrices to specified values One use of this

feature is to remove insignificant parameters by restricting them to 0

In the introductory example shown in the preceding section the F[23] parameter is

not significant (The parameters estimation output shown in Figure 186 gives the t

statistic for F[23] as -006 F[33] and F[31] also have low significance witht 2

)

The following statements reestimate this model with F[23] restricted to 0 The

FORM statement is used to specify the state vector and thus bypass the canonicalcorrelation analysis

proc statespace data=in out=out lead=10

var x(1) y(1)

id t

form x 2 y 1

restrict f(23)=0

run

The final estimates produced by these statements are shown in Figure 189

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Part 2 General Information

The STATESPACE Procedure

Selected Statespace Form and Fitted Model

State Vector

x(TT) y(TT) x(T+1T)

Estimate of Transition Matrix

0 0 1

0290051 0467468 0

0227051 0226139 026436

Input Matrix for Innovation

1 0

0 1

0256826 0202022

Variance Matrix for Innovation

0945175 01006960100696 1014733

The STATESPACE Procedure

Parameter Estimates

Standard

Parameter Estimate Error t Value

F(21) 0290051 0063904 454

F(22) 0467468 0060430 774

F(31) 0227051 0125221 181

F(32) 0226139 0111711 202

F(33) 0264360 0299537 088G(31) 0256826 0070994 362

G(32) 0202022 0068507 295

Figure 189 Results using RESTRICT Statement

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Chapter 18 Syntax

Syntax

The STATESPACE procedure uses the following statements

PROC STATESPACE options

BY variable FORM variable value

ID variable

INITIAL F(rowcolumn)=value

G(rowcolumn)=value

RESTRICT F(rowcolumn)=value

G(rowcolumn)=value

VAR variable (difference difference

)

Functional Summary

The statements and options used by PROC STATESPACE are summarized in the

following table

Description Statement Option

Input Data Set Options

specify the input data set PROC STATESPACE DATA=

prevent subtraction of sample mean PROC STATESPACE NOCENTER

specify the ID variable ID

specify the observed series and differencing VAR

Options for Autoregressive Estimates

specify the maximum order PROC STATESPACE ARMAX=

specify maximum lag for autocovariances PROC STATESPACE LAGMAX=

output only minimum AIC model PROC STATESPACE MINIC

specify the amount of detail printed PROC STATESPACE PRINTOUT=

write preliminary AR models to a data set PROC STATESPACE OUTAR=

Options for Canonical Correlation Analysis

print the sequence of canonical correlations PROC STATESPACE CANCORR

specify upper limit of dimension of state

vector

PROC STATESPACE DIMMAX=

specify the minimum number of lags PROC STATESPACE PASTMIN=

specify the multiplier of the degrees of

freedom

PROC STATESPACE SIGCORR=

Options for State Space Model Estimation

specify starting values INITIAL

print covariance matrix of parameter estimates PROC STATESPACE COVB

specify the convergence criterion PROC STATESPACE DETTOL=

specify the convergence criterion PROC STATESPACE PARMTOL=

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Part 2 General Information

Description Statement Option

print the details of the iterations PROC STATESPACE ITPRINT

specify an upper limit of the number of lags PROC STATESPACE KLAG=

specify maximum number of iterations

allowed

PROC STATESPACE MAXIT=

suppress the final estimation PROC STATESPACE NOEST

write the state space model parameter esti-

mates to an output data set

PROC STATESPACE OUTMODEL=

use conditional least squares for final estimates PROC STATESPACE RESIDEST

specify criterion for testing for singularity PROC STATESPACE SINGULAR=

Options for Forecasting

start forecasting before end of the input data PROC STATESPACE BACK=

specify the time interval between observations PROC STATESPACE INTERVAL=

specify multiple periods in the time series PROC STATESPACE INTPER=specify how many periods to forecast PROC STATESPACE LEAD=

specify the output data set for forecasts PROC STATESPACE OUT=

print forecasts PROC STATESPACE PRINT

Options to Specify the State Space Model

specify the state vector FORM

specify the parameter values RESTRICT

BY Groups

specify BY-group processing BY

Printing

suppresses all printed output NOPRINT

PROC STATESPACE Statement

PROC STATESPACE options

The following options can be specified in the PROC STATESPACE statement

Printing Options NOPRINT

suppresses all printed output

Input Data Options

DATA= SAS-data-set

specifies the name of the SAS data set to be used by the procedure If the DATA=

option is omitted the most recently created SAS data set is used

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Chapter 18 Syntax

LAGMAX= k

specifies the number of lags for which the sample autocovariance matrix is computed

The LAGMAX= option controls the number of lags printed in the schematic repre-

sentation of the autocorrelations

The sample autocovariance matrix of lag i denoted asC

i

is computed as

C

i

=

1

N 1

N

X

t = 1 + i

x

t

x

0

t i

wherex

t

is the differenced and centered data andN

is the number of observations

(If the NOCENTER option is specified 1 is not subtracted fromN

) LAGMAX= k

specifies thatC

0

throughC

k

are computed The default is LAGMAX=10

NOCENTER

prevents subtraction of the sample mean from the input series (after any specified

differencing) before the analysis

Options for Preliminary Autoregressive Models ARMAX= n

specifies the maximum order of the preliminary autoregressive models The AR-

MAX= option controls the autoregressive orders for which information criteria are

printed and controls the number of lags printed in the schematic representation of

partial autocorrelations The default is ARMAX=10 See Preliminary Autoregres-

sive Models later in this chapter for details

MINIC

writes to the OUTAR= data set only the preliminary Yule-Walker estimates for the

VAR model producing the minimum AIC See OUTAR= Data Set later in this chap-

ter for details

OUTAR= SAS-data-set

writes the Yule-Walker estimates of the preliminary autoregressive models to a SAS

data set See OUTAR= Data Set later in this chapter for details

PRINTOUT= SHORT | LONG | NONE

determines the amount of detail printed PRINTOUT=LONG prints the lagged co-

variance matrices the partial autoregressive matrices and estimates of the resid-

ual covariance matrices from the sequence of autoregressive models PRINT-

OUT=NONE suppresses the output for the preliminary autoregressive models The

descriptive statistics and state space model estimation output are still printed when

PRINTOUT=NONE is specified PRINTOUT=SHORT is the default

Canonical Correlation Analysis Options

CANCORR

prints the canonical correlations and information criterion for each candidate state

vector considered See Canonical Correlation Analysis later in this chapter for

details

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Part 2 General Information

DIMMAX= n

specifies the upper limit to the dimension of the state vector The DIMMAX= option

can be used to limit the size of the model selected The default is DIMMAX=10

PASTMIN= n

specifies the minimum number of lags to include in the canonical correlation analy-

sis The default is PASTMIN=0 See Canonical Correlation Analysis later in thischapter for details

SIGCORR= value

specifies the multiplier of the degrees of freedom for the penalty term in the informa-

tion criterion used to select the state space form The default is SIGCORR=2 The

larger the value of the SIGCORR= option the smaller the state vector tends to be

Hence a large value causes a simpler model to be fit See Canonical Correlations

Analysis later in this chapter for details

State Space Model Estimation Options

COVB

prints the inverse of the observed information matrix for the parameter estimatesThis matrix is an estimate of the covariance matrix for the parameter estimates

DETTOL= value

specifies the convergence criterion The DETTOL= and PARMTOL= option values

are used together to test for convergence of the estimation process If during an

iteration the relative change of the parameter estimates is less than the PARMTOL=

value and the relative change of the determinant of the innovation variance matrix

is less than the DETTOL= value then iteration ceases and the current estimates are

accepted The default is DETTOL=1E-5

ITPRINT

prints the iterations during the estimation process

KLAG= n

sets an upper limit for the number of lags of the sample autocovariance matrix used

in computing the approximate likelihood function If the data have a strong moving

average character a larger KLAG= value may be necessary to obtain good estimates

The default is KLAG=15 See Parameter Estimation later in this chapter for details

MAXIT= n

sets an upper limit to the number of iterations in the maximum likelihood or condi-

tional least-squares estimation The default is MAXIT=50

NOEST

suppresses the final maximum likelihood estimation of the selected model

OUTMODEL= SAS-data-set

writes the parameter estimates and their standard errors to a SAS data set See OUT-

MODEL= Data Set later in this chapter for details

PARMTOL= value

specifies the convergence criterion The DETTOL= and PARMTOL= option values

are used together to test for convergence of the estimation process If during an

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Chapter 18 Syntax

iteration the relative change of the parameter estimates is less than the PARMTOL=

value and the relative change of the determinant of the innovation variance matrix

is less than the DETTOL= value then iteration ceases and the current estimates are

accepted The default is PARMTOL=001

RESIDEST

computes the final estimates using conditional least squares on the raw data Thistype of estimation may be more stable than the default maximum likelihood method

but is usually more computationally expensive See Parameter Estimation later in

this chapter for details of the conditional least squares method

SINGULAR= value

specifies the criterion for testing for singularity of a matrix A matrix is declared

singular if a scaled pivot is less than the SINGULAR= value when sweeping the

matrix The default is SINGULAR=1E-7

Forecasting Options

BACK= n

starts forecasting n periods before the end of the input data The BACK= option valuemust not be greater than the number of observations The default is BACK=0

INTERVAL= interval

specifies the time interval between observations The INTERVAL= value is used

in conjunction with the ID variable to check that the input data are in order and

have no missing periods The INTERVAL= option is also used to extrapolate the ID

values past the end of the input data See Chapter 3 ldquoDate Intervals Formats and

Functionsrdquo for details on the INTERVAL= values allowed

INTPER= n

specifies that each input observation corresponds to n time periods For example

the options INTERVAL=MONTH and INTPER=2 specify bimonthly data and are

equivalent to specifying INTERVAL=MONTH2 If the INTERVAL= option is not

specified the INTPER= option controls the increment used to generate ID values for

the forecast observations The default is INTPER=1

LEAD= n

specifies how many forecast observations are produced The forecasts start at the

point set by the BACK= option The default is LEAD=0 which produces no fore-

casts

OUT= SAS-data-set

writes the residuals actual values forecasts and forecast standard errors to a SAS

data set See OUT= Data Set later in this chapter for details

PRINT

prints the forecasts

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Part 2 General Information

BY Statement

BY variable

A BY statement can be used with the STATESPACE procedure to obtain separateanalyses on observations in groups defined by the BY variables

FORM Statement

FORM variable value

The FORM statement specifies the number of times a variable is included in the state

vector Values can be specified for any variable listed in the VAR statement If a

value is specified for each variable in the VAR statement the state vector for the state

space model is entirely specified and automatic selection of the state space model is

not performed

The FORM statement forces the state vectorz

t

to contain a specific variable a given

number of times For example if Y is one of the variables inx

t

then the statement

form y 3

forces the state vector to containY

t

Y

t + 1 j t

andY

t + 2 j t

possibly along with other

variables

The following statements illustrate the use of the FORM statement

proc statespace data=in

var x y

f o r m x 3 y 2

run

These statements fit a state space model with the following state vector

z

t

=

2

6

6

6

6

4

x

t j t

y

t j t

x

t + 1 j t

y

t + 1 j t

x

t + 2 j t

3

7

7

7

7

5

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Chapter 18 Syntax

ID Statement

ID variable

The ID statement specifies a variable that identifies observations in the input data setThe variable specified in the ID statement is included in the OUT= data set The

values of the ID variable are extrapolated for the forecast observations based on the

values of the INTERVAL= and INTPER= options

INITIAL Statement

INITIAL F (rowcolumn)= value G(row column)= value

The INITIAL statement gives initial values to the specified elements of theF

andG

matrices These initial values are used as starting values for the iterative estimation

Parts of theF

andG

matrices represent fixed structural identities If an element

specified is a fixed structural element instead of a free parameter the corresponding

initialization is ignored

The following is an example of an INITIAL statement

initial f(32)=0 g(41)=0 g(51)=0

RESTRICT Statement

RESTRICT F(rowcolumn)= value G(rowcolumn)= value

The RESTRICT statement restricts the specified elements of theF

andG

matrices

to the specified values

To use the restrict statement you need to know the form of the model Either specify

the form of the model with the FORM statement or do a preliminary run perhaps

with the NOEST option to find the form of the model that PROC STATESPACE

selects for the data

The following is an example of a RESTRICT statement

restrict f(32)=0 g(41)=0 g(51)=0

Parts of theF

andG

matrices represent fixed structural identities If a restriction is

specified for an element that is a fixed structural element instead of a free parameter

the restriction is ignored

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Part 2 General Information

VAR Statement

VAR variable (difference difference )

The VAR statement specifies the variables in the input data set to model and fore-cast The VAR statement also specifies differencing of the input variables The VAR

statement is required

Differencing is specified by following the variable name with a list of difference

periods separated by commas See the section Stationarity and Differencing for

more information on differencing of input variables

The order in which variables are listed in the VAR statement controls the order in

which variables are included in the state vector Usually potential inputs should be

listed before potential outputs

For example assuming the input data are monthly the following VAR statement

specifies modeling and forecasting of the one period and seasonal second differenceof X and Y

var x(112) y(112)

In this example the vector time series analyzed is

x

t

=

1 B 1 B

1 2

X

t

x

1 B 1 B

1 2

Y

t

y

where B represents the back shift operator and x and y represent the means of thedifferenced series If the NOCENTER option is specified the mean differences are

not subtracted

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Chapter 18 Details

Details

Missing Values

The STATESPACE procedure does not support missing values The procedure uses

the first contiguous group of observations with no missing values for any of the VARstatement variables Observations at the beginning of the data set with missing values

for any VAR statement variable are not used or included in the output data set

Stationarity and Differencing

The state space model used by the STATESPACE procedure assumes that the time

series are stationary Hence the data should be checked for stationarity One way

to check for stationarity is to plot the series A graph of series over time can show a

time trend or variability changes

You can also check stationarity by using the sample autocorrelation functions dis-

played by the ARIMA procedure The autocorrelation functions of nonstationaryseries tend to decay slowly See Chapter 7 ldquoThe ARIMA Procedurerdquo for more in-

formation

Another alternative is to use the STATIONARITY= option on the IDENTIFY state-

ment in PROC ARIMA to apply Dickey-Fuller tests for unit roots in the time series

See Chapter 7 ldquoThe ARIMA Procedurerdquo for more information on Dickey-Fuller unit

root tests

The most popular way to transform a nonstationary series to stationarity is by dif-

ferencing Differencing of the time series is specified in the VAR statement For

example to take a simple first difference of the series X use this statement

var x(1)

In this example the change in X from one period to the next is analyzed When the se-

ries has a seasonal pattern differencing at a period equal to the length of the seasonal

cycle may be desirable For example suppose the variable X is measured quarterly

and shows a seasonal cycle over the year You can use the following statement to

analyze the series of changes from the same quarter in the previous year

var x(4)

To difference twice add another differencing period to the list For ex-

ample the following statement analyzes the series of second differences

X

t

X

t 1

X

t 1

X

t 2

= X

t

2 X

t 1

+ X

t 2

var x(11)

The following statement analyzes the seasonal second difference series

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Part 2 General Information

var x(14)

The series modeled is the 1-period difference of the 4-period difference

X

t

X

t 4

X

t 1

X

t 5

= X

t

X

t 1

X

t 4

+ X

t 5

Another way to obtain stationary series is to use a regression on time to de-trend the

data If the time series has a deterministic linear trend regressing the series on time

produces residuals that should be stationary The following statements write residuals

of X and Y to the variable RX and RY in the output data set DETREND

data a

set a

t=_n_

run

proc reg data=a

model x y = t

output out=detrend r=rx ry

run

You then use PROC STATESPACE to forecast the de-trended series RX and RY A

disadvantage of this method is that you need to add the trend back to the forecast

series in an additional step A more serious disadvantage of the de-trending method

is that it assumes a deterministic trend In practice most time series appear to have a

stochastic rather than a deterministic trend Differencing is a more flexible and often

more appropriate method

There are several other methods to handle nonstationary time series For more infor-

mation and examples refer to Brockwell and Davis (1991)

Preliminary Autoregressive Models

After computing the sample autocovariance matrices PROC STATESPACE fits a

sequence of vector autoregressive models These preliminary autoregressive models

are used to estimate the autoregressive order of the process and limit the order of the

autocovariances considered in the state vector selection process

Yule-Walker Equations for Forward and Backward Models

Unlike a univariate autoregressive model a multivariate autoregressive model has

different forms depending on whether the present observation is being predicted from

the past observations or from the future observations

Letx

t

be the r -component stationary time series given by the VAR statement after

differencing and subtracting the vector of sample means (If the NOCENTER option

is specified the mean is not subtracted) Let n be the number of observations of x

t

from the input data set

Lete

t

be a vector white noise sequence with mean vector 0 and variance matrix

p

and letn

t

be a vector white noise sequence with mean vector 0 and variance matrix

p

Let p be the order of the vector autoregressive model forx

t

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Chapter 18 Details

The forward autoregressive form based on the past observations is written as follows

x

t

=

p

X

i = 1

p

i

x

t i

+ e

t

The backward autoregressive form based on the future observations is written as fol-

lows

x

t

=

p

X

i = 1

p

i

x

t + i

+ n

t

LettingE

denote the expected value operator the autocovariance sequence for thex

t

series

i

is

i

= E x

t

x

0

t i

The Yule-Walker equations for the autoregressive model that matches the first p ele-

ments of the autocovariance sequence are

2

6

6

4

0

1

p 1

0

1

0

p 2

0

p 1

0

p 2

0

3

7

7

5

2

6

6

4

p

1

p

2

p

p

3

7

7

5

=

2

6

6

4

1

2

p

3

7

7

5

and

2

6

6

6

4

0

0

1

0

p 1

1

0

0

p 2

p 1

p 2

0

3

7

7

7

5

2

6

6

4

p

1

p

2

p

p

3

7

7

5

=

2

6

6

4

0

1

0

2

0

p

3

7

7

5

Here

p

i

are the coefficient matrices for the past observation form of the vector au-toregressive model and

p

i

are the coefficient matrices for the future observation

form More information on the Yule-Walker equations in the multivariate setting can

be found in Whittle (1963) and Ansley and Newbold (1979)

The innovation variance matrices for the two forms can be written as follows

p

=

0

p

X

i = 1

p

i

0

i

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Part 2 General Information

p

=

0

p

X

i = 1

p

i

i

The autoregressive models are fit to the data using the preceding Yule-Walker equa-

tions with

i

replaced by the sample covariance sequenceC

i

The covariance matri-

ces are calculated as

C

i

=

1

N 1

N

X

t = i + 1

x

t

x

0

t i

Let b

p

b

p

b

p

and b

p

represent the Yule-Walker estimates of

p

p

p

and

p

respectively These matrices are written to an output data set when the OUTAR=

option is specified

When the PRINTOUT=LONG option is specified the sequence of matrices b

p

and

the corresponding correlation matrices are printed The sequence of matricesb

p isused to compute Akaike information criteria for selection of the autoregressive order

of the process

Akaike Information Criterion

The Akaike information criterion or AIC is defined as -2( maximum of log likeli-

hood )+2(number of parameters) Since the vector autoregressive models are esti-

mates from the Yule-Walker equations not by maximum likelihood the exact likeli-

hood values are not available for computing the AIC However for the vector autore-

gressive model the maximum of the log likelihood can be approximated as

l n L

n

2

l n j

b

p

j

Thus the AIC for the order p model is computed as

A I C

p

= n l n j

b

p

j + 2 p r

2

You can use the printed AIC array to compute a likelihood ratio test of the autoregres-

sive order The log-likelihood ratio test statistic for testing the order p model against

the order p 1

model is

n l n j

b

p

j + n l n j

b

p 1

j

This quantity is asymptotically distributed as a

2 withr

2 degrees of freedom if the

series is autoregressive of order p 1

It can be computed from the AIC array as

A I C

p 1

A I C

p

+ 2 r

2

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Chapter 18 Details

You can evaluate the significance of these test statistics with the PROBCHI function

in a SAS DATA step or with a

2 table

Determining the Autoregressive Order

Although the autoregressive models can be used for prediction their primary value is

to aid in the selection of a suitable portion of the sample covariance matrix for use in

computing canonical correlations If the multivariate time series x

t is of autoregres-sive order p then the vector of past values to lag p is considered to contain essentially

all the information relevant for prediction of future values of the time series

By default PROC STATESPACE selects the order p producing the autoregressive

model with the smallestA I C

p

If the value p for the minimumA I C

p

is less than the

value of the PASTMIN= option then p is set to the PASTMIN= value Alternatively

you can use the ARMAX= and PASTMIN= options to force PROC STATESPACE to

use an order you select

Significance Limits for Partial Autocorrelations

The STATESPACE procedure prints a schematic representation of the partial autocor-

relation matrices indicating which partial autocorrelations are significantly greater orsignificantly less than 0 Figure 1810 shows an example of this table

The STATESPACE Procedure

Schematic Representation of Partial Autocorrelations

NameLag 1 2 3 4 5 6 7 8 9 10

x ++ +

y ++

+ is gt 2std error - is lt -2std error is between

Figure 1810 Significant Partial Autocorrelations

The partial autocorrelations are from the sample partial autoregressive matrices b

p

p

The standard errors used for the significance limits of the partial autocorrelations are

computed from the sequence of matrices

p

and

p

Under the assumption that the observed series arises from an autoregressive process

of order p 1

the pth sample partial autoregressive matrix b

p

p

has an asymptotic

variance matrix 1

n

1

p

p

The significance limits for b

p

p

used in the schematic plot of the sample partial autore-

gressive sequence are derived by replacing

p

and

p

with their sample estimators

to produce the variance estimate as follows

d

V a r

b

p

p

=

1

n r p

b

1

p

b

p

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Part 2 General Information

Canonical Correlation Analysis

Given the order p letp

t

be the vector of current and past values relevant to prediction

of x

t + 1

p

t

= x

0

t

x

0

t 1

x

0

t p

0

Letf

t

be the vector of current and future values

f

t

= x

0

t

x

0

t + 1

x

0

t + p

0

In the canonical correlation analysis consider submatrices of the sample covariance

matrix of p

t

andf

t

This covariance matrixV

has a block Hankel form

V =

2

6

6

6

4

C

0

C

0

1

C

0

2

C

0

p

C

0

1

C

0

2

C

0

3

C

0

p + 1

C

0

p

C

0

p + 1

C

0

p + 2

C

0

2 p

3

7

7

7

5

State Vector Selection Process

The canonical correlation analysis forms a sequence of potential state vectorsz

j

t

Examine a sequencef

j

t

of subvectors of f

t

and form the submatrixV

j consisting

of the rows and columns of V

corresponding to the components of f

j

t

and compute

its canonical correlations

The smallest canonical correlation of V

j is then used in the selection of the com-

ponents of the state vector The selection process is described in the following For

more details about this process refer to Akaike (1976)

In the following discussion the notationx

t + k j t

denotes the wide sense conditional

expectation (best linear predictor) of x

t + k

given allx

s

with s less than or equal to t

In the notationx

i t + 1

the first subscript denotes the ith component of x

t + 1

The initial state vectorz

1

t

is set tox

t

The sequencef

j

t

is initialized by setting

f

1

t

= z

1

0

t

x

1 t + 1 j t

0

= x

0

t

x

1 t + 1 j t

0

That is start by considering whether to addx

1 t + 1 j t

to the initial state vectorz

1

t

The procedure forms the submatrixV

1 corresponding tof

1

t

and computes its canon-

ical correlations Denote the smallest canonical correlation of V

1 as

m i n

If

m i n

is

significantly greater than 0x

1 t + 1 j t

is added to the state vector

If the smallest canonical correlation of V

1 is not significantly greater than 0 then a

linear combination of f

1

t

is uncorrelated with the pastp

t

Assuming that the determi-

nant of C

0

is not 0 (that is no input series is a constant) you can take the coefficient

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Chapter 18 Details

of x

1 t + 1 j t

in this linear combination to be 1 Denote the coefficients of z

1

t

in this

linear combination as

This gives the relationship

x

1 t + 1 j t

=

0

x

t

Therefore the current state vector already contains all the past information useful forpredicting

x

1 t + 1

and any greater leads of x

1 t

The variablex

1 t + 1 j t

is not added to

the state vector nor are any termsx

1 t + k j t

considered as possible components of the

state vector The variablex

1

is no longer active for state vector selection

The process described forx

1 t + 1 j t

is repeated for the remaining elements of f

t

The

next candidate for inclusion in the state vector is the next component of f

t

corre-

sponding to an active variable Components of f

t

corresponding to inactive variables

that produced a zero

m i n

in a previous step are skipped

Denote the next candidate asx

l t + k j t

The vectorf

j

t

is formed from the current state

vector andx

l t + k j t

as follows

f

j

t

= z

j

0

t

x

l t + k j t

0

The matrixV

j is formed fromf

j

t

and its canonical correlations are computed The

smallest canonical correlation of V

j is judged to be either greater than or equal to 0

If it is judged to be greater than 0x

l t + k j t

is added to the state vector If it is judged

to be 0 then a linear combination of f

j

t

is uncorrelated with thep

t

and the variable

x

l

is now inactive

The state vector selection process continues until no active variables remain

Testing Significance of Canonical Correlations For each step in the canonical correlation sequence the significance of the small-

est canonical correlation

m i n

is judged by an information criterion from Akaike

(1976) This information criterion is

n l n 1

2

m i n

r p + 1 q + 1

where q is the dimension of f

j

t

at the current step r is the order of the state vector p

is the order of the vector autoregressive process and

is the value of the SIGCORR=

option The default is SIGCORR=2 If this information criterion is less than or equal

to 0

m i n

is taken to be 0 otherwise it is taken to be significantly greater than 0

(Do not confuse this information criterion with the AIC)

Variables inx

t + p j t

are not added in the model even with positive information crite-

rion because of the singularity of V

You can force the consideration of more candi-

date state variables by increasing the size of theV

matrix by specifying a PASTMIN=

option value larger than p

Printing the Canonical Correlations

To print the details of the canonical correlation analysis process specify the CAN-

CORR option in the PROC STATESPACE statement The CANCORR option prints

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the candidate state vectors the canonical correlations and the information criteria for

testing the significance of the smallest canonical correlation

Bartlettrsquos

2 and its degrees of freedom are also printed when the CANCORR option

is specified The formula used for Bartlettrsquos

2 is

2

= n 5 r p + 1 q + 1 l n 1

2

m i n

withr p + 1 q + 1

degrees of freedom

Figure 1811 shows the output of the CANCORR option for the introductory example

shown in the Getting Started section of this chapter

The STATESPACE Procedure

Canonical Correlations Analysis

Information Chi

x(TT) y(TT) x(T+1T) Criterion Square DF

1 1 0237045 3566167 114505 4

Information Chi

x(TT) y(TT) x(T+1T) y(T+1T) Criterion Square DF

1 1 0238244 0056565 -535906 0636134 3

Information Chi

x(TT) y(TT) x(T+1T) x(T+2T) Criterion Square DF

1 1 0237602 0087493 -446312 1525353 3

Figure 1811 Canonical Correlations Analysis

New variables are added to the state vector if the information criteria are positive Inthis example

y

t + 1 j t

andx

t + 2 j t

are not added to the state space vector because the

information criteria for these models are negative

If the information criterion is nearly 0 then you may want to investigate models

that arise if the opposite decision is made regarding

m i n

This investigation can be

accomplished by using a FORM statement to specify part or all of the state vector

Preliminary Estimates of F

When a candidate variablex

l t + k j t

yields a zero

m i n

and is not added to the state

vector a linear combination of f

j

t

is uncorrelated with thep

t

Because of the method

used to construct thef

j

t

sequence the coefficient of x

l t + k j t

inl

can be taken as 1

Denote the coefficients of z

j

t

in this linear combination as l

This gives the relationship

x

l t + k j t

= l

0

z

j

t

The vectorl

is used as a preliminary estimate of the first r columns of the row of the

transition matrixF

corresponding tox

l t + k 1 j t

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Chapter 18 Details

Parameter Estimation

The model isz

t + 1

= F z

t

+ G e

t + 1

wheree

t

is a sequence of independent multivari-

ate normal innovations with mean vector 0 and variance

e e

The observed sequence

x

t

composes the first r components of z

t

and thusx

t

= H z

t

where H is ther s

matrix I

r

0

LetE

be ther n

matrix of innovations

E = e

1

e

n

If the number of observations n is reasonably large the log likelihood L can be

approximated up to an additive constant as follows

L =

n

2

l n j

e e

j

1

2

t r a c e

1

e e

E E

0

The elements of

e e

are taken as free parameters and are estimated as follows

S

0

=

1

n

E E

0

Replacing

e e

byS

0

in the likelihood equation the log likelihood up to an additive

constant is

L =

n

2

l n j S

0

j

Letting B be the backshift operator the formal relation betweenx

t

ande

t

is

x

t

= H I B F

1

G e

t

e

t

= H I B F

1

G

1

x

t

=

1

X

i = 0

i

x

t i

LettingC

i

be the ith lagged sample covariance of x

t

and neglecting end effects the

matrixS

0

is

S

0

=

1

X

i j = 0

i

C

i + j

0

j

For the computation of S

0

the infinite sum is truncated at the value of the KLAG=

option The value of the KLAG= option should be large enough that the sequence

i

is approximately 0 beyond that point

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Part 2 General Information

Let

be the vector of free parameters in theF

andG

matrices The derivative of the

log likelihood with respect to the parameter

is

L

=

n

2

t r a c e

S

1

0

S

0

The second derivative is

2

L

0

=

n

2

t r a c e

S

1

0

S

0

0

S

1

0

S

0

t r a c e

S

1

0

2

S

0

0

Near the maximum the first term is unimportant and the second term can be approx-

imated to give the following second derivative approximation

2

L

0

=

n t r a c e

S

1

0

E

E

0

0

The first derivative matrix and this second derivative matrix approximation are com-

puted from the sample covariance matrixC

0

and the truncated sequence

i

The

approximate likelihood function is maximized by a modified Newton-Raphson algo-

rithm employing these derivative matrices

The matrixS

0

is used as the estimate of the innovation covariance matrix

e e

The

negative of the inverse of the second derivative matrix at the maximum is used as an

approximate covariance matrix for the parameter estimates The standard errors of

the parameter estimates printed in the parameter estimates tables are taken from the

diagonal of this covariance matrix The parameter covariance matrix is printed when

the COVB option is specified

If the data are nearly nonstationary a better estimate of

e e

and the other parameters

can sometimes be obtained by specifying the RESIDEST option The RESIDEST

option estimates the parameters using conditional least squares instead of maximum

likelihood

The residuals are computed using the state space equation and the sample mean val-

ues of the variables in the model as start-up values The estimate of S

0

is then com-

puted using the residuals from the ith observation on where i is the maximum number

of times any variable occurs in the state vector A multivariate Gauss-Marquardt al-

gorithm is used to minimizej S

0

j

Refer to Harvey (1981a) for a further description

of this method

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Chapter 18 Details

Forecasting

Given estimates of F

G

and

e e

forecasts of x

t

are computed from the conditional

expectation of z

t

In forecasting the parameters F G and

e e

are replaced with the estimates or by val-

ues specified in the RESTRICT statement One-step-ahead forecasting is performedfor the observation

x

t

wheret n b

Heren

is the number of observations and

b is the value of the BACK= option For the observationx

t

wheret n b

m-

step-ahead forecasting is performed form = t n + b

The forecasts are generated

recursively with the initial conditionz

0

= 0

The m-step-ahead forecast of z

t + m

isz

t + m j t

wherez

t + m j t

denotes the conditional

expectation of z

t + m

given the information available at time t The m-step-ahead

forecast of x

t + m

isx

t + m j t

= H z

t + m j t

where the matrixH = I

r

0

Let

i

= F

i

G

Note that the lasts r

elements of z

t

consist of the elements of x

u j t

foru t

The state vector z

t + m

can be represented as

z

t + m

= F

m

z

t

+

m 1

X

i = 0

i

e

t + m i

Sincee

t + i j t

= 0

fori 0

the m-step-ahead forecastz

t + m j t

is

z

t + m j t

= F

m

z

t

= F z

t + m 1 j t

Therefore the m-step-ahead forecast of x

t + m

is

x

t + m j t

= H z

t + m j t

The m-step-ahead forecast error is

z

t + m

z

t + m j t

=

m 1

X

i = 0

i

e

t + m i

The variance of the m-step-ahead forecast error is

V

z m

=

m 1

X

i = 0

i

e e

0

i

LettingV

z 0

= 0

the variance of the m-step-ahead forecast error of z

t + m

V

z m

can

be computed recursively as follows

V

z m

= V

z m 1

+

m 1

e e

0

m 1

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Part 2 General Information

The variance of the m-step-ahead forecast error of x

t + m

is ther r

left upper sub-

matrix of V

z m

that is

V

x m

= H V

z m

H

0

Unless the NOCENTER option is specified the sample mean vector is added to the

forecast When differencing is specified the forecasts xt + m j t

plus the sample mean

vector are integrated back to produce forecasts for the original series

Lety

t

be the original series specified by the VAR statement with some 0 values

appended corresponding to the unobserved past observations Let B be the backshift

operator and let B

be thes s

matrix polynomial in the backshift operator

corresponding to the differencing specified by the VAR statement The off-diagonal

elements of

i

are 0 Note that

0

= I

s

whereI

s

is thes s

identity matrix Then

z

t

= B y

t

This gives the relationship

y

t

=

1

B z

t

=

1

X

i = 0

i

z

t i

where

1

B =

P

1

i = 0

i

B

i and

0

= I

s

The m-step-ahead forecast of y

t + m

is

y

t + m j t

=

m 1

X

i = 0

i

z

t + m i j t

+

1

X

i = m

i

z

t + m i

The m-step-ahead forecast error of y

t + m

is

m 1

X

i = 0

i

z

t + m i

z

t + m i j t

=

m 1

X

i = 0

i

X

u = 0

u

i u

e

t + m i

LettingV

y 0

= 0

the variance of the m-step-ahead forecast error of y

t + m

V

y m

is

V

y m

=

m 1

X

i = 0

i

X

u = 0

u

i u

e e

i

X

u = 0

u

i u

0

= V

y m 1

+

m 1

X

u = 0

u

m 1 u

e e

m 1

X

u = 0

u

m 1 u

0

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Chapter 18 Details

Relation of ARMA and State Space Forms

Every state space model has an ARMA representation and conversely every ARMA

model has a state space representation This section discusses this equivalence The

following material is adapted from Akaike (1974) where there is a more complete

discussion Pham-Dinh-Tuan (1978) also contains a discussion of this material

Suppose you are given the following ARMA model

B x

t

= B e

t

or in more detail

x

t

1

x

t 1

p

x

t p

= e

t

+

1

e

t 1

+ +

q

e

t q

(1)

wheree

t

is a sequence of independent multivariate normal random vectors with mean

0 and variance matrix

e e

B is the backshift operator (B x

t

= x

t 1

) B

and

B

are matrix polynomials in B andx

t

is the observed process

If the roots of the determinantial equationj B j = 0

are outside the unit circle in

the complex plane the model can also be written as

x

t

=

1

B B e

t

=

1

X

i = 0

i

e

t i

The

i

matrices are known as the impulse response matrices and can be computed

as

1

B B

You can assume p q

since if this is not initially true you can add more terms

i

that are identically 0 without changing the model

To write this set of equations in a state space form proceed as follows Letx

t + i j t

be

the conditional expectation of x

t + i

givenx

w

forw t

The following relations hold

x

t + i j t

=

1

X

j = i

j

e

t + i j

x

t + i j t + 1

= x

t + i j t

+

i 1

e

t + 1

However from equation (1) you can derive the following relationship

x

t + p j t

=

1

x

t + p 1 j t

+ +

p

x

t

(2)

Hence wheni = p

you can substitute forx

t + p j t

in the right-hand side of equation

(2) and close the system of equations

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Part 2 General Information

This substitution results in the following model in the state space form

z

t + 1

= F z

t

+ G e

t + 1

2

6

6

6

4

x

t + 1

x

t + 2 j t + 1

x

t + p j t + 1

3

7

7

7

5

=

2

6

6

4

0 I 0 0

0 0 I 0

p

p 1

1

3

7

7

5

2

6

6

6

4

x

t

x

t + 1 j t

x

t + p 1 j t

3

7

7

7

5

+

2

6

6

4

I

1

p 1

3

7

7

5

e

t + 1

Note that the state vectorz

t

is composed of conditional expectations of x

t

and the

first r components of z

t

are equal tox

t

The state space form can be cast into an ARMA form by solving the system of dif-

ference equations for the first r components

When converting from an ARMA form to a state space form you can generate a

state vector larger than needed that is the state space model may not be a minimal

representation When going from a state space form to an ARMA form you can have

nontrivial common factors in the autoregressive and moving average operators that

yield an ARMA model larger than necessary

If the state space form used is not a minimal representation some but not all compo-

nents of x

t + i j t

may be linearly dependent This situation corresponds to

p

p 1

being of less than full rank when B

and B

have no common nontrivial

left factors In this casez

t

consists of a subset of the possible components of

x

t + i j t

i = 1 2 p 1

However once a component of x

t + i j t

(for example the

jth one) is linearly dependent on the previous conditional expectations then all sub-

sequent jth components of x

t + k j t

fork i

must also be linearly dependent Note

that in this case equivalent but seemingly different structures can arise if the order of

the components withinx

t

is changed

OUT= Data Set

The forecasts are contained in the output data set specified by the OUT= option on the

PROC STATESPACE statement The OUT= data set contains the following variables

the BY variables

the ID variable

the VAR statement variables These variables contain the actual values from

the input data set

FORi numeric variables containing the forecasts The variable FORi contains

the forecasts for the ith variable in the VAR statement list Forecasts are one-

step-ahead predictions until the end of the data or until the observation specified

by the BACK= option

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Chapter 18 Details

RESi numeric variables containing the residual for the forecast of the ith vari-

able in the VAR statement list For forecast observations the actual values are

missing and the RESi variables contain missing values

STDi numeric variables containing the standard deviation for the forecast of

the ith variable in the VAR statement list The values of the STDi variables can

be used to construct univariate confidence limits for the corresponding fore-casts However such confidence limits do not take into account the covariance

of the forecasts

OUTAR= Data Set

The OUTAR= data set contains the estimates of the preliminary autoregressive mod-

els The OUTAR= data set contains the following variables

ORDER a numeric variable containing the order p of the autoregressive model

that the observation represents

AIC a numeric variable containing the value of the information criterionA I C

p

SIGFl numeric variables containing the estimate of the innovation covariance

matrices for the forward autoregressive models The variable SIGFl contains

the lth column of b

p

in the observations with ORDER= p

SIGBl numeric variables containing the estimate of the innovation covariance

matrices for the backward autoregressive models The variable SIGBl contains

the lth column of b

p

in the observations with ORDER= p

FORk ndashl numeric variables containing the estimates of the autoregressive pa-

rameter matrices for the forward models The variable FORk ndashl contains the

lth column of the lag k autoregressive parameter matrix b

p

k

in the observations

with ORDER= p

BACk ndashl numeric variables containing the estimates of the autoregressive pa-

rameter matrices for the backward models The variable BACk ndashl contains the

lth column of the lag k autoregressive parameter matrix b

p

k

in the observations

with ORDER= p

The estimates for the order p autoregressive model can be selected as those observa-

tions with ORDER= p Within these observations the k lth element of

p

i

is given by

the value of the FORindashl variable in the k th observation The k lth element of

p

i

is

given by the value of BACindashl variable in the k th observation The k lth element of

p

is given by SIGFl in the k th observation The k lth element of

p

is given by SIGBlin the k th observation

Table 181 shows an example of the OUTAR= data set with ARMAX=3 andx

t

of

dimension 2 In Table 181 i j

indicate the ijth element of the matrix

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Part 2 General Information

Table 181 Values in the OUTAR= Data Set

Obs ORDER AIC SIGF1 SIGF2 SIGB1 SIGB2 FOR1ndash 1 FOR1ndash 2 FOR2ndash 1 FOR2ndash 2 FOR3ndash 1

1 0 AIC0

0 1 1

0 1 2

0 1 1

0 1 2

2 0 AIC0

0 2 1

0 2 2

0 2 1

0 2 2

3 1 AIC1

1 1 1

1 1 2

1 1 1

1 1 2

1

1

1 1

1

1

1 2

4 1 AIC1

1 2 1

1 1 2

1 2 1

1 1 2

1

1

2 1

1

1

2 2

5 2 AIC2

2 1 1

2 1 2

2 1 1

2 1 2

2

1

1 1

2

1

1 2

2

2

1 1

2

2

1 2

6 2 AIC2

2 2 1

2 1 2

2 2 1

2 1 2

2

1

2 1

2

1

2 2

2

2

2 1

2

2

2 2

7 3 AIC3

3 1 1

3 1 2

3 1 1

3 1 2

3

1

1 1

3

1

1 2

3

2

1 1

3

2

1 2

3

3

1 1

8 3 AIC3

3 2 1

3 1 2

3 2 1

3 1 2

3

1

2 1

3

1

2 2

3

2

2 1

3

2

2 2

3

3

2 1

Obs FOR3ndash 2 BACK1ndash 1 BACK1ndash 2 BACK2ndash 1 BACK2ndash 2 BACK3ndash 1 BACK3ndash 2

1

2

3

1

1

1 1

1

1

1 2

4

1

1

2 1

1

1

2 2

5

2

1

1 1

2

1

1 2

2

2

1 1

2

2

1 2

6

2

1

2 1

2

1

2 2

2

2

2 1

2

2

2 2

7

3

3

1 2

3

1

1 1

3

1

1 2

3

2

1 1

3

2

1 2

3

3

1 1

3

3

1 2

8

3

3

2 2

3

1

2 1

3

1

2 2

3

2

2 1

3

2

2 2

3

3

2 1

3

3

2 2

The estimated autoregressive parameters can be used in the IML procedure to obtain

autoregressive estimates of the spectral density function or forecasts based on the

autoregressive models

OUTMODEL= Data Set

The OUTMODEL= data set contains the estimates of theF

andG

matrices and their

standard errors the names of the components of the state vector and the estimates of

the innovation covariance matrix The variables contained in the OUTMODEL= data

set are as follows

the BY variables

STATEVEC a character variable containing the name of the component of the

state vector corresponding to the observation The STATEVEC variable has

the value STD for standard deviations observations which contain the standarderrors for the estimates given in the preceding observation

Fndash j numeric variables containing the columns of theF

matrix The variable

Fndash j contains the jth column of F

The number of Fndash j variables is equal to

the value of the DIMMAX= option If the model is of smaller dimension the

extraneous variables are set to missing

Gndash j numeric variables containing the columns of theG

matrix The variable

Gndash j contains the jth column of G

The number of Gndash j variables is equal to r

the dimension of x

t

given by the number of variables in the VAR statement

SIGndash j numeric variables containing the columns of the innovation covariance

matrix The variable SIGndash j contains the jth column of

e e

There are r vari-ables SIGndash j

Table 182 shows an example of the OUTMODEL= data set withx

t

= x

t

y

t

0

z

t

= x

t

y

t

x

t + 1 j t

0 and DIMMAX=4 In Table 182F

i j

andG

i j

are the ijth

elements of F

andG

respectively Note that all elements for Fndash4 are missing because

F

is a3 3

matrix

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Chapter 18 Details

Table 182 Value in the OUTMODEL= Data Set

Obs STATEVEC Fndash 1 Fndash 2 Fndash 3 Fndash 4 Gndash 1 Gndash 2 SIGndash 1 SIGndash 2

1 X(TT) 0 0 1 1 0

1 1

1 2

2 STD

3 Y(TT) F2 1

F2 2

F2 3

0 1

2 1

2 2

4 STD std F2 1

std F2 2

std F2 3

5 X(T+1T) F3 1

F3 2

F3 3

G3 1

G3 2

6 STD std F3 1

std F3 2

std F3 3

std G3 1

std G3 2

Printed Output

The printed output produced by the STATESPACE procedure is described in the fol-

lowing

1 descriptive statistics which include the number of observations used the

names of the variables their means and standard deviations (Std) and the dif-

ferencing operations used

2 the Akaike information criteria for the sequence of preliminary autoregressive

models

3 if the PRINTOUT=LONG option is specified the sample autocovariance ma-

trices of the input series at various lags

4 if the PRINTOUT=LONG option is specified the sample autocorrelation ma-

trices of the input series

5 a schematic representation of the autocorrelation matrices showing the signif-

icant autocorrelations

6 if the PRINTOUT=LONG option is specified the partial autoregressive ma-

trices (These are

p

p

as described in Preliminary Autoregressive Modelsearlier in this chapter)

7 a schematic representation of the partial autocorrelation matrices showing the

significant partial autocorrelations

8 the Yule-Walker estimates of the autoregressive parameters for the autoregres-

sive model with the minimum AIC

9 if the PRINTOUT=LONG option is specified the autocovariance matrices of

the residuals of the minimum AIC model This is the sequence of estimated

innovation variance matrices for the solutions of the Yule-Walker equations

10 if the PRINTOUT=LONG option is specified the autocorrelation matrices of the residuals of the minimum AIC model

11 If the CANCORR option is specified the canonical correlations analysis for

each potential state vector considered in the state vector selection process

This includes the potential state vector the canonical correlations the informa-

tion criterion for the smallest canonical correlation Bartlettrsquos

2 statistic (ldquoChi

Squarerdquo) for the smallest canonical correlation and the degrees of freedom of

Bartlettrsquos

2

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Part 2 General Information

12 the components of the chosen state vector

13 the preliminary estimate of the transition matrixF

the input matrixG

and

the variance matrix for the innovations

e e

14 if the ITPRINT option is specified the iteration history of the likelihood max-

imization For each iteration this shows the iteration number the number of step halvings the determinant of the innovation variance matrix the damping

factor Lambda and the values of the parameters

15 the state vector printed again to aid interpretation of the following listing of F

andG

16 the final estimate of the transition matrixF

17 the final estimate of the input matrixG

18 the final estimate of the variance matrix for the innovations

e e

19 a table listing the estimates of the free parameters inF

andG

and their standarderrors and t statistics

20 if the COVB option is specified the covariance matrix of the parameter esti-

mates

21 if the COVB option is specified the correlation matrix of the parameter esti-

mates

22 if the PRINT option is specified the forecasts and their standard errors

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Chapter 18 Details

ODS Table Names

PROC STATESPACE assigns a name to each table it creates You can use these

names to reference the table when using the Output Delivery System (ODS) to select

tables and create output data sets These names are listed in the following table For

more information on ODS see Chapter 6 ldquoUsing the Output Delivery Systemrdquo

Table 183 ODS Tables Produced in PROC STATESPACE

ODS Table Name Description Option

NObs Number of observations default

Summary Simple summary statistics table default

InfoCriterion Information criterion table default

CovLags Covariance Matrices of Input Series PRINTOUT=LONG

CorrLags Correlation Matrices of Input Series PRINTOUT=LONG

PartialAR Partial Autoregressive Matrices PRINTOUT=LONG

YWEstimates Yule-Walker Estimates for Minimum AIC default

CovResiduals Covariance of Residuals PRINTOUT=LONGCorrResiduals Residual Correlations from AR Models PRINTOUT=LONG

StateVector State vector table default

CorrGraph Schematic Representation of Correlations default

TransitionMatrix Transition Matrix default

InputMatrix Input Matrix default

VarInnov Variance Matrix for the Innovation default

CovB Covariance of Parameter Estimates COVB

CorrB Correlation of Parameter Estimates COVB

CanCorr Canonical Correlation Analysis CANCORR

IterHistory Iterative Fitting table ITPRINT

ParameterEstimates Parameter Estimates Table default

Forecasts Forecasts Table PRINT

ConvergenceStatus Convergence Status Table default

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Part 2 General Information

Example

Example 181 Series J from Box and Jenkins

This example analyzes the gas furnace data (series J) from Box and Jenkins (The

data are not shown Refer to Box and Jenkins (1976) for the data)

First a model is selected and fit automatically using the following statements

title1 rsquoGas Furnace Datarsquo

title2 rsquoBox amp Jenkins Series Jrsquo

title3 rsquoAutomatically Selected Modelrsquo

proc statespace data=seriesj cancorr

var x y

run

The results for the automatically selected model are shown in Output 1811

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Chapter 18 Example

Output 1811 Results for Automatically Selected Model

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Number of Observations 296

Standard

Variable Mean Error

x -005683 1072766

y 5350912 3202121

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Information Criterion for Autoregressive Models

Lag=0 Lag=1 Lag=2 Lag=3 Lag=4 Lag=5 Lag=6 Lag=7 Lag=8

6513862 -103357 -163296 -164512 -165152 -164891 -164934 -164315 -163856

Information

Criterion for

Autoregressive

Models

Lag=9 Lag=10

-16348 -163359

Schematic Representation of Correlations

NameLag 0 1 2 3 4 5 6 7 8 9 10

x +- +- +- +- +- +- +- +- +- +- +-

y -+ -+ -+ -+ -+ -+ -+ -+ -+ -+ -+

+ is gt 2std error - is lt -2std error is between

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Part 2 General Information

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Schematic Representation of Partial Autocorrelations

NameLag 1 2 3 4 5 6 7 8 9 10

x + - + -

y -+ -- - + +

+ is gt 2std error - is lt -2std error is between

Yule-Walker Estimates for Minimum AIC

------Lag=1------ ------Lag=2------ ------Lag=3------ ------Lag=4------

x y x y x y x y

x 1925887 -000124 -120166 0004224 0116918 -000867 0104236 0003268

y 0050496 1299793 -002046 -03277 -071182 -025701 0195411 0133417

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Canonical Correlations Analysis

Information Chi

x(TT) y(TT) x(T+1T) Criterion Square DF

1 1 0804883 2929228 3047481 8

Information Chix(TT) y(TT) x(T+1T) y(T+1T) Criterion Square DF

1 1 0906681 0607529 1223358 1347237 7

Information Chi

x(TT) y(TT) x (T+1T) y (T+1T) x (T+2T) Criterion Square DF

1 1 0909434 0610278 0186274 -154701 1034705 6

Information Chi

x(TT) y(TT) x (T+1T) y (T+1T) y (T+2T) Criterion Square DF

1 1 091014 0618937 0206823 0940392 1280924 6

Information Chi

x(TT) y(TT) x(T+1T) y(T+1T) y(T+2T) y(T+3T) Criterion Square DF

1 1 0912963 0628785 0226598 0083258 -794103 2041584 5

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Chapter 18 Example

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Selected Statespace Form and Preliminary Estimates

State Vector

x(TT) y(TT) x(T+1T) y(T+1T) y(T+2T)

Estimate of Transition Matrix

0 0 1 0 0

0 0 0 1 0

-084718 0026794 1711715 -005019 0

0 0 0 0 1

-019785 0334274 -018174 -123557 1787475

Input Matrix for Innovation

1 0

0 11925887 -000124

0050496 1299793

0142421 1361696

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Selected Statespace Form and Preliminary Estimates

Variance Matrix for Innovation

0035274 -000734

-000734 0097569

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Part 2 General Information

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Selected Statespace Form and Fitted Model

State Vector

x(TT) y(TT) x(T+1T) y(T+1T) y(T+2T)

Estimate of Transition Matrix

0 0 1 0 0

0 0 0 1 0

-086192 0030609 1724235 -005483 0

0 0 0 0 1

-034839 0292124 -009435 -109823 1671418

Input Matrix for Innovation

1 0

0 1192442 -000416

0015621 1258495

008058 1353204

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Selected Statespace Form and Fitted Model

Variance Matrix for Innovation

0035579 -000728

-000728 0095577

Parameter Estimates

Standard

Parameter Estimate Error t Value

F(31) -086192 0072961 -1181

F(32) 0030609 0026167 117

F(33) 1724235 0061599 2799

F(34) -005483 0030169 -182

F(51) -034839 0135253 -258

F(52) 0292124 0046299 631

F(53) -009435 0096527 -098F(54) -109823 0109525 -1003

F(55) 1671418 0083737 1996

G(31) 1924420 0058162 3309

G(32) -000416 0035255 -012

G(41) 0015621 0095771 016

G(42) 1258495 0055742 2258

G(51) 0080580 0151622 053

G(52) 1353204 0091388 1481

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Chapter 18 Example

The two series are believed to have a transfer function relation with the gas rate

(variable X) as the input and the CO2

concentration (variable Y) as the output Since

the parameter estimates shown in Output 1811 support this kind of model the model

is reestimated with the feedback parameters restricted to 0 The following statements

fit the transfer function (no feedback) model

title3 rsquoTransfer Function Modelrsquo

proc statespace data=seriesj printout=none

var x y

restrict f(32)=0 f(34)=0

g(32)=0 g(41)=0 g(51)=0

run

The last 2 pages of the output are shown in Output 1812

Output 1812 STATESPACE Output for Transfer Function Model

Gas Furnace Data

Box amp Jenkins Series J

Transfer Function Model

The STATESPACE Procedure

Selected Statespace Form and Fitted Model

State Vector

x(TT) y(TT) x(T+1T) y(T+1T) y(T+2T)

Estimate of Transition Matrix

0 0 1 0 0

0 0 0 1 0

-068882 0 1598717 0 0

0 0 0 0 1

-035944 0284179 -00963 -107313 1650047

Input Matrix for Innovation

1 0

0 1

1923446 0

0 1260856

0 1346332

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Part 2 General Information

Gas Furnace Data

Box amp Jenkins Series J

Transfer Function Model

The STATESPACE Procedure

Selected Statespace Form and Fitted Model

Variance Matrix for Innovation

0036995 -00072

-00072 0095712

Parameter Estimates

Standard

Parameter Estimate Error t Value

F(31) -068882 0050549 -1363

F(33) 1598717 0050924 3139

F(51) -035944 0229044 -157

F(52) 0284179 0096944 293

F(53) -009630 0140876 -068

F(54) -107313 0250385 -429

F(55) 1650047 0188533 875G(31) 1923446 0056328 3415

G(42) 1260856 0056464 2233

G(52) 1346332 0091086 1478

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Chapter 18 References

References

Akaike H (1974) Markovian Representation of Stochastic Processes and Its Ap-

plication to the Analysis of Autoregressive Moving Average Processes Annals

of the Institute of Statistical Mathematics 26 363-387

Akaike H (1976) Canonical Correlations Analysis of Time Series and the Use of

an Information Criterion in Advances and Case Studies in System Identification

eds R Mehra and DG Lainiotis New York Academic Press

Anderson TW (1971) The Statistical Analysis of Time Series New York John

Wiley amp Sons

Ansley CF and Newbold P (1979) Multivariate Partial Autocorrelations Pro-

ceedings of the Business and Economic Statistics Section American Statistical

Association 349-353

Box GEP and Jenkins G (1976) Time Series Analysis Forecasting and Control

San Francisco Holden-Day

Brockwell PJ and Davis RA (1991) Time Series Theory and Methods 2nd Edi-

tion Springer-Verlag

Hannan EJ (1970) Multiple Time Series New York John Wiley amp Sons

Hannan EJ (1976) The Identification and Parameterization of ARMAX and State

Space Forms Econometrica 44 713-722

Harvey AC (1981a) The Econometric Analysis of Time Series New York John

Wiley amp Sons

Harvey AC (1981b) Time Series Models New York John Wiley amp Sons

Jones RH (1974) Identification and Autoregressive Spectrum Estimation IEEE Transactions on Automatic Control AC-19 894-897

Pham-Dinh-Tuan (1978) On the Fitting of Multivariate Processes of the

Autoregressive-Moving Average Type Biometrika 65 99-107

Priestley MB (1980) System Identification Kalman Filtering and Stochastic Con-

trol in Directions in Time Series eds DR Brillinger and GC Tiao Institute

of Mathematical Statistics

Whittle P (1963) On the Fitting of Multivariate Autoregressions and the Approxi-

mate Canonical Factorization of a Spectral Density Matrix Biometrika 50 129-

134

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The correct bibliographic citation for this manual is as follows SAS Institute Inc SAS

ET S U serrsquos Gui de Version 8 Cary NC SAS Institute Inc 1999 1546 pp

SASET S Userrsquos Guide Version 8

Copyright copy 1999 by S AS I ns t it u t e I nc Ca ry N C U S A

ISBN 1ndash58025ndash489ndash6

All r ight s res erved P r int ed in t he U n it ed St a t es of A merica N o pa rt of t his p u blica t ionma y b e reprodu ced s t ored in a ret r ieva l s ys t em or t ra ns mit t ed in a ny form or b y a nymeans electronic mechanical photocopying or otherwise without the prior writtenpermission of the publisher SAS Institute Inc

US Government Restricted Rights Notice Use du plicat ion or disclosure of the

software by the government is subject to restrictions as set forth in FAR 52227ndash19Commercial Computer Softw ar e-Restricted Rights (J une 1987)

SAS Institute Inc SAS Campus Drive Cary North Carolina 27513

1st print ing October 1999

S AS reg a nd a ll ot her SA S I ns t it u t e I nc p rodu ct or s ervice na m es a re regis t ered t ra dema rkst d k f SA S I t it t I i t h U S A d t h t i reg i di t U SA

Page 16: Statespace Model

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Part 2 General Information

The STATESPACE Procedure

Selected Statespace Form and Fitted Model

State Vector

x(TT) y(TT) x(T+1T)

Estimate of Transition Matrix

0 0 1

0290051 0467468 0

0227051 0226139 026436

Input Matrix for Innovation

1 0

0 1

0256826 0202022

Variance Matrix for Innovation

0945175 01006960100696 1014733

The STATESPACE Procedure

Parameter Estimates

Standard

Parameter Estimate Error t Value

F(21) 0290051 0063904 454

F(22) 0467468 0060430 774

F(31) 0227051 0125221 181

F(32) 0226139 0111711 202

F(33) 0264360 0299537 088G(31) 0256826 0070994 362

G(32) 0202022 0068507 295

Figure 189 Results using RESTRICT Statement

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Chapter 18 Syntax

Syntax

The STATESPACE procedure uses the following statements

PROC STATESPACE options

BY variable FORM variable value

ID variable

INITIAL F(rowcolumn)=value

G(rowcolumn)=value

RESTRICT F(rowcolumn)=value

G(rowcolumn)=value

VAR variable (difference difference

)

Functional Summary

The statements and options used by PROC STATESPACE are summarized in the

following table

Description Statement Option

Input Data Set Options

specify the input data set PROC STATESPACE DATA=

prevent subtraction of sample mean PROC STATESPACE NOCENTER

specify the ID variable ID

specify the observed series and differencing VAR

Options for Autoregressive Estimates

specify the maximum order PROC STATESPACE ARMAX=

specify maximum lag for autocovariances PROC STATESPACE LAGMAX=

output only minimum AIC model PROC STATESPACE MINIC

specify the amount of detail printed PROC STATESPACE PRINTOUT=

write preliminary AR models to a data set PROC STATESPACE OUTAR=

Options for Canonical Correlation Analysis

print the sequence of canonical correlations PROC STATESPACE CANCORR

specify upper limit of dimension of state

vector

PROC STATESPACE DIMMAX=

specify the minimum number of lags PROC STATESPACE PASTMIN=

specify the multiplier of the degrees of

freedom

PROC STATESPACE SIGCORR=

Options for State Space Model Estimation

specify starting values INITIAL

print covariance matrix of parameter estimates PROC STATESPACE COVB

specify the convergence criterion PROC STATESPACE DETTOL=

specify the convergence criterion PROC STATESPACE PARMTOL=

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Part 2 General Information

Description Statement Option

print the details of the iterations PROC STATESPACE ITPRINT

specify an upper limit of the number of lags PROC STATESPACE KLAG=

specify maximum number of iterations

allowed

PROC STATESPACE MAXIT=

suppress the final estimation PROC STATESPACE NOEST

write the state space model parameter esti-

mates to an output data set

PROC STATESPACE OUTMODEL=

use conditional least squares for final estimates PROC STATESPACE RESIDEST

specify criterion for testing for singularity PROC STATESPACE SINGULAR=

Options for Forecasting

start forecasting before end of the input data PROC STATESPACE BACK=

specify the time interval between observations PROC STATESPACE INTERVAL=

specify multiple periods in the time series PROC STATESPACE INTPER=specify how many periods to forecast PROC STATESPACE LEAD=

specify the output data set for forecasts PROC STATESPACE OUT=

print forecasts PROC STATESPACE PRINT

Options to Specify the State Space Model

specify the state vector FORM

specify the parameter values RESTRICT

BY Groups

specify BY-group processing BY

Printing

suppresses all printed output NOPRINT

PROC STATESPACE Statement

PROC STATESPACE options

The following options can be specified in the PROC STATESPACE statement

Printing Options NOPRINT

suppresses all printed output

Input Data Options

DATA= SAS-data-set

specifies the name of the SAS data set to be used by the procedure If the DATA=

option is omitted the most recently created SAS data set is used

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Chapter 18 Syntax

LAGMAX= k

specifies the number of lags for which the sample autocovariance matrix is computed

The LAGMAX= option controls the number of lags printed in the schematic repre-

sentation of the autocorrelations

The sample autocovariance matrix of lag i denoted asC

i

is computed as

C

i

=

1

N 1

N

X

t = 1 + i

x

t

x

0

t i

wherex

t

is the differenced and centered data andN

is the number of observations

(If the NOCENTER option is specified 1 is not subtracted fromN

) LAGMAX= k

specifies thatC

0

throughC

k

are computed The default is LAGMAX=10

NOCENTER

prevents subtraction of the sample mean from the input series (after any specified

differencing) before the analysis

Options for Preliminary Autoregressive Models ARMAX= n

specifies the maximum order of the preliminary autoregressive models The AR-

MAX= option controls the autoregressive orders for which information criteria are

printed and controls the number of lags printed in the schematic representation of

partial autocorrelations The default is ARMAX=10 See Preliminary Autoregres-

sive Models later in this chapter for details

MINIC

writes to the OUTAR= data set only the preliminary Yule-Walker estimates for the

VAR model producing the minimum AIC See OUTAR= Data Set later in this chap-

ter for details

OUTAR= SAS-data-set

writes the Yule-Walker estimates of the preliminary autoregressive models to a SAS

data set See OUTAR= Data Set later in this chapter for details

PRINTOUT= SHORT | LONG | NONE

determines the amount of detail printed PRINTOUT=LONG prints the lagged co-

variance matrices the partial autoregressive matrices and estimates of the resid-

ual covariance matrices from the sequence of autoregressive models PRINT-

OUT=NONE suppresses the output for the preliminary autoregressive models The

descriptive statistics and state space model estimation output are still printed when

PRINTOUT=NONE is specified PRINTOUT=SHORT is the default

Canonical Correlation Analysis Options

CANCORR

prints the canonical correlations and information criterion for each candidate state

vector considered See Canonical Correlation Analysis later in this chapter for

details

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Part 2 General Information

DIMMAX= n

specifies the upper limit to the dimension of the state vector The DIMMAX= option

can be used to limit the size of the model selected The default is DIMMAX=10

PASTMIN= n

specifies the minimum number of lags to include in the canonical correlation analy-

sis The default is PASTMIN=0 See Canonical Correlation Analysis later in thischapter for details

SIGCORR= value

specifies the multiplier of the degrees of freedom for the penalty term in the informa-

tion criterion used to select the state space form The default is SIGCORR=2 The

larger the value of the SIGCORR= option the smaller the state vector tends to be

Hence a large value causes a simpler model to be fit See Canonical Correlations

Analysis later in this chapter for details

State Space Model Estimation Options

COVB

prints the inverse of the observed information matrix for the parameter estimatesThis matrix is an estimate of the covariance matrix for the parameter estimates

DETTOL= value

specifies the convergence criterion The DETTOL= and PARMTOL= option values

are used together to test for convergence of the estimation process If during an

iteration the relative change of the parameter estimates is less than the PARMTOL=

value and the relative change of the determinant of the innovation variance matrix

is less than the DETTOL= value then iteration ceases and the current estimates are

accepted The default is DETTOL=1E-5

ITPRINT

prints the iterations during the estimation process

KLAG= n

sets an upper limit for the number of lags of the sample autocovariance matrix used

in computing the approximate likelihood function If the data have a strong moving

average character a larger KLAG= value may be necessary to obtain good estimates

The default is KLAG=15 See Parameter Estimation later in this chapter for details

MAXIT= n

sets an upper limit to the number of iterations in the maximum likelihood or condi-

tional least-squares estimation The default is MAXIT=50

NOEST

suppresses the final maximum likelihood estimation of the selected model

OUTMODEL= SAS-data-set

writes the parameter estimates and their standard errors to a SAS data set See OUT-

MODEL= Data Set later in this chapter for details

PARMTOL= value

specifies the convergence criterion The DETTOL= and PARMTOL= option values

are used together to test for convergence of the estimation process If during an

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Chapter 18 Syntax

iteration the relative change of the parameter estimates is less than the PARMTOL=

value and the relative change of the determinant of the innovation variance matrix

is less than the DETTOL= value then iteration ceases and the current estimates are

accepted The default is PARMTOL=001

RESIDEST

computes the final estimates using conditional least squares on the raw data Thistype of estimation may be more stable than the default maximum likelihood method

but is usually more computationally expensive See Parameter Estimation later in

this chapter for details of the conditional least squares method

SINGULAR= value

specifies the criterion for testing for singularity of a matrix A matrix is declared

singular if a scaled pivot is less than the SINGULAR= value when sweeping the

matrix The default is SINGULAR=1E-7

Forecasting Options

BACK= n

starts forecasting n periods before the end of the input data The BACK= option valuemust not be greater than the number of observations The default is BACK=0

INTERVAL= interval

specifies the time interval between observations The INTERVAL= value is used

in conjunction with the ID variable to check that the input data are in order and

have no missing periods The INTERVAL= option is also used to extrapolate the ID

values past the end of the input data See Chapter 3 ldquoDate Intervals Formats and

Functionsrdquo for details on the INTERVAL= values allowed

INTPER= n

specifies that each input observation corresponds to n time periods For example

the options INTERVAL=MONTH and INTPER=2 specify bimonthly data and are

equivalent to specifying INTERVAL=MONTH2 If the INTERVAL= option is not

specified the INTPER= option controls the increment used to generate ID values for

the forecast observations The default is INTPER=1

LEAD= n

specifies how many forecast observations are produced The forecasts start at the

point set by the BACK= option The default is LEAD=0 which produces no fore-

casts

OUT= SAS-data-set

writes the residuals actual values forecasts and forecast standard errors to a SAS

data set See OUT= Data Set later in this chapter for details

PRINT

prints the forecasts

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Part 2 General Information

BY Statement

BY variable

A BY statement can be used with the STATESPACE procedure to obtain separateanalyses on observations in groups defined by the BY variables

FORM Statement

FORM variable value

The FORM statement specifies the number of times a variable is included in the state

vector Values can be specified for any variable listed in the VAR statement If a

value is specified for each variable in the VAR statement the state vector for the state

space model is entirely specified and automatic selection of the state space model is

not performed

The FORM statement forces the state vectorz

t

to contain a specific variable a given

number of times For example if Y is one of the variables inx

t

then the statement

form y 3

forces the state vector to containY

t

Y

t + 1 j t

andY

t + 2 j t

possibly along with other

variables

The following statements illustrate the use of the FORM statement

proc statespace data=in

var x y

f o r m x 3 y 2

run

These statements fit a state space model with the following state vector

z

t

=

2

6

6

6

6

4

x

t j t

y

t j t

x

t + 1 j t

y

t + 1 j t

x

t + 2 j t

3

7

7

7

7

5

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Chapter 18 Syntax

ID Statement

ID variable

The ID statement specifies a variable that identifies observations in the input data setThe variable specified in the ID statement is included in the OUT= data set The

values of the ID variable are extrapolated for the forecast observations based on the

values of the INTERVAL= and INTPER= options

INITIAL Statement

INITIAL F (rowcolumn)= value G(row column)= value

The INITIAL statement gives initial values to the specified elements of theF

andG

matrices These initial values are used as starting values for the iterative estimation

Parts of theF

andG

matrices represent fixed structural identities If an element

specified is a fixed structural element instead of a free parameter the corresponding

initialization is ignored

The following is an example of an INITIAL statement

initial f(32)=0 g(41)=0 g(51)=0

RESTRICT Statement

RESTRICT F(rowcolumn)= value G(rowcolumn)= value

The RESTRICT statement restricts the specified elements of theF

andG

matrices

to the specified values

To use the restrict statement you need to know the form of the model Either specify

the form of the model with the FORM statement or do a preliminary run perhaps

with the NOEST option to find the form of the model that PROC STATESPACE

selects for the data

The following is an example of a RESTRICT statement

restrict f(32)=0 g(41)=0 g(51)=0

Parts of theF

andG

matrices represent fixed structural identities If a restriction is

specified for an element that is a fixed structural element instead of a free parameter

the restriction is ignored

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Part 2 General Information

VAR Statement

VAR variable (difference difference )

The VAR statement specifies the variables in the input data set to model and fore-cast The VAR statement also specifies differencing of the input variables The VAR

statement is required

Differencing is specified by following the variable name with a list of difference

periods separated by commas See the section Stationarity and Differencing for

more information on differencing of input variables

The order in which variables are listed in the VAR statement controls the order in

which variables are included in the state vector Usually potential inputs should be

listed before potential outputs

For example assuming the input data are monthly the following VAR statement

specifies modeling and forecasting of the one period and seasonal second differenceof X and Y

var x(112) y(112)

In this example the vector time series analyzed is

x

t

=

1 B 1 B

1 2

X

t

x

1 B 1 B

1 2

Y

t

y

where B represents the back shift operator and x and y represent the means of thedifferenced series If the NOCENTER option is specified the mean differences are

not subtracted

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Chapter 18 Details

Details

Missing Values

The STATESPACE procedure does not support missing values The procedure uses

the first contiguous group of observations with no missing values for any of the VARstatement variables Observations at the beginning of the data set with missing values

for any VAR statement variable are not used or included in the output data set

Stationarity and Differencing

The state space model used by the STATESPACE procedure assumes that the time

series are stationary Hence the data should be checked for stationarity One way

to check for stationarity is to plot the series A graph of series over time can show a

time trend or variability changes

You can also check stationarity by using the sample autocorrelation functions dis-

played by the ARIMA procedure The autocorrelation functions of nonstationaryseries tend to decay slowly See Chapter 7 ldquoThe ARIMA Procedurerdquo for more in-

formation

Another alternative is to use the STATIONARITY= option on the IDENTIFY state-

ment in PROC ARIMA to apply Dickey-Fuller tests for unit roots in the time series

See Chapter 7 ldquoThe ARIMA Procedurerdquo for more information on Dickey-Fuller unit

root tests

The most popular way to transform a nonstationary series to stationarity is by dif-

ferencing Differencing of the time series is specified in the VAR statement For

example to take a simple first difference of the series X use this statement

var x(1)

In this example the change in X from one period to the next is analyzed When the se-

ries has a seasonal pattern differencing at a period equal to the length of the seasonal

cycle may be desirable For example suppose the variable X is measured quarterly

and shows a seasonal cycle over the year You can use the following statement to

analyze the series of changes from the same quarter in the previous year

var x(4)

To difference twice add another differencing period to the list For ex-

ample the following statement analyzes the series of second differences

X

t

X

t 1

X

t 1

X

t 2

= X

t

2 X

t 1

+ X

t 2

var x(11)

The following statement analyzes the seasonal second difference series

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var x(14)

The series modeled is the 1-period difference of the 4-period difference

X

t

X

t 4

X

t 1

X

t 5

= X

t

X

t 1

X

t 4

+ X

t 5

Another way to obtain stationary series is to use a regression on time to de-trend the

data If the time series has a deterministic linear trend regressing the series on time

produces residuals that should be stationary The following statements write residuals

of X and Y to the variable RX and RY in the output data set DETREND

data a

set a

t=_n_

run

proc reg data=a

model x y = t

output out=detrend r=rx ry

run

You then use PROC STATESPACE to forecast the de-trended series RX and RY A

disadvantage of this method is that you need to add the trend back to the forecast

series in an additional step A more serious disadvantage of the de-trending method

is that it assumes a deterministic trend In practice most time series appear to have a

stochastic rather than a deterministic trend Differencing is a more flexible and often

more appropriate method

There are several other methods to handle nonstationary time series For more infor-

mation and examples refer to Brockwell and Davis (1991)

Preliminary Autoregressive Models

After computing the sample autocovariance matrices PROC STATESPACE fits a

sequence of vector autoregressive models These preliminary autoregressive models

are used to estimate the autoregressive order of the process and limit the order of the

autocovariances considered in the state vector selection process

Yule-Walker Equations for Forward and Backward Models

Unlike a univariate autoregressive model a multivariate autoregressive model has

different forms depending on whether the present observation is being predicted from

the past observations or from the future observations

Letx

t

be the r -component stationary time series given by the VAR statement after

differencing and subtracting the vector of sample means (If the NOCENTER option

is specified the mean is not subtracted) Let n be the number of observations of x

t

from the input data set

Lete

t

be a vector white noise sequence with mean vector 0 and variance matrix

p

and letn

t

be a vector white noise sequence with mean vector 0 and variance matrix

p

Let p be the order of the vector autoregressive model forx

t

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The forward autoregressive form based on the past observations is written as follows

x

t

=

p

X

i = 1

p

i

x

t i

+ e

t

The backward autoregressive form based on the future observations is written as fol-

lows

x

t

=

p

X

i = 1

p

i

x

t + i

+ n

t

LettingE

denote the expected value operator the autocovariance sequence for thex

t

series

i

is

i

= E x

t

x

0

t i

The Yule-Walker equations for the autoregressive model that matches the first p ele-

ments of the autocovariance sequence are

2

6

6

4

0

1

p 1

0

1

0

p 2

0

p 1

0

p 2

0

3

7

7

5

2

6

6

4

p

1

p

2

p

p

3

7

7

5

=

2

6

6

4

1

2

p

3

7

7

5

and

2

6

6

6

4

0

0

1

0

p 1

1

0

0

p 2

p 1

p 2

0

3

7

7

7

5

2

6

6

4

p

1

p

2

p

p

3

7

7

5

=

2

6

6

4

0

1

0

2

0

p

3

7

7

5

Here

p

i

are the coefficient matrices for the past observation form of the vector au-toregressive model and

p

i

are the coefficient matrices for the future observation

form More information on the Yule-Walker equations in the multivariate setting can

be found in Whittle (1963) and Ansley and Newbold (1979)

The innovation variance matrices for the two forms can be written as follows

p

=

0

p

X

i = 1

p

i

0

i

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p

=

0

p

X

i = 1

p

i

i

The autoregressive models are fit to the data using the preceding Yule-Walker equa-

tions with

i

replaced by the sample covariance sequenceC

i

The covariance matri-

ces are calculated as

C

i

=

1

N 1

N

X

t = i + 1

x

t

x

0

t i

Let b

p

b

p

b

p

and b

p

represent the Yule-Walker estimates of

p

p

p

and

p

respectively These matrices are written to an output data set when the OUTAR=

option is specified

When the PRINTOUT=LONG option is specified the sequence of matrices b

p

and

the corresponding correlation matrices are printed The sequence of matricesb

p isused to compute Akaike information criteria for selection of the autoregressive order

of the process

Akaike Information Criterion

The Akaike information criterion or AIC is defined as -2( maximum of log likeli-

hood )+2(number of parameters) Since the vector autoregressive models are esti-

mates from the Yule-Walker equations not by maximum likelihood the exact likeli-

hood values are not available for computing the AIC However for the vector autore-

gressive model the maximum of the log likelihood can be approximated as

l n L

n

2

l n j

b

p

j

Thus the AIC for the order p model is computed as

A I C

p

= n l n j

b

p

j + 2 p r

2

You can use the printed AIC array to compute a likelihood ratio test of the autoregres-

sive order The log-likelihood ratio test statistic for testing the order p model against

the order p 1

model is

n l n j

b

p

j + n l n j

b

p 1

j

This quantity is asymptotically distributed as a

2 withr

2 degrees of freedom if the

series is autoregressive of order p 1

It can be computed from the AIC array as

A I C

p 1

A I C

p

+ 2 r

2

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You can evaluate the significance of these test statistics with the PROBCHI function

in a SAS DATA step or with a

2 table

Determining the Autoregressive Order

Although the autoregressive models can be used for prediction their primary value is

to aid in the selection of a suitable portion of the sample covariance matrix for use in

computing canonical correlations If the multivariate time series x

t is of autoregres-sive order p then the vector of past values to lag p is considered to contain essentially

all the information relevant for prediction of future values of the time series

By default PROC STATESPACE selects the order p producing the autoregressive

model with the smallestA I C

p

If the value p for the minimumA I C

p

is less than the

value of the PASTMIN= option then p is set to the PASTMIN= value Alternatively

you can use the ARMAX= and PASTMIN= options to force PROC STATESPACE to

use an order you select

Significance Limits for Partial Autocorrelations

The STATESPACE procedure prints a schematic representation of the partial autocor-

relation matrices indicating which partial autocorrelations are significantly greater orsignificantly less than 0 Figure 1810 shows an example of this table

The STATESPACE Procedure

Schematic Representation of Partial Autocorrelations

NameLag 1 2 3 4 5 6 7 8 9 10

x ++ +

y ++

+ is gt 2std error - is lt -2std error is between

Figure 1810 Significant Partial Autocorrelations

The partial autocorrelations are from the sample partial autoregressive matrices b

p

p

The standard errors used for the significance limits of the partial autocorrelations are

computed from the sequence of matrices

p

and

p

Under the assumption that the observed series arises from an autoregressive process

of order p 1

the pth sample partial autoregressive matrix b

p

p

has an asymptotic

variance matrix 1

n

1

p

p

The significance limits for b

p

p

used in the schematic plot of the sample partial autore-

gressive sequence are derived by replacing

p

and

p

with their sample estimators

to produce the variance estimate as follows

d

V a r

b

p

p

=

1

n r p

b

1

p

b

p

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Canonical Correlation Analysis

Given the order p letp

t

be the vector of current and past values relevant to prediction

of x

t + 1

p

t

= x

0

t

x

0

t 1

x

0

t p

0

Letf

t

be the vector of current and future values

f

t

= x

0

t

x

0

t + 1

x

0

t + p

0

In the canonical correlation analysis consider submatrices of the sample covariance

matrix of p

t

andf

t

This covariance matrixV

has a block Hankel form

V =

2

6

6

6

4

C

0

C

0

1

C

0

2

C

0

p

C

0

1

C

0

2

C

0

3

C

0

p + 1

C

0

p

C

0

p + 1

C

0

p + 2

C

0

2 p

3

7

7

7

5

State Vector Selection Process

The canonical correlation analysis forms a sequence of potential state vectorsz

j

t

Examine a sequencef

j

t

of subvectors of f

t

and form the submatrixV

j consisting

of the rows and columns of V

corresponding to the components of f

j

t

and compute

its canonical correlations

The smallest canonical correlation of V

j is then used in the selection of the com-

ponents of the state vector The selection process is described in the following For

more details about this process refer to Akaike (1976)

In the following discussion the notationx

t + k j t

denotes the wide sense conditional

expectation (best linear predictor) of x

t + k

given allx

s

with s less than or equal to t

In the notationx

i t + 1

the first subscript denotes the ith component of x

t + 1

The initial state vectorz

1

t

is set tox

t

The sequencef

j

t

is initialized by setting

f

1

t

= z

1

0

t

x

1 t + 1 j t

0

= x

0

t

x

1 t + 1 j t

0

That is start by considering whether to addx

1 t + 1 j t

to the initial state vectorz

1

t

The procedure forms the submatrixV

1 corresponding tof

1

t

and computes its canon-

ical correlations Denote the smallest canonical correlation of V

1 as

m i n

If

m i n

is

significantly greater than 0x

1 t + 1 j t

is added to the state vector

If the smallest canonical correlation of V

1 is not significantly greater than 0 then a

linear combination of f

1

t

is uncorrelated with the pastp

t

Assuming that the determi-

nant of C

0

is not 0 (that is no input series is a constant) you can take the coefficient

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Chapter 18 Details

of x

1 t + 1 j t

in this linear combination to be 1 Denote the coefficients of z

1

t

in this

linear combination as

This gives the relationship

x

1 t + 1 j t

=

0

x

t

Therefore the current state vector already contains all the past information useful forpredicting

x

1 t + 1

and any greater leads of x

1 t

The variablex

1 t + 1 j t

is not added to

the state vector nor are any termsx

1 t + k j t

considered as possible components of the

state vector The variablex

1

is no longer active for state vector selection

The process described forx

1 t + 1 j t

is repeated for the remaining elements of f

t

The

next candidate for inclusion in the state vector is the next component of f

t

corre-

sponding to an active variable Components of f

t

corresponding to inactive variables

that produced a zero

m i n

in a previous step are skipped

Denote the next candidate asx

l t + k j t

The vectorf

j

t

is formed from the current state

vector andx

l t + k j t

as follows

f

j

t

= z

j

0

t

x

l t + k j t

0

The matrixV

j is formed fromf

j

t

and its canonical correlations are computed The

smallest canonical correlation of V

j is judged to be either greater than or equal to 0

If it is judged to be greater than 0x

l t + k j t

is added to the state vector If it is judged

to be 0 then a linear combination of f

j

t

is uncorrelated with thep

t

and the variable

x

l

is now inactive

The state vector selection process continues until no active variables remain

Testing Significance of Canonical Correlations For each step in the canonical correlation sequence the significance of the small-

est canonical correlation

m i n

is judged by an information criterion from Akaike

(1976) This information criterion is

n l n 1

2

m i n

r p + 1 q + 1

where q is the dimension of f

j

t

at the current step r is the order of the state vector p

is the order of the vector autoregressive process and

is the value of the SIGCORR=

option The default is SIGCORR=2 If this information criterion is less than or equal

to 0

m i n

is taken to be 0 otherwise it is taken to be significantly greater than 0

(Do not confuse this information criterion with the AIC)

Variables inx

t + p j t

are not added in the model even with positive information crite-

rion because of the singularity of V

You can force the consideration of more candi-

date state variables by increasing the size of theV

matrix by specifying a PASTMIN=

option value larger than p

Printing the Canonical Correlations

To print the details of the canonical correlation analysis process specify the CAN-

CORR option in the PROC STATESPACE statement The CANCORR option prints

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the candidate state vectors the canonical correlations and the information criteria for

testing the significance of the smallest canonical correlation

Bartlettrsquos

2 and its degrees of freedom are also printed when the CANCORR option

is specified The formula used for Bartlettrsquos

2 is

2

= n 5 r p + 1 q + 1 l n 1

2

m i n

withr p + 1 q + 1

degrees of freedom

Figure 1811 shows the output of the CANCORR option for the introductory example

shown in the Getting Started section of this chapter

The STATESPACE Procedure

Canonical Correlations Analysis

Information Chi

x(TT) y(TT) x(T+1T) Criterion Square DF

1 1 0237045 3566167 114505 4

Information Chi

x(TT) y(TT) x(T+1T) y(T+1T) Criterion Square DF

1 1 0238244 0056565 -535906 0636134 3

Information Chi

x(TT) y(TT) x(T+1T) x(T+2T) Criterion Square DF

1 1 0237602 0087493 -446312 1525353 3

Figure 1811 Canonical Correlations Analysis

New variables are added to the state vector if the information criteria are positive Inthis example

y

t + 1 j t

andx

t + 2 j t

are not added to the state space vector because the

information criteria for these models are negative

If the information criterion is nearly 0 then you may want to investigate models

that arise if the opposite decision is made regarding

m i n

This investigation can be

accomplished by using a FORM statement to specify part or all of the state vector

Preliminary Estimates of F

When a candidate variablex

l t + k j t

yields a zero

m i n

and is not added to the state

vector a linear combination of f

j

t

is uncorrelated with thep

t

Because of the method

used to construct thef

j

t

sequence the coefficient of x

l t + k j t

inl

can be taken as 1

Denote the coefficients of z

j

t

in this linear combination as l

This gives the relationship

x

l t + k j t

= l

0

z

j

t

The vectorl

is used as a preliminary estimate of the first r columns of the row of the

transition matrixF

corresponding tox

l t + k 1 j t

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Parameter Estimation

The model isz

t + 1

= F z

t

+ G e

t + 1

wheree

t

is a sequence of independent multivari-

ate normal innovations with mean vector 0 and variance

e e

The observed sequence

x

t

composes the first r components of z

t

and thusx

t

= H z

t

where H is ther s

matrix I

r

0

LetE

be ther n

matrix of innovations

E = e

1

e

n

If the number of observations n is reasonably large the log likelihood L can be

approximated up to an additive constant as follows

L =

n

2

l n j

e e

j

1

2

t r a c e

1

e e

E E

0

The elements of

e e

are taken as free parameters and are estimated as follows

S

0

=

1

n

E E

0

Replacing

e e

byS

0

in the likelihood equation the log likelihood up to an additive

constant is

L =

n

2

l n j S

0

j

Letting B be the backshift operator the formal relation betweenx

t

ande

t

is

x

t

= H I B F

1

G e

t

e

t

= H I B F

1

G

1

x

t

=

1

X

i = 0

i

x

t i

LettingC

i

be the ith lagged sample covariance of x

t

and neglecting end effects the

matrixS

0

is

S

0

=

1

X

i j = 0

i

C

i + j

0

j

For the computation of S

0

the infinite sum is truncated at the value of the KLAG=

option The value of the KLAG= option should be large enough that the sequence

i

is approximately 0 beyond that point

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Let

be the vector of free parameters in theF

andG

matrices The derivative of the

log likelihood with respect to the parameter

is

L

=

n

2

t r a c e

S

1

0

S

0

The second derivative is

2

L

0

=

n

2

t r a c e

S

1

0

S

0

0

S

1

0

S

0

t r a c e

S

1

0

2

S

0

0

Near the maximum the first term is unimportant and the second term can be approx-

imated to give the following second derivative approximation

2

L

0

=

n t r a c e

S

1

0

E

E

0

0

The first derivative matrix and this second derivative matrix approximation are com-

puted from the sample covariance matrixC

0

and the truncated sequence

i

The

approximate likelihood function is maximized by a modified Newton-Raphson algo-

rithm employing these derivative matrices

The matrixS

0

is used as the estimate of the innovation covariance matrix

e e

The

negative of the inverse of the second derivative matrix at the maximum is used as an

approximate covariance matrix for the parameter estimates The standard errors of

the parameter estimates printed in the parameter estimates tables are taken from the

diagonal of this covariance matrix The parameter covariance matrix is printed when

the COVB option is specified

If the data are nearly nonstationary a better estimate of

e e

and the other parameters

can sometimes be obtained by specifying the RESIDEST option The RESIDEST

option estimates the parameters using conditional least squares instead of maximum

likelihood

The residuals are computed using the state space equation and the sample mean val-

ues of the variables in the model as start-up values The estimate of S

0

is then com-

puted using the residuals from the ith observation on where i is the maximum number

of times any variable occurs in the state vector A multivariate Gauss-Marquardt al-

gorithm is used to minimizej S

0

j

Refer to Harvey (1981a) for a further description

of this method

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Forecasting

Given estimates of F

G

and

e e

forecasts of x

t

are computed from the conditional

expectation of z

t

In forecasting the parameters F G and

e e

are replaced with the estimates or by val-

ues specified in the RESTRICT statement One-step-ahead forecasting is performedfor the observation

x

t

wheret n b

Heren

is the number of observations and

b is the value of the BACK= option For the observationx

t

wheret n b

m-

step-ahead forecasting is performed form = t n + b

The forecasts are generated

recursively with the initial conditionz

0

= 0

The m-step-ahead forecast of z

t + m

isz

t + m j t

wherez

t + m j t

denotes the conditional

expectation of z

t + m

given the information available at time t The m-step-ahead

forecast of x

t + m

isx

t + m j t

= H z

t + m j t

where the matrixH = I

r

0

Let

i

= F

i

G

Note that the lasts r

elements of z

t

consist of the elements of x

u j t

foru t

The state vector z

t + m

can be represented as

z

t + m

= F

m

z

t

+

m 1

X

i = 0

i

e

t + m i

Sincee

t + i j t

= 0

fori 0

the m-step-ahead forecastz

t + m j t

is

z

t + m j t

= F

m

z

t

= F z

t + m 1 j t

Therefore the m-step-ahead forecast of x

t + m

is

x

t + m j t

= H z

t + m j t

The m-step-ahead forecast error is

z

t + m

z

t + m j t

=

m 1

X

i = 0

i

e

t + m i

The variance of the m-step-ahead forecast error is

V

z m

=

m 1

X

i = 0

i

e e

0

i

LettingV

z 0

= 0

the variance of the m-step-ahead forecast error of z

t + m

V

z m

can

be computed recursively as follows

V

z m

= V

z m 1

+

m 1

e e

0

m 1

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The variance of the m-step-ahead forecast error of x

t + m

is ther r

left upper sub-

matrix of V

z m

that is

V

x m

= H V

z m

H

0

Unless the NOCENTER option is specified the sample mean vector is added to the

forecast When differencing is specified the forecasts xt + m j t

plus the sample mean

vector are integrated back to produce forecasts for the original series

Lety

t

be the original series specified by the VAR statement with some 0 values

appended corresponding to the unobserved past observations Let B be the backshift

operator and let B

be thes s

matrix polynomial in the backshift operator

corresponding to the differencing specified by the VAR statement The off-diagonal

elements of

i

are 0 Note that

0

= I

s

whereI

s

is thes s

identity matrix Then

z

t

= B y

t

This gives the relationship

y

t

=

1

B z

t

=

1

X

i = 0

i

z

t i

where

1

B =

P

1

i = 0

i

B

i and

0

= I

s

The m-step-ahead forecast of y

t + m

is

y

t + m j t

=

m 1

X

i = 0

i

z

t + m i j t

+

1

X

i = m

i

z

t + m i

The m-step-ahead forecast error of y

t + m

is

m 1

X

i = 0

i

z

t + m i

z

t + m i j t

=

m 1

X

i = 0

i

X

u = 0

u

i u

e

t + m i

LettingV

y 0

= 0

the variance of the m-step-ahead forecast error of y

t + m

V

y m

is

V

y m

=

m 1

X

i = 0

i

X

u = 0

u

i u

e e

i

X

u = 0

u

i u

0

= V

y m 1

+

m 1

X

u = 0

u

m 1 u

e e

m 1

X

u = 0

u

m 1 u

0

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Relation of ARMA and State Space Forms

Every state space model has an ARMA representation and conversely every ARMA

model has a state space representation This section discusses this equivalence The

following material is adapted from Akaike (1974) where there is a more complete

discussion Pham-Dinh-Tuan (1978) also contains a discussion of this material

Suppose you are given the following ARMA model

B x

t

= B e

t

or in more detail

x

t

1

x

t 1

p

x

t p

= e

t

+

1

e

t 1

+ +

q

e

t q

(1)

wheree

t

is a sequence of independent multivariate normal random vectors with mean

0 and variance matrix

e e

B is the backshift operator (B x

t

= x

t 1

) B

and

B

are matrix polynomials in B andx

t

is the observed process

If the roots of the determinantial equationj B j = 0

are outside the unit circle in

the complex plane the model can also be written as

x

t

=

1

B B e

t

=

1

X

i = 0

i

e

t i

The

i

matrices are known as the impulse response matrices and can be computed

as

1

B B

You can assume p q

since if this is not initially true you can add more terms

i

that are identically 0 without changing the model

To write this set of equations in a state space form proceed as follows Letx

t + i j t

be

the conditional expectation of x

t + i

givenx

w

forw t

The following relations hold

x

t + i j t

=

1

X

j = i

j

e

t + i j

x

t + i j t + 1

= x

t + i j t

+

i 1

e

t + 1

However from equation (1) you can derive the following relationship

x

t + p j t

=

1

x

t + p 1 j t

+ +

p

x

t

(2)

Hence wheni = p

you can substitute forx

t + p j t

in the right-hand side of equation

(2) and close the system of equations

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Part 2 General Information

This substitution results in the following model in the state space form

z

t + 1

= F z

t

+ G e

t + 1

2

6

6

6

4

x

t + 1

x

t + 2 j t + 1

x

t + p j t + 1

3

7

7

7

5

=

2

6

6

4

0 I 0 0

0 0 I 0

p

p 1

1

3

7

7

5

2

6

6

6

4

x

t

x

t + 1 j t

x

t + p 1 j t

3

7

7

7

5

+

2

6

6

4

I

1

p 1

3

7

7

5

e

t + 1

Note that the state vectorz

t

is composed of conditional expectations of x

t

and the

first r components of z

t

are equal tox

t

The state space form can be cast into an ARMA form by solving the system of dif-

ference equations for the first r components

When converting from an ARMA form to a state space form you can generate a

state vector larger than needed that is the state space model may not be a minimal

representation When going from a state space form to an ARMA form you can have

nontrivial common factors in the autoregressive and moving average operators that

yield an ARMA model larger than necessary

If the state space form used is not a minimal representation some but not all compo-

nents of x

t + i j t

may be linearly dependent This situation corresponds to

p

p 1

being of less than full rank when B

and B

have no common nontrivial

left factors In this casez

t

consists of a subset of the possible components of

x

t + i j t

i = 1 2 p 1

However once a component of x

t + i j t

(for example the

jth one) is linearly dependent on the previous conditional expectations then all sub-

sequent jth components of x

t + k j t

fork i

must also be linearly dependent Note

that in this case equivalent but seemingly different structures can arise if the order of

the components withinx

t

is changed

OUT= Data Set

The forecasts are contained in the output data set specified by the OUT= option on the

PROC STATESPACE statement The OUT= data set contains the following variables

the BY variables

the ID variable

the VAR statement variables These variables contain the actual values from

the input data set

FORi numeric variables containing the forecasts The variable FORi contains

the forecasts for the ith variable in the VAR statement list Forecasts are one-

step-ahead predictions until the end of the data or until the observation specified

by the BACK= option

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Chapter 18 Details

RESi numeric variables containing the residual for the forecast of the ith vari-

able in the VAR statement list For forecast observations the actual values are

missing and the RESi variables contain missing values

STDi numeric variables containing the standard deviation for the forecast of

the ith variable in the VAR statement list The values of the STDi variables can

be used to construct univariate confidence limits for the corresponding fore-casts However such confidence limits do not take into account the covariance

of the forecasts

OUTAR= Data Set

The OUTAR= data set contains the estimates of the preliminary autoregressive mod-

els The OUTAR= data set contains the following variables

ORDER a numeric variable containing the order p of the autoregressive model

that the observation represents

AIC a numeric variable containing the value of the information criterionA I C

p

SIGFl numeric variables containing the estimate of the innovation covariance

matrices for the forward autoregressive models The variable SIGFl contains

the lth column of b

p

in the observations with ORDER= p

SIGBl numeric variables containing the estimate of the innovation covariance

matrices for the backward autoregressive models The variable SIGBl contains

the lth column of b

p

in the observations with ORDER= p

FORk ndashl numeric variables containing the estimates of the autoregressive pa-

rameter matrices for the forward models The variable FORk ndashl contains the

lth column of the lag k autoregressive parameter matrix b

p

k

in the observations

with ORDER= p

BACk ndashl numeric variables containing the estimates of the autoregressive pa-

rameter matrices for the backward models The variable BACk ndashl contains the

lth column of the lag k autoregressive parameter matrix b

p

k

in the observations

with ORDER= p

The estimates for the order p autoregressive model can be selected as those observa-

tions with ORDER= p Within these observations the k lth element of

p

i

is given by

the value of the FORindashl variable in the k th observation The k lth element of

p

i

is

given by the value of BACindashl variable in the k th observation The k lth element of

p

is given by SIGFl in the k th observation The k lth element of

p

is given by SIGBlin the k th observation

Table 181 shows an example of the OUTAR= data set with ARMAX=3 andx

t

of

dimension 2 In Table 181 i j

indicate the ijth element of the matrix

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Part 2 General Information

Table 181 Values in the OUTAR= Data Set

Obs ORDER AIC SIGF1 SIGF2 SIGB1 SIGB2 FOR1ndash 1 FOR1ndash 2 FOR2ndash 1 FOR2ndash 2 FOR3ndash 1

1 0 AIC0

0 1 1

0 1 2

0 1 1

0 1 2

2 0 AIC0

0 2 1

0 2 2

0 2 1

0 2 2

3 1 AIC1

1 1 1

1 1 2

1 1 1

1 1 2

1

1

1 1

1

1

1 2

4 1 AIC1

1 2 1

1 1 2

1 2 1

1 1 2

1

1

2 1

1

1

2 2

5 2 AIC2

2 1 1

2 1 2

2 1 1

2 1 2

2

1

1 1

2

1

1 2

2

2

1 1

2

2

1 2

6 2 AIC2

2 2 1

2 1 2

2 2 1

2 1 2

2

1

2 1

2

1

2 2

2

2

2 1

2

2

2 2

7 3 AIC3

3 1 1

3 1 2

3 1 1

3 1 2

3

1

1 1

3

1

1 2

3

2

1 1

3

2

1 2

3

3

1 1

8 3 AIC3

3 2 1

3 1 2

3 2 1

3 1 2

3

1

2 1

3

1

2 2

3

2

2 1

3

2

2 2

3

3

2 1

Obs FOR3ndash 2 BACK1ndash 1 BACK1ndash 2 BACK2ndash 1 BACK2ndash 2 BACK3ndash 1 BACK3ndash 2

1

2

3

1

1

1 1

1

1

1 2

4

1

1

2 1

1

1

2 2

5

2

1

1 1

2

1

1 2

2

2

1 1

2

2

1 2

6

2

1

2 1

2

1

2 2

2

2

2 1

2

2

2 2

7

3

3

1 2

3

1

1 1

3

1

1 2

3

2

1 1

3

2

1 2

3

3

1 1

3

3

1 2

8

3

3

2 2

3

1

2 1

3

1

2 2

3

2

2 1

3

2

2 2

3

3

2 1

3

3

2 2

The estimated autoregressive parameters can be used in the IML procedure to obtain

autoregressive estimates of the spectral density function or forecasts based on the

autoregressive models

OUTMODEL= Data Set

The OUTMODEL= data set contains the estimates of theF

andG

matrices and their

standard errors the names of the components of the state vector and the estimates of

the innovation covariance matrix The variables contained in the OUTMODEL= data

set are as follows

the BY variables

STATEVEC a character variable containing the name of the component of the

state vector corresponding to the observation The STATEVEC variable has

the value STD for standard deviations observations which contain the standarderrors for the estimates given in the preceding observation

Fndash j numeric variables containing the columns of theF

matrix The variable

Fndash j contains the jth column of F

The number of Fndash j variables is equal to

the value of the DIMMAX= option If the model is of smaller dimension the

extraneous variables are set to missing

Gndash j numeric variables containing the columns of theG

matrix The variable

Gndash j contains the jth column of G

The number of Gndash j variables is equal to r

the dimension of x

t

given by the number of variables in the VAR statement

SIGndash j numeric variables containing the columns of the innovation covariance

matrix The variable SIGndash j contains the jth column of

e e

There are r vari-ables SIGndash j

Table 182 shows an example of the OUTMODEL= data set withx

t

= x

t

y

t

0

z

t

= x

t

y

t

x

t + 1 j t

0 and DIMMAX=4 In Table 182F

i j

andG

i j

are the ijth

elements of F

andG

respectively Note that all elements for Fndash4 are missing because

F

is a3 3

matrix

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Chapter 18 Details

Table 182 Value in the OUTMODEL= Data Set

Obs STATEVEC Fndash 1 Fndash 2 Fndash 3 Fndash 4 Gndash 1 Gndash 2 SIGndash 1 SIGndash 2

1 X(TT) 0 0 1 1 0

1 1

1 2

2 STD

3 Y(TT) F2 1

F2 2

F2 3

0 1

2 1

2 2

4 STD std F2 1

std F2 2

std F2 3

5 X(T+1T) F3 1

F3 2

F3 3

G3 1

G3 2

6 STD std F3 1

std F3 2

std F3 3

std G3 1

std G3 2

Printed Output

The printed output produced by the STATESPACE procedure is described in the fol-

lowing

1 descriptive statistics which include the number of observations used the

names of the variables their means and standard deviations (Std) and the dif-

ferencing operations used

2 the Akaike information criteria for the sequence of preliminary autoregressive

models

3 if the PRINTOUT=LONG option is specified the sample autocovariance ma-

trices of the input series at various lags

4 if the PRINTOUT=LONG option is specified the sample autocorrelation ma-

trices of the input series

5 a schematic representation of the autocorrelation matrices showing the signif-

icant autocorrelations

6 if the PRINTOUT=LONG option is specified the partial autoregressive ma-

trices (These are

p

p

as described in Preliminary Autoregressive Modelsearlier in this chapter)

7 a schematic representation of the partial autocorrelation matrices showing the

significant partial autocorrelations

8 the Yule-Walker estimates of the autoregressive parameters for the autoregres-

sive model with the minimum AIC

9 if the PRINTOUT=LONG option is specified the autocovariance matrices of

the residuals of the minimum AIC model This is the sequence of estimated

innovation variance matrices for the solutions of the Yule-Walker equations

10 if the PRINTOUT=LONG option is specified the autocorrelation matrices of the residuals of the minimum AIC model

11 If the CANCORR option is specified the canonical correlations analysis for

each potential state vector considered in the state vector selection process

This includes the potential state vector the canonical correlations the informa-

tion criterion for the smallest canonical correlation Bartlettrsquos

2 statistic (ldquoChi

Squarerdquo) for the smallest canonical correlation and the degrees of freedom of

Bartlettrsquos

2

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Part 2 General Information

12 the components of the chosen state vector

13 the preliminary estimate of the transition matrixF

the input matrixG

and

the variance matrix for the innovations

e e

14 if the ITPRINT option is specified the iteration history of the likelihood max-

imization For each iteration this shows the iteration number the number of step halvings the determinant of the innovation variance matrix the damping

factor Lambda and the values of the parameters

15 the state vector printed again to aid interpretation of the following listing of F

andG

16 the final estimate of the transition matrixF

17 the final estimate of the input matrixG

18 the final estimate of the variance matrix for the innovations

e e

19 a table listing the estimates of the free parameters inF

andG

and their standarderrors and t statistics

20 if the COVB option is specified the covariance matrix of the parameter esti-

mates

21 if the COVB option is specified the correlation matrix of the parameter esti-

mates

22 if the PRINT option is specified the forecasts and their standard errors

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Chapter 18 Details

ODS Table Names

PROC STATESPACE assigns a name to each table it creates You can use these

names to reference the table when using the Output Delivery System (ODS) to select

tables and create output data sets These names are listed in the following table For

more information on ODS see Chapter 6 ldquoUsing the Output Delivery Systemrdquo

Table 183 ODS Tables Produced in PROC STATESPACE

ODS Table Name Description Option

NObs Number of observations default

Summary Simple summary statistics table default

InfoCriterion Information criterion table default

CovLags Covariance Matrices of Input Series PRINTOUT=LONG

CorrLags Correlation Matrices of Input Series PRINTOUT=LONG

PartialAR Partial Autoregressive Matrices PRINTOUT=LONG

YWEstimates Yule-Walker Estimates for Minimum AIC default

CovResiduals Covariance of Residuals PRINTOUT=LONGCorrResiduals Residual Correlations from AR Models PRINTOUT=LONG

StateVector State vector table default

CorrGraph Schematic Representation of Correlations default

TransitionMatrix Transition Matrix default

InputMatrix Input Matrix default

VarInnov Variance Matrix for the Innovation default

CovB Covariance of Parameter Estimates COVB

CorrB Correlation of Parameter Estimates COVB

CanCorr Canonical Correlation Analysis CANCORR

IterHistory Iterative Fitting table ITPRINT

ParameterEstimates Parameter Estimates Table default

Forecasts Forecasts Table PRINT

ConvergenceStatus Convergence Status Table default

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Part 2 General Information

Example

Example 181 Series J from Box and Jenkins

This example analyzes the gas furnace data (series J) from Box and Jenkins (The

data are not shown Refer to Box and Jenkins (1976) for the data)

First a model is selected and fit automatically using the following statements

title1 rsquoGas Furnace Datarsquo

title2 rsquoBox amp Jenkins Series Jrsquo

title3 rsquoAutomatically Selected Modelrsquo

proc statespace data=seriesj cancorr

var x y

run

The results for the automatically selected model are shown in Output 1811

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Chapter 18 Example

Output 1811 Results for Automatically Selected Model

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Number of Observations 296

Standard

Variable Mean Error

x -005683 1072766

y 5350912 3202121

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Information Criterion for Autoregressive Models

Lag=0 Lag=1 Lag=2 Lag=3 Lag=4 Lag=5 Lag=6 Lag=7 Lag=8

6513862 -103357 -163296 -164512 -165152 -164891 -164934 -164315 -163856

Information

Criterion for

Autoregressive

Models

Lag=9 Lag=10

-16348 -163359

Schematic Representation of Correlations

NameLag 0 1 2 3 4 5 6 7 8 9 10

x +- +- +- +- +- +- +- +- +- +- +-

y -+ -+ -+ -+ -+ -+ -+ -+ -+ -+ -+

+ is gt 2std error - is lt -2std error is between

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Part 2 General Information

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Schematic Representation of Partial Autocorrelations

NameLag 1 2 3 4 5 6 7 8 9 10

x + - + -

y -+ -- - + +

+ is gt 2std error - is lt -2std error is between

Yule-Walker Estimates for Minimum AIC

------Lag=1------ ------Lag=2------ ------Lag=3------ ------Lag=4------

x y x y x y x y

x 1925887 -000124 -120166 0004224 0116918 -000867 0104236 0003268

y 0050496 1299793 -002046 -03277 -071182 -025701 0195411 0133417

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Canonical Correlations Analysis

Information Chi

x(TT) y(TT) x(T+1T) Criterion Square DF

1 1 0804883 2929228 3047481 8

Information Chix(TT) y(TT) x(T+1T) y(T+1T) Criterion Square DF

1 1 0906681 0607529 1223358 1347237 7

Information Chi

x(TT) y(TT) x (T+1T) y (T+1T) x (T+2T) Criterion Square DF

1 1 0909434 0610278 0186274 -154701 1034705 6

Information Chi

x(TT) y(TT) x (T+1T) y (T+1T) y (T+2T) Criterion Square DF

1 1 091014 0618937 0206823 0940392 1280924 6

Information Chi

x(TT) y(TT) x(T+1T) y(T+1T) y(T+2T) y(T+3T) Criterion Square DF

1 1 0912963 0628785 0226598 0083258 -794103 2041584 5

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Chapter 18 Example

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Selected Statespace Form and Preliminary Estimates

State Vector

x(TT) y(TT) x(T+1T) y(T+1T) y(T+2T)

Estimate of Transition Matrix

0 0 1 0 0

0 0 0 1 0

-084718 0026794 1711715 -005019 0

0 0 0 0 1

-019785 0334274 -018174 -123557 1787475

Input Matrix for Innovation

1 0

0 11925887 -000124

0050496 1299793

0142421 1361696

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Selected Statespace Form and Preliminary Estimates

Variance Matrix for Innovation

0035274 -000734

-000734 0097569

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Part 2 General Information

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Selected Statespace Form and Fitted Model

State Vector

x(TT) y(TT) x(T+1T) y(T+1T) y(T+2T)

Estimate of Transition Matrix

0 0 1 0 0

0 0 0 1 0

-086192 0030609 1724235 -005483 0

0 0 0 0 1

-034839 0292124 -009435 -109823 1671418

Input Matrix for Innovation

1 0

0 1192442 -000416

0015621 1258495

008058 1353204

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Selected Statespace Form and Fitted Model

Variance Matrix for Innovation

0035579 -000728

-000728 0095577

Parameter Estimates

Standard

Parameter Estimate Error t Value

F(31) -086192 0072961 -1181

F(32) 0030609 0026167 117

F(33) 1724235 0061599 2799

F(34) -005483 0030169 -182

F(51) -034839 0135253 -258

F(52) 0292124 0046299 631

F(53) -009435 0096527 -098F(54) -109823 0109525 -1003

F(55) 1671418 0083737 1996

G(31) 1924420 0058162 3309

G(32) -000416 0035255 -012

G(41) 0015621 0095771 016

G(42) 1258495 0055742 2258

G(51) 0080580 0151622 053

G(52) 1353204 0091388 1481

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Chapter 18 Example

The two series are believed to have a transfer function relation with the gas rate

(variable X) as the input and the CO2

concentration (variable Y) as the output Since

the parameter estimates shown in Output 1811 support this kind of model the model

is reestimated with the feedback parameters restricted to 0 The following statements

fit the transfer function (no feedback) model

title3 rsquoTransfer Function Modelrsquo

proc statespace data=seriesj printout=none

var x y

restrict f(32)=0 f(34)=0

g(32)=0 g(41)=0 g(51)=0

run

The last 2 pages of the output are shown in Output 1812

Output 1812 STATESPACE Output for Transfer Function Model

Gas Furnace Data

Box amp Jenkins Series J

Transfer Function Model

The STATESPACE Procedure

Selected Statespace Form and Fitted Model

State Vector

x(TT) y(TT) x(T+1T) y(T+1T) y(T+2T)

Estimate of Transition Matrix

0 0 1 0 0

0 0 0 1 0

-068882 0 1598717 0 0

0 0 0 0 1

-035944 0284179 -00963 -107313 1650047

Input Matrix for Innovation

1 0

0 1

1923446 0

0 1260856

0 1346332

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Part 2 General Information

Gas Furnace Data

Box amp Jenkins Series J

Transfer Function Model

The STATESPACE Procedure

Selected Statespace Form and Fitted Model

Variance Matrix for Innovation

0036995 -00072

-00072 0095712

Parameter Estimates

Standard

Parameter Estimate Error t Value

F(31) -068882 0050549 -1363

F(33) 1598717 0050924 3139

F(51) -035944 0229044 -157

F(52) 0284179 0096944 293

F(53) -009630 0140876 -068

F(54) -107313 0250385 -429

F(55) 1650047 0188533 875G(31) 1923446 0056328 3415

G(42) 1260856 0056464 2233

G(52) 1346332 0091086 1478

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Chapter 18 References

References

Akaike H (1974) Markovian Representation of Stochastic Processes and Its Ap-

plication to the Analysis of Autoregressive Moving Average Processes Annals

of the Institute of Statistical Mathematics 26 363-387

Akaike H (1976) Canonical Correlations Analysis of Time Series and the Use of

an Information Criterion in Advances and Case Studies in System Identification

eds R Mehra and DG Lainiotis New York Academic Press

Anderson TW (1971) The Statistical Analysis of Time Series New York John

Wiley amp Sons

Ansley CF and Newbold P (1979) Multivariate Partial Autocorrelations Pro-

ceedings of the Business and Economic Statistics Section American Statistical

Association 349-353

Box GEP and Jenkins G (1976) Time Series Analysis Forecasting and Control

San Francisco Holden-Day

Brockwell PJ and Davis RA (1991) Time Series Theory and Methods 2nd Edi-

tion Springer-Verlag

Hannan EJ (1970) Multiple Time Series New York John Wiley amp Sons

Hannan EJ (1976) The Identification and Parameterization of ARMAX and State

Space Forms Econometrica 44 713-722

Harvey AC (1981a) The Econometric Analysis of Time Series New York John

Wiley amp Sons

Harvey AC (1981b) Time Series Models New York John Wiley amp Sons

Jones RH (1974) Identification and Autoregressive Spectrum Estimation IEEE Transactions on Automatic Control AC-19 894-897

Pham-Dinh-Tuan (1978) On the Fitting of Multivariate Processes of the

Autoregressive-Moving Average Type Biometrika 65 99-107

Priestley MB (1980) System Identification Kalman Filtering and Stochastic Con-

trol in Directions in Time Series eds DR Brillinger and GC Tiao Institute

of Mathematical Statistics

Whittle P (1963) On the Fitting of Multivariate Autoregressions and the Approxi-

mate Canonical Factorization of a Spectral Density Matrix Biometrika 50 129-

134

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The correct bibliographic citation for this manual is as follows SAS Institute Inc SAS

ET S U serrsquos Gui de Version 8 Cary NC SAS Institute Inc 1999 1546 pp

SASET S Userrsquos Guide Version 8

Copyright copy 1999 by S AS I ns t it u t e I nc Ca ry N C U S A

ISBN 1ndash58025ndash489ndash6

All r ight s res erved P r int ed in t he U n it ed St a t es of A merica N o pa rt of t his p u blica t ionma y b e reprodu ced s t ored in a ret r ieva l s ys t em or t ra ns mit t ed in a ny form or b y a nymeans electronic mechanical photocopying or otherwise without the prior writtenpermission of the publisher SAS Institute Inc

US Government Restricted Rights Notice Use du plicat ion or disclosure of the

software by the government is subject to restrictions as set forth in FAR 52227ndash19Commercial Computer Softw ar e-Restricted Rights (J une 1987)

SAS Institute Inc SAS Campus Drive Cary North Carolina 27513

1st print ing October 1999

S AS reg a nd a ll ot her SA S I ns t it u t e I nc p rodu ct or s ervice na m es a re regis t ered t ra dema rkst d k f SA S I t it t I i t h U S A d t h t i reg i di t U SA

Page 17: Statespace Model

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Chapter 18 Syntax

Syntax

The STATESPACE procedure uses the following statements

PROC STATESPACE options

BY variable FORM variable value

ID variable

INITIAL F(rowcolumn)=value

G(rowcolumn)=value

RESTRICT F(rowcolumn)=value

G(rowcolumn)=value

VAR variable (difference difference

)

Functional Summary

The statements and options used by PROC STATESPACE are summarized in the

following table

Description Statement Option

Input Data Set Options

specify the input data set PROC STATESPACE DATA=

prevent subtraction of sample mean PROC STATESPACE NOCENTER

specify the ID variable ID

specify the observed series and differencing VAR

Options for Autoregressive Estimates

specify the maximum order PROC STATESPACE ARMAX=

specify maximum lag for autocovariances PROC STATESPACE LAGMAX=

output only minimum AIC model PROC STATESPACE MINIC

specify the amount of detail printed PROC STATESPACE PRINTOUT=

write preliminary AR models to a data set PROC STATESPACE OUTAR=

Options for Canonical Correlation Analysis

print the sequence of canonical correlations PROC STATESPACE CANCORR

specify upper limit of dimension of state

vector

PROC STATESPACE DIMMAX=

specify the minimum number of lags PROC STATESPACE PASTMIN=

specify the multiplier of the degrees of

freedom

PROC STATESPACE SIGCORR=

Options for State Space Model Estimation

specify starting values INITIAL

print covariance matrix of parameter estimates PROC STATESPACE COVB

specify the convergence criterion PROC STATESPACE DETTOL=

specify the convergence criterion PROC STATESPACE PARMTOL=

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Part 2 General Information

Description Statement Option

print the details of the iterations PROC STATESPACE ITPRINT

specify an upper limit of the number of lags PROC STATESPACE KLAG=

specify maximum number of iterations

allowed

PROC STATESPACE MAXIT=

suppress the final estimation PROC STATESPACE NOEST

write the state space model parameter esti-

mates to an output data set

PROC STATESPACE OUTMODEL=

use conditional least squares for final estimates PROC STATESPACE RESIDEST

specify criterion for testing for singularity PROC STATESPACE SINGULAR=

Options for Forecasting

start forecasting before end of the input data PROC STATESPACE BACK=

specify the time interval between observations PROC STATESPACE INTERVAL=

specify multiple periods in the time series PROC STATESPACE INTPER=specify how many periods to forecast PROC STATESPACE LEAD=

specify the output data set for forecasts PROC STATESPACE OUT=

print forecasts PROC STATESPACE PRINT

Options to Specify the State Space Model

specify the state vector FORM

specify the parameter values RESTRICT

BY Groups

specify BY-group processing BY

Printing

suppresses all printed output NOPRINT

PROC STATESPACE Statement

PROC STATESPACE options

The following options can be specified in the PROC STATESPACE statement

Printing Options NOPRINT

suppresses all printed output

Input Data Options

DATA= SAS-data-set

specifies the name of the SAS data set to be used by the procedure If the DATA=

option is omitted the most recently created SAS data set is used

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Chapter 18 Syntax

LAGMAX= k

specifies the number of lags for which the sample autocovariance matrix is computed

The LAGMAX= option controls the number of lags printed in the schematic repre-

sentation of the autocorrelations

The sample autocovariance matrix of lag i denoted asC

i

is computed as

C

i

=

1

N 1

N

X

t = 1 + i

x

t

x

0

t i

wherex

t

is the differenced and centered data andN

is the number of observations

(If the NOCENTER option is specified 1 is not subtracted fromN

) LAGMAX= k

specifies thatC

0

throughC

k

are computed The default is LAGMAX=10

NOCENTER

prevents subtraction of the sample mean from the input series (after any specified

differencing) before the analysis

Options for Preliminary Autoregressive Models ARMAX= n

specifies the maximum order of the preliminary autoregressive models The AR-

MAX= option controls the autoregressive orders for which information criteria are

printed and controls the number of lags printed in the schematic representation of

partial autocorrelations The default is ARMAX=10 See Preliminary Autoregres-

sive Models later in this chapter for details

MINIC

writes to the OUTAR= data set only the preliminary Yule-Walker estimates for the

VAR model producing the minimum AIC See OUTAR= Data Set later in this chap-

ter for details

OUTAR= SAS-data-set

writes the Yule-Walker estimates of the preliminary autoregressive models to a SAS

data set See OUTAR= Data Set later in this chapter for details

PRINTOUT= SHORT | LONG | NONE

determines the amount of detail printed PRINTOUT=LONG prints the lagged co-

variance matrices the partial autoregressive matrices and estimates of the resid-

ual covariance matrices from the sequence of autoregressive models PRINT-

OUT=NONE suppresses the output for the preliminary autoregressive models The

descriptive statistics and state space model estimation output are still printed when

PRINTOUT=NONE is specified PRINTOUT=SHORT is the default

Canonical Correlation Analysis Options

CANCORR

prints the canonical correlations and information criterion for each candidate state

vector considered See Canonical Correlation Analysis later in this chapter for

details

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Part 2 General Information

DIMMAX= n

specifies the upper limit to the dimension of the state vector The DIMMAX= option

can be used to limit the size of the model selected The default is DIMMAX=10

PASTMIN= n

specifies the minimum number of lags to include in the canonical correlation analy-

sis The default is PASTMIN=0 See Canonical Correlation Analysis later in thischapter for details

SIGCORR= value

specifies the multiplier of the degrees of freedom for the penalty term in the informa-

tion criterion used to select the state space form The default is SIGCORR=2 The

larger the value of the SIGCORR= option the smaller the state vector tends to be

Hence a large value causes a simpler model to be fit See Canonical Correlations

Analysis later in this chapter for details

State Space Model Estimation Options

COVB

prints the inverse of the observed information matrix for the parameter estimatesThis matrix is an estimate of the covariance matrix for the parameter estimates

DETTOL= value

specifies the convergence criterion The DETTOL= and PARMTOL= option values

are used together to test for convergence of the estimation process If during an

iteration the relative change of the parameter estimates is less than the PARMTOL=

value and the relative change of the determinant of the innovation variance matrix

is less than the DETTOL= value then iteration ceases and the current estimates are

accepted The default is DETTOL=1E-5

ITPRINT

prints the iterations during the estimation process

KLAG= n

sets an upper limit for the number of lags of the sample autocovariance matrix used

in computing the approximate likelihood function If the data have a strong moving

average character a larger KLAG= value may be necessary to obtain good estimates

The default is KLAG=15 See Parameter Estimation later in this chapter for details

MAXIT= n

sets an upper limit to the number of iterations in the maximum likelihood or condi-

tional least-squares estimation The default is MAXIT=50

NOEST

suppresses the final maximum likelihood estimation of the selected model

OUTMODEL= SAS-data-set

writes the parameter estimates and their standard errors to a SAS data set See OUT-

MODEL= Data Set later in this chapter for details

PARMTOL= value

specifies the convergence criterion The DETTOL= and PARMTOL= option values

are used together to test for convergence of the estimation process If during an

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Chapter 18 Syntax

iteration the relative change of the parameter estimates is less than the PARMTOL=

value and the relative change of the determinant of the innovation variance matrix

is less than the DETTOL= value then iteration ceases and the current estimates are

accepted The default is PARMTOL=001

RESIDEST

computes the final estimates using conditional least squares on the raw data Thistype of estimation may be more stable than the default maximum likelihood method

but is usually more computationally expensive See Parameter Estimation later in

this chapter for details of the conditional least squares method

SINGULAR= value

specifies the criterion for testing for singularity of a matrix A matrix is declared

singular if a scaled pivot is less than the SINGULAR= value when sweeping the

matrix The default is SINGULAR=1E-7

Forecasting Options

BACK= n

starts forecasting n periods before the end of the input data The BACK= option valuemust not be greater than the number of observations The default is BACK=0

INTERVAL= interval

specifies the time interval between observations The INTERVAL= value is used

in conjunction with the ID variable to check that the input data are in order and

have no missing periods The INTERVAL= option is also used to extrapolate the ID

values past the end of the input data See Chapter 3 ldquoDate Intervals Formats and

Functionsrdquo for details on the INTERVAL= values allowed

INTPER= n

specifies that each input observation corresponds to n time periods For example

the options INTERVAL=MONTH and INTPER=2 specify bimonthly data and are

equivalent to specifying INTERVAL=MONTH2 If the INTERVAL= option is not

specified the INTPER= option controls the increment used to generate ID values for

the forecast observations The default is INTPER=1

LEAD= n

specifies how many forecast observations are produced The forecasts start at the

point set by the BACK= option The default is LEAD=0 which produces no fore-

casts

OUT= SAS-data-set

writes the residuals actual values forecasts and forecast standard errors to a SAS

data set See OUT= Data Set later in this chapter for details

PRINT

prints the forecasts

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Part 2 General Information

BY Statement

BY variable

A BY statement can be used with the STATESPACE procedure to obtain separateanalyses on observations in groups defined by the BY variables

FORM Statement

FORM variable value

The FORM statement specifies the number of times a variable is included in the state

vector Values can be specified for any variable listed in the VAR statement If a

value is specified for each variable in the VAR statement the state vector for the state

space model is entirely specified and automatic selection of the state space model is

not performed

The FORM statement forces the state vectorz

t

to contain a specific variable a given

number of times For example if Y is one of the variables inx

t

then the statement

form y 3

forces the state vector to containY

t

Y

t + 1 j t

andY

t + 2 j t

possibly along with other

variables

The following statements illustrate the use of the FORM statement

proc statespace data=in

var x y

f o r m x 3 y 2

run

These statements fit a state space model with the following state vector

z

t

=

2

6

6

6

6

4

x

t j t

y

t j t

x

t + 1 j t

y

t + 1 j t

x

t + 2 j t

3

7

7

7

7

5

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Chapter 18 Syntax

ID Statement

ID variable

The ID statement specifies a variable that identifies observations in the input data setThe variable specified in the ID statement is included in the OUT= data set The

values of the ID variable are extrapolated for the forecast observations based on the

values of the INTERVAL= and INTPER= options

INITIAL Statement

INITIAL F (rowcolumn)= value G(row column)= value

The INITIAL statement gives initial values to the specified elements of theF

andG

matrices These initial values are used as starting values for the iterative estimation

Parts of theF

andG

matrices represent fixed structural identities If an element

specified is a fixed structural element instead of a free parameter the corresponding

initialization is ignored

The following is an example of an INITIAL statement

initial f(32)=0 g(41)=0 g(51)=0

RESTRICT Statement

RESTRICT F(rowcolumn)= value G(rowcolumn)= value

The RESTRICT statement restricts the specified elements of theF

andG

matrices

to the specified values

To use the restrict statement you need to know the form of the model Either specify

the form of the model with the FORM statement or do a preliminary run perhaps

with the NOEST option to find the form of the model that PROC STATESPACE

selects for the data

The following is an example of a RESTRICT statement

restrict f(32)=0 g(41)=0 g(51)=0

Parts of theF

andG

matrices represent fixed structural identities If a restriction is

specified for an element that is a fixed structural element instead of a free parameter

the restriction is ignored

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Part 2 General Information

VAR Statement

VAR variable (difference difference )

The VAR statement specifies the variables in the input data set to model and fore-cast The VAR statement also specifies differencing of the input variables The VAR

statement is required

Differencing is specified by following the variable name with a list of difference

periods separated by commas See the section Stationarity and Differencing for

more information on differencing of input variables

The order in which variables are listed in the VAR statement controls the order in

which variables are included in the state vector Usually potential inputs should be

listed before potential outputs

For example assuming the input data are monthly the following VAR statement

specifies modeling and forecasting of the one period and seasonal second differenceof X and Y

var x(112) y(112)

In this example the vector time series analyzed is

x

t

=

1 B 1 B

1 2

X

t

x

1 B 1 B

1 2

Y

t

y

where B represents the back shift operator and x and y represent the means of thedifferenced series If the NOCENTER option is specified the mean differences are

not subtracted

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Chapter 18 Details

Details

Missing Values

The STATESPACE procedure does not support missing values The procedure uses

the first contiguous group of observations with no missing values for any of the VARstatement variables Observations at the beginning of the data set with missing values

for any VAR statement variable are not used or included in the output data set

Stationarity and Differencing

The state space model used by the STATESPACE procedure assumes that the time

series are stationary Hence the data should be checked for stationarity One way

to check for stationarity is to plot the series A graph of series over time can show a

time trend or variability changes

You can also check stationarity by using the sample autocorrelation functions dis-

played by the ARIMA procedure The autocorrelation functions of nonstationaryseries tend to decay slowly See Chapter 7 ldquoThe ARIMA Procedurerdquo for more in-

formation

Another alternative is to use the STATIONARITY= option on the IDENTIFY state-

ment in PROC ARIMA to apply Dickey-Fuller tests for unit roots in the time series

See Chapter 7 ldquoThe ARIMA Procedurerdquo for more information on Dickey-Fuller unit

root tests

The most popular way to transform a nonstationary series to stationarity is by dif-

ferencing Differencing of the time series is specified in the VAR statement For

example to take a simple first difference of the series X use this statement

var x(1)

In this example the change in X from one period to the next is analyzed When the se-

ries has a seasonal pattern differencing at a period equal to the length of the seasonal

cycle may be desirable For example suppose the variable X is measured quarterly

and shows a seasonal cycle over the year You can use the following statement to

analyze the series of changes from the same quarter in the previous year

var x(4)

To difference twice add another differencing period to the list For ex-

ample the following statement analyzes the series of second differences

X

t

X

t 1

X

t 1

X

t 2

= X

t

2 X

t 1

+ X

t 2

var x(11)

The following statement analyzes the seasonal second difference series

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Part 2 General Information

var x(14)

The series modeled is the 1-period difference of the 4-period difference

X

t

X

t 4

X

t 1

X

t 5

= X

t

X

t 1

X

t 4

+ X

t 5

Another way to obtain stationary series is to use a regression on time to de-trend the

data If the time series has a deterministic linear trend regressing the series on time

produces residuals that should be stationary The following statements write residuals

of X and Y to the variable RX and RY in the output data set DETREND

data a

set a

t=_n_

run

proc reg data=a

model x y = t

output out=detrend r=rx ry

run

You then use PROC STATESPACE to forecast the de-trended series RX and RY A

disadvantage of this method is that you need to add the trend back to the forecast

series in an additional step A more serious disadvantage of the de-trending method

is that it assumes a deterministic trend In practice most time series appear to have a

stochastic rather than a deterministic trend Differencing is a more flexible and often

more appropriate method

There are several other methods to handle nonstationary time series For more infor-

mation and examples refer to Brockwell and Davis (1991)

Preliminary Autoregressive Models

After computing the sample autocovariance matrices PROC STATESPACE fits a

sequence of vector autoregressive models These preliminary autoregressive models

are used to estimate the autoregressive order of the process and limit the order of the

autocovariances considered in the state vector selection process

Yule-Walker Equations for Forward and Backward Models

Unlike a univariate autoregressive model a multivariate autoregressive model has

different forms depending on whether the present observation is being predicted from

the past observations or from the future observations

Letx

t

be the r -component stationary time series given by the VAR statement after

differencing and subtracting the vector of sample means (If the NOCENTER option

is specified the mean is not subtracted) Let n be the number of observations of x

t

from the input data set

Lete

t

be a vector white noise sequence with mean vector 0 and variance matrix

p

and letn

t

be a vector white noise sequence with mean vector 0 and variance matrix

p

Let p be the order of the vector autoregressive model forx

t

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Chapter 18 Details

The forward autoregressive form based on the past observations is written as follows

x

t

=

p

X

i = 1

p

i

x

t i

+ e

t

The backward autoregressive form based on the future observations is written as fol-

lows

x

t

=

p

X

i = 1

p

i

x

t + i

+ n

t

LettingE

denote the expected value operator the autocovariance sequence for thex

t

series

i

is

i

= E x

t

x

0

t i

The Yule-Walker equations for the autoregressive model that matches the first p ele-

ments of the autocovariance sequence are

2

6

6

4

0

1

p 1

0

1

0

p 2

0

p 1

0

p 2

0

3

7

7

5

2

6

6

4

p

1

p

2

p

p

3

7

7

5

=

2

6

6

4

1

2

p

3

7

7

5

and

2

6

6

6

4

0

0

1

0

p 1

1

0

0

p 2

p 1

p 2

0

3

7

7

7

5

2

6

6

4

p

1

p

2

p

p

3

7

7

5

=

2

6

6

4

0

1

0

2

0

p

3

7

7

5

Here

p

i

are the coefficient matrices for the past observation form of the vector au-toregressive model and

p

i

are the coefficient matrices for the future observation

form More information on the Yule-Walker equations in the multivariate setting can

be found in Whittle (1963) and Ansley and Newbold (1979)

The innovation variance matrices for the two forms can be written as follows

p

=

0

p

X

i = 1

p

i

0

i

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Part 2 General Information

p

=

0

p

X

i = 1

p

i

i

The autoregressive models are fit to the data using the preceding Yule-Walker equa-

tions with

i

replaced by the sample covariance sequenceC

i

The covariance matri-

ces are calculated as

C

i

=

1

N 1

N

X

t = i + 1

x

t

x

0

t i

Let b

p

b

p

b

p

and b

p

represent the Yule-Walker estimates of

p

p

p

and

p

respectively These matrices are written to an output data set when the OUTAR=

option is specified

When the PRINTOUT=LONG option is specified the sequence of matrices b

p

and

the corresponding correlation matrices are printed The sequence of matricesb

p isused to compute Akaike information criteria for selection of the autoregressive order

of the process

Akaike Information Criterion

The Akaike information criterion or AIC is defined as -2( maximum of log likeli-

hood )+2(number of parameters) Since the vector autoregressive models are esti-

mates from the Yule-Walker equations not by maximum likelihood the exact likeli-

hood values are not available for computing the AIC However for the vector autore-

gressive model the maximum of the log likelihood can be approximated as

l n L

n

2

l n j

b

p

j

Thus the AIC for the order p model is computed as

A I C

p

= n l n j

b

p

j + 2 p r

2

You can use the printed AIC array to compute a likelihood ratio test of the autoregres-

sive order The log-likelihood ratio test statistic for testing the order p model against

the order p 1

model is

n l n j

b

p

j + n l n j

b

p 1

j

This quantity is asymptotically distributed as a

2 withr

2 degrees of freedom if the

series is autoregressive of order p 1

It can be computed from the AIC array as

A I C

p 1

A I C

p

+ 2 r

2

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Chapter 18 Details

You can evaluate the significance of these test statistics with the PROBCHI function

in a SAS DATA step or with a

2 table

Determining the Autoregressive Order

Although the autoregressive models can be used for prediction their primary value is

to aid in the selection of a suitable portion of the sample covariance matrix for use in

computing canonical correlations If the multivariate time series x

t is of autoregres-sive order p then the vector of past values to lag p is considered to contain essentially

all the information relevant for prediction of future values of the time series

By default PROC STATESPACE selects the order p producing the autoregressive

model with the smallestA I C

p

If the value p for the minimumA I C

p

is less than the

value of the PASTMIN= option then p is set to the PASTMIN= value Alternatively

you can use the ARMAX= and PASTMIN= options to force PROC STATESPACE to

use an order you select

Significance Limits for Partial Autocorrelations

The STATESPACE procedure prints a schematic representation of the partial autocor-

relation matrices indicating which partial autocorrelations are significantly greater orsignificantly less than 0 Figure 1810 shows an example of this table

The STATESPACE Procedure

Schematic Representation of Partial Autocorrelations

NameLag 1 2 3 4 5 6 7 8 9 10

x ++ +

y ++

+ is gt 2std error - is lt -2std error is between

Figure 1810 Significant Partial Autocorrelations

The partial autocorrelations are from the sample partial autoregressive matrices b

p

p

The standard errors used for the significance limits of the partial autocorrelations are

computed from the sequence of matrices

p

and

p

Under the assumption that the observed series arises from an autoregressive process

of order p 1

the pth sample partial autoregressive matrix b

p

p

has an asymptotic

variance matrix 1

n

1

p

p

The significance limits for b

p

p

used in the schematic plot of the sample partial autore-

gressive sequence are derived by replacing

p

and

p

with their sample estimators

to produce the variance estimate as follows

d

V a r

b

p

p

=

1

n r p

b

1

p

b

p

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Part 2 General Information

Canonical Correlation Analysis

Given the order p letp

t

be the vector of current and past values relevant to prediction

of x

t + 1

p

t

= x

0

t

x

0

t 1

x

0

t p

0

Letf

t

be the vector of current and future values

f

t

= x

0

t

x

0

t + 1

x

0

t + p

0

In the canonical correlation analysis consider submatrices of the sample covariance

matrix of p

t

andf

t

This covariance matrixV

has a block Hankel form

V =

2

6

6

6

4

C

0

C

0

1

C

0

2

C

0

p

C

0

1

C

0

2

C

0

3

C

0

p + 1

C

0

p

C

0

p + 1

C

0

p + 2

C

0

2 p

3

7

7

7

5

State Vector Selection Process

The canonical correlation analysis forms a sequence of potential state vectorsz

j

t

Examine a sequencef

j

t

of subvectors of f

t

and form the submatrixV

j consisting

of the rows and columns of V

corresponding to the components of f

j

t

and compute

its canonical correlations

The smallest canonical correlation of V

j is then used in the selection of the com-

ponents of the state vector The selection process is described in the following For

more details about this process refer to Akaike (1976)

In the following discussion the notationx

t + k j t

denotes the wide sense conditional

expectation (best linear predictor) of x

t + k

given allx

s

with s less than or equal to t

In the notationx

i t + 1

the first subscript denotes the ith component of x

t + 1

The initial state vectorz

1

t

is set tox

t

The sequencef

j

t

is initialized by setting

f

1

t

= z

1

0

t

x

1 t + 1 j t

0

= x

0

t

x

1 t + 1 j t

0

That is start by considering whether to addx

1 t + 1 j t

to the initial state vectorz

1

t

The procedure forms the submatrixV

1 corresponding tof

1

t

and computes its canon-

ical correlations Denote the smallest canonical correlation of V

1 as

m i n

If

m i n

is

significantly greater than 0x

1 t + 1 j t

is added to the state vector

If the smallest canonical correlation of V

1 is not significantly greater than 0 then a

linear combination of f

1

t

is uncorrelated with the pastp

t

Assuming that the determi-

nant of C

0

is not 0 (that is no input series is a constant) you can take the coefficient

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Chapter 18 Details

of x

1 t + 1 j t

in this linear combination to be 1 Denote the coefficients of z

1

t

in this

linear combination as

This gives the relationship

x

1 t + 1 j t

=

0

x

t

Therefore the current state vector already contains all the past information useful forpredicting

x

1 t + 1

and any greater leads of x

1 t

The variablex

1 t + 1 j t

is not added to

the state vector nor are any termsx

1 t + k j t

considered as possible components of the

state vector The variablex

1

is no longer active for state vector selection

The process described forx

1 t + 1 j t

is repeated for the remaining elements of f

t

The

next candidate for inclusion in the state vector is the next component of f

t

corre-

sponding to an active variable Components of f

t

corresponding to inactive variables

that produced a zero

m i n

in a previous step are skipped

Denote the next candidate asx

l t + k j t

The vectorf

j

t

is formed from the current state

vector andx

l t + k j t

as follows

f

j

t

= z

j

0

t

x

l t + k j t

0

The matrixV

j is formed fromf

j

t

and its canonical correlations are computed The

smallest canonical correlation of V

j is judged to be either greater than or equal to 0

If it is judged to be greater than 0x

l t + k j t

is added to the state vector If it is judged

to be 0 then a linear combination of f

j

t

is uncorrelated with thep

t

and the variable

x

l

is now inactive

The state vector selection process continues until no active variables remain

Testing Significance of Canonical Correlations For each step in the canonical correlation sequence the significance of the small-

est canonical correlation

m i n

is judged by an information criterion from Akaike

(1976) This information criterion is

n l n 1

2

m i n

r p + 1 q + 1

where q is the dimension of f

j

t

at the current step r is the order of the state vector p

is the order of the vector autoregressive process and

is the value of the SIGCORR=

option The default is SIGCORR=2 If this information criterion is less than or equal

to 0

m i n

is taken to be 0 otherwise it is taken to be significantly greater than 0

(Do not confuse this information criterion with the AIC)

Variables inx

t + p j t

are not added in the model even with positive information crite-

rion because of the singularity of V

You can force the consideration of more candi-

date state variables by increasing the size of theV

matrix by specifying a PASTMIN=

option value larger than p

Printing the Canonical Correlations

To print the details of the canonical correlation analysis process specify the CAN-

CORR option in the PROC STATESPACE statement The CANCORR option prints

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Part 2 General Information

the candidate state vectors the canonical correlations and the information criteria for

testing the significance of the smallest canonical correlation

Bartlettrsquos

2 and its degrees of freedom are also printed when the CANCORR option

is specified The formula used for Bartlettrsquos

2 is

2

= n 5 r p + 1 q + 1 l n 1

2

m i n

withr p + 1 q + 1

degrees of freedom

Figure 1811 shows the output of the CANCORR option for the introductory example

shown in the Getting Started section of this chapter

The STATESPACE Procedure

Canonical Correlations Analysis

Information Chi

x(TT) y(TT) x(T+1T) Criterion Square DF

1 1 0237045 3566167 114505 4

Information Chi

x(TT) y(TT) x(T+1T) y(T+1T) Criterion Square DF

1 1 0238244 0056565 -535906 0636134 3

Information Chi

x(TT) y(TT) x(T+1T) x(T+2T) Criterion Square DF

1 1 0237602 0087493 -446312 1525353 3

Figure 1811 Canonical Correlations Analysis

New variables are added to the state vector if the information criteria are positive Inthis example

y

t + 1 j t

andx

t + 2 j t

are not added to the state space vector because the

information criteria for these models are negative

If the information criterion is nearly 0 then you may want to investigate models

that arise if the opposite decision is made regarding

m i n

This investigation can be

accomplished by using a FORM statement to specify part or all of the state vector

Preliminary Estimates of F

When a candidate variablex

l t + k j t

yields a zero

m i n

and is not added to the state

vector a linear combination of f

j

t

is uncorrelated with thep

t

Because of the method

used to construct thef

j

t

sequence the coefficient of x

l t + k j t

inl

can be taken as 1

Denote the coefficients of z

j

t

in this linear combination as l

This gives the relationship

x

l t + k j t

= l

0

z

j

t

The vectorl

is used as a preliminary estimate of the first r columns of the row of the

transition matrixF

corresponding tox

l t + k 1 j t

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Chapter 18 Details

Parameter Estimation

The model isz

t + 1

= F z

t

+ G e

t + 1

wheree

t

is a sequence of independent multivari-

ate normal innovations with mean vector 0 and variance

e e

The observed sequence

x

t

composes the first r components of z

t

and thusx

t

= H z

t

where H is ther s

matrix I

r

0

LetE

be ther n

matrix of innovations

E = e

1

e

n

If the number of observations n is reasonably large the log likelihood L can be

approximated up to an additive constant as follows

L =

n

2

l n j

e e

j

1

2

t r a c e

1

e e

E E

0

The elements of

e e

are taken as free parameters and are estimated as follows

S

0

=

1

n

E E

0

Replacing

e e

byS

0

in the likelihood equation the log likelihood up to an additive

constant is

L =

n

2

l n j S

0

j

Letting B be the backshift operator the formal relation betweenx

t

ande

t

is

x

t

= H I B F

1

G e

t

e

t

= H I B F

1

G

1

x

t

=

1

X

i = 0

i

x

t i

LettingC

i

be the ith lagged sample covariance of x

t

and neglecting end effects the

matrixS

0

is

S

0

=

1

X

i j = 0

i

C

i + j

0

j

For the computation of S

0

the infinite sum is truncated at the value of the KLAG=

option The value of the KLAG= option should be large enough that the sequence

i

is approximately 0 beyond that point

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Part 2 General Information

Let

be the vector of free parameters in theF

andG

matrices The derivative of the

log likelihood with respect to the parameter

is

L

=

n

2

t r a c e

S

1

0

S

0

The second derivative is

2

L

0

=

n

2

t r a c e

S

1

0

S

0

0

S

1

0

S

0

t r a c e

S

1

0

2

S

0

0

Near the maximum the first term is unimportant and the second term can be approx-

imated to give the following second derivative approximation

2

L

0

=

n t r a c e

S

1

0

E

E

0

0

The first derivative matrix and this second derivative matrix approximation are com-

puted from the sample covariance matrixC

0

and the truncated sequence

i

The

approximate likelihood function is maximized by a modified Newton-Raphson algo-

rithm employing these derivative matrices

The matrixS

0

is used as the estimate of the innovation covariance matrix

e e

The

negative of the inverse of the second derivative matrix at the maximum is used as an

approximate covariance matrix for the parameter estimates The standard errors of

the parameter estimates printed in the parameter estimates tables are taken from the

diagonal of this covariance matrix The parameter covariance matrix is printed when

the COVB option is specified

If the data are nearly nonstationary a better estimate of

e e

and the other parameters

can sometimes be obtained by specifying the RESIDEST option The RESIDEST

option estimates the parameters using conditional least squares instead of maximum

likelihood

The residuals are computed using the state space equation and the sample mean val-

ues of the variables in the model as start-up values The estimate of S

0

is then com-

puted using the residuals from the ith observation on where i is the maximum number

of times any variable occurs in the state vector A multivariate Gauss-Marquardt al-

gorithm is used to minimizej S

0

j

Refer to Harvey (1981a) for a further description

of this method

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Chapter 18 Details

Forecasting

Given estimates of F

G

and

e e

forecasts of x

t

are computed from the conditional

expectation of z

t

In forecasting the parameters F G and

e e

are replaced with the estimates or by val-

ues specified in the RESTRICT statement One-step-ahead forecasting is performedfor the observation

x

t

wheret n b

Heren

is the number of observations and

b is the value of the BACK= option For the observationx

t

wheret n b

m-

step-ahead forecasting is performed form = t n + b

The forecasts are generated

recursively with the initial conditionz

0

= 0

The m-step-ahead forecast of z

t + m

isz

t + m j t

wherez

t + m j t

denotes the conditional

expectation of z

t + m

given the information available at time t The m-step-ahead

forecast of x

t + m

isx

t + m j t

= H z

t + m j t

where the matrixH = I

r

0

Let

i

= F

i

G

Note that the lasts r

elements of z

t

consist of the elements of x

u j t

foru t

The state vector z

t + m

can be represented as

z

t + m

= F

m

z

t

+

m 1

X

i = 0

i

e

t + m i

Sincee

t + i j t

= 0

fori 0

the m-step-ahead forecastz

t + m j t

is

z

t + m j t

= F

m

z

t

= F z

t + m 1 j t

Therefore the m-step-ahead forecast of x

t + m

is

x

t + m j t

= H z

t + m j t

The m-step-ahead forecast error is

z

t + m

z

t + m j t

=

m 1

X

i = 0

i

e

t + m i

The variance of the m-step-ahead forecast error is

V

z m

=

m 1

X

i = 0

i

e e

0

i

LettingV

z 0

= 0

the variance of the m-step-ahead forecast error of z

t + m

V

z m

can

be computed recursively as follows

V

z m

= V

z m 1

+

m 1

e e

0

m 1

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Part 2 General Information

The variance of the m-step-ahead forecast error of x

t + m

is ther r

left upper sub-

matrix of V

z m

that is

V

x m

= H V

z m

H

0

Unless the NOCENTER option is specified the sample mean vector is added to the

forecast When differencing is specified the forecasts xt + m j t

plus the sample mean

vector are integrated back to produce forecasts for the original series

Lety

t

be the original series specified by the VAR statement with some 0 values

appended corresponding to the unobserved past observations Let B be the backshift

operator and let B

be thes s

matrix polynomial in the backshift operator

corresponding to the differencing specified by the VAR statement The off-diagonal

elements of

i

are 0 Note that

0

= I

s

whereI

s

is thes s

identity matrix Then

z

t

= B y

t

This gives the relationship

y

t

=

1

B z

t

=

1

X

i = 0

i

z

t i

where

1

B =

P

1

i = 0

i

B

i and

0

= I

s

The m-step-ahead forecast of y

t + m

is

y

t + m j t

=

m 1

X

i = 0

i

z

t + m i j t

+

1

X

i = m

i

z

t + m i

The m-step-ahead forecast error of y

t + m

is

m 1

X

i = 0

i

z

t + m i

z

t + m i j t

=

m 1

X

i = 0

i

X

u = 0

u

i u

e

t + m i

LettingV

y 0

= 0

the variance of the m-step-ahead forecast error of y

t + m

V

y m

is

V

y m

=

m 1

X

i = 0

i

X

u = 0

u

i u

e e

i

X

u = 0

u

i u

0

= V

y m 1

+

m 1

X

u = 0

u

m 1 u

e e

m 1

X

u = 0

u

m 1 u

0

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Chapter 18 Details

Relation of ARMA and State Space Forms

Every state space model has an ARMA representation and conversely every ARMA

model has a state space representation This section discusses this equivalence The

following material is adapted from Akaike (1974) where there is a more complete

discussion Pham-Dinh-Tuan (1978) also contains a discussion of this material

Suppose you are given the following ARMA model

B x

t

= B e

t

or in more detail

x

t

1

x

t 1

p

x

t p

= e

t

+

1

e

t 1

+ +

q

e

t q

(1)

wheree

t

is a sequence of independent multivariate normal random vectors with mean

0 and variance matrix

e e

B is the backshift operator (B x

t

= x

t 1

) B

and

B

are matrix polynomials in B andx

t

is the observed process

If the roots of the determinantial equationj B j = 0

are outside the unit circle in

the complex plane the model can also be written as

x

t

=

1

B B e

t

=

1

X

i = 0

i

e

t i

The

i

matrices are known as the impulse response matrices and can be computed

as

1

B B

You can assume p q

since if this is not initially true you can add more terms

i

that are identically 0 without changing the model

To write this set of equations in a state space form proceed as follows Letx

t + i j t

be

the conditional expectation of x

t + i

givenx

w

forw t

The following relations hold

x

t + i j t

=

1

X

j = i

j

e

t + i j

x

t + i j t + 1

= x

t + i j t

+

i 1

e

t + 1

However from equation (1) you can derive the following relationship

x

t + p j t

=

1

x

t + p 1 j t

+ +

p

x

t

(2)

Hence wheni = p

you can substitute forx

t + p j t

in the right-hand side of equation

(2) and close the system of equations

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Part 2 General Information

This substitution results in the following model in the state space form

z

t + 1

= F z

t

+ G e

t + 1

2

6

6

6

4

x

t + 1

x

t + 2 j t + 1

x

t + p j t + 1

3

7

7

7

5

=

2

6

6

4

0 I 0 0

0 0 I 0

p

p 1

1

3

7

7

5

2

6

6

6

4

x

t

x

t + 1 j t

x

t + p 1 j t

3

7

7

7

5

+

2

6

6

4

I

1

p 1

3

7

7

5

e

t + 1

Note that the state vectorz

t

is composed of conditional expectations of x

t

and the

first r components of z

t

are equal tox

t

The state space form can be cast into an ARMA form by solving the system of dif-

ference equations for the first r components

When converting from an ARMA form to a state space form you can generate a

state vector larger than needed that is the state space model may not be a minimal

representation When going from a state space form to an ARMA form you can have

nontrivial common factors in the autoregressive and moving average operators that

yield an ARMA model larger than necessary

If the state space form used is not a minimal representation some but not all compo-

nents of x

t + i j t

may be linearly dependent This situation corresponds to

p

p 1

being of less than full rank when B

and B

have no common nontrivial

left factors In this casez

t

consists of a subset of the possible components of

x

t + i j t

i = 1 2 p 1

However once a component of x

t + i j t

(for example the

jth one) is linearly dependent on the previous conditional expectations then all sub-

sequent jth components of x

t + k j t

fork i

must also be linearly dependent Note

that in this case equivalent but seemingly different structures can arise if the order of

the components withinx

t

is changed

OUT= Data Set

The forecasts are contained in the output data set specified by the OUT= option on the

PROC STATESPACE statement The OUT= data set contains the following variables

the BY variables

the ID variable

the VAR statement variables These variables contain the actual values from

the input data set

FORi numeric variables containing the forecasts The variable FORi contains

the forecasts for the ith variable in the VAR statement list Forecasts are one-

step-ahead predictions until the end of the data or until the observation specified

by the BACK= option

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Chapter 18 Details

RESi numeric variables containing the residual for the forecast of the ith vari-

able in the VAR statement list For forecast observations the actual values are

missing and the RESi variables contain missing values

STDi numeric variables containing the standard deviation for the forecast of

the ith variable in the VAR statement list The values of the STDi variables can

be used to construct univariate confidence limits for the corresponding fore-casts However such confidence limits do not take into account the covariance

of the forecasts

OUTAR= Data Set

The OUTAR= data set contains the estimates of the preliminary autoregressive mod-

els The OUTAR= data set contains the following variables

ORDER a numeric variable containing the order p of the autoregressive model

that the observation represents

AIC a numeric variable containing the value of the information criterionA I C

p

SIGFl numeric variables containing the estimate of the innovation covariance

matrices for the forward autoregressive models The variable SIGFl contains

the lth column of b

p

in the observations with ORDER= p

SIGBl numeric variables containing the estimate of the innovation covariance

matrices for the backward autoregressive models The variable SIGBl contains

the lth column of b

p

in the observations with ORDER= p

FORk ndashl numeric variables containing the estimates of the autoregressive pa-

rameter matrices for the forward models The variable FORk ndashl contains the

lth column of the lag k autoregressive parameter matrix b

p

k

in the observations

with ORDER= p

BACk ndashl numeric variables containing the estimates of the autoregressive pa-

rameter matrices for the backward models The variable BACk ndashl contains the

lth column of the lag k autoregressive parameter matrix b

p

k

in the observations

with ORDER= p

The estimates for the order p autoregressive model can be selected as those observa-

tions with ORDER= p Within these observations the k lth element of

p

i

is given by

the value of the FORindashl variable in the k th observation The k lth element of

p

i

is

given by the value of BACindashl variable in the k th observation The k lth element of

p

is given by SIGFl in the k th observation The k lth element of

p

is given by SIGBlin the k th observation

Table 181 shows an example of the OUTAR= data set with ARMAX=3 andx

t

of

dimension 2 In Table 181 i j

indicate the ijth element of the matrix

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Part 2 General Information

Table 181 Values in the OUTAR= Data Set

Obs ORDER AIC SIGF1 SIGF2 SIGB1 SIGB2 FOR1ndash 1 FOR1ndash 2 FOR2ndash 1 FOR2ndash 2 FOR3ndash 1

1 0 AIC0

0 1 1

0 1 2

0 1 1

0 1 2

2 0 AIC0

0 2 1

0 2 2

0 2 1

0 2 2

3 1 AIC1

1 1 1

1 1 2

1 1 1

1 1 2

1

1

1 1

1

1

1 2

4 1 AIC1

1 2 1

1 1 2

1 2 1

1 1 2

1

1

2 1

1

1

2 2

5 2 AIC2

2 1 1

2 1 2

2 1 1

2 1 2

2

1

1 1

2

1

1 2

2

2

1 1

2

2

1 2

6 2 AIC2

2 2 1

2 1 2

2 2 1

2 1 2

2

1

2 1

2

1

2 2

2

2

2 1

2

2

2 2

7 3 AIC3

3 1 1

3 1 2

3 1 1

3 1 2

3

1

1 1

3

1

1 2

3

2

1 1

3

2

1 2

3

3

1 1

8 3 AIC3

3 2 1

3 1 2

3 2 1

3 1 2

3

1

2 1

3

1

2 2

3

2

2 1

3

2

2 2

3

3

2 1

Obs FOR3ndash 2 BACK1ndash 1 BACK1ndash 2 BACK2ndash 1 BACK2ndash 2 BACK3ndash 1 BACK3ndash 2

1

2

3

1

1

1 1

1

1

1 2

4

1

1

2 1

1

1

2 2

5

2

1

1 1

2

1

1 2

2

2

1 1

2

2

1 2

6

2

1

2 1

2

1

2 2

2

2

2 1

2

2

2 2

7

3

3

1 2

3

1

1 1

3

1

1 2

3

2

1 1

3

2

1 2

3

3

1 1

3

3

1 2

8

3

3

2 2

3

1

2 1

3

1

2 2

3

2

2 1

3

2

2 2

3

3

2 1

3

3

2 2

The estimated autoregressive parameters can be used in the IML procedure to obtain

autoregressive estimates of the spectral density function or forecasts based on the

autoregressive models

OUTMODEL= Data Set

The OUTMODEL= data set contains the estimates of theF

andG

matrices and their

standard errors the names of the components of the state vector and the estimates of

the innovation covariance matrix The variables contained in the OUTMODEL= data

set are as follows

the BY variables

STATEVEC a character variable containing the name of the component of the

state vector corresponding to the observation The STATEVEC variable has

the value STD for standard deviations observations which contain the standarderrors for the estimates given in the preceding observation

Fndash j numeric variables containing the columns of theF

matrix The variable

Fndash j contains the jth column of F

The number of Fndash j variables is equal to

the value of the DIMMAX= option If the model is of smaller dimension the

extraneous variables are set to missing

Gndash j numeric variables containing the columns of theG

matrix The variable

Gndash j contains the jth column of G

The number of Gndash j variables is equal to r

the dimension of x

t

given by the number of variables in the VAR statement

SIGndash j numeric variables containing the columns of the innovation covariance

matrix The variable SIGndash j contains the jth column of

e e

There are r vari-ables SIGndash j

Table 182 shows an example of the OUTMODEL= data set withx

t

= x

t

y

t

0

z

t

= x

t

y

t

x

t + 1 j t

0 and DIMMAX=4 In Table 182F

i j

andG

i j

are the ijth

elements of F

andG

respectively Note that all elements for Fndash4 are missing because

F

is a3 3

matrix

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Chapter 18 Details

Table 182 Value in the OUTMODEL= Data Set

Obs STATEVEC Fndash 1 Fndash 2 Fndash 3 Fndash 4 Gndash 1 Gndash 2 SIGndash 1 SIGndash 2

1 X(TT) 0 0 1 1 0

1 1

1 2

2 STD

3 Y(TT) F2 1

F2 2

F2 3

0 1

2 1

2 2

4 STD std F2 1

std F2 2

std F2 3

5 X(T+1T) F3 1

F3 2

F3 3

G3 1

G3 2

6 STD std F3 1

std F3 2

std F3 3

std G3 1

std G3 2

Printed Output

The printed output produced by the STATESPACE procedure is described in the fol-

lowing

1 descriptive statistics which include the number of observations used the

names of the variables their means and standard deviations (Std) and the dif-

ferencing operations used

2 the Akaike information criteria for the sequence of preliminary autoregressive

models

3 if the PRINTOUT=LONG option is specified the sample autocovariance ma-

trices of the input series at various lags

4 if the PRINTOUT=LONG option is specified the sample autocorrelation ma-

trices of the input series

5 a schematic representation of the autocorrelation matrices showing the signif-

icant autocorrelations

6 if the PRINTOUT=LONG option is specified the partial autoregressive ma-

trices (These are

p

p

as described in Preliminary Autoregressive Modelsearlier in this chapter)

7 a schematic representation of the partial autocorrelation matrices showing the

significant partial autocorrelations

8 the Yule-Walker estimates of the autoregressive parameters for the autoregres-

sive model with the minimum AIC

9 if the PRINTOUT=LONG option is specified the autocovariance matrices of

the residuals of the minimum AIC model This is the sequence of estimated

innovation variance matrices for the solutions of the Yule-Walker equations

10 if the PRINTOUT=LONG option is specified the autocorrelation matrices of the residuals of the minimum AIC model

11 If the CANCORR option is specified the canonical correlations analysis for

each potential state vector considered in the state vector selection process

This includes the potential state vector the canonical correlations the informa-

tion criterion for the smallest canonical correlation Bartlettrsquos

2 statistic (ldquoChi

Squarerdquo) for the smallest canonical correlation and the degrees of freedom of

Bartlettrsquos

2

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Part 2 General Information

12 the components of the chosen state vector

13 the preliminary estimate of the transition matrixF

the input matrixG

and

the variance matrix for the innovations

e e

14 if the ITPRINT option is specified the iteration history of the likelihood max-

imization For each iteration this shows the iteration number the number of step halvings the determinant of the innovation variance matrix the damping

factor Lambda and the values of the parameters

15 the state vector printed again to aid interpretation of the following listing of F

andG

16 the final estimate of the transition matrixF

17 the final estimate of the input matrixG

18 the final estimate of the variance matrix for the innovations

e e

19 a table listing the estimates of the free parameters inF

andG

and their standarderrors and t statistics

20 if the COVB option is specified the covariance matrix of the parameter esti-

mates

21 if the COVB option is specified the correlation matrix of the parameter esti-

mates

22 if the PRINT option is specified the forecasts and their standard errors

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Chapter 18 Details

ODS Table Names

PROC STATESPACE assigns a name to each table it creates You can use these

names to reference the table when using the Output Delivery System (ODS) to select

tables and create output data sets These names are listed in the following table For

more information on ODS see Chapter 6 ldquoUsing the Output Delivery Systemrdquo

Table 183 ODS Tables Produced in PROC STATESPACE

ODS Table Name Description Option

NObs Number of observations default

Summary Simple summary statistics table default

InfoCriterion Information criterion table default

CovLags Covariance Matrices of Input Series PRINTOUT=LONG

CorrLags Correlation Matrices of Input Series PRINTOUT=LONG

PartialAR Partial Autoregressive Matrices PRINTOUT=LONG

YWEstimates Yule-Walker Estimates for Minimum AIC default

CovResiduals Covariance of Residuals PRINTOUT=LONGCorrResiduals Residual Correlations from AR Models PRINTOUT=LONG

StateVector State vector table default

CorrGraph Schematic Representation of Correlations default

TransitionMatrix Transition Matrix default

InputMatrix Input Matrix default

VarInnov Variance Matrix for the Innovation default

CovB Covariance of Parameter Estimates COVB

CorrB Correlation of Parameter Estimates COVB

CanCorr Canonical Correlation Analysis CANCORR

IterHistory Iterative Fitting table ITPRINT

ParameterEstimates Parameter Estimates Table default

Forecasts Forecasts Table PRINT

ConvergenceStatus Convergence Status Table default

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Part 2 General Information

Example

Example 181 Series J from Box and Jenkins

This example analyzes the gas furnace data (series J) from Box and Jenkins (The

data are not shown Refer to Box and Jenkins (1976) for the data)

First a model is selected and fit automatically using the following statements

title1 rsquoGas Furnace Datarsquo

title2 rsquoBox amp Jenkins Series Jrsquo

title3 rsquoAutomatically Selected Modelrsquo

proc statespace data=seriesj cancorr

var x y

run

The results for the automatically selected model are shown in Output 1811

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Chapter 18 Example

Output 1811 Results for Automatically Selected Model

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Number of Observations 296

Standard

Variable Mean Error

x -005683 1072766

y 5350912 3202121

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Information Criterion for Autoregressive Models

Lag=0 Lag=1 Lag=2 Lag=3 Lag=4 Lag=5 Lag=6 Lag=7 Lag=8

6513862 -103357 -163296 -164512 -165152 -164891 -164934 -164315 -163856

Information

Criterion for

Autoregressive

Models

Lag=9 Lag=10

-16348 -163359

Schematic Representation of Correlations

NameLag 0 1 2 3 4 5 6 7 8 9 10

x +- +- +- +- +- +- +- +- +- +- +-

y -+ -+ -+ -+ -+ -+ -+ -+ -+ -+ -+

+ is gt 2std error - is lt -2std error is between

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Part 2 General Information

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Schematic Representation of Partial Autocorrelations

NameLag 1 2 3 4 5 6 7 8 9 10

x + - + -

y -+ -- - + +

+ is gt 2std error - is lt -2std error is between

Yule-Walker Estimates for Minimum AIC

------Lag=1------ ------Lag=2------ ------Lag=3------ ------Lag=4------

x y x y x y x y

x 1925887 -000124 -120166 0004224 0116918 -000867 0104236 0003268

y 0050496 1299793 -002046 -03277 -071182 -025701 0195411 0133417

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Canonical Correlations Analysis

Information Chi

x(TT) y(TT) x(T+1T) Criterion Square DF

1 1 0804883 2929228 3047481 8

Information Chix(TT) y(TT) x(T+1T) y(T+1T) Criterion Square DF

1 1 0906681 0607529 1223358 1347237 7

Information Chi

x(TT) y(TT) x (T+1T) y (T+1T) x (T+2T) Criterion Square DF

1 1 0909434 0610278 0186274 -154701 1034705 6

Information Chi

x(TT) y(TT) x (T+1T) y (T+1T) y (T+2T) Criterion Square DF

1 1 091014 0618937 0206823 0940392 1280924 6

Information Chi

x(TT) y(TT) x(T+1T) y(T+1T) y(T+2T) y(T+3T) Criterion Square DF

1 1 0912963 0628785 0226598 0083258 -794103 2041584 5

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Chapter 18 Example

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Selected Statespace Form and Preliminary Estimates

State Vector

x(TT) y(TT) x(T+1T) y(T+1T) y(T+2T)

Estimate of Transition Matrix

0 0 1 0 0

0 0 0 1 0

-084718 0026794 1711715 -005019 0

0 0 0 0 1

-019785 0334274 -018174 -123557 1787475

Input Matrix for Innovation

1 0

0 11925887 -000124

0050496 1299793

0142421 1361696

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Selected Statespace Form and Preliminary Estimates

Variance Matrix for Innovation

0035274 -000734

-000734 0097569

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Part 2 General Information

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Selected Statespace Form and Fitted Model

State Vector

x(TT) y(TT) x(T+1T) y(T+1T) y(T+2T)

Estimate of Transition Matrix

0 0 1 0 0

0 0 0 1 0

-086192 0030609 1724235 -005483 0

0 0 0 0 1

-034839 0292124 -009435 -109823 1671418

Input Matrix for Innovation

1 0

0 1192442 -000416

0015621 1258495

008058 1353204

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Selected Statespace Form and Fitted Model

Variance Matrix for Innovation

0035579 -000728

-000728 0095577

Parameter Estimates

Standard

Parameter Estimate Error t Value

F(31) -086192 0072961 -1181

F(32) 0030609 0026167 117

F(33) 1724235 0061599 2799

F(34) -005483 0030169 -182

F(51) -034839 0135253 -258

F(52) 0292124 0046299 631

F(53) -009435 0096527 -098F(54) -109823 0109525 -1003

F(55) 1671418 0083737 1996

G(31) 1924420 0058162 3309

G(32) -000416 0035255 -012

G(41) 0015621 0095771 016

G(42) 1258495 0055742 2258

G(51) 0080580 0151622 053

G(52) 1353204 0091388 1481

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Chapter 18 Example

The two series are believed to have a transfer function relation with the gas rate

(variable X) as the input and the CO2

concentration (variable Y) as the output Since

the parameter estimates shown in Output 1811 support this kind of model the model

is reestimated with the feedback parameters restricted to 0 The following statements

fit the transfer function (no feedback) model

title3 rsquoTransfer Function Modelrsquo

proc statespace data=seriesj printout=none

var x y

restrict f(32)=0 f(34)=0

g(32)=0 g(41)=0 g(51)=0

run

The last 2 pages of the output are shown in Output 1812

Output 1812 STATESPACE Output for Transfer Function Model

Gas Furnace Data

Box amp Jenkins Series J

Transfer Function Model

The STATESPACE Procedure

Selected Statespace Form and Fitted Model

State Vector

x(TT) y(TT) x(T+1T) y(T+1T) y(T+2T)

Estimate of Transition Matrix

0 0 1 0 0

0 0 0 1 0

-068882 0 1598717 0 0

0 0 0 0 1

-035944 0284179 -00963 -107313 1650047

Input Matrix for Innovation

1 0

0 1

1923446 0

0 1260856

0 1346332

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Part 2 General Information

Gas Furnace Data

Box amp Jenkins Series J

Transfer Function Model

The STATESPACE Procedure

Selected Statespace Form and Fitted Model

Variance Matrix for Innovation

0036995 -00072

-00072 0095712

Parameter Estimates

Standard

Parameter Estimate Error t Value

F(31) -068882 0050549 -1363

F(33) 1598717 0050924 3139

F(51) -035944 0229044 -157

F(52) 0284179 0096944 293

F(53) -009630 0140876 -068

F(54) -107313 0250385 -429

F(55) 1650047 0188533 875G(31) 1923446 0056328 3415

G(42) 1260856 0056464 2233

G(52) 1346332 0091086 1478

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Chapter 18 References

References

Akaike H (1974) Markovian Representation of Stochastic Processes and Its Ap-

plication to the Analysis of Autoregressive Moving Average Processes Annals

of the Institute of Statistical Mathematics 26 363-387

Akaike H (1976) Canonical Correlations Analysis of Time Series and the Use of

an Information Criterion in Advances and Case Studies in System Identification

eds R Mehra and DG Lainiotis New York Academic Press

Anderson TW (1971) The Statistical Analysis of Time Series New York John

Wiley amp Sons

Ansley CF and Newbold P (1979) Multivariate Partial Autocorrelations Pro-

ceedings of the Business and Economic Statistics Section American Statistical

Association 349-353

Box GEP and Jenkins G (1976) Time Series Analysis Forecasting and Control

San Francisco Holden-Day

Brockwell PJ and Davis RA (1991) Time Series Theory and Methods 2nd Edi-

tion Springer-Verlag

Hannan EJ (1970) Multiple Time Series New York John Wiley amp Sons

Hannan EJ (1976) The Identification and Parameterization of ARMAX and State

Space Forms Econometrica 44 713-722

Harvey AC (1981a) The Econometric Analysis of Time Series New York John

Wiley amp Sons

Harvey AC (1981b) Time Series Models New York John Wiley amp Sons

Jones RH (1974) Identification and Autoregressive Spectrum Estimation IEEE Transactions on Automatic Control AC-19 894-897

Pham-Dinh-Tuan (1978) On the Fitting of Multivariate Processes of the

Autoregressive-Moving Average Type Biometrika 65 99-107

Priestley MB (1980) System Identification Kalman Filtering and Stochastic Con-

trol in Directions in Time Series eds DR Brillinger and GC Tiao Institute

of Mathematical Statistics

Whittle P (1963) On the Fitting of Multivariate Autoregressions and the Approxi-

mate Canonical Factorization of a Spectral Density Matrix Biometrika 50 129-

134

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The correct bibliographic citation for this manual is as follows SAS Institute Inc SAS

ET S U serrsquos Gui de Version 8 Cary NC SAS Institute Inc 1999 1546 pp

SASET S Userrsquos Guide Version 8

Copyright copy 1999 by S AS I ns t it u t e I nc Ca ry N C U S A

ISBN 1ndash58025ndash489ndash6

All r ight s res erved P r int ed in t he U n it ed St a t es of A merica N o pa rt of t his p u blica t ionma y b e reprodu ced s t ored in a ret r ieva l s ys t em or t ra ns mit t ed in a ny form or b y a nymeans electronic mechanical photocopying or otherwise without the prior writtenpermission of the publisher SAS Institute Inc

US Government Restricted Rights Notice Use du plicat ion or disclosure of the

software by the government is subject to restrictions as set forth in FAR 52227ndash19Commercial Computer Softw ar e-Restricted Rights (J une 1987)

SAS Institute Inc SAS Campus Drive Cary North Carolina 27513

1st print ing October 1999

S AS reg a nd a ll ot her SA S I ns t it u t e I nc p rodu ct or s ervice na m es a re regis t ered t ra dema rkst d k f SA S I t it t I i t h U S A d t h t i reg i di t U SA

Page 18: Statespace Model

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Part 2 General Information

Description Statement Option

print the details of the iterations PROC STATESPACE ITPRINT

specify an upper limit of the number of lags PROC STATESPACE KLAG=

specify maximum number of iterations

allowed

PROC STATESPACE MAXIT=

suppress the final estimation PROC STATESPACE NOEST

write the state space model parameter esti-

mates to an output data set

PROC STATESPACE OUTMODEL=

use conditional least squares for final estimates PROC STATESPACE RESIDEST

specify criterion for testing for singularity PROC STATESPACE SINGULAR=

Options for Forecasting

start forecasting before end of the input data PROC STATESPACE BACK=

specify the time interval between observations PROC STATESPACE INTERVAL=

specify multiple periods in the time series PROC STATESPACE INTPER=specify how many periods to forecast PROC STATESPACE LEAD=

specify the output data set for forecasts PROC STATESPACE OUT=

print forecasts PROC STATESPACE PRINT

Options to Specify the State Space Model

specify the state vector FORM

specify the parameter values RESTRICT

BY Groups

specify BY-group processing BY

Printing

suppresses all printed output NOPRINT

PROC STATESPACE Statement

PROC STATESPACE options

The following options can be specified in the PROC STATESPACE statement

Printing Options NOPRINT

suppresses all printed output

Input Data Options

DATA= SAS-data-set

specifies the name of the SAS data set to be used by the procedure If the DATA=

option is omitted the most recently created SAS data set is used

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Chapter 18 Syntax

LAGMAX= k

specifies the number of lags for which the sample autocovariance matrix is computed

The LAGMAX= option controls the number of lags printed in the schematic repre-

sentation of the autocorrelations

The sample autocovariance matrix of lag i denoted asC

i

is computed as

C

i

=

1

N 1

N

X

t = 1 + i

x

t

x

0

t i

wherex

t

is the differenced and centered data andN

is the number of observations

(If the NOCENTER option is specified 1 is not subtracted fromN

) LAGMAX= k

specifies thatC

0

throughC

k

are computed The default is LAGMAX=10

NOCENTER

prevents subtraction of the sample mean from the input series (after any specified

differencing) before the analysis

Options for Preliminary Autoregressive Models ARMAX= n

specifies the maximum order of the preliminary autoregressive models The AR-

MAX= option controls the autoregressive orders for which information criteria are

printed and controls the number of lags printed in the schematic representation of

partial autocorrelations The default is ARMAX=10 See Preliminary Autoregres-

sive Models later in this chapter for details

MINIC

writes to the OUTAR= data set only the preliminary Yule-Walker estimates for the

VAR model producing the minimum AIC See OUTAR= Data Set later in this chap-

ter for details

OUTAR= SAS-data-set

writes the Yule-Walker estimates of the preliminary autoregressive models to a SAS

data set See OUTAR= Data Set later in this chapter for details

PRINTOUT= SHORT | LONG | NONE

determines the amount of detail printed PRINTOUT=LONG prints the lagged co-

variance matrices the partial autoregressive matrices and estimates of the resid-

ual covariance matrices from the sequence of autoregressive models PRINT-

OUT=NONE suppresses the output for the preliminary autoregressive models The

descriptive statistics and state space model estimation output are still printed when

PRINTOUT=NONE is specified PRINTOUT=SHORT is the default

Canonical Correlation Analysis Options

CANCORR

prints the canonical correlations and information criterion for each candidate state

vector considered See Canonical Correlation Analysis later in this chapter for

details

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Part 2 General Information

DIMMAX= n

specifies the upper limit to the dimension of the state vector The DIMMAX= option

can be used to limit the size of the model selected The default is DIMMAX=10

PASTMIN= n

specifies the minimum number of lags to include in the canonical correlation analy-

sis The default is PASTMIN=0 See Canonical Correlation Analysis later in thischapter for details

SIGCORR= value

specifies the multiplier of the degrees of freedom for the penalty term in the informa-

tion criterion used to select the state space form The default is SIGCORR=2 The

larger the value of the SIGCORR= option the smaller the state vector tends to be

Hence a large value causes a simpler model to be fit See Canonical Correlations

Analysis later in this chapter for details

State Space Model Estimation Options

COVB

prints the inverse of the observed information matrix for the parameter estimatesThis matrix is an estimate of the covariance matrix for the parameter estimates

DETTOL= value

specifies the convergence criterion The DETTOL= and PARMTOL= option values

are used together to test for convergence of the estimation process If during an

iteration the relative change of the parameter estimates is less than the PARMTOL=

value and the relative change of the determinant of the innovation variance matrix

is less than the DETTOL= value then iteration ceases and the current estimates are

accepted The default is DETTOL=1E-5

ITPRINT

prints the iterations during the estimation process

KLAG= n

sets an upper limit for the number of lags of the sample autocovariance matrix used

in computing the approximate likelihood function If the data have a strong moving

average character a larger KLAG= value may be necessary to obtain good estimates

The default is KLAG=15 See Parameter Estimation later in this chapter for details

MAXIT= n

sets an upper limit to the number of iterations in the maximum likelihood or condi-

tional least-squares estimation The default is MAXIT=50

NOEST

suppresses the final maximum likelihood estimation of the selected model

OUTMODEL= SAS-data-set

writes the parameter estimates and their standard errors to a SAS data set See OUT-

MODEL= Data Set later in this chapter for details

PARMTOL= value

specifies the convergence criterion The DETTOL= and PARMTOL= option values

are used together to test for convergence of the estimation process If during an

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Chapter 18 Syntax

iteration the relative change of the parameter estimates is less than the PARMTOL=

value and the relative change of the determinant of the innovation variance matrix

is less than the DETTOL= value then iteration ceases and the current estimates are

accepted The default is PARMTOL=001

RESIDEST

computes the final estimates using conditional least squares on the raw data Thistype of estimation may be more stable than the default maximum likelihood method

but is usually more computationally expensive See Parameter Estimation later in

this chapter for details of the conditional least squares method

SINGULAR= value

specifies the criterion for testing for singularity of a matrix A matrix is declared

singular if a scaled pivot is less than the SINGULAR= value when sweeping the

matrix The default is SINGULAR=1E-7

Forecasting Options

BACK= n

starts forecasting n periods before the end of the input data The BACK= option valuemust not be greater than the number of observations The default is BACK=0

INTERVAL= interval

specifies the time interval between observations The INTERVAL= value is used

in conjunction with the ID variable to check that the input data are in order and

have no missing periods The INTERVAL= option is also used to extrapolate the ID

values past the end of the input data See Chapter 3 ldquoDate Intervals Formats and

Functionsrdquo for details on the INTERVAL= values allowed

INTPER= n

specifies that each input observation corresponds to n time periods For example

the options INTERVAL=MONTH and INTPER=2 specify bimonthly data and are

equivalent to specifying INTERVAL=MONTH2 If the INTERVAL= option is not

specified the INTPER= option controls the increment used to generate ID values for

the forecast observations The default is INTPER=1

LEAD= n

specifies how many forecast observations are produced The forecasts start at the

point set by the BACK= option The default is LEAD=0 which produces no fore-

casts

OUT= SAS-data-set

writes the residuals actual values forecasts and forecast standard errors to a SAS

data set See OUT= Data Set later in this chapter for details

PRINT

prints the forecasts

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Part 2 General Information

BY Statement

BY variable

A BY statement can be used with the STATESPACE procedure to obtain separateanalyses on observations in groups defined by the BY variables

FORM Statement

FORM variable value

The FORM statement specifies the number of times a variable is included in the state

vector Values can be specified for any variable listed in the VAR statement If a

value is specified for each variable in the VAR statement the state vector for the state

space model is entirely specified and automatic selection of the state space model is

not performed

The FORM statement forces the state vectorz

t

to contain a specific variable a given

number of times For example if Y is one of the variables inx

t

then the statement

form y 3

forces the state vector to containY

t

Y

t + 1 j t

andY

t + 2 j t

possibly along with other

variables

The following statements illustrate the use of the FORM statement

proc statespace data=in

var x y

f o r m x 3 y 2

run

These statements fit a state space model with the following state vector

z

t

=

2

6

6

6

6

4

x

t j t

y

t j t

x

t + 1 j t

y

t + 1 j t

x

t + 2 j t

3

7

7

7

7

5

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Chapter 18 Syntax

ID Statement

ID variable

The ID statement specifies a variable that identifies observations in the input data setThe variable specified in the ID statement is included in the OUT= data set The

values of the ID variable are extrapolated for the forecast observations based on the

values of the INTERVAL= and INTPER= options

INITIAL Statement

INITIAL F (rowcolumn)= value G(row column)= value

The INITIAL statement gives initial values to the specified elements of theF

andG

matrices These initial values are used as starting values for the iterative estimation

Parts of theF

andG

matrices represent fixed structural identities If an element

specified is a fixed structural element instead of a free parameter the corresponding

initialization is ignored

The following is an example of an INITIAL statement

initial f(32)=0 g(41)=0 g(51)=0

RESTRICT Statement

RESTRICT F(rowcolumn)= value G(rowcolumn)= value

The RESTRICT statement restricts the specified elements of theF

andG

matrices

to the specified values

To use the restrict statement you need to know the form of the model Either specify

the form of the model with the FORM statement or do a preliminary run perhaps

with the NOEST option to find the form of the model that PROC STATESPACE

selects for the data

The following is an example of a RESTRICT statement

restrict f(32)=0 g(41)=0 g(51)=0

Parts of theF

andG

matrices represent fixed structural identities If a restriction is

specified for an element that is a fixed structural element instead of a free parameter

the restriction is ignored

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Part 2 General Information

VAR Statement

VAR variable (difference difference )

The VAR statement specifies the variables in the input data set to model and fore-cast The VAR statement also specifies differencing of the input variables The VAR

statement is required

Differencing is specified by following the variable name with a list of difference

periods separated by commas See the section Stationarity and Differencing for

more information on differencing of input variables

The order in which variables are listed in the VAR statement controls the order in

which variables are included in the state vector Usually potential inputs should be

listed before potential outputs

For example assuming the input data are monthly the following VAR statement

specifies modeling and forecasting of the one period and seasonal second differenceof X and Y

var x(112) y(112)

In this example the vector time series analyzed is

x

t

=

1 B 1 B

1 2

X

t

x

1 B 1 B

1 2

Y

t

y

where B represents the back shift operator and x and y represent the means of thedifferenced series If the NOCENTER option is specified the mean differences are

not subtracted

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Chapter 18 Details

Details

Missing Values

The STATESPACE procedure does not support missing values The procedure uses

the first contiguous group of observations with no missing values for any of the VARstatement variables Observations at the beginning of the data set with missing values

for any VAR statement variable are not used or included in the output data set

Stationarity and Differencing

The state space model used by the STATESPACE procedure assumes that the time

series are stationary Hence the data should be checked for stationarity One way

to check for stationarity is to plot the series A graph of series over time can show a

time trend or variability changes

You can also check stationarity by using the sample autocorrelation functions dis-

played by the ARIMA procedure The autocorrelation functions of nonstationaryseries tend to decay slowly See Chapter 7 ldquoThe ARIMA Procedurerdquo for more in-

formation

Another alternative is to use the STATIONARITY= option on the IDENTIFY state-

ment in PROC ARIMA to apply Dickey-Fuller tests for unit roots in the time series

See Chapter 7 ldquoThe ARIMA Procedurerdquo for more information on Dickey-Fuller unit

root tests

The most popular way to transform a nonstationary series to stationarity is by dif-

ferencing Differencing of the time series is specified in the VAR statement For

example to take a simple first difference of the series X use this statement

var x(1)

In this example the change in X from one period to the next is analyzed When the se-

ries has a seasonal pattern differencing at a period equal to the length of the seasonal

cycle may be desirable For example suppose the variable X is measured quarterly

and shows a seasonal cycle over the year You can use the following statement to

analyze the series of changes from the same quarter in the previous year

var x(4)

To difference twice add another differencing period to the list For ex-

ample the following statement analyzes the series of second differences

X

t

X

t 1

X

t 1

X

t 2

= X

t

2 X

t 1

+ X

t 2

var x(11)

The following statement analyzes the seasonal second difference series

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Part 2 General Information

var x(14)

The series modeled is the 1-period difference of the 4-period difference

X

t

X

t 4

X

t 1

X

t 5

= X

t

X

t 1

X

t 4

+ X

t 5

Another way to obtain stationary series is to use a regression on time to de-trend the

data If the time series has a deterministic linear trend regressing the series on time

produces residuals that should be stationary The following statements write residuals

of X and Y to the variable RX and RY in the output data set DETREND

data a

set a

t=_n_

run

proc reg data=a

model x y = t

output out=detrend r=rx ry

run

You then use PROC STATESPACE to forecast the de-trended series RX and RY A

disadvantage of this method is that you need to add the trend back to the forecast

series in an additional step A more serious disadvantage of the de-trending method

is that it assumes a deterministic trend In practice most time series appear to have a

stochastic rather than a deterministic trend Differencing is a more flexible and often

more appropriate method

There are several other methods to handle nonstationary time series For more infor-

mation and examples refer to Brockwell and Davis (1991)

Preliminary Autoregressive Models

After computing the sample autocovariance matrices PROC STATESPACE fits a

sequence of vector autoregressive models These preliminary autoregressive models

are used to estimate the autoregressive order of the process and limit the order of the

autocovariances considered in the state vector selection process

Yule-Walker Equations for Forward and Backward Models

Unlike a univariate autoregressive model a multivariate autoregressive model has

different forms depending on whether the present observation is being predicted from

the past observations or from the future observations

Letx

t

be the r -component stationary time series given by the VAR statement after

differencing and subtracting the vector of sample means (If the NOCENTER option

is specified the mean is not subtracted) Let n be the number of observations of x

t

from the input data set

Lete

t

be a vector white noise sequence with mean vector 0 and variance matrix

p

and letn

t

be a vector white noise sequence with mean vector 0 and variance matrix

p

Let p be the order of the vector autoregressive model forx

t

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Chapter 18 Details

The forward autoregressive form based on the past observations is written as follows

x

t

=

p

X

i = 1

p

i

x

t i

+ e

t

The backward autoregressive form based on the future observations is written as fol-

lows

x

t

=

p

X

i = 1

p

i

x

t + i

+ n

t

LettingE

denote the expected value operator the autocovariance sequence for thex

t

series

i

is

i

= E x

t

x

0

t i

The Yule-Walker equations for the autoregressive model that matches the first p ele-

ments of the autocovariance sequence are

2

6

6

4

0

1

p 1

0

1

0

p 2

0

p 1

0

p 2

0

3

7

7

5

2

6

6

4

p

1

p

2

p

p

3

7

7

5

=

2

6

6

4

1

2

p

3

7

7

5

and

2

6

6

6

4

0

0

1

0

p 1

1

0

0

p 2

p 1

p 2

0

3

7

7

7

5

2

6

6

4

p

1

p

2

p

p

3

7

7

5

=

2

6

6

4

0

1

0

2

0

p

3

7

7

5

Here

p

i

are the coefficient matrices for the past observation form of the vector au-toregressive model and

p

i

are the coefficient matrices for the future observation

form More information on the Yule-Walker equations in the multivariate setting can

be found in Whittle (1963) and Ansley and Newbold (1979)

The innovation variance matrices for the two forms can be written as follows

p

=

0

p

X

i = 1

p

i

0

i

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p

=

0

p

X

i = 1

p

i

i

The autoregressive models are fit to the data using the preceding Yule-Walker equa-

tions with

i

replaced by the sample covariance sequenceC

i

The covariance matri-

ces are calculated as

C

i

=

1

N 1

N

X

t = i + 1

x

t

x

0

t i

Let b

p

b

p

b

p

and b

p

represent the Yule-Walker estimates of

p

p

p

and

p

respectively These matrices are written to an output data set when the OUTAR=

option is specified

When the PRINTOUT=LONG option is specified the sequence of matrices b

p

and

the corresponding correlation matrices are printed The sequence of matricesb

p isused to compute Akaike information criteria for selection of the autoregressive order

of the process

Akaike Information Criterion

The Akaike information criterion or AIC is defined as -2( maximum of log likeli-

hood )+2(number of parameters) Since the vector autoregressive models are esti-

mates from the Yule-Walker equations not by maximum likelihood the exact likeli-

hood values are not available for computing the AIC However for the vector autore-

gressive model the maximum of the log likelihood can be approximated as

l n L

n

2

l n j

b

p

j

Thus the AIC for the order p model is computed as

A I C

p

= n l n j

b

p

j + 2 p r

2

You can use the printed AIC array to compute a likelihood ratio test of the autoregres-

sive order The log-likelihood ratio test statistic for testing the order p model against

the order p 1

model is

n l n j

b

p

j + n l n j

b

p 1

j

This quantity is asymptotically distributed as a

2 withr

2 degrees of freedom if the

series is autoregressive of order p 1

It can be computed from the AIC array as

A I C

p 1

A I C

p

+ 2 r

2

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You can evaluate the significance of these test statistics with the PROBCHI function

in a SAS DATA step or with a

2 table

Determining the Autoregressive Order

Although the autoregressive models can be used for prediction their primary value is

to aid in the selection of a suitable portion of the sample covariance matrix for use in

computing canonical correlations If the multivariate time series x

t is of autoregres-sive order p then the vector of past values to lag p is considered to contain essentially

all the information relevant for prediction of future values of the time series

By default PROC STATESPACE selects the order p producing the autoregressive

model with the smallestA I C

p

If the value p for the minimumA I C

p

is less than the

value of the PASTMIN= option then p is set to the PASTMIN= value Alternatively

you can use the ARMAX= and PASTMIN= options to force PROC STATESPACE to

use an order you select

Significance Limits for Partial Autocorrelations

The STATESPACE procedure prints a schematic representation of the partial autocor-

relation matrices indicating which partial autocorrelations are significantly greater orsignificantly less than 0 Figure 1810 shows an example of this table

The STATESPACE Procedure

Schematic Representation of Partial Autocorrelations

NameLag 1 2 3 4 5 6 7 8 9 10

x ++ +

y ++

+ is gt 2std error - is lt -2std error is between

Figure 1810 Significant Partial Autocorrelations

The partial autocorrelations are from the sample partial autoregressive matrices b

p

p

The standard errors used for the significance limits of the partial autocorrelations are

computed from the sequence of matrices

p

and

p

Under the assumption that the observed series arises from an autoregressive process

of order p 1

the pth sample partial autoregressive matrix b

p

p

has an asymptotic

variance matrix 1

n

1

p

p

The significance limits for b

p

p

used in the schematic plot of the sample partial autore-

gressive sequence are derived by replacing

p

and

p

with their sample estimators

to produce the variance estimate as follows

d

V a r

b

p

p

=

1

n r p

b

1

p

b

p

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Canonical Correlation Analysis

Given the order p letp

t

be the vector of current and past values relevant to prediction

of x

t + 1

p

t

= x

0

t

x

0

t 1

x

0

t p

0

Letf

t

be the vector of current and future values

f

t

= x

0

t

x

0

t + 1

x

0

t + p

0

In the canonical correlation analysis consider submatrices of the sample covariance

matrix of p

t

andf

t

This covariance matrixV

has a block Hankel form

V =

2

6

6

6

4

C

0

C

0

1

C

0

2

C

0

p

C

0

1

C

0

2

C

0

3

C

0

p + 1

C

0

p

C

0

p + 1

C

0

p + 2

C

0

2 p

3

7

7

7

5

State Vector Selection Process

The canonical correlation analysis forms a sequence of potential state vectorsz

j

t

Examine a sequencef

j

t

of subvectors of f

t

and form the submatrixV

j consisting

of the rows and columns of V

corresponding to the components of f

j

t

and compute

its canonical correlations

The smallest canonical correlation of V

j is then used in the selection of the com-

ponents of the state vector The selection process is described in the following For

more details about this process refer to Akaike (1976)

In the following discussion the notationx

t + k j t

denotes the wide sense conditional

expectation (best linear predictor) of x

t + k

given allx

s

with s less than or equal to t

In the notationx

i t + 1

the first subscript denotes the ith component of x

t + 1

The initial state vectorz

1

t

is set tox

t

The sequencef

j

t

is initialized by setting

f

1

t

= z

1

0

t

x

1 t + 1 j t

0

= x

0

t

x

1 t + 1 j t

0

That is start by considering whether to addx

1 t + 1 j t

to the initial state vectorz

1

t

The procedure forms the submatrixV

1 corresponding tof

1

t

and computes its canon-

ical correlations Denote the smallest canonical correlation of V

1 as

m i n

If

m i n

is

significantly greater than 0x

1 t + 1 j t

is added to the state vector

If the smallest canonical correlation of V

1 is not significantly greater than 0 then a

linear combination of f

1

t

is uncorrelated with the pastp

t

Assuming that the determi-

nant of C

0

is not 0 (that is no input series is a constant) you can take the coefficient

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Chapter 18 Details

of x

1 t + 1 j t

in this linear combination to be 1 Denote the coefficients of z

1

t

in this

linear combination as

This gives the relationship

x

1 t + 1 j t

=

0

x

t

Therefore the current state vector already contains all the past information useful forpredicting

x

1 t + 1

and any greater leads of x

1 t

The variablex

1 t + 1 j t

is not added to

the state vector nor are any termsx

1 t + k j t

considered as possible components of the

state vector The variablex

1

is no longer active for state vector selection

The process described forx

1 t + 1 j t

is repeated for the remaining elements of f

t

The

next candidate for inclusion in the state vector is the next component of f

t

corre-

sponding to an active variable Components of f

t

corresponding to inactive variables

that produced a zero

m i n

in a previous step are skipped

Denote the next candidate asx

l t + k j t

The vectorf

j

t

is formed from the current state

vector andx

l t + k j t

as follows

f

j

t

= z

j

0

t

x

l t + k j t

0

The matrixV

j is formed fromf

j

t

and its canonical correlations are computed The

smallest canonical correlation of V

j is judged to be either greater than or equal to 0

If it is judged to be greater than 0x

l t + k j t

is added to the state vector If it is judged

to be 0 then a linear combination of f

j

t

is uncorrelated with thep

t

and the variable

x

l

is now inactive

The state vector selection process continues until no active variables remain

Testing Significance of Canonical Correlations For each step in the canonical correlation sequence the significance of the small-

est canonical correlation

m i n

is judged by an information criterion from Akaike

(1976) This information criterion is

n l n 1

2

m i n

r p + 1 q + 1

where q is the dimension of f

j

t

at the current step r is the order of the state vector p

is the order of the vector autoregressive process and

is the value of the SIGCORR=

option The default is SIGCORR=2 If this information criterion is less than or equal

to 0

m i n

is taken to be 0 otherwise it is taken to be significantly greater than 0

(Do not confuse this information criterion with the AIC)

Variables inx

t + p j t

are not added in the model even with positive information crite-

rion because of the singularity of V

You can force the consideration of more candi-

date state variables by increasing the size of theV

matrix by specifying a PASTMIN=

option value larger than p

Printing the Canonical Correlations

To print the details of the canonical correlation analysis process specify the CAN-

CORR option in the PROC STATESPACE statement The CANCORR option prints

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the candidate state vectors the canonical correlations and the information criteria for

testing the significance of the smallest canonical correlation

Bartlettrsquos

2 and its degrees of freedom are also printed when the CANCORR option

is specified The formula used for Bartlettrsquos

2 is

2

= n 5 r p + 1 q + 1 l n 1

2

m i n

withr p + 1 q + 1

degrees of freedom

Figure 1811 shows the output of the CANCORR option for the introductory example

shown in the Getting Started section of this chapter

The STATESPACE Procedure

Canonical Correlations Analysis

Information Chi

x(TT) y(TT) x(T+1T) Criterion Square DF

1 1 0237045 3566167 114505 4

Information Chi

x(TT) y(TT) x(T+1T) y(T+1T) Criterion Square DF

1 1 0238244 0056565 -535906 0636134 3

Information Chi

x(TT) y(TT) x(T+1T) x(T+2T) Criterion Square DF

1 1 0237602 0087493 -446312 1525353 3

Figure 1811 Canonical Correlations Analysis

New variables are added to the state vector if the information criteria are positive Inthis example

y

t + 1 j t

andx

t + 2 j t

are not added to the state space vector because the

information criteria for these models are negative

If the information criterion is nearly 0 then you may want to investigate models

that arise if the opposite decision is made regarding

m i n

This investigation can be

accomplished by using a FORM statement to specify part or all of the state vector

Preliminary Estimates of F

When a candidate variablex

l t + k j t

yields a zero

m i n

and is not added to the state

vector a linear combination of f

j

t

is uncorrelated with thep

t

Because of the method

used to construct thef

j

t

sequence the coefficient of x

l t + k j t

inl

can be taken as 1

Denote the coefficients of z

j

t

in this linear combination as l

This gives the relationship

x

l t + k j t

= l

0

z

j

t

The vectorl

is used as a preliminary estimate of the first r columns of the row of the

transition matrixF

corresponding tox

l t + k 1 j t

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Chapter 18 Details

Parameter Estimation

The model isz

t + 1

= F z

t

+ G e

t + 1

wheree

t

is a sequence of independent multivari-

ate normal innovations with mean vector 0 and variance

e e

The observed sequence

x

t

composes the first r components of z

t

and thusx

t

= H z

t

where H is ther s

matrix I

r

0

LetE

be ther n

matrix of innovations

E = e

1

e

n

If the number of observations n is reasonably large the log likelihood L can be

approximated up to an additive constant as follows

L =

n

2

l n j

e e

j

1

2

t r a c e

1

e e

E E

0

The elements of

e e

are taken as free parameters and are estimated as follows

S

0

=

1

n

E E

0

Replacing

e e

byS

0

in the likelihood equation the log likelihood up to an additive

constant is

L =

n

2

l n j S

0

j

Letting B be the backshift operator the formal relation betweenx

t

ande

t

is

x

t

= H I B F

1

G e

t

e

t

= H I B F

1

G

1

x

t

=

1

X

i = 0

i

x

t i

LettingC

i

be the ith lagged sample covariance of x

t

and neglecting end effects the

matrixS

0

is

S

0

=

1

X

i j = 0

i

C

i + j

0

j

For the computation of S

0

the infinite sum is truncated at the value of the KLAG=

option The value of the KLAG= option should be large enough that the sequence

i

is approximately 0 beyond that point

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Let

be the vector of free parameters in theF

andG

matrices The derivative of the

log likelihood with respect to the parameter

is

L

=

n

2

t r a c e

S

1

0

S

0

The second derivative is

2

L

0

=

n

2

t r a c e

S

1

0

S

0

0

S

1

0

S

0

t r a c e

S

1

0

2

S

0

0

Near the maximum the first term is unimportant and the second term can be approx-

imated to give the following second derivative approximation

2

L

0

=

n t r a c e

S

1

0

E

E

0

0

The first derivative matrix and this second derivative matrix approximation are com-

puted from the sample covariance matrixC

0

and the truncated sequence

i

The

approximate likelihood function is maximized by a modified Newton-Raphson algo-

rithm employing these derivative matrices

The matrixS

0

is used as the estimate of the innovation covariance matrix

e e

The

negative of the inverse of the second derivative matrix at the maximum is used as an

approximate covariance matrix for the parameter estimates The standard errors of

the parameter estimates printed in the parameter estimates tables are taken from the

diagonal of this covariance matrix The parameter covariance matrix is printed when

the COVB option is specified

If the data are nearly nonstationary a better estimate of

e e

and the other parameters

can sometimes be obtained by specifying the RESIDEST option The RESIDEST

option estimates the parameters using conditional least squares instead of maximum

likelihood

The residuals are computed using the state space equation and the sample mean val-

ues of the variables in the model as start-up values The estimate of S

0

is then com-

puted using the residuals from the ith observation on where i is the maximum number

of times any variable occurs in the state vector A multivariate Gauss-Marquardt al-

gorithm is used to minimizej S

0

j

Refer to Harvey (1981a) for a further description

of this method

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Forecasting

Given estimates of F

G

and

e e

forecasts of x

t

are computed from the conditional

expectation of z

t

In forecasting the parameters F G and

e e

are replaced with the estimates or by val-

ues specified in the RESTRICT statement One-step-ahead forecasting is performedfor the observation

x

t

wheret n b

Heren

is the number of observations and

b is the value of the BACK= option For the observationx

t

wheret n b

m-

step-ahead forecasting is performed form = t n + b

The forecasts are generated

recursively with the initial conditionz

0

= 0

The m-step-ahead forecast of z

t + m

isz

t + m j t

wherez

t + m j t

denotes the conditional

expectation of z

t + m

given the information available at time t The m-step-ahead

forecast of x

t + m

isx

t + m j t

= H z

t + m j t

where the matrixH = I

r

0

Let

i

= F

i

G

Note that the lasts r

elements of z

t

consist of the elements of x

u j t

foru t

The state vector z

t + m

can be represented as

z

t + m

= F

m

z

t

+

m 1

X

i = 0

i

e

t + m i

Sincee

t + i j t

= 0

fori 0

the m-step-ahead forecastz

t + m j t

is

z

t + m j t

= F

m

z

t

= F z

t + m 1 j t

Therefore the m-step-ahead forecast of x

t + m

is

x

t + m j t

= H z

t + m j t

The m-step-ahead forecast error is

z

t + m

z

t + m j t

=

m 1

X

i = 0

i

e

t + m i

The variance of the m-step-ahead forecast error is

V

z m

=

m 1

X

i = 0

i

e e

0

i

LettingV

z 0

= 0

the variance of the m-step-ahead forecast error of z

t + m

V

z m

can

be computed recursively as follows

V

z m

= V

z m 1

+

m 1

e e

0

m 1

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The variance of the m-step-ahead forecast error of x

t + m

is ther r

left upper sub-

matrix of V

z m

that is

V

x m

= H V

z m

H

0

Unless the NOCENTER option is specified the sample mean vector is added to the

forecast When differencing is specified the forecasts xt + m j t

plus the sample mean

vector are integrated back to produce forecasts for the original series

Lety

t

be the original series specified by the VAR statement with some 0 values

appended corresponding to the unobserved past observations Let B be the backshift

operator and let B

be thes s

matrix polynomial in the backshift operator

corresponding to the differencing specified by the VAR statement The off-diagonal

elements of

i

are 0 Note that

0

= I

s

whereI

s

is thes s

identity matrix Then

z

t

= B y

t

This gives the relationship

y

t

=

1

B z

t

=

1

X

i = 0

i

z

t i

where

1

B =

P

1

i = 0

i

B

i and

0

= I

s

The m-step-ahead forecast of y

t + m

is

y

t + m j t

=

m 1

X

i = 0

i

z

t + m i j t

+

1

X

i = m

i

z

t + m i

The m-step-ahead forecast error of y

t + m

is

m 1

X

i = 0

i

z

t + m i

z

t + m i j t

=

m 1

X

i = 0

i

X

u = 0

u

i u

e

t + m i

LettingV

y 0

= 0

the variance of the m-step-ahead forecast error of y

t + m

V

y m

is

V

y m

=

m 1

X

i = 0

i

X

u = 0

u

i u

e e

i

X

u = 0

u

i u

0

= V

y m 1

+

m 1

X

u = 0

u

m 1 u

e e

m 1

X

u = 0

u

m 1 u

0

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Relation of ARMA and State Space Forms

Every state space model has an ARMA representation and conversely every ARMA

model has a state space representation This section discusses this equivalence The

following material is adapted from Akaike (1974) where there is a more complete

discussion Pham-Dinh-Tuan (1978) also contains a discussion of this material

Suppose you are given the following ARMA model

B x

t

= B e

t

or in more detail

x

t

1

x

t 1

p

x

t p

= e

t

+

1

e

t 1

+ +

q

e

t q

(1)

wheree

t

is a sequence of independent multivariate normal random vectors with mean

0 and variance matrix

e e

B is the backshift operator (B x

t

= x

t 1

) B

and

B

are matrix polynomials in B andx

t

is the observed process

If the roots of the determinantial equationj B j = 0

are outside the unit circle in

the complex plane the model can also be written as

x

t

=

1

B B e

t

=

1

X

i = 0

i

e

t i

The

i

matrices are known as the impulse response matrices and can be computed

as

1

B B

You can assume p q

since if this is not initially true you can add more terms

i

that are identically 0 without changing the model

To write this set of equations in a state space form proceed as follows Letx

t + i j t

be

the conditional expectation of x

t + i

givenx

w

forw t

The following relations hold

x

t + i j t

=

1

X

j = i

j

e

t + i j

x

t + i j t + 1

= x

t + i j t

+

i 1

e

t + 1

However from equation (1) you can derive the following relationship

x

t + p j t

=

1

x

t + p 1 j t

+ +

p

x

t

(2)

Hence wheni = p

you can substitute forx

t + p j t

in the right-hand side of equation

(2) and close the system of equations

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This substitution results in the following model in the state space form

z

t + 1

= F z

t

+ G e

t + 1

2

6

6

6

4

x

t + 1

x

t + 2 j t + 1

x

t + p j t + 1

3

7

7

7

5

=

2

6

6

4

0 I 0 0

0 0 I 0

p

p 1

1

3

7

7

5

2

6

6

6

4

x

t

x

t + 1 j t

x

t + p 1 j t

3

7

7

7

5

+

2

6

6

4

I

1

p 1

3

7

7

5

e

t + 1

Note that the state vectorz

t

is composed of conditional expectations of x

t

and the

first r components of z

t

are equal tox

t

The state space form can be cast into an ARMA form by solving the system of dif-

ference equations for the first r components

When converting from an ARMA form to a state space form you can generate a

state vector larger than needed that is the state space model may not be a minimal

representation When going from a state space form to an ARMA form you can have

nontrivial common factors in the autoregressive and moving average operators that

yield an ARMA model larger than necessary

If the state space form used is not a minimal representation some but not all compo-

nents of x

t + i j t

may be linearly dependent This situation corresponds to

p

p 1

being of less than full rank when B

and B

have no common nontrivial

left factors In this casez

t

consists of a subset of the possible components of

x

t + i j t

i = 1 2 p 1

However once a component of x

t + i j t

(for example the

jth one) is linearly dependent on the previous conditional expectations then all sub-

sequent jth components of x

t + k j t

fork i

must also be linearly dependent Note

that in this case equivalent but seemingly different structures can arise if the order of

the components withinx

t

is changed

OUT= Data Set

The forecasts are contained in the output data set specified by the OUT= option on the

PROC STATESPACE statement The OUT= data set contains the following variables

the BY variables

the ID variable

the VAR statement variables These variables contain the actual values from

the input data set

FORi numeric variables containing the forecasts The variable FORi contains

the forecasts for the ith variable in the VAR statement list Forecasts are one-

step-ahead predictions until the end of the data or until the observation specified

by the BACK= option

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Chapter 18 Details

RESi numeric variables containing the residual for the forecast of the ith vari-

able in the VAR statement list For forecast observations the actual values are

missing and the RESi variables contain missing values

STDi numeric variables containing the standard deviation for the forecast of

the ith variable in the VAR statement list The values of the STDi variables can

be used to construct univariate confidence limits for the corresponding fore-casts However such confidence limits do not take into account the covariance

of the forecasts

OUTAR= Data Set

The OUTAR= data set contains the estimates of the preliminary autoregressive mod-

els The OUTAR= data set contains the following variables

ORDER a numeric variable containing the order p of the autoregressive model

that the observation represents

AIC a numeric variable containing the value of the information criterionA I C

p

SIGFl numeric variables containing the estimate of the innovation covariance

matrices for the forward autoregressive models The variable SIGFl contains

the lth column of b

p

in the observations with ORDER= p

SIGBl numeric variables containing the estimate of the innovation covariance

matrices for the backward autoregressive models The variable SIGBl contains

the lth column of b

p

in the observations with ORDER= p

FORk ndashl numeric variables containing the estimates of the autoregressive pa-

rameter matrices for the forward models The variable FORk ndashl contains the

lth column of the lag k autoregressive parameter matrix b

p

k

in the observations

with ORDER= p

BACk ndashl numeric variables containing the estimates of the autoregressive pa-

rameter matrices for the backward models The variable BACk ndashl contains the

lth column of the lag k autoregressive parameter matrix b

p

k

in the observations

with ORDER= p

The estimates for the order p autoregressive model can be selected as those observa-

tions with ORDER= p Within these observations the k lth element of

p

i

is given by

the value of the FORindashl variable in the k th observation The k lth element of

p

i

is

given by the value of BACindashl variable in the k th observation The k lth element of

p

is given by SIGFl in the k th observation The k lth element of

p

is given by SIGBlin the k th observation

Table 181 shows an example of the OUTAR= data set with ARMAX=3 andx

t

of

dimension 2 In Table 181 i j

indicate the ijth element of the matrix

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Part 2 General Information

Table 181 Values in the OUTAR= Data Set

Obs ORDER AIC SIGF1 SIGF2 SIGB1 SIGB2 FOR1ndash 1 FOR1ndash 2 FOR2ndash 1 FOR2ndash 2 FOR3ndash 1

1 0 AIC0

0 1 1

0 1 2

0 1 1

0 1 2

2 0 AIC0

0 2 1

0 2 2

0 2 1

0 2 2

3 1 AIC1

1 1 1

1 1 2

1 1 1

1 1 2

1

1

1 1

1

1

1 2

4 1 AIC1

1 2 1

1 1 2

1 2 1

1 1 2

1

1

2 1

1

1

2 2

5 2 AIC2

2 1 1

2 1 2

2 1 1

2 1 2

2

1

1 1

2

1

1 2

2

2

1 1

2

2

1 2

6 2 AIC2

2 2 1

2 1 2

2 2 1

2 1 2

2

1

2 1

2

1

2 2

2

2

2 1

2

2

2 2

7 3 AIC3

3 1 1

3 1 2

3 1 1

3 1 2

3

1

1 1

3

1

1 2

3

2

1 1

3

2

1 2

3

3

1 1

8 3 AIC3

3 2 1

3 1 2

3 2 1

3 1 2

3

1

2 1

3

1

2 2

3

2

2 1

3

2

2 2

3

3

2 1

Obs FOR3ndash 2 BACK1ndash 1 BACK1ndash 2 BACK2ndash 1 BACK2ndash 2 BACK3ndash 1 BACK3ndash 2

1

2

3

1

1

1 1

1

1

1 2

4

1

1

2 1

1

1

2 2

5

2

1

1 1

2

1

1 2

2

2

1 1

2

2

1 2

6

2

1

2 1

2

1

2 2

2

2

2 1

2

2

2 2

7

3

3

1 2

3

1

1 1

3

1

1 2

3

2

1 1

3

2

1 2

3

3

1 1

3

3

1 2

8

3

3

2 2

3

1

2 1

3

1

2 2

3

2

2 1

3

2

2 2

3

3

2 1

3

3

2 2

The estimated autoregressive parameters can be used in the IML procedure to obtain

autoregressive estimates of the spectral density function or forecasts based on the

autoregressive models

OUTMODEL= Data Set

The OUTMODEL= data set contains the estimates of theF

andG

matrices and their

standard errors the names of the components of the state vector and the estimates of

the innovation covariance matrix The variables contained in the OUTMODEL= data

set are as follows

the BY variables

STATEVEC a character variable containing the name of the component of the

state vector corresponding to the observation The STATEVEC variable has

the value STD for standard deviations observations which contain the standarderrors for the estimates given in the preceding observation

Fndash j numeric variables containing the columns of theF

matrix The variable

Fndash j contains the jth column of F

The number of Fndash j variables is equal to

the value of the DIMMAX= option If the model is of smaller dimension the

extraneous variables are set to missing

Gndash j numeric variables containing the columns of theG

matrix The variable

Gndash j contains the jth column of G

The number of Gndash j variables is equal to r

the dimension of x

t

given by the number of variables in the VAR statement

SIGndash j numeric variables containing the columns of the innovation covariance

matrix The variable SIGndash j contains the jth column of

e e

There are r vari-ables SIGndash j

Table 182 shows an example of the OUTMODEL= data set withx

t

= x

t

y

t

0

z

t

= x

t

y

t

x

t + 1 j t

0 and DIMMAX=4 In Table 182F

i j

andG

i j

are the ijth

elements of F

andG

respectively Note that all elements for Fndash4 are missing because

F

is a3 3

matrix

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Chapter 18 Details

Table 182 Value in the OUTMODEL= Data Set

Obs STATEVEC Fndash 1 Fndash 2 Fndash 3 Fndash 4 Gndash 1 Gndash 2 SIGndash 1 SIGndash 2

1 X(TT) 0 0 1 1 0

1 1

1 2

2 STD

3 Y(TT) F2 1

F2 2

F2 3

0 1

2 1

2 2

4 STD std F2 1

std F2 2

std F2 3

5 X(T+1T) F3 1

F3 2

F3 3

G3 1

G3 2

6 STD std F3 1

std F3 2

std F3 3

std G3 1

std G3 2

Printed Output

The printed output produced by the STATESPACE procedure is described in the fol-

lowing

1 descriptive statistics which include the number of observations used the

names of the variables their means and standard deviations (Std) and the dif-

ferencing operations used

2 the Akaike information criteria for the sequence of preliminary autoregressive

models

3 if the PRINTOUT=LONG option is specified the sample autocovariance ma-

trices of the input series at various lags

4 if the PRINTOUT=LONG option is specified the sample autocorrelation ma-

trices of the input series

5 a schematic representation of the autocorrelation matrices showing the signif-

icant autocorrelations

6 if the PRINTOUT=LONG option is specified the partial autoregressive ma-

trices (These are

p

p

as described in Preliminary Autoregressive Modelsearlier in this chapter)

7 a schematic representation of the partial autocorrelation matrices showing the

significant partial autocorrelations

8 the Yule-Walker estimates of the autoregressive parameters for the autoregres-

sive model with the minimum AIC

9 if the PRINTOUT=LONG option is specified the autocovariance matrices of

the residuals of the minimum AIC model This is the sequence of estimated

innovation variance matrices for the solutions of the Yule-Walker equations

10 if the PRINTOUT=LONG option is specified the autocorrelation matrices of the residuals of the minimum AIC model

11 If the CANCORR option is specified the canonical correlations analysis for

each potential state vector considered in the state vector selection process

This includes the potential state vector the canonical correlations the informa-

tion criterion for the smallest canonical correlation Bartlettrsquos

2 statistic (ldquoChi

Squarerdquo) for the smallest canonical correlation and the degrees of freedom of

Bartlettrsquos

2

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Part 2 General Information

12 the components of the chosen state vector

13 the preliminary estimate of the transition matrixF

the input matrixG

and

the variance matrix for the innovations

e e

14 if the ITPRINT option is specified the iteration history of the likelihood max-

imization For each iteration this shows the iteration number the number of step halvings the determinant of the innovation variance matrix the damping

factor Lambda and the values of the parameters

15 the state vector printed again to aid interpretation of the following listing of F

andG

16 the final estimate of the transition matrixF

17 the final estimate of the input matrixG

18 the final estimate of the variance matrix for the innovations

e e

19 a table listing the estimates of the free parameters inF

andG

and their standarderrors and t statistics

20 if the COVB option is specified the covariance matrix of the parameter esti-

mates

21 if the COVB option is specified the correlation matrix of the parameter esti-

mates

22 if the PRINT option is specified the forecasts and their standard errors

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Chapter 18 Details

ODS Table Names

PROC STATESPACE assigns a name to each table it creates You can use these

names to reference the table when using the Output Delivery System (ODS) to select

tables and create output data sets These names are listed in the following table For

more information on ODS see Chapter 6 ldquoUsing the Output Delivery Systemrdquo

Table 183 ODS Tables Produced in PROC STATESPACE

ODS Table Name Description Option

NObs Number of observations default

Summary Simple summary statistics table default

InfoCriterion Information criterion table default

CovLags Covariance Matrices of Input Series PRINTOUT=LONG

CorrLags Correlation Matrices of Input Series PRINTOUT=LONG

PartialAR Partial Autoregressive Matrices PRINTOUT=LONG

YWEstimates Yule-Walker Estimates for Minimum AIC default

CovResiduals Covariance of Residuals PRINTOUT=LONGCorrResiduals Residual Correlations from AR Models PRINTOUT=LONG

StateVector State vector table default

CorrGraph Schematic Representation of Correlations default

TransitionMatrix Transition Matrix default

InputMatrix Input Matrix default

VarInnov Variance Matrix for the Innovation default

CovB Covariance of Parameter Estimates COVB

CorrB Correlation of Parameter Estimates COVB

CanCorr Canonical Correlation Analysis CANCORR

IterHistory Iterative Fitting table ITPRINT

ParameterEstimates Parameter Estimates Table default

Forecasts Forecasts Table PRINT

ConvergenceStatus Convergence Status Table default

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Part 2 General Information

Example

Example 181 Series J from Box and Jenkins

This example analyzes the gas furnace data (series J) from Box and Jenkins (The

data are not shown Refer to Box and Jenkins (1976) for the data)

First a model is selected and fit automatically using the following statements

title1 rsquoGas Furnace Datarsquo

title2 rsquoBox amp Jenkins Series Jrsquo

title3 rsquoAutomatically Selected Modelrsquo

proc statespace data=seriesj cancorr

var x y

run

The results for the automatically selected model are shown in Output 1811

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Chapter 18 Example

Output 1811 Results for Automatically Selected Model

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Number of Observations 296

Standard

Variable Mean Error

x -005683 1072766

y 5350912 3202121

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Information Criterion for Autoregressive Models

Lag=0 Lag=1 Lag=2 Lag=3 Lag=4 Lag=5 Lag=6 Lag=7 Lag=8

6513862 -103357 -163296 -164512 -165152 -164891 -164934 -164315 -163856

Information

Criterion for

Autoregressive

Models

Lag=9 Lag=10

-16348 -163359

Schematic Representation of Correlations

NameLag 0 1 2 3 4 5 6 7 8 9 10

x +- +- +- +- +- +- +- +- +- +- +-

y -+ -+ -+ -+ -+ -+ -+ -+ -+ -+ -+

+ is gt 2std error - is lt -2std error is between

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Part 2 General Information

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Schematic Representation of Partial Autocorrelations

NameLag 1 2 3 4 5 6 7 8 9 10

x + - + -

y -+ -- - + +

+ is gt 2std error - is lt -2std error is between

Yule-Walker Estimates for Minimum AIC

------Lag=1------ ------Lag=2------ ------Lag=3------ ------Lag=4------

x y x y x y x y

x 1925887 -000124 -120166 0004224 0116918 -000867 0104236 0003268

y 0050496 1299793 -002046 -03277 -071182 -025701 0195411 0133417

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Canonical Correlations Analysis

Information Chi

x(TT) y(TT) x(T+1T) Criterion Square DF

1 1 0804883 2929228 3047481 8

Information Chix(TT) y(TT) x(T+1T) y(T+1T) Criterion Square DF

1 1 0906681 0607529 1223358 1347237 7

Information Chi

x(TT) y(TT) x (T+1T) y (T+1T) x (T+2T) Criterion Square DF

1 1 0909434 0610278 0186274 -154701 1034705 6

Information Chi

x(TT) y(TT) x (T+1T) y (T+1T) y (T+2T) Criterion Square DF

1 1 091014 0618937 0206823 0940392 1280924 6

Information Chi

x(TT) y(TT) x(T+1T) y(T+1T) y(T+2T) y(T+3T) Criterion Square DF

1 1 0912963 0628785 0226598 0083258 -794103 2041584 5

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Chapter 18 Example

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Selected Statespace Form and Preliminary Estimates

State Vector

x(TT) y(TT) x(T+1T) y(T+1T) y(T+2T)

Estimate of Transition Matrix

0 0 1 0 0

0 0 0 1 0

-084718 0026794 1711715 -005019 0

0 0 0 0 1

-019785 0334274 -018174 -123557 1787475

Input Matrix for Innovation

1 0

0 11925887 -000124

0050496 1299793

0142421 1361696

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Selected Statespace Form and Preliminary Estimates

Variance Matrix for Innovation

0035274 -000734

-000734 0097569

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Part 2 General Information

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Selected Statespace Form and Fitted Model

State Vector

x(TT) y(TT) x(T+1T) y(T+1T) y(T+2T)

Estimate of Transition Matrix

0 0 1 0 0

0 0 0 1 0

-086192 0030609 1724235 -005483 0

0 0 0 0 1

-034839 0292124 -009435 -109823 1671418

Input Matrix for Innovation

1 0

0 1192442 -000416

0015621 1258495

008058 1353204

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Selected Statespace Form and Fitted Model

Variance Matrix for Innovation

0035579 -000728

-000728 0095577

Parameter Estimates

Standard

Parameter Estimate Error t Value

F(31) -086192 0072961 -1181

F(32) 0030609 0026167 117

F(33) 1724235 0061599 2799

F(34) -005483 0030169 -182

F(51) -034839 0135253 -258

F(52) 0292124 0046299 631

F(53) -009435 0096527 -098F(54) -109823 0109525 -1003

F(55) 1671418 0083737 1996

G(31) 1924420 0058162 3309

G(32) -000416 0035255 -012

G(41) 0015621 0095771 016

G(42) 1258495 0055742 2258

G(51) 0080580 0151622 053

G(52) 1353204 0091388 1481

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Chapter 18 Example

The two series are believed to have a transfer function relation with the gas rate

(variable X) as the input and the CO2

concentration (variable Y) as the output Since

the parameter estimates shown in Output 1811 support this kind of model the model

is reestimated with the feedback parameters restricted to 0 The following statements

fit the transfer function (no feedback) model

title3 rsquoTransfer Function Modelrsquo

proc statespace data=seriesj printout=none

var x y

restrict f(32)=0 f(34)=0

g(32)=0 g(41)=0 g(51)=0

run

The last 2 pages of the output are shown in Output 1812

Output 1812 STATESPACE Output for Transfer Function Model

Gas Furnace Data

Box amp Jenkins Series J

Transfer Function Model

The STATESPACE Procedure

Selected Statespace Form and Fitted Model

State Vector

x(TT) y(TT) x(T+1T) y(T+1T) y(T+2T)

Estimate of Transition Matrix

0 0 1 0 0

0 0 0 1 0

-068882 0 1598717 0 0

0 0 0 0 1

-035944 0284179 -00963 -107313 1650047

Input Matrix for Innovation

1 0

0 1

1923446 0

0 1260856

0 1346332

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Part 2 General Information

Gas Furnace Data

Box amp Jenkins Series J

Transfer Function Model

The STATESPACE Procedure

Selected Statespace Form and Fitted Model

Variance Matrix for Innovation

0036995 -00072

-00072 0095712

Parameter Estimates

Standard

Parameter Estimate Error t Value

F(31) -068882 0050549 -1363

F(33) 1598717 0050924 3139

F(51) -035944 0229044 -157

F(52) 0284179 0096944 293

F(53) -009630 0140876 -068

F(54) -107313 0250385 -429

F(55) 1650047 0188533 875G(31) 1923446 0056328 3415

G(42) 1260856 0056464 2233

G(52) 1346332 0091086 1478

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Chapter 18 References

References

Akaike H (1974) Markovian Representation of Stochastic Processes and Its Ap-

plication to the Analysis of Autoregressive Moving Average Processes Annals

of the Institute of Statistical Mathematics 26 363-387

Akaike H (1976) Canonical Correlations Analysis of Time Series and the Use of

an Information Criterion in Advances and Case Studies in System Identification

eds R Mehra and DG Lainiotis New York Academic Press

Anderson TW (1971) The Statistical Analysis of Time Series New York John

Wiley amp Sons

Ansley CF and Newbold P (1979) Multivariate Partial Autocorrelations Pro-

ceedings of the Business and Economic Statistics Section American Statistical

Association 349-353

Box GEP and Jenkins G (1976) Time Series Analysis Forecasting and Control

San Francisco Holden-Day

Brockwell PJ and Davis RA (1991) Time Series Theory and Methods 2nd Edi-

tion Springer-Verlag

Hannan EJ (1970) Multiple Time Series New York John Wiley amp Sons

Hannan EJ (1976) The Identification and Parameterization of ARMAX and State

Space Forms Econometrica 44 713-722

Harvey AC (1981a) The Econometric Analysis of Time Series New York John

Wiley amp Sons

Harvey AC (1981b) Time Series Models New York John Wiley amp Sons

Jones RH (1974) Identification and Autoregressive Spectrum Estimation IEEE Transactions on Automatic Control AC-19 894-897

Pham-Dinh-Tuan (1978) On the Fitting of Multivariate Processes of the

Autoregressive-Moving Average Type Biometrika 65 99-107

Priestley MB (1980) System Identification Kalman Filtering and Stochastic Con-

trol in Directions in Time Series eds DR Brillinger and GC Tiao Institute

of Mathematical Statistics

Whittle P (1963) On the Fitting of Multivariate Autoregressions and the Approxi-

mate Canonical Factorization of a Spectral Density Matrix Biometrika 50 129-

134

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The correct bibliographic citation for this manual is as follows SAS Institute Inc SAS

ET S U serrsquos Gui de Version 8 Cary NC SAS Institute Inc 1999 1546 pp

SASET S Userrsquos Guide Version 8

Copyright copy 1999 by S AS I ns t it u t e I nc Ca ry N C U S A

ISBN 1ndash58025ndash489ndash6

All r ight s res erved P r int ed in t he U n it ed St a t es of A merica N o pa rt of t his p u blica t ionma y b e reprodu ced s t ored in a ret r ieva l s ys t em or t ra ns mit t ed in a ny form or b y a nymeans electronic mechanical photocopying or otherwise without the prior writtenpermission of the publisher SAS Institute Inc

US Government Restricted Rights Notice Use du plicat ion or disclosure of the

software by the government is subject to restrictions as set forth in FAR 52227ndash19Commercial Computer Softw ar e-Restricted Rights (J une 1987)

SAS Institute Inc SAS Campus Drive Cary North Carolina 27513

1st print ing October 1999

S AS reg a nd a ll ot her SA S I ns t it u t e I nc p rodu ct or s ervice na m es a re regis t ered t ra dema rkst d k f SA S I t it t I i t h U S A d t h t i reg i di t U SA

Page 19: Statespace Model

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Chapter 18 Syntax

LAGMAX= k

specifies the number of lags for which the sample autocovariance matrix is computed

The LAGMAX= option controls the number of lags printed in the schematic repre-

sentation of the autocorrelations

The sample autocovariance matrix of lag i denoted asC

i

is computed as

C

i

=

1

N 1

N

X

t = 1 + i

x

t

x

0

t i

wherex

t

is the differenced and centered data andN

is the number of observations

(If the NOCENTER option is specified 1 is not subtracted fromN

) LAGMAX= k

specifies thatC

0

throughC

k

are computed The default is LAGMAX=10

NOCENTER

prevents subtraction of the sample mean from the input series (after any specified

differencing) before the analysis

Options for Preliminary Autoregressive Models ARMAX= n

specifies the maximum order of the preliminary autoregressive models The AR-

MAX= option controls the autoregressive orders for which information criteria are

printed and controls the number of lags printed in the schematic representation of

partial autocorrelations The default is ARMAX=10 See Preliminary Autoregres-

sive Models later in this chapter for details

MINIC

writes to the OUTAR= data set only the preliminary Yule-Walker estimates for the

VAR model producing the minimum AIC See OUTAR= Data Set later in this chap-

ter for details

OUTAR= SAS-data-set

writes the Yule-Walker estimates of the preliminary autoregressive models to a SAS

data set See OUTAR= Data Set later in this chapter for details

PRINTOUT= SHORT | LONG | NONE

determines the amount of detail printed PRINTOUT=LONG prints the lagged co-

variance matrices the partial autoregressive matrices and estimates of the resid-

ual covariance matrices from the sequence of autoregressive models PRINT-

OUT=NONE suppresses the output for the preliminary autoregressive models The

descriptive statistics and state space model estimation output are still printed when

PRINTOUT=NONE is specified PRINTOUT=SHORT is the default

Canonical Correlation Analysis Options

CANCORR

prints the canonical correlations and information criterion for each candidate state

vector considered See Canonical Correlation Analysis later in this chapter for

details

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Part 2 General Information

DIMMAX= n

specifies the upper limit to the dimension of the state vector The DIMMAX= option

can be used to limit the size of the model selected The default is DIMMAX=10

PASTMIN= n

specifies the minimum number of lags to include in the canonical correlation analy-

sis The default is PASTMIN=0 See Canonical Correlation Analysis later in thischapter for details

SIGCORR= value

specifies the multiplier of the degrees of freedom for the penalty term in the informa-

tion criterion used to select the state space form The default is SIGCORR=2 The

larger the value of the SIGCORR= option the smaller the state vector tends to be

Hence a large value causes a simpler model to be fit See Canonical Correlations

Analysis later in this chapter for details

State Space Model Estimation Options

COVB

prints the inverse of the observed information matrix for the parameter estimatesThis matrix is an estimate of the covariance matrix for the parameter estimates

DETTOL= value

specifies the convergence criterion The DETTOL= and PARMTOL= option values

are used together to test for convergence of the estimation process If during an

iteration the relative change of the parameter estimates is less than the PARMTOL=

value and the relative change of the determinant of the innovation variance matrix

is less than the DETTOL= value then iteration ceases and the current estimates are

accepted The default is DETTOL=1E-5

ITPRINT

prints the iterations during the estimation process

KLAG= n

sets an upper limit for the number of lags of the sample autocovariance matrix used

in computing the approximate likelihood function If the data have a strong moving

average character a larger KLAG= value may be necessary to obtain good estimates

The default is KLAG=15 See Parameter Estimation later in this chapter for details

MAXIT= n

sets an upper limit to the number of iterations in the maximum likelihood or condi-

tional least-squares estimation The default is MAXIT=50

NOEST

suppresses the final maximum likelihood estimation of the selected model

OUTMODEL= SAS-data-set

writes the parameter estimates and their standard errors to a SAS data set See OUT-

MODEL= Data Set later in this chapter for details

PARMTOL= value

specifies the convergence criterion The DETTOL= and PARMTOL= option values

are used together to test for convergence of the estimation process If during an

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Chapter 18 Syntax

iteration the relative change of the parameter estimates is less than the PARMTOL=

value and the relative change of the determinant of the innovation variance matrix

is less than the DETTOL= value then iteration ceases and the current estimates are

accepted The default is PARMTOL=001

RESIDEST

computes the final estimates using conditional least squares on the raw data Thistype of estimation may be more stable than the default maximum likelihood method

but is usually more computationally expensive See Parameter Estimation later in

this chapter for details of the conditional least squares method

SINGULAR= value

specifies the criterion for testing for singularity of a matrix A matrix is declared

singular if a scaled pivot is less than the SINGULAR= value when sweeping the

matrix The default is SINGULAR=1E-7

Forecasting Options

BACK= n

starts forecasting n periods before the end of the input data The BACK= option valuemust not be greater than the number of observations The default is BACK=0

INTERVAL= interval

specifies the time interval between observations The INTERVAL= value is used

in conjunction with the ID variable to check that the input data are in order and

have no missing periods The INTERVAL= option is also used to extrapolate the ID

values past the end of the input data See Chapter 3 ldquoDate Intervals Formats and

Functionsrdquo for details on the INTERVAL= values allowed

INTPER= n

specifies that each input observation corresponds to n time periods For example

the options INTERVAL=MONTH and INTPER=2 specify bimonthly data and are

equivalent to specifying INTERVAL=MONTH2 If the INTERVAL= option is not

specified the INTPER= option controls the increment used to generate ID values for

the forecast observations The default is INTPER=1

LEAD= n

specifies how many forecast observations are produced The forecasts start at the

point set by the BACK= option The default is LEAD=0 which produces no fore-

casts

OUT= SAS-data-set

writes the residuals actual values forecasts and forecast standard errors to a SAS

data set See OUT= Data Set later in this chapter for details

PRINT

prints the forecasts

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Part 2 General Information

BY Statement

BY variable

A BY statement can be used with the STATESPACE procedure to obtain separateanalyses on observations in groups defined by the BY variables

FORM Statement

FORM variable value

The FORM statement specifies the number of times a variable is included in the state

vector Values can be specified for any variable listed in the VAR statement If a

value is specified for each variable in the VAR statement the state vector for the state

space model is entirely specified and automatic selection of the state space model is

not performed

The FORM statement forces the state vectorz

t

to contain a specific variable a given

number of times For example if Y is one of the variables inx

t

then the statement

form y 3

forces the state vector to containY

t

Y

t + 1 j t

andY

t + 2 j t

possibly along with other

variables

The following statements illustrate the use of the FORM statement

proc statespace data=in

var x y

f o r m x 3 y 2

run

These statements fit a state space model with the following state vector

z

t

=

2

6

6

6

6

4

x

t j t

y

t j t

x

t + 1 j t

y

t + 1 j t

x

t + 2 j t

3

7

7

7

7

5

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Chapter 18 Syntax

ID Statement

ID variable

The ID statement specifies a variable that identifies observations in the input data setThe variable specified in the ID statement is included in the OUT= data set The

values of the ID variable are extrapolated for the forecast observations based on the

values of the INTERVAL= and INTPER= options

INITIAL Statement

INITIAL F (rowcolumn)= value G(row column)= value

The INITIAL statement gives initial values to the specified elements of theF

andG

matrices These initial values are used as starting values for the iterative estimation

Parts of theF

andG

matrices represent fixed structural identities If an element

specified is a fixed structural element instead of a free parameter the corresponding

initialization is ignored

The following is an example of an INITIAL statement

initial f(32)=0 g(41)=0 g(51)=0

RESTRICT Statement

RESTRICT F(rowcolumn)= value G(rowcolumn)= value

The RESTRICT statement restricts the specified elements of theF

andG

matrices

to the specified values

To use the restrict statement you need to know the form of the model Either specify

the form of the model with the FORM statement or do a preliminary run perhaps

with the NOEST option to find the form of the model that PROC STATESPACE

selects for the data

The following is an example of a RESTRICT statement

restrict f(32)=0 g(41)=0 g(51)=0

Parts of theF

andG

matrices represent fixed structural identities If a restriction is

specified for an element that is a fixed structural element instead of a free parameter

the restriction is ignored

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Part 2 General Information

VAR Statement

VAR variable (difference difference )

The VAR statement specifies the variables in the input data set to model and fore-cast The VAR statement also specifies differencing of the input variables The VAR

statement is required

Differencing is specified by following the variable name with a list of difference

periods separated by commas See the section Stationarity and Differencing for

more information on differencing of input variables

The order in which variables are listed in the VAR statement controls the order in

which variables are included in the state vector Usually potential inputs should be

listed before potential outputs

For example assuming the input data are monthly the following VAR statement

specifies modeling and forecasting of the one period and seasonal second differenceof X and Y

var x(112) y(112)

In this example the vector time series analyzed is

x

t

=

1 B 1 B

1 2

X

t

x

1 B 1 B

1 2

Y

t

y

where B represents the back shift operator and x and y represent the means of thedifferenced series If the NOCENTER option is specified the mean differences are

not subtracted

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Chapter 18 Details

Details

Missing Values

The STATESPACE procedure does not support missing values The procedure uses

the first contiguous group of observations with no missing values for any of the VARstatement variables Observations at the beginning of the data set with missing values

for any VAR statement variable are not used or included in the output data set

Stationarity and Differencing

The state space model used by the STATESPACE procedure assumes that the time

series are stationary Hence the data should be checked for stationarity One way

to check for stationarity is to plot the series A graph of series over time can show a

time trend or variability changes

You can also check stationarity by using the sample autocorrelation functions dis-

played by the ARIMA procedure The autocorrelation functions of nonstationaryseries tend to decay slowly See Chapter 7 ldquoThe ARIMA Procedurerdquo for more in-

formation

Another alternative is to use the STATIONARITY= option on the IDENTIFY state-

ment in PROC ARIMA to apply Dickey-Fuller tests for unit roots in the time series

See Chapter 7 ldquoThe ARIMA Procedurerdquo for more information on Dickey-Fuller unit

root tests

The most popular way to transform a nonstationary series to stationarity is by dif-

ferencing Differencing of the time series is specified in the VAR statement For

example to take a simple first difference of the series X use this statement

var x(1)

In this example the change in X from one period to the next is analyzed When the se-

ries has a seasonal pattern differencing at a period equal to the length of the seasonal

cycle may be desirable For example suppose the variable X is measured quarterly

and shows a seasonal cycle over the year You can use the following statement to

analyze the series of changes from the same quarter in the previous year

var x(4)

To difference twice add another differencing period to the list For ex-

ample the following statement analyzes the series of second differences

X

t

X

t 1

X

t 1

X

t 2

= X

t

2 X

t 1

+ X

t 2

var x(11)

The following statement analyzes the seasonal second difference series

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Part 2 General Information

var x(14)

The series modeled is the 1-period difference of the 4-period difference

X

t

X

t 4

X

t 1

X

t 5

= X

t

X

t 1

X

t 4

+ X

t 5

Another way to obtain stationary series is to use a regression on time to de-trend the

data If the time series has a deterministic linear trend regressing the series on time

produces residuals that should be stationary The following statements write residuals

of X and Y to the variable RX and RY in the output data set DETREND

data a

set a

t=_n_

run

proc reg data=a

model x y = t

output out=detrend r=rx ry

run

You then use PROC STATESPACE to forecast the de-trended series RX and RY A

disadvantage of this method is that you need to add the trend back to the forecast

series in an additional step A more serious disadvantage of the de-trending method

is that it assumes a deterministic trend In practice most time series appear to have a

stochastic rather than a deterministic trend Differencing is a more flexible and often

more appropriate method

There are several other methods to handle nonstationary time series For more infor-

mation and examples refer to Brockwell and Davis (1991)

Preliminary Autoregressive Models

After computing the sample autocovariance matrices PROC STATESPACE fits a

sequence of vector autoregressive models These preliminary autoregressive models

are used to estimate the autoregressive order of the process and limit the order of the

autocovariances considered in the state vector selection process

Yule-Walker Equations for Forward and Backward Models

Unlike a univariate autoregressive model a multivariate autoregressive model has

different forms depending on whether the present observation is being predicted from

the past observations or from the future observations

Letx

t

be the r -component stationary time series given by the VAR statement after

differencing and subtracting the vector of sample means (If the NOCENTER option

is specified the mean is not subtracted) Let n be the number of observations of x

t

from the input data set

Lete

t

be a vector white noise sequence with mean vector 0 and variance matrix

p

and letn

t

be a vector white noise sequence with mean vector 0 and variance matrix

p

Let p be the order of the vector autoregressive model forx

t

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Chapter 18 Details

The forward autoregressive form based on the past observations is written as follows

x

t

=

p

X

i = 1

p

i

x

t i

+ e

t

The backward autoregressive form based on the future observations is written as fol-

lows

x

t

=

p

X

i = 1

p

i

x

t + i

+ n

t

LettingE

denote the expected value operator the autocovariance sequence for thex

t

series

i

is

i

= E x

t

x

0

t i

The Yule-Walker equations for the autoregressive model that matches the first p ele-

ments of the autocovariance sequence are

2

6

6

4

0

1

p 1

0

1

0

p 2

0

p 1

0

p 2

0

3

7

7

5

2

6

6

4

p

1

p

2

p

p

3

7

7

5

=

2

6

6

4

1

2

p

3

7

7

5

and

2

6

6

6

4

0

0

1

0

p 1

1

0

0

p 2

p 1

p 2

0

3

7

7

7

5

2

6

6

4

p

1

p

2

p

p

3

7

7

5

=

2

6

6

4

0

1

0

2

0

p

3

7

7

5

Here

p

i

are the coefficient matrices for the past observation form of the vector au-toregressive model and

p

i

are the coefficient matrices for the future observation

form More information on the Yule-Walker equations in the multivariate setting can

be found in Whittle (1963) and Ansley and Newbold (1979)

The innovation variance matrices for the two forms can be written as follows

p

=

0

p

X

i = 1

p

i

0

i

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Part 2 General Information

p

=

0

p

X

i = 1

p

i

i

The autoregressive models are fit to the data using the preceding Yule-Walker equa-

tions with

i

replaced by the sample covariance sequenceC

i

The covariance matri-

ces are calculated as

C

i

=

1

N 1

N

X

t = i + 1

x

t

x

0

t i

Let b

p

b

p

b

p

and b

p

represent the Yule-Walker estimates of

p

p

p

and

p

respectively These matrices are written to an output data set when the OUTAR=

option is specified

When the PRINTOUT=LONG option is specified the sequence of matrices b

p

and

the corresponding correlation matrices are printed The sequence of matricesb

p isused to compute Akaike information criteria for selection of the autoregressive order

of the process

Akaike Information Criterion

The Akaike information criterion or AIC is defined as -2( maximum of log likeli-

hood )+2(number of parameters) Since the vector autoregressive models are esti-

mates from the Yule-Walker equations not by maximum likelihood the exact likeli-

hood values are not available for computing the AIC However for the vector autore-

gressive model the maximum of the log likelihood can be approximated as

l n L

n

2

l n j

b

p

j

Thus the AIC for the order p model is computed as

A I C

p

= n l n j

b

p

j + 2 p r

2

You can use the printed AIC array to compute a likelihood ratio test of the autoregres-

sive order The log-likelihood ratio test statistic for testing the order p model against

the order p 1

model is

n l n j

b

p

j + n l n j

b

p 1

j

This quantity is asymptotically distributed as a

2 withr

2 degrees of freedom if the

series is autoregressive of order p 1

It can be computed from the AIC array as

A I C

p 1

A I C

p

+ 2 r

2

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Chapter 18 Details

You can evaluate the significance of these test statistics with the PROBCHI function

in a SAS DATA step or with a

2 table

Determining the Autoregressive Order

Although the autoregressive models can be used for prediction their primary value is

to aid in the selection of a suitable portion of the sample covariance matrix for use in

computing canonical correlations If the multivariate time series x

t is of autoregres-sive order p then the vector of past values to lag p is considered to contain essentially

all the information relevant for prediction of future values of the time series

By default PROC STATESPACE selects the order p producing the autoregressive

model with the smallestA I C

p

If the value p for the minimumA I C

p

is less than the

value of the PASTMIN= option then p is set to the PASTMIN= value Alternatively

you can use the ARMAX= and PASTMIN= options to force PROC STATESPACE to

use an order you select

Significance Limits for Partial Autocorrelations

The STATESPACE procedure prints a schematic representation of the partial autocor-

relation matrices indicating which partial autocorrelations are significantly greater orsignificantly less than 0 Figure 1810 shows an example of this table

The STATESPACE Procedure

Schematic Representation of Partial Autocorrelations

NameLag 1 2 3 4 5 6 7 8 9 10

x ++ +

y ++

+ is gt 2std error - is lt -2std error is between

Figure 1810 Significant Partial Autocorrelations

The partial autocorrelations are from the sample partial autoregressive matrices b

p

p

The standard errors used for the significance limits of the partial autocorrelations are

computed from the sequence of matrices

p

and

p

Under the assumption that the observed series arises from an autoregressive process

of order p 1

the pth sample partial autoregressive matrix b

p

p

has an asymptotic

variance matrix 1

n

1

p

p

The significance limits for b

p

p

used in the schematic plot of the sample partial autore-

gressive sequence are derived by replacing

p

and

p

with their sample estimators

to produce the variance estimate as follows

d

V a r

b

p

p

=

1

n r p

b

1

p

b

p

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Part 2 General Information

Canonical Correlation Analysis

Given the order p letp

t

be the vector of current and past values relevant to prediction

of x

t + 1

p

t

= x

0

t

x

0

t 1

x

0

t p

0

Letf

t

be the vector of current and future values

f

t

= x

0

t

x

0

t + 1

x

0

t + p

0

In the canonical correlation analysis consider submatrices of the sample covariance

matrix of p

t

andf

t

This covariance matrixV

has a block Hankel form

V =

2

6

6

6

4

C

0

C

0

1

C

0

2

C

0

p

C

0

1

C

0

2

C

0

3

C

0

p + 1

C

0

p

C

0

p + 1

C

0

p + 2

C

0

2 p

3

7

7

7

5

State Vector Selection Process

The canonical correlation analysis forms a sequence of potential state vectorsz

j

t

Examine a sequencef

j

t

of subvectors of f

t

and form the submatrixV

j consisting

of the rows and columns of V

corresponding to the components of f

j

t

and compute

its canonical correlations

The smallest canonical correlation of V

j is then used in the selection of the com-

ponents of the state vector The selection process is described in the following For

more details about this process refer to Akaike (1976)

In the following discussion the notationx

t + k j t

denotes the wide sense conditional

expectation (best linear predictor) of x

t + k

given allx

s

with s less than or equal to t

In the notationx

i t + 1

the first subscript denotes the ith component of x

t + 1

The initial state vectorz

1

t

is set tox

t

The sequencef

j

t

is initialized by setting

f

1

t

= z

1

0

t

x

1 t + 1 j t

0

= x

0

t

x

1 t + 1 j t

0

That is start by considering whether to addx

1 t + 1 j t

to the initial state vectorz

1

t

The procedure forms the submatrixV

1 corresponding tof

1

t

and computes its canon-

ical correlations Denote the smallest canonical correlation of V

1 as

m i n

If

m i n

is

significantly greater than 0x

1 t + 1 j t

is added to the state vector

If the smallest canonical correlation of V

1 is not significantly greater than 0 then a

linear combination of f

1

t

is uncorrelated with the pastp

t

Assuming that the determi-

nant of C

0

is not 0 (that is no input series is a constant) you can take the coefficient

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Chapter 18 Details

of x

1 t + 1 j t

in this linear combination to be 1 Denote the coefficients of z

1

t

in this

linear combination as

This gives the relationship

x

1 t + 1 j t

=

0

x

t

Therefore the current state vector already contains all the past information useful forpredicting

x

1 t + 1

and any greater leads of x

1 t

The variablex

1 t + 1 j t

is not added to

the state vector nor are any termsx

1 t + k j t

considered as possible components of the

state vector The variablex

1

is no longer active for state vector selection

The process described forx

1 t + 1 j t

is repeated for the remaining elements of f

t

The

next candidate for inclusion in the state vector is the next component of f

t

corre-

sponding to an active variable Components of f

t

corresponding to inactive variables

that produced a zero

m i n

in a previous step are skipped

Denote the next candidate asx

l t + k j t

The vectorf

j

t

is formed from the current state

vector andx

l t + k j t

as follows

f

j

t

= z

j

0

t

x

l t + k j t

0

The matrixV

j is formed fromf

j

t

and its canonical correlations are computed The

smallest canonical correlation of V

j is judged to be either greater than or equal to 0

If it is judged to be greater than 0x

l t + k j t

is added to the state vector If it is judged

to be 0 then a linear combination of f

j

t

is uncorrelated with thep

t

and the variable

x

l

is now inactive

The state vector selection process continues until no active variables remain

Testing Significance of Canonical Correlations For each step in the canonical correlation sequence the significance of the small-

est canonical correlation

m i n

is judged by an information criterion from Akaike

(1976) This information criterion is

n l n 1

2

m i n

r p + 1 q + 1

where q is the dimension of f

j

t

at the current step r is the order of the state vector p

is the order of the vector autoregressive process and

is the value of the SIGCORR=

option The default is SIGCORR=2 If this information criterion is less than or equal

to 0

m i n

is taken to be 0 otherwise it is taken to be significantly greater than 0

(Do not confuse this information criterion with the AIC)

Variables inx

t + p j t

are not added in the model even with positive information crite-

rion because of the singularity of V

You can force the consideration of more candi-

date state variables by increasing the size of theV

matrix by specifying a PASTMIN=

option value larger than p

Printing the Canonical Correlations

To print the details of the canonical correlation analysis process specify the CAN-

CORR option in the PROC STATESPACE statement The CANCORR option prints

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Part 2 General Information

the candidate state vectors the canonical correlations and the information criteria for

testing the significance of the smallest canonical correlation

Bartlettrsquos

2 and its degrees of freedom are also printed when the CANCORR option

is specified The formula used for Bartlettrsquos

2 is

2

= n 5 r p + 1 q + 1 l n 1

2

m i n

withr p + 1 q + 1

degrees of freedom

Figure 1811 shows the output of the CANCORR option for the introductory example

shown in the Getting Started section of this chapter

The STATESPACE Procedure

Canonical Correlations Analysis

Information Chi

x(TT) y(TT) x(T+1T) Criterion Square DF

1 1 0237045 3566167 114505 4

Information Chi

x(TT) y(TT) x(T+1T) y(T+1T) Criterion Square DF

1 1 0238244 0056565 -535906 0636134 3

Information Chi

x(TT) y(TT) x(T+1T) x(T+2T) Criterion Square DF

1 1 0237602 0087493 -446312 1525353 3

Figure 1811 Canonical Correlations Analysis

New variables are added to the state vector if the information criteria are positive Inthis example

y

t + 1 j t

andx

t + 2 j t

are not added to the state space vector because the

information criteria for these models are negative

If the information criterion is nearly 0 then you may want to investigate models

that arise if the opposite decision is made regarding

m i n

This investigation can be

accomplished by using a FORM statement to specify part or all of the state vector

Preliminary Estimates of F

When a candidate variablex

l t + k j t

yields a zero

m i n

and is not added to the state

vector a linear combination of f

j

t

is uncorrelated with thep

t

Because of the method

used to construct thef

j

t

sequence the coefficient of x

l t + k j t

inl

can be taken as 1

Denote the coefficients of z

j

t

in this linear combination as l

This gives the relationship

x

l t + k j t

= l

0

z

j

t

The vectorl

is used as a preliminary estimate of the first r columns of the row of the

transition matrixF

corresponding tox

l t + k 1 j t

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Chapter 18 Details

Parameter Estimation

The model isz

t + 1

= F z

t

+ G e

t + 1

wheree

t

is a sequence of independent multivari-

ate normal innovations with mean vector 0 and variance

e e

The observed sequence

x

t

composes the first r components of z

t

and thusx

t

= H z

t

where H is ther s

matrix I

r

0

LetE

be ther n

matrix of innovations

E = e

1

e

n

If the number of observations n is reasonably large the log likelihood L can be

approximated up to an additive constant as follows

L =

n

2

l n j

e e

j

1

2

t r a c e

1

e e

E E

0

The elements of

e e

are taken as free parameters and are estimated as follows

S

0

=

1

n

E E

0

Replacing

e e

byS

0

in the likelihood equation the log likelihood up to an additive

constant is

L =

n

2

l n j S

0

j

Letting B be the backshift operator the formal relation betweenx

t

ande

t

is

x

t

= H I B F

1

G e

t

e

t

= H I B F

1

G

1

x

t

=

1

X

i = 0

i

x

t i

LettingC

i

be the ith lagged sample covariance of x

t

and neglecting end effects the

matrixS

0

is

S

0

=

1

X

i j = 0

i

C

i + j

0

j

For the computation of S

0

the infinite sum is truncated at the value of the KLAG=

option The value of the KLAG= option should be large enough that the sequence

i

is approximately 0 beyond that point

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Part 2 General Information

Let

be the vector of free parameters in theF

andG

matrices The derivative of the

log likelihood with respect to the parameter

is

L

=

n

2

t r a c e

S

1

0

S

0

The second derivative is

2

L

0

=

n

2

t r a c e

S

1

0

S

0

0

S

1

0

S

0

t r a c e

S

1

0

2

S

0

0

Near the maximum the first term is unimportant and the second term can be approx-

imated to give the following second derivative approximation

2

L

0

=

n t r a c e

S

1

0

E

E

0

0

The first derivative matrix and this second derivative matrix approximation are com-

puted from the sample covariance matrixC

0

and the truncated sequence

i

The

approximate likelihood function is maximized by a modified Newton-Raphson algo-

rithm employing these derivative matrices

The matrixS

0

is used as the estimate of the innovation covariance matrix

e e

The

negative of the inverse of the second derivative matrix at the maximum is used as an

approximate covariance matrix for the parameter estimates The standard errors of

the parameter estimates printed in the parameter estimates tables are taken from the

diagonal of this covariance matrix The parameter covariance matrix is printed when

the COVB option is specified

If the data are nearly nonstationary a better estimate of

e e

and the other parameters

can sometimes be obtained by specifying the RESIDEST option The RESIDEST

option estimates the parameters using conditional least squares instead of maximum

likelihood

The residuals are computed using the state space equation and the sample mean val-

ues of the variables in the model as start-up values The estimate of S

0

is then com-

puted using the residuals from the ith observation on where i is the maximum number

of times any variable occurs in the state vector A multivariate Gauss-Marquardt al-

gorithm is used to minimizej S

0

j

Refer to Harvey (1981a) for a further description

of this method

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Chapter 18 Details

Forecasting

Given estimates of F

G

and

e e

forecasts of x

t

are computed from the conditional

expectation of z

t

In forecasting the parameters F G and

e e

are replaced with the estimates or by val-

ues specified in the RESTRICT statement One-step-ahead forecasting is performedfor the observation

x

t

wheret n b

Heren

is the number of observations and

b is the value of the BACK= option For the observationx

t

wheret n b

m-

step-ahead forecasting is performed form = t n + b

The forecasts are generated

recursively with the initial conditionz

0

= 0

The m-step-ahead forecast of z

t + m

isz

t + m j t

wherez

t + m j t

denotes the conditional

expectation of z

t + m

given the information available at time t The m-step-ahead

forecast of x

t + m

isx

t + m j t

= H z

t + m j t

where the matrixH = I

r

0

Let

i

= F

i

G

Note that the lasts r

elements of z

t

consist of the elements of x

u j t

foru t

The state vector z

t + m

can be represented as

z

t + m

= F

m

z

t

+

m 1

X

i = 0

i

e

t + m i

Sincee

t + i j t

= 0

fori 0

the m-step-ahead forecastz

t + m j t

is

z

t + m j t

= F

m

z

t

= F z

t + m 1 j t

Therefore the m-step-ahead forecast of x

t + m

is

x

t + m j t

= H z

t + m j t

The m-step-ahead forecast error is

z

t + m

z

t + m j t

=

m 1

X

i = 0

i

e

t + m i

The variance of the m-step-ahead forecast error is

V

z m

=

m 1

X

i = 0

i

e e

0

i

LettingV

z 0

= 0

the variance of the m-step-ahead forecast error of z

t + m

V

z m

can

be computed recursively as follows

V

z m

= V

z m 1

+

m 1

e e

0

m 1

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Part 2 General Information

The variance of the m-step-ahead forecast error of x

t + m

is ther r

left upper sub-

matrix of V

z m

that is

V

x m

= H V

z m

H

0

Unless the NOCENTER option is specified the sample mean vector is added to the

forecast When differencing is specified the forecasts xt + m j t

plus the sample mean

vector are integrated back to produce forecasts for the original series

Lety

t

be the original series specified by the VAR statement with some 0 values

appended corresponding to the unobserved past observations Let B be the backshift

operator and let B

be thes s

matrix polynomial in the backshift operator

corresponding to the differencing specified by the VAR statement The off-diagonal

elements of

i

are 0 Note that

0

= I

s

whereI

s

is thes s

identity matrix Then

z

t

= B y

t

This gives the relationship

y

t

=

1

B z

t

=

1

X

i = 0

i

z

t i

where

1

B =

P

1

i = 0

i

B

i and

0

= I

s

The m-step-ahead forecast of y

t + m

is

y

t + m j t

=

m 1

X

i = 0

i

z

t + m i j t

+

1

X

i = m

i

z

t + m i

The m-step-ahead forecast error of y

t + m

is

m 1

X

i = 0

i

z

t + m i

z

t + m i j t

=

m 1

X

i = 0

i

X

u = 0

u

i u

e

t + m i

LettingV

y 0

= 0

the variance of the m-step-ahead forecast error of y

t + m

V

y m

is

V

y m

=

m 1

X

i = 0

i

X

u = 0

u

i u

e e

i

X

u = 0

u

i u

0

= V

y m 1

+

m 1

X

u = 0

u

m 1 u

e e

m 1

X

u = 0

u

m 1 u

0

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Chapter 18 Details

Relation of ARMA and State Space Forms

Every state space model has an ARMA representation and conversely every ARMA

model has a state space representation This section discusses this equivalence The

following material is adapted from Akaike (1974) where there is a more complete

discussion Pham-Dinh-Tuan (1978) also contains a discussion of this material

Suppose you are given the following ARMA model

B x

t

= B e

t

or in more detail

x

t

1

x

t 1

p

x

t p

= e

t

+

1

e

t 1

+ +

q

e

t q

(1)

wheree

t

is a sequence of independent multivariate normal random vectors with mean

0 and variance matrix

e e

B is the backshift operator (B x

t

= x

t 1

) B

and

B

are matrix polynomials in B andx

t

is the observed process

If the roots of the determinantial equationj B j = 0

are outside the unit circle in

the complex plane the model can also be written as

x

t

=

1

B B e

t

=

1

X

i = 0

i

e

t i

The

i

matrices are known as the impulse response matrices and can be computed

as

1

B B

You can assume p q

since if this is not initially true you can add more terms

i

that are identically 0 without changing the model

To write this set of equations in a state space form proceed as follows Letx

t + i j t

be

the conditional expectation of x

t + i

givenx

w

forw t

The following relations hold

x

t + i j t

=

1

X

j = i

j

e

t + i j

x

t + i j t + 1

= x

t + i j t

+

i 1

e

t + 1

However from equation (1) you can derive the following relationship

x

t + p j t

=

1

x

t + p 1 j t

+ +

p

x

t

(2)

Hence wheni = p

you can substitute forx

t + p j t

in the right-hand side of equation

(2) and close the system of equations

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Part 2 General Information

This substitution results in the following model in the state space form

z

t + 1

= F z

t

+ G e

t + 1

2

6

6

6

4

x

t + 1

x

t + 2 j t + 1

x

t + p j t + 1

3

7

7

7

5

=

2

6

6

4

0 I 0 0

0 0 I 0

p

p 1

1

3

7

7

5

2

6

6

6

4

x

t

x

t + 1 j t

x

t + p 1 j t

3

7

7

7

5

+

2

6

6

4

I

1

p 1

3

7

7

5

e

t + 1

Note that the state vectorz

t

is composed of conditional expectations of x

t

and the

first r components of z

t

are equal tox

t

The state space form can be cast into an ARMA form by solving the system of dif-

ference equations for the first r components

When converting from an ARMA form to a state space form you can generate a

state vector larger than needed that is the state space model may not be a minimal

representation When going from a state space form to an ARMA form you can have

nontrivial common factors in the autoregressive and moving average operators that

yield an ARMA model larger than necessary

If the state space form used is not a minimal representation some but not all compo-

nents of x

t + i j t

may be linearly dependent This situation corresponds to

p

p 1

being of less than full rank when B

and B

have no common nontrivial

left factors In this casez

t

consists of a subset of the possible components of

x

t + i j t

i = 1 2 p 1

However once a component of x

t + i j t

(for example the

jth one) is linearly dependent on the previous conditional expectations then all sub-

sequent jth components of x

t + k j t

fork i

must also be linearly dependent Note

that in this case equivalent but seemingly different structures can arise if the order of

the components withinx

t

is changed

OUT= Data Set

The forecasts are contained in the output data set specified by the OUT= option on the

PROC STATESPACE statement The OUT= data set contains the following variables

the BY variables

the ID variable

the VAR statement variables These variables contain the actual values from

the input data set

FORi numeric variables containing the forecasts The variable FORi contains

the forecasts for the ith variable in the VAR statement list Forecasts are one-

step-ahead predictions until the end of the data or until the observation specified

by the BACK= option

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Chapter 18 Details

RESi numeric variables containing the residual for the forecast of the ith vari-

able in the VAR statement list For forecast observations the actual values are

missing and the RESi variables contain missing values

STDi numeric variables containing the standard deviation for the forecast of

the ith variable in the VAR statement list The values of the STDi variables can

be used to construct univariate confidence limits for the corresponding fore-casts However such confidence limits do not take into account the covariance

of the forecasts

OUTAR= Data Set

The OUTAR= data set contains the estimates of the preliminary autoregressive mod-

els The OUTAR= data set contains the following variables

ORDER a numeric variable containing the order p of the autoregressive model

that the observation represents

AIC a numeric variable containing the value of the information criterionA I C

p

SIGFl numeric variables containing the estimate of the innovation covariance

matrices for the forward autoregressive models The variable SIGFl contains

the lth column of b

p

in the observations with ORDER= p

SIGBl numeric variables containing the estimate of the innovation covariance

matrices for the backward autoregressive models The variable SIGBl contains

the lth column of b

p

in the observations with ORDER= p

FORk ndashl numeric variables containing the estimates of the autoregressive pa-

rameter matrices for the forward models The variable FORk ndashl contains the

lth column of the lag k autoregressive parameter matrix b

p

k

in the observations

with ORDER= p

BACk ndashl numeric variables containing the estimates of the autoregressive pa-

rameter matrices for the backward models The variable BACk ndashl contains the

lth column of the lag k autoregressive parameter matrix b

p

k

in the observations

with ORDER= p

The estimates for the order p autoregressive model can be selected as those observa-

tions with ORDER= p Within these observations the k lth element of

p

i

is given by

the value of the FORindashl variable in the k th observation The k lth element of

p

i

is

given by the value of BACindashl variable in the k th observation The k lth element of

p

is given by SIGFl in the k th observation The k lth element of

p

is given by SIGBlin the k th observation

Table 181 shows an example of the OUTAR= data set with ARMAX=3 andx

t

of

dimension 2 In Table 181 i j

indicate the ijth element of the matrix

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Part 2 General Information

Table 181 Values in the OUTAR= Data Set

Obs ORDER AIC SIGF1 SIGF2 SIGB1 SIGB2 FOR1ndash 1 FOR1ndash 2 FOR2ndash 1 FOR2ndash 2 FOR3ndash 1

1 0 AIC0

0 1 1

0 1 2

0 1 1

0 1 2

2 0 AIC0

0 2 1

0 2 2

0 2 1

0 2 2

3 1 AIC1

1 1 1

1 1 2

1 1 1

1 1 2

1

1

1 1

1

1

1 2

4 1 AIC1

1 2 1

1 1 2

1 2 1

1 1 2

1

1

2 1

1

1

2 2

5 2 AIC2

2 1 1

2 1 2

2 1 1

2 1 2

2

1

1 1

2

1

1 2

2

2

1 1

2

2

1 2

6 2 AIC2

2 2 1

2 1 2

2 2 1

2 1 2

2

1

2 1

2

1

2 2

2

2

2 1

2

2

2 2

7 3 AIC3

3 1 1

3 1 2

3 1 1

3 1 2

3

1

1 1

3

1

1 2

3

2

1 1

3

2

1 2

3

3

1 1

8 3 AIC3

3 2 1

3 1 2

3 2 1

3 1 2

3

1

2 1

3

1

2 2

3

2

2 1

3

2

2 2

3

3

2 1

Obs FOR3ndash 2 BACK1ndash 1 BACK1ndash 2 BACK2ndash 1 BACK2ndash 2 BACK3ndash 1 BACK3ndash 2

1

2

3

1

1

1 1

1

1

1 2

4

1

1

2 1

1

1

2 2

5

2

1

1 1

2

1

1 2

2

2

1 1

2

2

1 2

6

2

1

2 1

2

1

2 2

2

2

2 1

2

2

2 2

7

3

3

1 2

3

1

1 1

3

1

1 2

3

2

1 1

3

2

1 2

3

3

1 1

3

3

1 2

8

3

3

2 2

3

1

2 1

3

1

2 2

3

2

2 1

3

2

2 2

3

3

2 1

3

3

2 2

The estimated autoregressive parameters can be used in the IML procedure to obtain

autoregressive estimates of the spectral density function or forecasts based on the

autoregressive models

OUTMODEL= Data Set

The OUTMODEL= data set contains the estimates of theF

andG

matrices and their

standard errors the names of the components of the state vector and the estimates of

the innovation covariance matrix The variables contained in the OUTMODEL= data

set are as follows

the BY variables

STATEVEC a character variable containing the name of the component of the

state vector corresponding to the observation The STATEVEC variable has

the value STD for standard deviations observations which contain the standarderrors for the estimates given in the preceding observation

Fndash j numeric variables containing the columns of theF

matrix The variable

Fndash j contains the jth column of F

The number of Fndash j variables is equal to

the value of the DIMMAX= option If the model is of smaller dimension the

extraneous variables are set to missing

Gndash j numeric variables containing the columns of theG

matrix The variable

Gndash j contains the jth column of G

The number of Gndash j variables is equal to r

the dimension of x

t

given by the number of variables in the VAR statement

SIGndash j numeric variables containing the columns of the innovation covariance

matrix The variable SIGndash j contains the jth column of

e e

There are r vari-ables SIGndash j

Table 182 shows an example of the OUTMODEL= data set withx

t

= x

t

y

t

0

z

t

= x

t

y

t

x

t + 1 j t

0 and DIMMAX=4 In Table 182F

i j

andG

i j

are the ijth

elements of F

andG

respectively Note that all elements for Fndash4 are missing because

F

is a3 3

matrix

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Chapter 18 Details

Table 182 Value in the OUTMODEL= Data Set

Obs STATEVEC Fndash 1 Fndash 2 Fndash 3 Fndash 4 Gndash 1 Gndash 2 SIGndash 1 SIGndash 2

1 X(TT) 0 0 1 1 0

1 1

1 2

2 STD

3 Y(TT) F2 1

F2 2

F2 3

0 1

2 1

2 2

4 STD std F2 1

std F2 2

std F2 3

5 X(T+1T) F3 1

F3 2

F3 3

G3 1

G3 2

6 STD std F3 1

std F3 2

std F3 3

std G3 1

std G3 2

Printed Output

The printed output produced by the STATESPACE procedure is described in the fol-

lowing

1 descriptive statistics which include the number of observations used the

names of the variables their means and standard deviations (Std) and the dif-

ferencing operations used

2 the Akaike information criteria for the sequence of preliminary autoregressive

models

3 if the PRINTOUT=LONG option is specified the sample autocovariance ma-

trices of the input series at various lags

4 if the PRINTOUT=LONG option is specified the sample autocorrelation ma-

trices of the input series

5 a schematic representation of the autocorrelation matrices showing the signif-

icant autocorrelations

6 if the PRINTOUT=LONG option is specified the partial autoregressive ma-

trices (These are

p

p

as described in Preliminary Autoregressive Modelsearlier in this chapter)

7 a schematic representation of the partial autocorrelation matrices showing the

significant partial autocorrelations

8 the Yule-Walker estimates of the autoregressive parameters for the autoregres-

sive model with the minimum AIC

9 if the PRINTOUT=LONG option is specified the autocovariance matrices of

the residuals of the minimum AIC model This is the sequence of estimated

innovation variance matrices for the solutions of the Yule-Walker equations

10 if the PRINTOUT=LONG option is specified the autocorrelation matrices of the residuals of the minimum AIC model

11 If the CANCORR option is specified the canonical correlations analysis for

each potential state vector considered in the state vector selection process

This includes the potential state vector the canonical correlations the informa-

tion criterion for the smallest canonical correlation Bartlettrsquos

2 statistic (ldquoChi

Squarerdquo) for the smallest canonical correlation and the degrees of freedom of

Bartlettrsquos

2

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Part 2 General Information

12 the components of the chosen state vector

13 the preliminary estimate of the transition matrixF

the input matrixG

and

the variance matrix for the innovations

e e

14 if the ITPRINT option is specified the iteration history of the likelihood max-

imization For each iteration this shows the iteration number the number of step halvings the determinant of the innovation variance matrix the damping

factor Lambda and the values of the parameters

15 the state vector printed again to aid interpretation of the following listing of F

andG

16 the final estimate of the transition matrixF

17 the final estimate of the input matrixG

18 the final estimate of the variance matrix for the innovations

e e

19 a table listing the estimates of the free parameters inF

andG

and their standarderrors and t statistics

20 if the COVB option is specified the covariance matrix of the parameter esti-

mates

21 if the COVB option is specified the correlation matrix of the parameter esti-

mates

22 if the PRINT option is specified the forecasts and their standard errors

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Chapter 18 Details

ODS Table Names

PROC STATESPACE assigns a name to each table it creates You can use these

names to reference the table when using the Output Delivery System (ODS) to select

tables and create output data sets These names are listed in the following table For

more information on ODS see Chapter 6 ldquoUsing the Output Delivery Systemrdquo

Table 183 ODS Tables Produced in PROC STATESPACE

ODS Table Name Description Option

NObs Number of observations default

Summary Simple summary statistics table default

InfoCriterion Information criterion table default

CovLags Covariance Matrices of Input Series PRINTOUT=LONG

CorrLags Correlation Matrices of Input Series PRINTOUT=LONG

PartialAR Partial Autoregressive Matrices PRINTOUT=LONG

YWEstimates Yule-Walker Estimates for Minimum AIC default

CovResiduals Covariance of Residuals PRINTOUT=LONGCorrResiduals Residual Correlations from AR Models PRINTOUT=LONG

StateVector State vector table default

CorrGraph Schematic Representation of Correlations default

TransitionMatrix Transition Matrix default

InputMatrix Input Matrix default

VarInnov Variance Matrix for the Innovation default

CovB Covariance of Parameter Estimates COVB

CorrB Correlation of Parameter Estimates COVB

CanCorr Canonical Correlation Analysis CANCORR

IterHistory Iterative Fitting table ITPRINT

ParameterEstimates Parameter Estimates Table default

Forecasts Forecasts Table PRINT

ConvergenceStatus Convergence Status Table default

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Part 2 General Information

Example

Example 181 Series J from Box and Jenkins

This example analyzes the gas furnace data (series J) from Box and Jenkins (The

data are not shown Refer to Box and Jenkins (1976) for the data)

First a model is selected and fit automatically using the following statements

title1 rsquoGas Furnace Datarsquo

title2 rsquoBox amp Jenkins Series Jrsquo

title3 rsquoAutomatically Selected Modelrsquo

proc statespace data=seriesj cancorr

var x y

run

The results for the automatically selected model are shown in Output 1811

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Chapter 18 Example

Output 1811 Results for Automatically Selected Model

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Number of Observations 296

Standard

Variable Mean Error

x -005683 1072766

y 5350912 3202121

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Information Criterion for Autoregressive Models

Lag=0 Lag=1 Lag=2 Lag=3 Lag=4 Lag=5 Lag=6 Lag=7 Lag=8

6513862 -103357 -163296 -164512 -165152 -164891 -164934 -164315 -163856

Information

Criterion for

Autoregressive

Models

Lag=9 Lag=10

-16348 -163359

Schematic Representation of Correlations

NameLag 0 1 2 3 4 5 6 7 8 9 10

x +- +- +- +- +- +- +- +- +- +- +-

y -+ -+ -+ -+ -+ -+ -+ -+ -+ -+ -+

+ is gt 2std error - is lt -2std error is between

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Part 2 General Information

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Schematic Representation of Partial Autocorrelations

NameLag 1 2 3 4 5 6 7 8 9 10

x + - + -

y -+ -- - + +

+ is gt 2std error - is lt -2std error is between

Yule-Walker Estimates for Minimum AIC

------Lag=1------ ------Lag=2------ ------Lag=3------ ------Lag=4------

x y x y x y x y

x 1925887 -000124 -120166 0004224 0116918 -000867 0104236 0003268

y 0050496 1299793 -002046 -03277 -071182 -025701 0195411 0133417

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Canonical Correlations Analysis

Information Chi

x(TT) y(TT) x(T+1T) Criterion Square DF

1 1 0804883 2929228 3047481 8

Information Chix(TT) y(TT) x(T+1T) y(T+1T) Criterion Square DF

1 1 0906681 0607529 1223358 1347237 7

Information Chi

x(TT) y(TT) x (T+1T) y (T+1T) x (T+2T) Criterion Square DF

1 1 0909434 0610278 0186274 -154701 1034705 6

Information Chi

x(TT) y(TT) x (T+1T) y (T+1T) y (T+2T) Criterion Square DF

1 1 091014 0618937 0206823 0940392 1280924 6

Information Chi

x(TT) y(TT) x(T+1T) y(T+1T) y(T+2T) y(T+3T) Criterion Square DF

1 1 0912963 0628785 0226598 0083258 -794103 2041584 5

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Chapter 18 Example

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Selected Statespace Form and Preliminary Estimates

State Vector

x(TT) y(TT) x(T+1T) y(T+1T) y(T+2T)

Estimate of Transition Matrix

0 0 1 0 0

0 0 0 1 0

-084718 0026794 1711715 -005019 0

0 0 0 0 1

-019785 0334274 -018174 -123557 1787475

Input Matrix for Innovation

1 0

0 11925887 -000124

0050496 1299793

0142421 1361696

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Selected Statespace Form and Preliminary Estimates

Variance Matrix for Innovation

0035274 -000734

-000734 0097569

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Part 2 General Information

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Selected Statespace Form and Fitted Model

State Vector

x(TT) y(TT) x(T+1T) y(T+1T) y(T+2T)

Estimate of Transition Matrix

0 0 1 0 0

0 0 0 1 0

-086192 0030609 1724235 -005483 0

0 0 0 0 1

-034839 0292124 -009435 -109823 1671418

Input Matrix for Innovation

1 0

0 1192442 -000416

0015621 1258495

008058 1353204

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Selected Statespace Form and Fitted Model

Variance Matrix for Innovation

0035579 -000728

-000728 0095577

Parameter Estimates

Standard

Parameter Estimate Error t Value

F(31) -086192 0072961 -1181

F(32) 0030609 0026167 117

F(33) 1724235 0061599 2799

F(34) -005483 0030169 -182

F(51) -034839 0135253 -258

F(52) 0292124 0046299 631

F(53) -009435 0096527 -098F(54) -109823 0109525 -1003

F(55) 1671418 0083737 1996

G(31) 1924420 0058162 3309

G(32) -000416 0035255 -012

G(41) 0015621 0095771 016

G(42) 1258495 0055742 2258

G(51) 0080580 0151622 053

G(52) 1353204 0091388 1481

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Chapter 18 Example

The two series are believed to have a transfer function relation with the gas rate

(variable X) as the input and the CO2

concentration (variable Y) as the output Since

the parameter estimates shown in Output 1811 support this kind of model the model

is reestimated with the feedback parameters restricted to 0 The following statements

fit the transfer function (no feedback) model

title3 rsquoTransfer Function Modelrsquo

proc statespace data=seriesj printout=none

var x y

restrict f(32)=0 f(34)=0

g(32)=0 g(41)=0 g(51)=0

run

The last 2 pages of the output are shown in Output 1812

Output 1812 STATESPACE Output for Transfer Function Model

Gas Furnace Data

Box amp Jenkins Series J

Transfer Function Model

The STATESPACE Procedure

Selected Statespace Form and Fitted Model

State Vector

x(TT) y(TT) x(T+1T) y(T+1T) y(T+2T)

Estimate of Transition Matrix

0 0 1 0 0

0 0 0 1 0

-068882 0 1598717 0 0

0 0 0 0 1

-035944 0284179 -00963 -107313 1650047

Input Matrix for Innovation

1 0

0 1

1923446 0

0 1260856

0 1346332

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Part 2 General Information

Gas Furnace Data

Box amp Jenkins Series J

Transfer Function Model

The STATESPACE Procedure

Selected Statespace Form and Fitted Model

Variance Matrix for Innovation

0036995 -00072

-00072 0095712

Parameter Estimates

Standard

Parameter Estimate Error t Value

F(31) -068882 0050549 -1363

F(33) 1598717 0050924 3139

F(51) -035944 0229044 -157

F(52) 0284179 0096944 293

F(53) -009630 0140876 -068

F(54) -107313 0250385 -429

F(55) 1650047 0188533 875G(31) 1923446 0056328 3415

G(42) 1260856 0056464 2233

G(52) 1346332 0091086 1478

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Chapter 18 References

References

Akaike H (1974) Markovian Representation of Stochastic Processes and Its Ap-

plication to the Analysis of Autoregressive Moving Average Processes Annals

of the Institute of Statistical Mathematics 26 363-387

Akaike H (1976) Canonical Correlations Analysis of Time Series and the Use of

an Information Criterion in Advances and Case Studies in System Identification

eds R Mehra and DG Lainiotis New York Academic Press

Anderson TW (1971) The Statistical Analysis of Time Series New York John

Wiley amp Sons

Ansley CF and Newbold P (1979) Multivariate Partial Autocorrelations Pro-

ceedings of the Business and Economic Statistics Section American Statistical

Association 349-353

Box GEP and Jenkins G (1976) Time Series Analysis Forecasting and Control

San Francisco Holden-Day

Brockwell PJ and Davis RA (1991) Time Series Theory and Methods 2nd Edi-

tion Springer-Verlag

Hannan EJ (1970) Multiple Time Series New York John Wiley amp Sons

Hannan EJ (1976) The Identification and Parameterization of ARMAX and State

Space Forms Econometrica 44 713-722

Harvey AC (1981a) The Econometric Analysis of Time Series New York John

Wiley amp Sons

Harvey AC (1981b) Time Series Models New York John Wiley amp Sons

Jones RH (1974) Identification and Autoregressive Spectrum Estimation IEEE Transactions on Automatic Control AC-19 894-897

Pham-Dinh-Tuan (1978) On the Fitting of Multivariate Processes of the

Autoregressive-Moving Average Type Biometrika 65 99-107

Priestley MB (1980) System Identification Kalman Filtering and Stochastic Con-

trol in Directions in Time Series eds DR Brillinger and GC Tiao Institute

of Mathematical Statistics

Whittle P (1963) On the Fitting of Multivariate Autoregressions and the Approxi-

mate Canonical Factorization of a Spectral Density Matrix Biometrika 50 129-

134

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The correct bibliographic citation for this manual is as follows SAS Institute Inc SAS

ET S U serrsquos Gui de Version 8 Cary NC SAS Institute Inc 1999 1546 pp

SASET S Userrsquos Guide Version 8

Copyright copy 1999 by S AS I ns t it u t e I nc Ca ry N C U S A

ISBN 1ndash58025ndash489ndash6

All r ight s res erved P r int ed in t he U n it ed St a t es of A merica N o pa rt of t his p u blica t ionma y b e reprodu ced s t ored in a ret r ieva l s ys t em or t ra ns mit t ed in a ny form or b y a nymeans electronic mechanical photocopying or otherwise without the prior writtenpermission of the publisher SAS Institute Inc

US Government Restricted Rights Notice Use du plicat ion or disclosure of the

software by the government is subject to restrictions as set forth in FAR 52227ndash19Commercial Computer Softw ar e-Restricted Rights (J une 1987)

SAS Institute Inc SAS Campus Drive Cary North Carolina 27513

1st print ing October 1999

S AS reg a nd a ll ot her SA S I ns t it u t e I nc p rodu ct or s ervice na m es a re regis t ered t ra dema rkst d k f SA S I t it t I i t h U S A d t h t i reg i di t U SA

Page 20: Statespace Model

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Part 2 General Information

DIMMAX= n

specifies the upper limit to the dimension of the state vector The DIMMAX= option

can be used to limit the size of the model selected The default is DIMMAX=10

PASTMIN= n

specifies the minimum number of lags to include in the canonical correlation analy-

sis The default is PASTMIN=0 See Canonical Correlation Analysis later in thischapter for details

SIGCORR= value

specifies the multiplier of the degrees of freedom for the penalty term in the informa-

tion criterion used to select the state space form The default is SIGCORR=2 The

larger the value of the SIGCORR= option the smaller the state vector tends to be

Hence a large value causes a simpler model to be fit See Canonical Correlations

Analysis later in this chapter for details

State Space Model Estimation Options

COVB

prints the inverse of the observed information matrix for the parameter estimatesThis matrix is an estimate of the covariance matrix for the parameter estimates

DETTOL= value

specifies the convergence criterion The DETTOL= and PARMTOL= option values

are used together to test for convergence of the estimation process If during an

iteration the relative change of the parameter estimates is less than the PARMTOL=

value and the relative change of the determinant of the innovation variance matrix

is less than the DETTOL= value then iteration ceases and the current estimates are

accepted The default is DETTOL=1E-5

ITPRINT

prints the iterations during the estimation process

KLAG= n

sets an upper limit for the number of lags of the sample autocovariance matrix used

in computing the approximate likelihood function If the data have a strong moving

average character a larger KLAG= value may be necessary to obtain good estimates

The default is KLAG=15 See Parameter Estimation later in this chapter for details

MAXIT= n

sets an upper limit to the number of iterations in the maximum likelihood or condi-

tional least-squares estimation The default is MAXIT=50

NOEST

suppresses the final maximum likelihood estimation of the selected model

OUTMODEL= SAS-data-set

writes the parameter estimates and their standard errors to a SAS data set See OUT-

MODEL= Data Set later in this chapter for details

PARMTOL= value

specifies the convergence criterion The DETTOL= and PARMTOL= option values

are used together to test for convergence of the estimation process If during an

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Chapter 18 Syntax

iteration the relative change of the parameter estimates is less than the PARMTOL=

value and the relative change of the determinant of the innovation variance matrix

is less than the DETTOL= value then iteration ceases and the current estimates are

accepted The default is PARMTOL=001

RESIDEST

computes the final estimates using conditional least squares on the raw data Thistype of estimation may be more stable than the default maximum likelihood method

but is usually more computationally expensive See Parameter Estimation later in

this chapter for details of the conditional least squares method

SINGULAR= value

specifies the criterion for testing for singularity of a matrix A matrix is declared

singular if a scaled pivot is less than the SINGULAR= value when sweeping the

matrix The default is SINGULAR=1E-7

Forecasting Options

BACK= n

starts forecasting n periods before the end of the input data The BACK= option valuemust not be greater than the number of observations The default is BACK=0

INTERVAL= interval

specifies the time interval between observations The INTERVAL= value is used

in conjunction with the ID variable to check that the input data are in order and

have no missing periods The INTERVAL= option is also used to extrapolate the ID

values past the end of the input data See Chapter 3 ldquoDate Intervals Formats and

Functionsrdquo for details on the INTERVAL= values allowed

INTPER= n

specifies that each input observation corresponds to n time periods For example

the options INTERVAL=MONTH and INTPER=2 specify bimonthly data and are

equivalent to specifying INTERVAL=MONTH2 If the INTERVAL= option is not

specified the INTPER= option controls the increment used to generate ID values for

the forecast observations The default is INTPER=1

LEAD= n

specifies how many forecast observations are produced The forecasts start at the

point set by the BACK= option The default is LEAD=0 which produces no fore-

casts

OUT= SAS-data-set

writes the residuals actual values forecasts and forecast standard errors to a SAS

data set See OUT= Data Set later in this chapter for details

PRINT

prints the forecasts

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Part 2 General Information

BY Statement

BY variable

A BY statement can be used with the STATESPACE procedure to obtain separateanalyses on observations in groups defined by the BY variables

FORM Statement

FORM variable value

The FORM statement specifies the number of times a variable is included in the state

vector Values can be specified for any variable listed in the VAR statement If a

value is specified for each variable in the VAR statement the state vector for the state

space model is entirely specified and automatic selection of the state space model is

not performed

The FORM statement forces the state vectorz

t

to contain a specific variable a given

number of times For example if Y is one of the variables inx

t

then the statement

form y 3

forces the state vector to containY

t

Y

t + 1 j t

andY

t + 2 j t

possibly along with other

variables

The following statements illustrate the use of the FORM statement

proc statespace data=in

var x y

f o r m x 3 y 2

run

These statements fit a state space model with the following state vector

z

t

=

2

6

6

6

6

4

x

t j t

y

t j t

x

t + 1 j t

y

t + 1 j t

x

t + 2 j t

3

7

7

7

7

5

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Chapter 18 Syntax

ID Statement

ID variable

The ID statement specifies a variable that identifies observations in the input data setThe variable specified in the ID statement is included in the OUT= data set The

values of the ID variable are extrapolated for the forecast observations based on the

values of the INTERVAL= and INTPER= options

INITIAL Statement

INITIAL F (rowcolumn)= value G(row column)= value

The INITIAL statement gives initial values to the specified elements of theF

andG

matrices These initial values are used as starting values for the iterative estimation

Parts of theF

andG

matrices represent fixed structural identities If an element

specified is a fixed structural element instead of a free parameter the corresponding

initialization is ignored

The following is an example of an INITIAL statement

initial f(32)=0 g(41)=0 g(51)=0

RESTRICT Statement

RESTRICT F(rowcolumn)= value G(rowcolumn)= value

The RESTRICT statement restricts the specified elements of theF

andG

matrices

to the specified values

To use the restrict statement you need to know the form of the model Either specify

the form of the model with the FORM statement or do a preliminary run perhaps

with the NOEST option to find the form of the model that PROC STATESPACE

selects for the data

The following is an example of a RESTRICT statement

restrict f(32)=0 g(41)=0 g(51)=0

Parts of theF

andG

matrices represent fixed structural identities If a restriction is

specified for an element that is a fixed structural element instead of a free parameter

the restriction is ignored

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Part 2 General Information

VAR Statement

VAR variable (difference difference )

The VAR statement specifies the variables in the input data set to model and fore-cast The VAR statement also specifies differencing of the input variables The VAR

statement is required

Differencing is specified by following the variable name with a list of difference

periods separated by commas See the section Stationarity and Differencing for

more information on differencing of input variables

The order in which variables are listed in the VAR statement controls the order in

which variables are included in the state vector Usually potential inputs should be

listed before potential outputs

For example assuming the input data are monthly the following VAR statement

specifies modeling and forecasting of the one period and seasonal second differenceof X and Y

var x(112) y(112)

In this example the vector time series analyzed is

x

t

=

1 B 1 B

1 2

X

t

x

1 B 1 B

1 2

Y

t

y

where B represents the back shift operator and x and y represent the means of thedifferenced series If the NOCENTER option is specified the mean differences are

not subtracted

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Chapter 18 Details

Details

Missing Values

The STATESPACE procedure does not support missing values The procedure uses

the first contiguous group of observations with no missing values for any of the VARstatement variables Observations at the beginning of the data set with missing values

for any VAR statement variable are not used or included in the output data set

Stationarity and Differencing

The state space model used by the STATESPACE procedure assumes that the time

series are stationary Hence the data should be checked for stationarity One way

to check for stationarity is to plot the series A graph of series over time can show a

time trend or variability changes

You can also check stationarity by using the sample autocorrelation functions dis-

played by the ARIMA procedure The autocorrelation functions of nonstationaryseries tend to decay slowly See Chapter 7 ldquoThe ARIMA Procedurerdquo for more in-

formation

Another alternative is to use the STATIONARITY= option on the IDENTIFY state-

ment in PROC ARIMA to apply Dickey-Fuller tests for unit roots in the time series

See Chapter 7 ldquoThe ARIMA Procedurerdquo for more information on Dickey-Fuller unit

root tests

The most popular way to transform a nonstationary series to stationarity is by dif-

ferencing Differencing of the time series is specified in the VAR statement For

example to take a simple first difference of the series X use this statement

var x(1)

In this example the change in X from one period to the next is analyzed When the se-

ries has a seasonal pattern differencing at a period equal to the length of the seasonal

cycle may be desirable For example suppose the variable X is measured quarterly

and shows a seasonal cycle over the year You can use the following statement to

analyze the series of changes from the same quarter in the previous year

var x(4)

To difference twice add another differencing period to the list For ex-

ample the following statement analyzes the series of second differences

X

t

X

t 1

X

t 1

X

t 2

= X

t

2 X

t 1

+ X

t 2

var x(11)

The following statement analyzes the seasonal second difference series

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Part 2 General Information

var x(14)

The series modeled is the 1-period difference of the 4-period difference

X

t

X

t 4

X

t 1

X

t 5

= X

t

X

t 1

X

t 4

+ X

t 5

Another way to obtain stationary series is to use a regression on time to de-trend the

data If the time series has a deterministic linear trend regressing the series on time

produces residuals that should be stationary The following statements write residuals

of X and Y to the variable RX and RY in the output data set DETREND

data a

set a

t=_n_

run

proc reg data=a

model x y = t

output out=detrend r=rx ry

run

You then use PROC STATESPACE to forecast the de-trended series RX and RY A

disadvantage of this method is that you need to add the trend back to the forecast

series in an additional step A more serious disadvantage of the de-trending method

is that it assumes a deterministic trend In practice most time series appear to have a

stochastic rather than a deterministic trend Differencing is a more flexible and often

more appropriate method

There are several other methods to handle nonstationary time series For more infor-

mation and examples refer to Brockwell and Davis (1991)

Preliminary Autoregressive Models

After computing the sample autocovariance matrices PROC STATESPACE fits a

sequence of vector autoregressive models These preliminary autoregressive models

are used to estimate the autoregressive order of the process and limit the order of the

autocovariances considered in the state vector selection process

Yule-Walker Equations for Forward and Backward Models

Unlike a univariate autoregressive model a multivariate autoregressive model has

different forms depending on whether the present observation is being predicted from

the past observations or from the future observations

Letx

t

be the r -component stationary time series given by the VAR statement after

differencing and subtracting the vector of sample means (If the NOCENTER option

is specified the mean is not subtracted) Let n be the number of observations of x

t

from the input data set

Lete

t

be a vector white noise sequence with mean vector 0 and variance matrix

p

and letn

t

be a vector white noise sequence with mean vector 0 and variance matrix

p

Let p be the order of the vector autoregressive model forx

t

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Chapter 18 Details

The forward autoregressive form based on the past observations is written as follows

x

t

=

p

X

i = 1

p

i

x

t i

+ e

t

The backward autoregressive form based on the future observations is written as fol-

lows

x

t

=

p

X

i = 1

p

i

x

t + i

+ n

t

LettingE

denote the expected value operator the autocovariance sequence for thex

t

series

i

is

i

= E x

t

x

0

t i

The Yule-Walker equations for the autoregressive model that matches the first p ele-

ments of the autocovariance sequence are

2

6

6

4

0

1

p 1

0

1

0

p 2

0

p 1

0

p 2

0

3

7

7

5

2

6

6

4

p

1

p

2

p

p

3

7

7

5

=

2

6

6

4

1

2

p

3

7

7

5

and

2

6

6

6

4

0

0

1

0

p 1

1

0

0

p 2

p 1

p 2

0

3

7

7

7

5

2

6

6

4

p

1

p

2

p

p

3

7

7

5

=

2

6

6

4

0

1

0

2

0

p

3

7

7

5

Here

p

i

are the coefficient matrices for the past observation form of the vector au-toregressive model and

p

i

are the coefficient matrices for the future observation

form More information on the Yule-Walker equations in the multivariate setting can

be found in Whittle (1963) and Ansley and Newbold (1979)

The innovation variance matrices for the two forms can be written as follows

p

=

0

p

X

i = 1

p

i

0

i

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Part 2 General Information

p

=

0

p

X

i = 1

p

i

i

The autoregressive models are fit to the data using the preceding Yule-Walker equa-

tions with

i

replaced by the sample covariance sequenceC

i

The covariance matri-

ces are calculated as

C

i

=

1

N 1

N

X

t = i + 1

x

t

x

0

t i

Let b

p

b

p

b

p

and b

p

represent the Yule-Walker estimates of

p

p

p

and

p

respectively These matrices are written to an output data set when the OUTAR=

option is specified

When the PRINTOUT=LONG option is specified the sequence of matrices b

p

and

the corresponding correlation matrices are printed The sequence of matricesb

p isused to compute Akaike information criteria for selection of the autoregressive order

of the process

Akaike Information Criterion

The Akaike information criterion or AIC is defined as -2( maximum of log likeli-

hood )+2(number of parameters) Since the vector autoregressive models are esti-

mates from the Yule-Walker equations not by maximum likelihood the exact likeli-

hood values are not available for computing the AIC However for the vector autore-

gressive model the maximum of the log likelihood can be approximated as

l n L

n

2

l n j

b

p

j

Thus the AIC for the order p model is computed as

A I C

p

= n l n j

b

p

j + 2 p r

2

You can use the printed AIC array to compute a likelihood ratio test of the autoregres-

sive order The log-likelihood ratio test statistic for testing the order p model against

the order p 1

model is

n l n j

b

p

j + n l n j

b

p 1

j

This quantity is asymptotically distributed as a

2 withr

2 degrees of freedom if the

series is autoregressive of order p 1

It can be computed from the AIC array as

A I C

p 1

A I C

p

+ 2 r

2

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Chapter 18 Details

You can evaluate the significance of these test statistics with the PROBCHI function

in a SAS DATA step or with a

2 table

Determining the Autoregressive Order

Although the autoregressive models can be used for prediction their primary value is

to aid in the selection of a suitable portion of the sample covariance matrix for use in

computing canonical correlations If the multivariate time series x

t is of autoregres-sive order p then the vector of past values to lag p is considered to contain essentially

all the information relevant for prediction of future values of the time series

By default PROC STATESPACE selects the order p producing the autoregressive

model with the smallestA I C

p

If the value p for the minimumA I C

p

is less than the

value of the PASTMIN= option then p is set to the PASTMIN= value Alternatively

you can use the ARMAX= and PASTMIN= options to force PROC STATESPACE to

use an order you select

Significance Limits for Partial Autocorrelations

The STATESPACE procedure prints a schematic representation of the partial autocor-

relation matrices indicating which partial autocorrelations are significantly greater orsignificantly less than 0 Figure 1810 shows an example of this table

The STATESPACE Procedure

Schematic Representation of Partial Autocorrelations

NameLag 1 2 3 4 5 6 7 8 9 10

x ++ +

y ++

+ is gt 2std error - is lt -2std error is between

Figure 1810 Significant Partial Autocorrelations

The partial autocorrelations are from the sample partial autoregressive matrices b

p

p

The standard errors used for the significance limits of the partial autocorrelations are

computed from the sequence of matrices

p

and

p

Under the assumption that the observed series arises from an autoregressive process

of order p 1

the pth sample partial autoregressive matrix b

p

p

has an asymptotic

variance matrix 1

n

1

p

p

The significance limits for b

p

p

used in the schematic plot of the sample partial autore-

gressive sequence are derived by replacing

p

and

p

with their sample estimators

to produce the variance estimate as follows

d

V a r

b

p

p

=

1

n r p

b

1

p

b

p

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Part 2 General Information

Canonical Correlation Analysis

Given the order p letp

t

be the vector of current and past values relevant to prediction

of x

t + 1

p

t

= x

0

t

x

0

t 1

x

0

t p

0

Letf

t

be the vector of current and future values

f

t

= x

0

t

x

0

t + 1

x

0

t + p

0

In the canonical correlation analysis consider submatrices of the sample covariance

matrix of p

t

andf

t

This covariance matrixV

has a block Hankel form

V =

2

6

6

6

4

C

0

C

0

1

C

0

2

C

0

p

C

0

1

C

0

2

C

0

3

C

0

p + 1

C

0

p

C

0

p + 1

C

0

p + 2

C

0

2 p

3

7

7

7

5

State Vector Selection Process

The canonical correlation analysis forms a sequence of potential state vectorsz

j

t

Examine a sequencef

j

t

of subvectors of f

t

and form the submatrixV

j consisting

of the rows and columns of V

corresponding to the components of f

j

t

and compute

its canonical correlations

The smallest canonical correlation of V

j is then used in the selection of the com-

ponents of the state vector The selection process is described in the following For

more details about this process refer to Akaike (1976)

In the following discussion the notationx

t + k j t

denotes the wide sense conditional

expectation (best linear predictor) of x

t + k

given allx

s

with s less than or equal to t

In the notationx

i t + 1

the first subscript denotes the ith component of x

t + 1

The initial state vectorz

1

t

is set tox

t

The sequencef

j

t

is initialized by setting

f

1

t

= z

1

0

t

x

1 t + 1 j t

0

= x

0

t

x

1 t + 1 j t

0

That is start by considering whether to addx

1 t + 1 j t

to the initial state vectorz

1

t

The procedure forms the submatrixV

1 corresponding tof

1

t

and computes its canon-

ical correlations Denote the smallest canonical correlation of V

1 as

m i n

If

m i n

is

significantly greater than 0x

1 t + 1 j t

is added to the state vector

If the smallest canonical correlation of V

1 is not significantly greater than 0 then a

linear combination of f

1

t

is uncorrelated with the pastp

t

Assuming that the determi-

nant of C

0

is not 0 (that is no input series is a constant) you can take the coefficient

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Chapter 18 Details

of x

1 t + 1 j t

in this linear combination to be 1 Denote the coefficients of z

1

t

in this

linear combination as

This gives the relationship

x

1 t + 1 j t

=

0

x

t

Therefore the current state vector already contains all the past information useful forpredicting

x

1 t + 1

and any greater leads of x

1 t

The variablex

1 t + 1 j t

is not added to

the state vector nor are any termsx

1 t + k j t

considered as possible components of the

state vector The variablex

1

is no longer active for state vector selection

The process described forx

1 t + 1 j t

is repeated for the remaining elements of f

t

The

next candidate for inclusion in the state vector is the next component of f

t

corre-

sponding to an active variable Components of f

t

corresponding to inactive variables

that produced a zero

m i n

in a previous step are skipped

Denote the next candidate asx

l t + k j t

The vectorf

j

t

is formed from the current state

vector andx

l t + k j t

as follows

f

j

t

= z

j

0

t

x

l t + k j t

0

The matrixV

j is formed fromf

j

t

and its canonical correlations are computed The

smallest canonical correlation of V

j is judged to be either greater than or equal to 0

If it is judged to be greater than 0x

l t + k j t

is added to the state vector If it is judged

to be 0 then a linear combination of f

j

t

is uncorrelated with thep

t

and the variable

x

l

is now inactive

The state vector selection process continues until no active variables remain

Testing Significance of Canonical Correlations For each step in the canonical correlation sequence the significance of the small-

est canonical correlation

m i n

is judged by an information criterion from Akaike

(1976) This information criterion is

n l n 1

2

m i n

r p + 1 q + 1

where q is the dimension of f

j

t

at the current step r is the order of the state vector p

is the order of the vector autoregressive process and

is the value of the SIGCORR=

option The default is SIGCORR=2 If this information criterion is less than or equal

to 0

m i n

is taken to be 0 otherwise it is taken to be significantly greater than 0

(Do not confuse this information criterion with the AIC)

Variables inx

t + p j t

are not added in the model even with positive information crite-

rion because of the singularity of V

You can force the consideration of more candi-

date state variables by increasing the size of theV

matrix by specifying a PASTMIN=

option value larger than p

Printing the Canonical Correlations

To print the details of the canonical correlation analysis process specify the CAN-

CORR option in the PROC STATESPACE statement The CANCORR option prints

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the candidate state vectors the canonical correlations and the information criteria for

testing the significance of the smallest canonical correlation

Bartlettrsquos

2 and its degrees of freedom are also printed when the CANCORR option

is specified The formula used for Bartlettrsquos

2 is

2

= n 5 r p + 1 q + 1 l n 1

2

m i n

withr p + 1 q + 1

degrees of freedom

Figure 1811 shows the output of the CANCORR option for the introductory example

shown in the Getting Started section of this chapter

The STATESPACE Procedure

Canonical Correlations Analysis

Information Chi

x(TT) y(TT) x(T+1T) Criterion Square DF

1 1 0237045 3566167 114505 4

Information Chi

x(TT) y(TT) x(T+1T) y(T+1T) Criterion Square DF

1 1 0238244 0056565 -535906 0636134 3

Information Chi

x(TT) y(TT) x(T+1T) x(T+2T) Criterion Square DF

1 1 0237602 0087493 -446312 1525353 3

Figure 1811 Canonical Correlations Analysis

New variables are added to the state vector if the information criteria are positive Inthis example

y

t + 1 j t

andx

t + 2 j t

are not added to the state space vector because the

information criteria for these models are negative

If the information criterion is nearly 0 then you may want to investigate models

that arise if the opposite decision is made regarding

m i n

This investigation can be

accomplished by using a FORM statement to specify part or all of the state vector

Preliminary Estimates of F

When a candidate variablex

l t + k j t

yields a zero

m i n

and is not added to the state

vector a linear combination of f

j

t

is uncorrelated with thep

t

Because of the method

used to construct thef

j

t

sequence the coefficient of x

l t + k j t

inl

can be taken as 1

Denote the coefficients of z

j

t

in this linear combination as l

This gives the relationship

x

l t + k j t

= l

0

z

j

t

The vectorl

is used as a preliminary estimate of the first r columns of the row of the

transition matrixF

corresponding tox

l t + k 1 j t

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Chapter 18 Details

Parameter Estimation

The model isz

t + 1

= F z

t

+ G e

t + 1

wheree

t

is a sequence of independent multivari-

ate normal innovations with mean vector 0 and variance

e e

The observed sequence

x

t

composes the first r components of z

t

and thusx

t

= H z

t

where H is ther s

matrix I

r

0

LetE

be ther n

matrix of innovations

E = e

1

e

n

If the number of observations n is reasonably large the log likelihood L can be

approximated up to an additive constant as follows

L =

n

2

l n j

e e

j

1

2

t r a c e

1

e e

E E

0

The elements of

e e

are taken as free parameters and are estimated as follows

S

0

=

1

n

E E

0

Replacing

e e

byS

0

in the likelihood equation the log likelihood up to an additive

constant is

L =

n

2

l n j S

0

j

Letting B be the backshift operator the formal relation betweenx

t

ande

t

is

x

t

= H I B F

1

G e

t

e

t

= H I B F

1

G

1

x

t

=

1

X

i = 0

i

x

t i

LettingC

i

be the ith lagged sample covariance of x

t

and neglecting end effects the

matrixS

0

is

S

0

=

1

X

i j = 0

i

C

i + j

0

j

For the computation of S

0

the infinite sum is truncated at the value of the KLAG=

option The value of the KLAG= option should be large enough that the sequence

i

is approximately 0 beyond that point

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Let

be the vector of free parameters in theF

andG

matrices The derivative of the

log likelihood with respect to the parameter

is

L

=

n

2

t r a c e

S

1

0

S

0

The second derivative is

2

L

0

=

n

2

t r a c e

S

1

0

S

0

0

S

1

0

S

0

t r a c e

S

1

0

2

S

0

0

Near the maximum the first term is unimportant and the second term can be approx-

imated to give the following second derivative approximation

2

L

0

=

n t r a c e

S

1

0

E

E

0

0

The first derivative matrix and this second derivative matrix approximation are com-

puted from the sample covariance matrixC

0

and the truncated sequence

i

The

approximate likelihood function is maximized by a modified Newton-Raphson algo-

rithm employing these derivative matrices

The matrixS

0

is used as the estimate of the innovation covariance matrix

e e

The

negative of the inverse of the second derivative matrix at the maximum is used as an

approximate covariance matrix for the parameter estimates The standard errors of

the parameter estimates printed in the parameter estimates tables are taken from the

diagonal of this covariance matrix The parameter covariance matrix is printed when

the COVB option is specified

If the data are nearly nonstationary a better estimate of

e e

and the other parameters

can sometimes be obtained by specifying the RESIDEST option The RESIDEST

option estimates the parameters using conditional least squares instead of maximum

likelihood

The residuals are computed using the state space equation and the sample mean val-

ues of the variables in the model as start-up values The estimate of S

0

is then com-

puted using the residuals from the ith observation on where i is the maximum number

of times any variable occurs in the state vector A multivariate Gauss-Marquardt al-

gorithm is used to minimizej S

0

j

Refer to Harvey (1981a) for a further description

of this method

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Chapter 18 Details

Forecasting

Given estimates of F

G

and

e e

forecasts of x

t

are computed from the conditional

expectation of z

t

In forecasting the parameters F G and

e e

are replaced with the estimates or by val-

ues specified in the RESTRICT statement One-step-ahead forecasting is performedfor the observation

x

t

wheret n b

Heren

is the number of observations and

b is the value of the BACK= option For the observationx

t

wheret n b

m-

step-ahead forecasting is performed form = t n + b

The forecasts are generated

recursively with the initial conditionz

0

= 0

The m-step-ahead forecast of z

t + m

isz

t + m j t

wherez

t + m j t

denotes the conditional

expectation of z

t + m

given the information available at time t The m-step-ahead

forecast of x

t + m

isx

t + m j t

= H z

t + m j t

where the matrixH = I

r

0

Let

i

= F

i

G

Note that the lasts r

elements of z

t

consist of the elements of x

u j t

foru t

The state vector z

t + m

can be represented as

z

t + m

= F

m

z

t

+

m 1

X

i = 0

i

e

t + m i

Sincee

t + i j t

= 0

fori 0

the m-step-ahead forecastz

t + m j t

is

z

t + m j t

= F

m

z

t

= F z

t + m 1 j t

Therefore the m-step-ahead forecast of x

t + m

is

x

t + m j t

= H z

t + m j t

The m-step-ahead forecast error is

z

t + m

z

t + m j t

=

m 1

X

i = 0

i

e

t + m i

The variance of the m-step-ahead forecast error is

V

z m

=

m 1

X

i = 0

i

e e

0

i

LettingV

z 0

= 0

the variance of the m-step-ahead forecast error of z

t + m

V

z m

can

be computed recursively as follows

V

z m

= V

z m 1

+

m 1

e e

0

m 1

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The variance of the m-step-ahead forecast error of x

t + m

is ther r

left upper sub-

matrix of V

z m

that is

V

x m

= H V

z m

H

0

Unless the NOCENTER option is specified the sample mean vector is added to the

forecast When differencing is specified the forecasts xt + m j t

plus the sample mean

vector are integrated back to produce forecasts for the original series

Lety

t

be the original series specified by the VAR statement with some 0 values

appended corresponding to the unobserved past observations Let B be the backshift

operator and let B

be thes s

matrix polynomial in the backshift operator

corresponding to the differencing specified by the VAR statement The off-diagonal

elements of

i

are 0 Note that

0

= I

s

whereI

s

is thes s

identity matrix Then

z

t

= B y

t

This gives the relationship

y

t

=

1

B z

t

=

1

X

i = 0

i

z

t i

where

1

B =

P

1

i = 0

i

B

i and

0

= I

s

The m-step-ahead forecast of y

t + m

is

y

t + m j t

=

m 1

X

i = 0

i

z

t + m i j t

+

1

X

i = m

i

z

t + m i

The m-step-ahead forecast error of y

t + m

is

m 1

X

i = 0

i

z

t + m i

z

t + m i j t

=

m 1

X

i = 0

i

X

u = 0

u

i u

e

t + m i

LettingV

y 0

= 0

the variance of the m-step-ahead forecast error of y

t + m

V

y m

is

V

y m

=

m 1

X

i = 0

i

X

u = 0

u

i u

e e

i

X

u = 0

u

i u

0

= V

y m 1

+

m 1

X

u = 0

u

m 1 u

e e

m 1

X

u = 0

u

m 1 u

0

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Chapter 18 Details

Relation of ARMA and State Space Forms

Every state space model has an ARMA representation and conversely every ARMA

model has a state space representation This section discusses this equivalence The

following material is adapted from Akaike (1974) where there is a more complete

discussion Pham-Dinh-Tuan (1978) also contains a discussion of this material

Suppose you are given the following ARMA model

B x

t

= B e

t

or in more detail

x

t

1

x

t 1

p

x

t p

= e

t

+

1

e

t 1

+ +

q

e

t q

(1)

wheree

t

is a sequence of independent multivariate normal random vectors with mean

0 and variance matrix

e e

B is the backshift operator (B x

t

= x

t 1

) B

and

B

are matrix polynomials in B andx

t

is the observed process

If the roots of the determinantial equationj B j = 0

are outside the unit circle in

the complex plane the model can also be written as

x

t

=

1

B B e

t

=

1

X

i = 0

i

e

t i

The

i

matrices are known as the impulse response matrices and can be computed

as

1

B B

You can assume p q

since if this is not initially true you can add more terms

i

that are identically 0 without changing the model

To write this set of equations in a state space form proceed as follows Letx

t + i j t

be

the conditional expectation of x

t + i

givenx

w

forw t

The following relations hold

x

t + i j t

=

1

X

j = i

j

e

t + i j

x

t + i j t + 1

= x

t + i j t

+

i 1

e

t + 1

However from equation (1) you can derive the following relationship

x

t + p j t

=

1

x

t + p 1 j t

+ +

p

x

t

(2)

Hence wheni = p

you can substitute forx

t + p j t

in the right-hand side of equation

(2) and close the system of equations

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This substitution results in the following model in the state space form

z

t + 1

= F z

t

+ G e

t + 1

2

6

6

6

4

x

t + 1

x

t + 2 j t + 1

x

t + p j t + 1

3

7

7

7

5

=

2

6

6

4

0 I 0 0

0 0 I 0

p

p 1

1

3

7

7

5

2

6

6

6

4

x

t

x

t + 1 j t

x

t + p 1 j t

3

7

7

7

5

+

2

6

6

4

I

1

p 1

3

7

7

5

e

t + 1

Note that the state vectorz

t

is composed of conditional expectations of x

t

and the

first r components of z

t

are equal tox

t

The state space form can be cast into an ARMA form by solving the system of dif-

ference equations for the first r components

When converting from an ARMA form to a state space form you can generate a

state vector larger than needed that is the state space model may not be a minimal

representation When going from a state space form to an ARMA form you can have

nontrivial common factors in the autoregressive and moving average operators that

yield an ARMA model larger than necessary

If the state space form used is not a minimal representation some but not all compo-

nents of x

t + i j t

may be linearly dependent This situation corresponds to

p

p 1

being of less than full rank when B

and B

have no common nontrivial

left factors In this casez

t

consists of a subset of the possible components of

x

t + i j t

i = 1 2 p 1

However once a component of x

t + i j t

(for example the

jth one) is linearly dependent on the previous conditional expectations then all sub-

sequent jth components of x

t + k j t

fork i

must also be linearly dependent Note

that in this case equivalent but seemingly different structures can arise if the order of

the components withinx

t

is changed

OUT= Data Set

The forecasts are contained in the output data set specified by the OUT= option on the

PROC STATESPACE statement The OUT= data set contains the following variables

the BY variables

the ID variable

the VAR statement variables These variables contain the actual values from

the input data set

FORi numeric variables containing the forecasts The variable FORi contains

the forecasts for the ith variable in the VAR statement list Forecasts are one-

step-ahead predictions until the end of the data or until the observation specified

by the BACK= option

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Chapter 18 Details

RESi numeric variables containing the residual for the forecast of the ith vari-

able in the VAR statement list For forecast observations the actual values are

missing and the RESi variables contain missing values

STDi numeric variables containing the standard deviation for the forecast of

the ith variable in the VAR statement list The values of the STDi variables can

be used to construct univariate confidence limits for the corresponding fore-casts However such confidence limits do not take into account the covariance

of the forecasts

OUTAR= Data Set

The OUTAR= data set contains the estimates of the preliminary autoregressive mod-

els The OUTAR= data set contains the following variables

ORDER a numeric variable containing the order p of the autoregressive model

that the observation represents

AIC a numeric variable containing the value of the information criterionA I C

p

SIGFl numeric variables containing the estimate of the innovation covariance

matrices for the forward autoregressive models The variable SIGFl contains

the lth column of b

p

in the observations with ORDER= p

SIGBl numeric variables containing the estimate of the innovation covariance

matrices for the backward autoregressive models The variable SIGBl contains

the lth column of b

p

in the observations with ORDER= p

FORk ndashl numeric variables containing the estimates of the autoregressive pa-

rameter matrices for the forward models The variable FORk ndashl contains the

lth column of the lag k autoregressive parameter matrix b

p

k

in the observations

with ORDER= p

BACk ndashl numeric variables containing the estimates of the autoregressive pa-

rameter matrices for the backward models The variable BACk ndashl contains the

lth column of the lag k autoregressive parameter matrix b

p

k

in the observations

with ORDER= p

The estimates for the order p autoregressive model can be selected as those observa-

tions with ORDER= p Within these observations the k lth element of

p

i

is given by

the value of the FORindashl variable in the k th observation The k lth element of

p

i

is

given by the value of BACindashl variable in the k th observation The k lth element of

p

is given by SIGFl in the k th observation The k lth element of

p

is given by SIGBlin the k th observation

Table 181 shows an example of the OUTAR= data set with ARMAX=3 andx

t

of

dimension 2 In Table 181 i j

indicate the ijth element of the matrix

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Table 181 Values in the OUTAR= Data Set

Obs ORDER AIC SIGF1 SIGF2 SIGB1 SIGB2 FOR1ndash 1 FOR1ndash 2 FOR2ndash 1 FOR2ndash 2 FOR3ndash 1

1 0 AIC0

0 1 1

0 1 2

0 1 1

0 1 2

2 0 AIC0

0 2 1

0 2 2

0 2 1

0 2 2

3 1 AIC1

1 1 1

1 1 2

1 1 1

1 1 2

1

1

1 1

1

1

1 2

4 1 AIC1

1 2 1

1 1 2

1 2 1

1 1 2

1

1

2 1

1

1

2 2

5 2 AIC2

2 1 1

2 1 2

2 1 1

2 1 2

2

1

1 1

2

1

1 2

2

2

1 1

2

2

1 2

6 2 AIC2

2 2 1

2 1 2

2 2 1

2 1 2

2

1

2 1

2

1

2 2

2

2

2 1

2

2

2 2

7 3 AIC3

3 1 1

3 1 2

3 1 1

3 1 2

3

1

1 1

3

1

1 2

3

2

1 1

3

2

1 2

3

3

1 1

8 3 AIC3

3 2 1

3 1 2

3 2 1

3 1 2

3

1

2 1

3

1

2 2

3

2

2 1

3

2

2 2

3

3

2 1

Obs FOR3ndash 2 BACK1ndash 1 BACK1ndash 2 BACK2ndash 1 BACK2ndash 2 BACK3ndash 1 BACK3ndash 2

1

2

3

1

1

1 1

1

1

1 2

4

1

1

2 1

1

1

2 2

5

2

1

1 1

2

1

1 2

2

2

1 1

2

2

1 2

6

2

1

2 1

2

1

2 2

2

2

2 1

2

2

2 2

7

3

3

1 2

3

1

1 1

3

1

1 2

3

2

1 1

3

2

1 2

3

3

1 1

3

3

1 2

8

3

3

2 2

3

1

2 1

3

1

2 2

3

2

2 1

3

2

2 2

3

3

2 1

3

3

2 2

The estimated autoregressive parameters can be used in the IML procedure to obtain

autoregressive estimates of the spectral density function or forecasts based on the

autoregressive models

OUTMODEL= Data Set

The OUTMODEL= data set contains the estimates of theF

andG

matrices and their

standard errors the names of the components of the state vector and the estimates of

the innovation covariance matrix The variables contained in the OUTMODEL= data

set are as follows

the BY variables

STATEVEC a character variable containing the name of the component of the

state vector corresponding to the observation The STATEVEC variable has

the value STD for standard deviations observations which contain the standarderrors for the estimates given in the preceding observation

Fndash j numeric variables containing the columns of theF

matrix The variable

Fndash j contains the jth column of F

The number of Fndash j variables is equal to

the value of the DIMMAX= option If the model is of smaller dimension the

extraneous variables are set to missing

Gndash j numeric variables containing the columns of theG

matrix The variable

Gndash j contains the jth column of G

The number of Gndash j variables is equal to r

the dimension of x

t

given by the number of variables in the VAR statement

SIGndash j numeric variables containing the columns of the innovation covariance

matrix The variable SIGndash j contains the jth column of

e e

There are r vari-ables SIGndash j

Table 182 shows an example of the OUTMODEL= data set withx

t

= x

t

y

t

0

z

t

= x

t

y

t

x

t + 1 j t

0 and DIMMAX=4 In Table 182F

i j

andG

i j

are the ijth

elements of F

andG

respectively Note that all elements for Fndash4 are missing because

F

is a3 3

matrix

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Table 182 Value in the OUTMODEL= Data Set

Obs STATEVEC Fndash 1 Fndash 2 Fndash 3 Fndash 4 Gndash 1 Gndash 2 SIGndash 1 SIGndash 2

1 X(TT) 0 0 1 1 0

1 1

1 2

2 STD

3 Y(TT) F2 1

F2 2

F2 3

0 1

2 1

2 2

4 STD std F2 1

std F2 2

std F2 3

5 X(T+1T) F3 1

F3 2

F3 3

G3 1

G3 2

6 STD std F3 1

std F3 2

std F3 3

std G3 1

std G3 2

Printed Output

The printed output produced by the STATESPACE procedure is described in the fol-

lowing

1 descriptive statistics which include the number of observations used the

names of the variables their means and standard deviations (Std) and the dif-

ferencing operations used

2 the Akaike information criteria for the sequence of preliminary autoregressive

models

3 if the PRINTOUT=LONG option is specified the sample autocovariance ma-

trices of the input series at various lags

4 if the PRINTOUT=LONG option is specified the sample autocorrelation ma-

trices of the input series

5 a schematic representation of the autocorrelation matrices showing the signif-

icant autocorrelations

6 if the PRINTOUT=LONG option is specified the partial autoregressive ma-

trices (These are

p

p

as described in Preliminary Autoregressive Modelsearlier in this chapter)

7 a schematic representation of the partial autocorrelation matrices showing the

significant partial autocorrelations

8 the Yule-Walker estimates of the autoregressive parameters for the autoregres-

sive model with the minimum AIC

9 if the PRINTOUT=LONG option is specified the autocovariance matrices of

the residuals of the minimum AIC model This is the sequence of estimated

innovation variance matrices for the solutions of the Yule-Walker equations

10 if the PRINTOUT=LONG option is specified the autocorrelation matrices of the residuals of the minimum AIC model

11 If the CANCORR option is specified the canonical correlations analysis for

each potential state vector considered in the state vector selection process

This includes the potential state vector the canonical correlations the informa-

tion criterion for the smallest canonical correlation Bartlettrsquos

2 statistic (ldquoChi

Squarerdquo) for the smallest canonical correlation and the degrees of freedom of

Bartlettrsquos

2

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Part 2 General Information

12 the components of the chosen state vector

13 the preliminary estimate of the transition matrixF

the input matrixG

and

the variance matrix for the innovations

e e

14 if the ITPRINT option is specified the iteration history of the likelihood max-

imization For each iteration this shows the iteration number the number of step halvings the determinant of the innovation variance matrix the damping

factor Lambda and the values of the parameters

15 the state vector printed again to aid interpretation of the following listing of F

andG

16 the final estimate of the transition matrixF

17 the final estimate of the input matrixG

18 the final estimate of the variance matrix for the innovations

e e

19 a table listing the estimates of the free parameters inF

andG

and their standarderrors and t statistics

20 if the COVB option is specified the covariance matrix of the parameter esti-

mates

21 if the COVB option is specified the correlation matrix of the parameter esti-

mates

22 if the PRINT option is specified the forecasts and their standard errors

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Chapter 18 Details

ODS Table Names

PROC STATESPACE assigns a name to each table it creates You can use these

names to reference the table when using the Output Delivery System (ODS) to select

tables and create output data sets These names are listed in the following table For

more information on ODS see Chapter 6 ldquoUsing the Output Delivery Systemrdquo

Table 183 ODS Tables Produced in PROC STATESPACE

ODS Table Name Description Option

NObs Number of observations default

Summary Simple summary statistics table default

InfoCriterion Information criterion table default

CovLags Covariance Matrices of Input Series PRINTOUT=LONG

CorrLags Correlation Matrices of Input Series PRINTOUT=LONG

PartialAR Partial Autoregressive Matrices PRINTOUT=LONG

YWEstimates Yule-Walker Estimates for Minimum AIC default

CovResiduals Covariance of Residuals PRINTOUT=LONGCorrResiduals Residual Correlations from AR Models PRINTOUT=LONG

StateVector State vector table default

CorrGraph Schematic Representation of Correlations default

TransitionMatrix Transition Matrix default

InputMatrix Input Matrix default

VarInnov Variance Matrix for the Innovation default

CovB Covariance of Parameter Estimates COVB

CorrB Correlation of Parameter Estimates COVB

CanCorr Canonical Correlation Analysis CANCORR

IterHistory Iterative Fitting table ITPRINT

ParameterEstimates Parameter Estimates Table default

Forecasts Forecasts Table PRINT

ConvergenceStatus Convergence Status Table default

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Part 2 General Information

Example

Example 181 Series J from Box and Jenkins

This example analyzes the gas furnace data (series J) from Box and Jenkins (The

data are not shown Refer to Box and Jenkins (1976) for the data)

First a model is selected and fit automatically using the following statements

title1 rsquoGas Furnace Datarsquo

title2 rsquoBox amp Jenkins Series Jrsquo

title3 rsquoAutomatically Selected Modelrsquo

proc statespace data=seriesj cancorr

var x y

run

The results for the automatically selected model are shown in Output 1811

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Chapter 18 Example

Output 1811 Results for Automatically Selected Model

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Number of Observations 296

Standard

Variable Mean Error

x -005683 1072766

y 5350912 3202121

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Information Criterion for Autoregressive Models

Lag=0 Lag=1 Lag=2 Lag=3 Lag=4 Lag=5 Lag=6 Lag=7 Lag=8

6513862 -103357 -163296 -164512 -165152 -164891 -164934 -164315 -163856

Information

Criterion for

Autoregressive

Models

Lag=9 Lag=10

-16348 -163359

Schematic Representation of Correlations

NameLag 0 1 2 3 4 5 6 7 8 9 10

x +- +- +- +- +- +- +- +- +- +- +-

y -+ -+ -+ -+ -+ -+ -+ -+ -+ -+ -+

+ is gt 2std error - is lt -2std error is between

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Part 2 General Information

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Schematic Representation of Partial Autocorrelations

NameLag 1 2 3 4 5 6 7 8 9 10

x + - + -

y -+ -- - + +

+ is gt 2std error - is lt -2std error is between

Yule-Walker Estimates for Minimum AIC

------Lag=1------ ------Lag=2------ ------Lag=3------ ------Lag=4------

x y x y x y x y

x 1925887 -000124 -120166 0004224 0116918 -000867 0104236 0003268

y 0050496 1299793 -002046 -03277 -071182 -025701 0195411 0133417

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Canonical Correlations Analysis

Information Chi

x(TT) y(TT) x(T+1T) Criterion Square DF

1 1 0804883 2929228 3047481 8

Information Chix(TT) y(TT) x(T+1T) y(T+1T) Criterion Square DF

1 1 0906681 0607529 1223358 1347237 7

Information Chi

x(TT) y(TT) x (T+1T) y (T+1T) x (T+2T) Criterion Square DF

1 1 0909434 0610278 0186274 -154701 1034705 6

Information Chi

x(TT) y(TT) x (T+1T) y (T+1T) y (T+2T) Criterion Square DF

1 1 091014 0618937 0206823 0940392 1280924 6

Information Chi

x(TT) y(TT) x(T+1T) y(T+1T) y(T+2T) y(T+3T) Criterion Square DF

1 1 0912963 0628785 0226598 0083258 -794103 2041584 5

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Chapter 18 Example

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Selected Statespace Form and Preliminary Estimates

State Vector

x(TT) y(TT) x(T+1T) y(T+1T) y(T+2T)

Estimate of Transition Matrix

0 0 1 0 0

0 0 0 1 0

-084718 0026794 1711715 -005019 0

0 0 0 0 1

-019785 0334274 -018174 -123557 1787475

Input Matrix for Innovation

1 0

0 11925887 -000124

0050496 1299793

0142421 1361696

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Selected Statespace Form and Preliminary Estimates

Variance Matrix for Innovation

0035274 -000734

-000734 0097569

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Part 2 General Information

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Selected Statespace Form and Fitted Model

State Vector

x(TT) y(TT) x(T+1T) y(T+1T) y(T+2T)

Estimate of Transition Matrix

0 0 1 0 0

0 0 0 1 0

-086192 0030609 1724235 -005483 0

0 0 0 0 1

-034839 0292124 -009435 -109823 1671418

Input Matrix for Innovation

1 0

0 1192442 -000416

0015621 1258495

008058 1353204

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Selected Statespace Form and Fitted Model

Variance Matrix for Innovation

0035579 -000728

-000728 0095577

Parameter Estimates

Standard

Parameter Estimate Error t Value

F(31) -086192 0072961 -1181

F(32) 0030609 0026167 117

F(33) 1724235 0061599 2799

F(34) -005483 0030169 -182

F(51) -034839 0135253 -258

F(52) 0292124 0046299 631

F(53) -009435 0096527 -098F(54) -109823 0109525 -1003

F(55) 1671418 0083737 1996

G(31) 1924420 0058162 3309

G(32) -000416 0035255 -012

G(41) 0015621 0095771 016

G(42) 1258495 0055742 2258

G(51) 0080580 0151622 053

G(52) 1353204 0091388 1481

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Chapter 18 Example

The two series are believed to have a transfer function relation with the gas rate

(variable X) as the input and the CO2

concentration (variable Y) as the output Since

the parameter estimates shown in Output 1811 support this kind of model the model

is reestimated with the feedback parameters restricted to 0 The following statements

fit the transfer function (no feedback) model

title3 rsquoTransfer Function Modelrsquo

proc statespace data=seriesj printout=none

var x y

restrict f(32)=0 f(34)=0

g(32)=0 g(41)=0 g(51)=0

run

The last 2 pages of the output are shown in Output 1812

Output 1812 STATESPACE Output for Transfer Function Model

Gas Furnace Data

Box amp Jenkins Series J

Transfer Function Model

The STATESPACE Procedure

Selected Statespace Form and Fitted Model

State Vector

x(TT) y(TT) x(T+1T) y(T+1T) y(T+2T)

Estimate of Transition Matrix

0 0 1 0 0

0 0 0 1 0

-068882 0 1598717 0 0

0 0 0 0 1

-035944 0284179 -00963 -107313 1650047

Input Matrix for Innovation

1 0

0 1

1923446 0

0 1260856

0 1346332

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Part 2 General Information

Gas Furnace Data

Box amp Jenkins Series J

Transfer Function Model

The STATESPACE Procedure

Selected Statespace Form and Fitted Model

Variance Matrix for Innovation

0036995 -00072

-00072 0095712

Parameter Estimates

Standard

Parameter Estimate Error t Value

F(31) -068882 0050549 -1363

F(33) 1598717 0050924 3139

F(51) -035944 0229044 -157

F(52) 0284179 0096944 293

F(53) -009630 0140876 -068

F(54) -107313 0250385 -429

F(55) 1650047 0188533 875G(31) 1923446 0056328 3415

G(42) 1260856 0056464 2233

G(52) 1346332 0091086 1478

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Chapter 18 References

References

Akaike H (1974) Markovian Representation of Stochastic Processes and Its Ap-

plication to the Analysis of Autoregressive Moving Average Processes Annals

of the Institute of Statistical Mathematics 26 363-387

Akaike H (1976) Canonical Correlations Analysis of Time Series and the Use of

an Information Criterion in Advances and Case Studies in System Identification

eds R Mehra and DG Lainiotis New York Academic Press

Anderson TW (1971) The Statistical Analysis of Time Series New York John

Wiley amp Sons

Ansley CF and Newbold P (1979) Multivariate Partial Autocorrelations Pro-

ceedings of the Business and Economic Statistics Section American Statistical

Association 349-353

Box GEP and Jenkins G (1976) Time Series Analysis Forecasting and Control

San Francisco Holden-Day

Brockwell PJ and Davis RA (1991) Time Series Theory and Methods 2nd Edi-

tion Springer-Verlag

Hannan EJ (1970) Multiple Time Series New York John Wiley amp Sons

Hannan EJ (1976) The Identification and Parameterization of ARMAX and State

Space Forms Econometrica 44 713-722

Harvey AC (1981a) The Econometric Analysis of Time Series New York John

Wiley amp Sons

Harvey AC (1981b) Time Series Models New York John Wiley amp Sons

Jones RH (1974) Identification and Autoregressive Spectrum Estimation IEEE Transactions on Automatic Control AC-19 894-897

Pham-Dinh-Tuan (1978) On the Fitting of Multivariate Processes of the

Autoregressive-Moving Average Type Biometrika 65 99-107

Priestley MB (1980) System Identification Kalman Filtering and Stochastic Con-

trol in Directions in Time Series eds DR Brillinger and GC Tiao Institute

of Mathematical Statistics

Whittle P (1963) On the Fitting of Multivariate Autoregressions and the Approxi-

mate Canonical Factorization of a Spectral Density Matrix Biometrika 50 129-

134

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The correct bibliographic citation for this manual is as follows SAS Institute Inc SAS

ET S U serrsquos Gui de Version 8 Cary NC SAS Institute Inc 1999 1546 pp

SASET S Userrsquos Guide Version 8

Copyright copy 1999 by S AS I ns t it u t e I nc Ca ry N C U S A

ISBN 1ndash58025ndash489ndash6

All r ight s res erved P r int ed in t he U n it ed St a t es of A merica N o pa rt of t his p u blica t ionma y b e reprodu ced s t ored in a ret r ieva l s ys t em or t ra ns mit t ed in a ny form or b y a nymeans electronic mechanical photocopying or otherwise without the prior writtenpermission of the publisher SAS Institute Inc

US Government Restricted Rights Notice Use du plicat ion or disclosure of the

software by the government is subject to restrictions as set forth in FAR 52227ndash19Commercial Computer Softw ar e-Restricted Rights (J une 1987)

SAS Institute Inc SAS Campus Drive Cary North Carolina 27513

1st print ing October 1999

S AS reg a nd a ll ot her SA S I ns t it u t e I nc p rodu ct or s ervice na m es a re regis t ered t ra dema rkst d k f SA S I t it t I i t h U S A d t h t i reg i di t U SA

Page 21: Statespace Model

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Chapter 18 Syntax

iteration the relative change of the parameter estimates is less than the PARMTOL=

value and the relative change of the determinant of the innovation variance matrix

is less than the DETTOL= value then iteration ceases and the current estimates are

accepted The default is PARMTOL=001

RESIDEST

computes the final estimates using conditional least squares on the raw data Thistype of estimation may be more stable than the default maximum likelihood method

but is usually more computationally expensive See Parameter Estimation later in

this chapter for details of the conditional least squares method

SINGULAR= value

specifies the criterion for testing for singularity of a matrix A matrix is declared

singular if a scaled pivot is less than the SINGULAR= value when sweeping the

matrix The default is SINGULAR=1E-7

Forecasting Options

BACK= n

starts forecasting n periods before the end of the input data The BACK= option valuemust not be greater than the number of observations The default is BACK=0

INTERVAL= interval

specifies the time interval between observations The INTERVAL= value is used

in conjunction with the ID variable to check that the input data are in order and

have no missing periods The INTERVAL= option is also used to extrapolate the ID

values past the end of the input data See Chapter 3 ldquoDate Intervals Formats and

Functionsrdquo for details on the INTERVAL= values allowed

INTPER= n

specifies that each input observation corresponds to n time periods For example

the options INTERVAL=MONTH and INTPER=2 specify bimonthly data and are

equivalent to specifying INTERVAL=MONTH2 If the INTERVAL= option is not

specified the INTPER= option controls the increment used to generate ID values for

the forecast observations The default is INTPER=1

LEAD= n

specifies how many forecast observations are produced The forecasts start at the

point set by the BACK= option The default is LEAD=0 which produces no fore-

casts

OUT= SAS-data-set

writes the residuals actual values forecasts and forecast standard errors to a SAS

data set See OUT= Data Set later in this chapter for details

PRINT

prints the forecasts

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Part 2 General Information

BY Statement

BY variable

A BY statement can be used with the STATESPACE procedure to obtain separateanalyses on observations in groups defined by the BY variables

FORM Statement

FORM variable value

The FORM statement specifies the number of times a variable is included in the state

vector Values can be specified for any variable listed in the VAR statement If a

value is specified for each variable in the VAR statement the state vector for the state

space model is entirely specified and automatic selection of the state space model is

not performed

The FORM statement forces the state vectorz

t

to contain a specific variable a given

number of times For example if Y is one of the variables inx

t

then the statement

form y 3

forces the state vector to containY

t

Y

t + 1 j t

andY

t + 2 j t

possibly along with other

variables

The following statements illustrate the use of the FORM statement

proc statespace data=in

var x y

f o r m x 3 y 2

run

These statements fit a state space model with the following state vector

z

t

=

2

6

6

6

6

4

x

t j t

y

t j t

x

t + 1 j t

y

t + 1 j t

x

t + 2 j t

3

7

7

7

7

5

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Chapter 18 Syntax

ID Statement

ID variable

The ID statement specifies a variable that identifies observations in the input data setThe variable specified in the ID statement is included in the OUT= data set The

values of the ID variable are extrapolated for the forecast observations based on the

values of the INTERVAL= and INTPER= options

INITIAL Statement

INITIAL F (rowcolumn)= value G(row column)= value

The INITIAL statement gives initial values to the specified elements of theF

andG

matrices These initial values are used as starting values for the iterative estimation

Parts of theF

andG

matrices represent fixed structural identities If an element

specified is a fixed structural element instead of a free parameter the corresponding

initialization is ignored

The following is an example of an INITIAL statement

initial f(32)=0 g(41)=0 g(51)=0

RESTRICT Statement

RESTRICT F(rowcolumn)= value G(rowcolumn)= value

The RESTRICT statement restricts the specified elements of theF

andG

matrices

to the specified values

To use the restrict statement you need to know the form of the model Either specify

the form of the model with the FORM statement or do a preliminary run perhaps

with the NOEST option to find the form of the model that PROC STATESPACE

selects for the data

The following is an example of a RESTRICT statement

restrict f(32)=0 g(41)=0 g(51)=0

Parts of theF

andG

matrices represent fixed structural identities If a restriction is

specified for an element that is a fixed structural element instead of a free parameter

the restriction is ignored

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Part 2 General Information

VAR Statement

VAR variable (difference difference )

The VAR statement specifies the variables in the input data set to model and fore-cast The VAR statement also specifies differencing of the input variables The VAR

statement is required

Differencing is specified by following the variable name with a list of difference

periods separated by commas See the section Stationarity and Differencing for

more information on differencing of input variables

The order in which variables are listed in the VAR statement controls the order in

which variables are included in the state vector Usually potential inputs should be

listed before potential outputs

For example assuming the input data are monthly the following VAR statement

specifies modeling and forecasting of the one period and seasonal second differenceof X and Y

var x(112) y(112)

In this example the vector time series analyzed is

x

t

=

1 B 1 B

1 2

X

t

x

1 B 1 B

1 2

Y

t

y

where B represents the back shift operator and x and y represent the means of thedifferenced series If the NOCENTER option is specified the mean differences are

not subtracted

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Chapter 18 Details

Details

Missing Values

The STATESPACE procedure does not support missing values The procedure uses

the first contiguous group of observations with no missing values for any of the VARstatement variables Observations at the beginning of the data set with missing values

for any VAR statement variable are not used or included in the output data set

Stationarity and Differencing

The state space model used by the STATESPACE procedure assumes that the time

series are stationary Hence the data should be checked for stationarity One way

to check for stationarity is to plot the series A graph of series over time can show a

time trend or variability changes

You can also check stationarity by using the sample autocorrelation functions dis-

played by the ARIMA procedure The autocorrelation functions of nonstationaryseries tend to decay slowly See Chapter 7 ldquoThe ARIMA Procedurerdquo for more in-

formation

Another alternative is to use the STATIONARITY= option on the IDENTIFY state-

ment in PROC ARIMA to apply Dickey-Fuller tests for unit roots in the time series

See Chapter 7 ldquoThe ARIMA Procedurerdquo for more information on Dickey-Fuller unit

root tests

The most popular way to transform a nonstationary series to stationarity is by dif-

ferencing Differencing of the time series is specified in the VAR statement For

example to take a simple first difference of the series X use this statement

var x(1)

In this example the change in X from one period to the next is analyzed When the se-

ries has a seasonal pattern differencing at a period equal to the length of the seasonal

cycle may be desirable For example suppose the variable X is measured quarterly

and shows a seasonal cycle over the year You can use the following statement to

analyze the series of changes from the same quarter in the previous year

var x(4)

To difference twice add another differencing period to the list For ex-

ample the following statement analyzes the series of second differences

X

t

X

t 1

X

t 1

X

t 2

= X

t

2 X

t 1

+ X

t 2

var x(11)

The following statement analyzes the seasonal second difference series

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Part 2 General Information

var x(14)

The series modeled is the 1-period difference of the 4-period difference

X

t

X

t 4

X

t 1

X

t 5

= X

t

X

t 1

X

t 4

+ X

t 5

Another way to obtain stationary series is to use a regression on time to de-trend the

data If the time series has a deterministic linear trend regressing the series on time

produces residuals that should be stationary The following statements write residuals

of X and Y to the variable RX and RY in the output data set DETREND

data a

set a

t=_n_

run

proc reg data=a

model x y = t

output out=detrend r=rx ry

run

You then use PROC STATESPACE to forecast the de-trended series RX and RY A

disadvantage of this method is that you need to add the trend back to the forecast

series in an additional step A more serious disadvantage of the de-trending method

is that it assumes a deterministic trend In practice most time series appear to have a

stochastic rather than a deterministic trend Differencing is a more flexible and often

more appropriate method

There are several other methods to handle nonstationary time series For more infor-

mation and examples refer to Brockwell and Davis (1991)

Preliminary Autoregressive Models

After computing the sample autocovariance matrices PROC STATESPACE fits a

sequence of vector autoregressive models These preliminary autoregressive models

are used to estimate the autoregressive order of the process and limit the order of the

autocovariances considered in the state vector selection process

Yule-Walker Equations for Forward and Backward Models

Unlike a univariate autoregressive model a multivariate autoregressive model has

different forms depending on whether the present observation is being predicted from

the past observations or from the future observations

Letx

t

be the r -component stationary time series given by the VAR statement after

differencing and subtracting the vector of sample means (If the NOCENTER option

is specified the mean is not subtracted) Let n be the number of observations of x

t

from the input data set

Lete

t

be a vector white noise sequence with mean vector 0 and variance matrix

p

and letn

t

be a vector white noise sequence with mean vector 0 and variance matrix

p

Let p be the order of the vector autoregressive model forx

t

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Chapter 18 Details

The forward autoregressive form based on the past observations is written as follows

x

t

=

p

X

i = 1

p

i

x

t i

+ e

t

The backward autoregressive form based on the future observations is written as fol-

lows

x

t

=

p

X

i = 1

p

i

x

t + i

+ n

t

LettingE

denote the expected value operator the autocovariance sequence for thex

t

series

i

is

i

= E x

t

x

0

t i

The Yule-Walker equations for the autoregressive model that matches the first p ele-

ments of the autocovariance sequence are

2

6

6

4

0

1

p 1

0

1

0

p 2

0

p 1

0

p 2

0

3

7

7

5

2

6

6

4

p

1

p

2

p

p

3

7

7

5

=

2

6

6

4

1

2

p

3

7

7

5

and

2

6

6

6

4

0

0

1

0

p 1

1

0

0

p 2

p 1

p 2

0

3

7

7

7

5

2

6

6

4

p

1

p

2

p

p

3

7

7

5

=

2

6

6

4

0

1

0

2

0

p

3

7

7

5

Here

p

i

are the coefficient matrices for the past observation form of the vector au-toregressive model and

p

i

are the coefficient matrices for the future observation

form More information on the Yule-Walker equations in the multivariate setting can

be found in Whittle (1963) and Ansley and Newbold (1979)

The innovation variance matrices for the two forms can be written as follows

p

=

0

p

X

i = 1

p

i

0

i

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Part 2 General Information

p

=

0

p

X

i = 1

p

i

i

The autoregressive models are fit to the data using the preceding Yule-Walker equa-

tions with

i

replaced by the sample covariance sequenceC

i

The covariance matri-

ces are calculated as

C

i

=

1

N 1

N

X

t = i + 1

x

t

x

0

t i

Let b

p

b

p

b

p

and b

p

represent the Yule-Walker estimates of

p

p

p

and

p

respectively These matrices are written to an output data set when the OUTAR=

option is specified

When the PRINTOUT=LONG option is specified the sequence of matrices b

p

and

the corresponding correlation matrices are printed The sequence of matricesb

p isused to compute Akaike information criteria for selection of the autoregressive order

of the process

Akaike Information Criterion

The Akaike information criterion or AIC is defined as -2( maximum of log likeli-

hood )+2(number of parameters) Since the vector autoregressive models are esti-

mates from the Yule-Walker equations not by maximum likelihood the exact likeli-

hood values are not available for computing the AIC However for the vector autore-

gressive model the maximum of the log likelihood can be approximated as

l n L

n

2

l n j

b

p

j

Thus the AIC for the order p model is computed as

A I C

p

= n l n j

b

p

j + 2 p r

2

You can use the printed AIC array to compute a likelihood ratio test of the autoregres-

sive order The log-likelihood ratio test statistic for testing the order p model against

the order p 1

model is

n l n j

b

p

j + n l n j

b

p 1

j

This quantity is asymptotically distributed as a

2 withr

2 degrees of freedom if the

series is autoregressive of order p 1

It can be computed from the AIC array as

A I C

p 1

A I C

p

+ 2 r

2

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You can evaluate the significance of these test statistics with the PROBCHI function

in a SAS DATA step or with a

2 table

Determining the Autoregressive Order

Although the autoregressive models can be used for prediction their primary value is

to aid in the selection of a suitable portion of the sample covariance matrix for use in

computing canonical correlations If the multivariate time series x

t is of autoregres-sive order p then the vector of past values to lag p is considered to contain essentially

all the information relevant for prediction of future values of the time series

By default PROC STATESPACE selects the order p producing the autoregressive

model with the smallestA I C

p

If the value p for the minimumA I C

p

is less than the

value of the PASTMIN= option then p is set to the PASTMIN= value Alternatively

you can use the ARMAX= and PASTMIN= options to force PROC STATESPACE to

use an order you select

Significance Limits for Partial Autocorrelations

The STATESPACE procedure prints a schematic representation of the partial autocor-

relation matrices indicating which partial autocorrelations are significantly greater orsignificantly less than 0 Figure 1810 shows an example of this table

The STATESPACE Procedure

Schematic Representation of Partial Autocorrelations

NameLag 1 2 3 4 5 6 7 8 9 10

x ++ +

y ++

+ is gt 2std error - is lt -2std error is between

Figure 1810 Significant Partial Autocorrelations

The partial autocorrelations are from the sample partial autoregressive matrices b

p

p

The standard errors used for the significance limits of the partial autocorrelations are

computed from the sequence of matrices

p

and

p

Under the assumption that the observed series arises from an autoregressive process

of order p 1

the pth sample partial autoregressive matrix b

p

p

has an asymptotic

variance matrix 1

n

1

p

p

The significance limits for b

p

p

used in the schematic plot of the sample partial autore-

gressive sequence are derived by replacing

p

and

p

with their sample estimators

to produce the variance estimate as follows

d

V a r

b

p

p

=

1

n r p

b

1

p

b

p

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Canonical Correlation Analysis

Given the order p letp

t

be the vector of current and past values relevant to prediction

of x

t + 1

p

t

= x

0

t

x

0

t 1

x

0

t p

0

Letf

t

be the vector of current and future values

f

t

= x

0

t

x

0

t + 1

x

0

t + p

0

In the canonical correlation analysis consider submatrices of the sample covariance

matrix of p

t

andf

t

This covariance matrixV

has a block Hankel form

V =

2

6

6

6

4

C

0

C

0

1

C

0

2

C

0

p

C

0

1

C

0

2

C

0

3

C

0

p + 1

C

0

p

C

0

p + 1

C

0

p + 2

C

0

2 p

3

7

7

7

5

State Vector Selection Process

The canonical correlation analysis forms a sequence of potential state vectorsz

j

t

Examine a sequencef

j

t

of subvectors of f

t

and form the submatrixV

j consisting

of the rows and columns of V

corresponding to the components of f

j

t

and compute

its canonical correlations

The smallest canonical correlation of V

j is then used in the selection of the com-

ponents of the state vector The selection process is described in the following For

more details about this process refer to Akaike (1976)

In the following discussion the notationx

t + k j t

denotes the wide sense conditional

expectation (best linear predictor) of x

t + k

given allx

s

with s less than or equal to t

In the notationx

i t + 1

the first subscript denotes the ith component of x

t + 1

The initial state vectorz

1

t

is set tox

t

The sequencef

j

t

is initialized by setting

f

1

t

= z

1

0

t

x

1 t + 1 j t

0

= x

0

t

x

1 t + 1 j t

0

That is start by considering whether to addx

1 t + 1 j t

to the initial state vectorz

1

t

The procedure forms the submatrixV

1 corresponding tof

1

t

and computes its canon-

ical correlations Denote the smallest canonical correlation of V

1 as

m i n

If

m i n

is

significantly greater than 0x

1 t + 1 j t

is added to the state vector

If the smallest canonical correlation of V

1 is not significantly greater than 0 then a

linear combination of f

1

t

is uncorrelated with the pastp

t

Assuming that the determi-

nant of C

0

is not 0 (that is no input series is a constant) you can take the coefficient

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of x

1 t + 1 j t

in this linear combination to be 1 Denote the coefficients of z

1

t

in this

linear combination as

This gives the relationship

x

1 t + 1 j t

=

0

x

t

Therefore the current state vector already contains all the past information useful forpredicting

x

1 t + 1

and any greater leads of x

1 t

The variablex

1 t + 1 j t

is not added to

the state vector nor are any termsx

1 t + k j t

considered as possible components of the

state vector The variablex

1

is no longer active for state vector selection

The process described forx

1 t + 1 j t

is repeated for the remaining elements of f

t

The

next candidate for inclusion in the state vector is the next component of f

t

corre-

sponding to an active variable Components of f

t

corresponding to inactive variables

that produced a zero

m i n

in a previous step are skipped

Denote the next candidate asx

l t + k j t

The vectorf

j

t

is formed from the current state

vector andx

l t + k j t

as follows

f

j

t

= z

j

0

t

x

l t + k j t

0

The matrixV

j is formed fromf

j

t

and its canonical correlations are computed The

smallest canonical correlation of V

j is judged to be either greater than or equal to 0

If it is judged to be greater than 0x

l t + k j t

is added to the state vector If it is judged

to be 0 then a linear combination of f

j

t

is uncorrelated with thep

t

and the variable

x

l

is now inactive

The state vector selection process continues until no active variables remain

Testing Significance of Canonical Correlations For each step in the canonical correlation sequence the significance of the small-

est canonical correlation

m i n

is judged by an information criterion from Akaike

(1976) This information criterion is

n l n 1

2

m i n

r p + 1 q + 1

where q is the dimension of f

j

t

at the current step r is the order of the state vector p

is the order of the vector autoregressive process and

is the value of the SIGCORR=

option The default is SIGCORR=2 If this information criterion is less than or equal

to 0

m i n

is taken to be 0 otherwise it is taken to be significantly greater than 0

(Do not confuse this information criterion with the AIC)

Variables inx

t + p j t

are not added in the model even with positive information crite-

rion because of the singularity of V

You can force the consideration of more candi-

date state variables by increasing the size of theV

matrix by specifying a PASTMIN=

option value larger than p

Printing the Canonical Correlations

To print the details of the canonical correlation analysis process specify the CAN-

CORR option in the PROC STATESPACE statement The CANCORR option prints

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the candidate state vectors the canonical correlations and the information criteria for

testing the significance of the smallest canonical correlation

Bartlettrsquos

2 and its degrees of freedom are also printed when the CANCORR option

is specified The formula used for Bartlettrsquos

2 is

2

= n 5 r p + 1 q + 1 l n 1

2

m i n

withr p + 1 q + 1

degrees of freedom

Figure 1811 shows the output of the CANCORR option for the introductory example

shown in the Getting Started section of this chapter

The STATESPACE Procedure

Canonical Correlations Analysis

Information Chi

x(TT) y(TT) x(T+1T) Criterion Square DF

1 1 0237045 3566167 114505 4

Information Chi

x(TT) y(TT) x(T+1T) y(T+1T) Criterion Square DF

1 1 0238244 0056565 -535906 0636134 3

Information Chi

x(TT) y(TT) x(T+1T) x(T+2T) Criterion Square DF

1 1 0237602 0087493 -446312 1525353 3

Figure 1811 Canonical Correlations Analysis

New variables are added to the state vector if the information criteria are positive Inthis example

y

t + 1 j t

andx

t + 2 j t

are not added to the state space vector because the

information criteria for these models are negative

If the information criterion is nearly 0 then you may want to investigate models

that arise if the opposite decision is made regarding

m i n

This investigation can be

accomplished by using a FORM statement to specify part or all of the state vector

Preliminary Estimates of F

When a candidate variablex

l t + k j t

yields a zero

m i n

and is not added to the state

vector a linear combination of f

j

t

is uncorrelated with thep

t

Because of the method

used to construct thef

j

t

sequence the coefficient of x

l t + k j t

inl

can be taken as 1

Denote the coefficients of z

j

t

in this linear combination as l

This gives the relationship

x

l t + k j t

= l

0

z

j

t

The vectorl

is used as a preliminary estimate of the first r columns of the row of the

transition matrixF

corresponding tox

l t + k 1 j t

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Parameter Estimation

The model isz

t + 1

= F z

t

+ G e

t + 1

wheree

t

is a sequence of independent multivari-

ate normal innovations with mean vector 0 and variance

e e

The observed sequence

x

t

composes the first r components of z

t

and thusx

t

= H z

t

where H is ther s

matrix I

r

0

LetE

be ther n

matrix of innovations

E = e

1

e

n

If the number of observations n is reasonably large the log likelihood L can be

approximated up to an additive constant as follows

L =

n

2

l n j

e e

j

1

2

t r a c e

1

e e

E E

0

The elements of

e e

are taken as free parameters and are estimated as follows

S

0

=

1

n

E E

0

Replacing

e e

byS

0

in the likelihood equation the log likelihood up to an additive

constant is

L =

n

2

l n j S

0

j

Letting B be the backshift operator the formal relation betweenx

t

ande

t

is

x

t

= H I B F

1

G e

t

e

t

= H I B F

1

G

1

x

t

=

1

X

i = 0

i

x

t i

LettingC

i

be the ith lagged sample covariance of x

t

and neglecting end effects the

matrixS

0

is

S

0

=

1

X

i j = 0

i

C

i + j

0

j

For the computation of S

0

the infinite sum is truncated at the value of the KLAG=

option The value of the KLAG= option should be large enough that the sequence

i

is approximately 0 beyond that point

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Let

be the vector of free parameters in theF

andG

matrices The derivative of the

log likelihood with respect to the parameter

is

L

=

n

2

t r a c e

S

1

0

S

0

The second derivative is

2

L

0

=

n

2

t r a c e

S

1

0

S

0

0

S

1

0

S

0

t r a c e

S

1

0

2

S

0

0

Near the maximum the first term is unimportant and the second term can be approx-

imated to give the following second derivative approximation

2

L

0

=

n t r a c e

S

1

0

E

E

0

0

The first derivative matrix and this second derivative matrix approximation are com-

puted from the sample covariance matrixC

0

and the truncated sequence

i

The

approximate likelihood function is maximized by a modified Newton-Raphson algo-

rithm employing these derivative matrices

The matrixS

0

is used as the estimate of the innovation covariance matrix

e e

The

negative of the inverse of the second derivative matrix at the maximum is used as an

approximate covariance matrix for the parameter estimates The standard errors of

the parameter estimates printed in the parameter estimates tables are taken from the

diagonal of this covariance matrix The parameter covariance matrix is printed when

the COVB option is specified

If the data are nearly nonstationary a better estimate of

e e

and the other parameters

can sometimes be obtained by specifying the RESIDEST option The RESIDEST

option estimates the parameters using conditional least squares instead of maximum

likelihood

The residuals are computed using the state space equation and the sample mean val-

ues of the variables in the model as start-up values The estimate of S

0

is then com-

puted using the residuals from the ith observation on where i is the maximum number

of times any variable occurs in the state vector A multivariate Gauss-Marquardt al-

gorithm is used to minimizej S

0

j

Refer to Harvey (1981a) for a further description

of this method

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Forecasting

Given estimates of F

G

and

e e

forecasts of x

t

are computed from the conditional

expectation of z

t

In forecasting the parameters F G and

e e

are replaced with the estimates or by val-

ues specified in the RESTRICT statement One-step-ahead forecasting is performedfor the observation

x

t

wheret n b

Heren

is the number of observations and

b is the value of the BACK= option For the observationx

t

wheret n b

m-

step-ahead forecasting is performed form = t n + b

The forecasts are generated

recursively with the initial conditionz

0

= 0

The m-step-ahead forecast of z

t + m

isz

t + m j t

wherez

t + m j t

denotes the conditional

expectation of z

t + m

given the information available at time t The m-step-ahead

forecast of x

t + m

isx

t + m j t

= H z

t + m j t

where the matrixH = I

r

0

Let

i

= F

i

G

Note that the lasts r

elements of z

t

consist of the elements of x

u j t

foru t

The state vector z

t + m

can be represented as

z

t + m

= F

m

z

t

+

m 1

X

i = 0

i

e

t + m i

Sincee

t + i j t

= 0

fori 0

the m-step-ahead forecastz

t + m j t

is

z

t + m j t

= F

m

z

t

= F z

t + m 1 j t

Therefore the m-step-ahead forecast of x

t + m

is

x

t + m j t

= H z

t + m j t

The m-step-ahead forecast error is

z

t + m

z

t + m j t

=

m 1

X

i = 0

i

e

t + m i

The variance of the m-step-ahead forecast error is

V

z m

=

m 1

X

i = 0

i

e e

0

i

LettingV

z 0

= 0

the variance of the m-step-ahead forecast error of z

t + m

V

z m

can

be computed recursively as follows

V

z m

= V

z m 1

+

m 1

e e

0

m 1

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The variance of the m-step-ahead forecast error of x

t + m

is ther r

left upper sub-

matrix of V

z m

that is

V

x m

= H V

z m

H

0

Unless the NOCENTER option is specified the sample mean vector is added to the

forecast When differencing is specified the forecasts xt + m j t

plus the sample mean

vector are integrated back to produce forecasts for the original series

Lety

t

be the original series specified by the VAR statement with some 0 values

appended corresponding to the unobserved past observations Let B be the backshift

operator and let B

be thes s

matrix polynomial in the backshift operator

corresponding to the differencing specified by the VAR statement The off-diagonal

elements of

i

are 0 Note that

0

= I

s

whereI

s

is thes s

identity matrix Then

z

t

= B y

t

This gives the relationship

y

t

=

1

B z

t

=

1

X

i = 0

i

z

t i

where

1

B =

P

1

i = 0

i

B

i and

0

= I

s

The m-step-ahead forecast of y

t + m

is

y

t + m j t

=

m 1

X

i = 0

i

z

t + m i j t

+

1

X

i = m

i

z

t + m i

The m-step-ahead forecast error of y

t + m

is

m 1

X

i = 0

i

z

t + m i

z

t + m i j t

=

m 1

X

i = 0

i

X

u = 0

u

i u

e

t + m i

LettingV

y 0

= 0

the variance of the m-step-ahead forecast error of y

t + m

V

y m

is

V

y m

=

m 1

X

i = 0

i

X

u = 0

u

i u

e e

i

X

u = 0

u

i u

0

= V

y m 1

+

m 1

X

u = 0

u

m 1 u

e e

m 1

X

u = 0

u

m 1 u

0

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Relation of ARMA and State Space Forms

Every state space model has an ARMA representation and conversely every ARMA

model has a state space representation This section discusses this equivalence The

following material is adapted from Akaike (1974) where there is a more complete

discussion Pham-Dinh-Tuan (1978) also contains a discussion of this material

Suppose you are given the following ARMA model

B x

t

= B e

t

or in more detail

x

t

1

x

t 1

p

x

t p

= e

t

+

1

e

t 1

+ +

q

e

t q

(1)

wheree

t

is a sequence of independent multivariate normal random vectors with mean

0 and variance matrix

e e

B is the backshift operator (B x

t

= x

t 1

) B

and

B

are matrix polynomials in B andx

t

is the observed process

If the roots of the determinantial equationj B j = 0

are outside the unit circle in

the complex plane the model can also be written as

x

t

=

1

B B e

t

=

1

X

i = 0

i

e

t i

The

i

matrices are known as the impulse response matrices and can be computed

as

1

B B

You can assume p q

since if this is not initially true you can add more terms

i

that are identically 0 without changing the model

To write this set of equations in a state space form proceed as follows Letx

t + i j t

be

the conditional expectation of x

t + i

givenx

w

forw t

The following relations hold

x

t + i j t

=

1

X

j = i

j

e

t + i j

x

t + i j t + 1

= x

t + i j t

+

i 1

e

t + 1

However from equation (1) you can derive the following relationship

x

t + p j t

=

1

x

t + p 1 j t

+ +

p

x

t

(2)

Hence wheni = p

you can substitute forx

t + p j t

in the right-hand side of equation

(2) and close the system of equations

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This substitution results in the following model in the state space form

z

t + 1

= F z

t

+ G e

t + 1

2

6

6

6

4

x

t + 1

x

t + 2 j t + 1

x

t + p j t + 1

3

7

7

7

5

=

2

6

6

4

0 I 0 0

0 0 I 0

p

p 1

1

3

7

7

5

2

6

6

6

4

x

t

x

t + 1 j t

x

t + p 1 j t

3

7

7

7

5

+

2

6

6

4

I

1

p 1

3

7

7

5

e

t + 1

Note that the state vectorz

t

is composed of conditional expectations of x

t

and the

first r components of z

t

are equal tox

t

The state space form can be cast into an ARMA form by solving the system of dif-

ference equations for the first r components

When converting from an ARMA form to a state space form you can generate a

state vector larger than needed that is the state space model may not be a minimal

representation When going from a state space form to an ARMA form you can have

nontrivial common factors in the autoregressive and moving average operators that

yield an ARMA model larger than necessary

If the state space form used is not a minimal representation some but not all compo-

nents of x

t + i j t

may be linearly dependent This situation corresponds to

p

p 1

being of less than full rank when B

and B

have no common nontrivial

left factors In this casez

t

consists of a subset of the possible components of

x

t + i j t

i = 1 2 p 1

However once a component of x

t + i j t

(for example the

jth one) is linearly dependent on the previous conditional expectations then all sub-

sequent jth components of x

t + k j t

fork i

must also be linearly dependent Note

that in this case equivalent but seemingly different structures can arise if the order of

the components withinx

t

is changed

OUT= Data Set

The forecasts are contained in the output data set specified by the OUT= option on the

PROC STATESPACE statement The OUT= data set contains the following variables

the BY variables

the ID variable

the VAR statement variables These variables contain the actual values from

the input data set

FORi numeric variables containing the forecasts The variable FORi contains

the forecasts for the ith variable in the VAR statement list Forecasts are one-

step-ahead predictions until the end of the data or until the observation specified

by the BACK= option

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RESi numeric variables containing the residual for the forecast of the ith vari-

able in the VAR statement list For forecast observations the actual values are

missing and the RESi variables contain missing values

STDi numeric variables containing the standard deviation for the forecast of

the ith variable in the VAR statement list The values of the STDi variables can

be used to construct univariate confidence limits for the corresponding fore-casts However such confidence limits do not take into account the covariance

of the forecasts

OUTAR= Data Set

The OUTAR= data set contains the estimates of the preliminary autoregressive mod-

els The OUTAR= data set contains the following variables

ORDER a numeric variable containing the order p of the autoregressive model

that the observation represents

AIC a numeric variable containing the value of the information criterionA I C

p

SIGFl numeric variables containing the estimate of the innovation covariance

matrices for the forward autoregressive models The variable SIGFl contains

the lth column of b

p

in the observations with ORDER= p

SIGBl numeric variables containing the estimate of the innovation covariance

matrices for the backward autoregressive models The variable SIGBl contains

the lth column of b

p

in the observations with ORDER= p

FORk ndashl numeric variables containing the estimates of the autoregressive pa-

rameter matrices for the forward models The variable FORk ndashl contains the

lth column of the lag k autoregressive parameter matrix b

p

k

in the observations

with ORDER= p

BACk ndashl numeric variables containing the estimates of the autoregressive pa-

rameter matrices for the backward models The variable BACk ndashl contains the

lth column of the lag k autoregressive parameter matrix b

p

k

in the observations

with ORDER= p

The estimates for the order p autoregressive model can be selected as those observa-

tions with ORDER= p Within these observations the k lth element of

p

i

is given by

the value of the FORindashl variable in the k th observation The k lth element of

p

i

is

given by the value of BACindashl variable in the k th observation The k lth element of

p

is given by SIGFl in the k th observation The k lth element of

p

is given by SIGBlin the k th observation

Table 181 shows an example of the OUTAR= data set with ARMAX=3 andx

t

of

dimension 2 In Table 181 i j

indicate the ijth element of the matrix

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Table 181 Values in the OUTAR= Data Set

Obs ORDER AIC SIGF1 SIGF2 SIGB1 SIGB2 FOR1ndash 1 FOR1ndash 2 FOR2ndash 1 FOR2ndash 2 FOR3ndash 1

1 0 AIC0

0 1 1

0 1 2

0 1 1

0 1 2

2 0 AIC0

0 2 1

0 2 2

0 2 1

0 2 2

3 1 AIC1

1 1 1

1 1 2

1 1 1

1 1 2

1

1

1 1

1

1

1 2

4 1 AIC1

1 2 1

1 1 2

1 2 1

1 1 2

1

1

2 1

1

1

2 2

5 2 AIC2

2 1 1

2 1 2

2 1 1

2 1 2

2

1

1 1

2

1

1 2

2

2

1 1

2

2

1 2

6 2 AIC2

2 2 1

2 1 2

2 2 1

2 1 2

2

1

2 1

2

1

2 2

2

2

2 1

2

2

2 2

7 3 AIC3

3 1 1

3 1 2

3 1 1

3 1 2

3

1

1 1

3

1

1 2

3

2

1 1

3

2

1 2

3

3

1 1

8 3 AIC3

3 2 1

3 1 2

3 2 1

3 1 2

3

1

2 1

3

1

2 2

3

2

2 1

3

2

2 2

3

3

2 1

Obs FOR3ndash 2 BACK1ndash 1 BACK1ndash 2 BACK2ndash 1 BACK2ndash 2 BACK3ndash 1 BACK3ndash 2

1

2

3

1

1

1 1

1

1

1 2

4

1

1

2 1

1

1

2 2

5

2

1

1 1

2

1

1 2

2

2

1 1

2

2

1 2

6

2

1

2 1

2

1

2 2

2

2

2 1

2

2

2 2

7

3

3

1 2

3

1

1 1

3

1

1 2

3

2

1 1

3

2

1 2

3

3

1 1

3

3

1 2

8

3

3

2 2

3

1

2 1

3

1

2 2

3

2

2 1

3

2

2 2

3

3

2 1

3

3

2 2

The estimated autoregressive parameters can be used in the IML procedure to obtain

autoregressive estimates of the spectral density function or forecasts based on the

autoregressive models

OUTMODEL= Data Set

The OUTMODEL= data set contains the estimates of theF

andG

matrices and their

standard errors the names of the components of the state vector and the estimates of

the innovation covariance matrix The variables contained in the OUTMODEL= data

set are as follows

the BY variables

STATEVEC a character variable containing the name of the component of the

state vector corresponding to the observation The STATEVEC variable has

the value STD for standard deviations observations which contain the standarderrors for the estimates given in the preceding observation

Fndash j numeric variables containing the columns of theF

matrix The variable

Fndash j contains the jth column of F

The number of Fndash j variables is equal to

the value of the DIMMAX= option If the model is of smaller dimension the

extraneous variables are set to missing

Gndash j numeric variables containing the columns of theG

matrix The variable

Gndash j contains the jth column of G

The number of Gndash j variables is equal to r

the dimension of x

t

given by the number of variables in the VAR statement

SIGndash j numeric variables containing the columns of the innovation covariance

matrix The variable SIGndash j contains the jth column of

e e

There are r vari-ables SIGndash j

Table 182 shows an example of the OUTMODEL= data set withx

t

= x

t

y

t

0

z

t

= x

t

y

t

x

t + 1 j t

0 and DIMMAX=4 In Table 182F

i j

andG

i j

are the ijth

elements of F

andG

respectively Note that all elements for Fndash4 are missing because

F

is a3 3

matrix

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Chapter 18 Details

Table 182 Value in the OUTMODEL= Data Set

Obs STATEVEC Fndash 1 Fndash 2 Fndash 3 Fndash 4 Gndash 1 Gndash 2 SIGndash 1 SIGndash 2

1 X(TT) 0 0 1 1 0

1 1

1 2

2 STD

3 Y(TT) F2 1

F2 2

F2 3

0 1

2 1

2 2

4 STD std F2 1

std F2 2

std F2 3

5 X(T+1T) F3 1

F3 2

F3 3

G3 1

G3 2

6 STD std F3 1

std F3 2

std F3 3

std G3 1

std G3 2

Printed Output

The printed output produced by the STATESPACE procedure is described in the fol-

lowing

1 descriptive statistics which include the number of observations used the

names of the variables their means and standard deviations (Std) and the dif-

ferencing operations used

2 the Akaike information criteria for the sequence of preliminary autoregressive

models

3 if the PRINTOUT=LONG option is specified the sample autocovariance ma-

trices of the input series at various lags

4 if the PRINTOUT=LONG option is specified the sample autocorrelation ma-

trices of the input series

5 a schematic representation of the autocorrelation matrices showing the signif-

icant autocorrelations

6 if the PRINTOUT=LONG option is specified the partial autoregressive ma-

trices (These are

p

p

as described in Preliminary Autoregressive Modelsearlier in this chapter)

7 a schematic representation of the partial autocorrelation matrices showing the

significant partial autocorrelations

8 the Yule-Walker estimates of the autoregressive parameters for the autoregres-

sive model with the minimum AIC

9 if the PRINTOUT=LONG option is specified the autocovariance matrices of

the residuals of the minimum AIC model This is the sequence of estimated

innovation variance matrices for the solutions of the Yule-Walker equations

10 if the PRINTOUT=LONG option is specified the autocorrelation matrices of the residuals of the minimum AIC model

11 If the CANCORR option is specified the canonical correlations analysis for

each potential state vector considered in the state vector selection process

This includes the potential state vector the canonical correlations the informa-

tion criterion for the smallest canonical correlation Bartlettrsquos

2 statistic (ldquoChi

Squarerdquo) for the smallest canonical correlation and the degrees of freedom of

Bartlettrsquos

2

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Part 2 General Information

12 the components of the chosen state vector

13 the preliminary estimate of the transition matrixF

the input matrixG

and

the variance matrix for the innovations

e e

14 if the ITPRINT option is specified the iteration history of the likelihood max-

imization For each iteration this shows the iteration number the number of step halvings the determinant of the innovation variance matrix the damping

factor Lambda and the values of the parameters

15 the state vector printed again to aid interpretation of the following listing of F

andG

16 the final estimate of the transition matrixF

17 the final estimate of the input matrixG

18 the final estimate of the variance matrix for the innovations

e e

19 a table listing the estimates of the free parameters inF

andG

and their standarderrors and t statistics

20 if the COVB option is specified the covariance matrix of the parameter esti-

mates

21 if the COVB option is specified the correlation matrix of the parameter esti-

mates

22 if the PRINT option is specified the forecasts and their standard errors

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Chapter 18 Details

ODS Table Names

PROC STATESPACE assigns a name to each table it creates You can use these

names to reference the table when using the Output Delivery System (ODS) to select

tables and create output data sets These names are listed in the following table For

more information on ODS see Chapter 6 ldquoUsing the Output Delivery Systemrdquo

Table 183 ODS Tables Produced in PROC STATESPACE

ODS Table Name Description Option

NObs Number of observations default

Summary Simple summary statistics table default

InfoCriterion Information criterion table default

CovLags Covariance Matrices of Input Series PRINTOUT=LONG

CorrLags Correlation Matrices of Input Series PRINTOUT=LONG

PartialAR Partial Autoregressive Matrices PRINTOUT=LONG

YWEstimates Yule-Walker Estimates for Minimum AIC default

CovResiduals Covariance of Residuals PRINTOUT=LONGCorrResiduals Residual Correlations from AR Models PRINTOUT=LONG

StateVector State vector table default

CorrGraph Schematic Representation of Correlations default

TransitionMatrix Transition Matrix default

InputMatrix Input Matrix default

VarInnov Variance Matrix for the Innovation default

CovB Covariance of Parameter Estimates COVB

CorrB Correlation of Parameter Estimates COVB

CanCorr Canonical Correlation Analysis CANCORR

IterHistory Iterative Fitting table ITPRINT

ParameterEstimates Parameter Estimates Table default

Forecasts Forecasts Table PRINT

ConvergenceStatus Convergence Status Table default

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Part 2 General Information

Example

Example 181 Series J from Box and Jenkins

This example analyzes the gas furnace data (series J) from Box and Jenkins (The

data are not shown Refer to Box and Jenkins (1976) for the data)

First a model is selected and fit automatically using the following statements

title1 rsquoGas Furnace Datarsquo

title2 rsquoBox amp Jenkins Series Jrsquo

title3 rsquoAutomatically Selected Modelrsquo

proc statespace data=seriesj cancorr

var x y

run

The results for the automatically selected model are shown in Output 1811

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Chapter 18 Example

Output 1811 Results for Automatically Selected Model

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Number of Observations 296

Standard

Variable Mean Error

x -005683 1072766

y 5350912 3202121

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Information Criterion for Autoregressive Models

Lag=0 Lag=1 Lag=2 Lag=3 Lag=4 Lag=5 Lag=6 Lag=7 Lag=8

6513862 -103357 -163296 -164512 -165152 -164891 -164934 -164315 -163856

Information

Criterion for

Autoregressive

Models

Lag=9 Lag=10

-16348 -163359

Schematic Representation of Correlations

NameLag 0 1 2 3 4 5 6 7 8 9 10

x +- +- +- +- +- +- +- +- +- +- +-

y -+ -+ -+ -+ -+ -+ -+ -+ -+ -+ -+

+ is gt 2std error - is lt -2std error is between

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Part 2 General Information

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Schematic Representation of Partial Autocorrelations

NameLag 1 2 3 4 5 6 7 8 9 10

x + - + -

y -+ -- - + +

+ is gt 2std error - is lt -2std error is between

Yule-Walker Estimates for Minimum AIC

------Lag=1------ ------Lag=2------ ------Lag=3------ ------Lag=4------

x y x y x y x y

x 1925887 -000124 -120166 0004224 0116918 -000867 0104236 0003268

y 0050496 1299793 -002046 -03277 -071182 -025701 0195411 0133417

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Canonical Correlations Analysis

Information Chi

x(TT) y(TT) x(T+1T) Criterion Square DF

1 1 0804883 2929228 3047481 8

Information Chix(TT) y(TT) x(T+1T) y(T+1T) Criterion Square DF

1 1 0906681 0607529 1223358 1347237 7

Information Chi

x(TT) y(TT) x (T+1T) y (T+1T) x (T+2T) Criterion Square DF

1 1 0909434 0610278 0186274 -154701 1034705 6

Information Chi

x(TT) y(TT) x (T+1T) y (T+1T) y (T+2T) Criterion Square DF

1 1 091014 0618937 0206823 0940392 1280924 6

Information Chi

x(TT) y(TT) x(T+1T) y(T+1T) y(T+2T) y(T+3T) Criterion Square DF

1 1 0912963 0628785 0226598 0083258 -794103 2041584 5

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Chapter 18 Example

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Selected Statespace Form and Preliminary Estimates

State Vector

x(TT) y(TT) x(T+1T) y(T+1T) y(T+2T)

Estimate of Transition Matrix

0 0 1 0 0

0 0 0 1 0

-084718 0026794 1711715 -005019 0

0 0 0 0 1

-019785 0334274 -018174 -123557 1787475

Input Matrix for Innovation

1 0

0 11925887 -000124

0050496 1299793

0142421 1361696

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Selected Statespace Form and Preliminary Estimates

Variance Matrix for Innovation

0035274 -000734

-000734 0097569

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Part 2 General Information

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Selected Statespace Form and Fitted Model

State Vector

x(TT) y(TT) x(T+1T) y(T+1T) y(T+2T)

Estimate of Transition Matrix

0 0 1 0 0

0 0 0 1 0

-086192 0030609 1724235 -005483 0

0 0 0 0 1

-034839 0292124 -009435 -109823 1671418

Input Matrix for Innovation

1 0

0 1192442 -000416

0015621 1258495

008058 1353204

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Selected Statespace Form and Fitted Model

Variance Matrix for Innovation

0035579 -000728

-000728 0095577

Parameter Estimates

Standard

Parameter Estimate Error t Value

F(31) -086192 0072961 -1181

F(32) 0030609 0026167 117

F(33) 1724235 0061599 2799

F(34) -005483 0030169 -182

F(51) -034839 0135253 -258

F(52) 0292124 0046299 631

F(53) -009435 0096527 -098F(54) -109823 0109525 -1003

F(55) 1671418 0083737 1996

G(31) 1924420 0058162 3309

G(32) -000416 0035255 -012

G(41) 0015621 0095771 016

G(42) 1258495 0055742 2258

G(51) 0080580 0151622 053

G(52) 1353204 0091388 1481

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Chapter 18 Example

The two series are believed to have a transfer function relation with the gas rate

(variable X) as the input and the CO2

concentration (variable Y) as the output Since

the parameter estimates shown in Output 1811 support this kind of model the model

is reestimated with the feedback parameters restricted to 0 The following statements

fit the transfer function (no feedback) model

title3 rsquoTransfer Function Modelrsquo

proc statespace data=seriesj printout=none

var x y

restrict f(32)=0 f(34)=0

g(32)=0 g(41)=0 g(51)=0

run

The last 2 pages of the output are shown in Output 1812

Output 1812 STATESPACE Output for Transfer Function Model

Gas Furnace Data

Box amp Jenkins Series J

Transfer Function Model

The STATESPACE Procedure

Selected Statespace Form and Fitted Model

State Vector

x(TT) y(TT) x(T+1T) y(T+1T) y(T+2T)

Estimate of Transition Matrix

0 0 1 0 0

0 0 0 1 0

-068882 0 1598717 0 0

0 0 0 0 1

-035944 0284179 -00963 -107313 1650047

Input Matrix for Innovation

1 0

0 1

1923446 0

0 1260856

0 1346332

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Part 2 General Information

Gas Furnace Data

Box amp Jenkins Series J

Transfer Function Model

The STATESPACE Procedure

Selected Statespace Form and Fitted Model

Variance Matrix for Innovation

0036995 -00072

-00072 0095712

Parameter Estimates

Standard

Parameter Estimate Error t Value

F(31) -068882 0050549 -1363

F(33) 1598717 0050924 3139

F(51) -035944 0229044 -157

F(52) 0284179 0096944 293

F(53) -009630 0140876 -068

F(54) -107313 0250385 -429

F(55) 1650047 0188533 875G(31) 1923446 0056328 3415

G(42) 1260856 0056464 2233

G(52) 1346332 0091086 1478

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Chapter 18 References

References

Akaike H (1974) Markovian Representation of Stochastic Processes and Its Ap-

plication to the Analysis of Autoregressive Moving Average Processes Annals

of the Institute of Statistical Mathematics 26 363-387

Akaike H (1976) Canonical Correlations Analysis of Time Series and the Use of

an Information Criterion in Advances and Case Studies in System Identification

eds R Mehra and DG Lainiotis New York Academic Press

Anderson TW (1971) The Statistical Analysis of Time Series New York John

Wiley amp Sons

Ansley CF and Newbold P (1979) Multivariate Partial Autocorrelations Pro-

ceedings of the Business and Economic Statistics Section American Statistical

Association 349-353

Box GEP and Jenkins G (1976) Time Series Analysis Forecasting and Control

San Francisco Holden-Day

Brockwell PJ and Davis RA (1991) Time Series Theory and Methods 2nd Edi-

tion Springer-Verlag

Hannan EJ (1970) Multiple Time Series New York John Wiley amp Sons

Hannan EJ (1976) The Identification and Parameterization of ARMAX and State

Space Forms Econometrica 44 713-722

Harvey AC (1981a) The Econometric Analysis of Time Series New York John

Wiley amp Sons

Harvey AC (1981b) Time Series Models New York John Wiley amp Sons

Jones RH (1974) Identification and Autoregressive Spectrum Estimation IEEE Transactions on Automatic Control AC-19 894-897

Pham-Dinh-Tuan (1978) On the Fitting of Multivariate Processes of the

Autoregressive-Moving Average Type Biometrika 65 99-107

Priestley MB (1980) System Identification Kalman Filtering and Stochastic Con-

trol in Directions in Time Series eds DR Brillinger and GC Tiao Institute

of Mathematical Statistics

Whittle P (1963) On the Fitting of Multivariate Autoregressions and the Approxi-

mate Canonical Factorization of a Spectral Density Matrix Biometrika 50 129-

134

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The correct bibliographic citation for this manual is as follows SAS Institute Inc SAS

ET S U serrsquos Gui de Version 8 Cary NC SAS Institute Inc 1999 1546 pp

SASET S Userrsquos Guide Version 8

Copyright copy 1999 by S AS I ns t it u t e I nc Ca ry N C U S A

ISBN 1ndash58025ndash489ndash6

All r ight s res erved P r int ed in t he U n it ed St a t es of A merica N o pa rt of t his p u blica t ionma y b e reprodu ced s t ored in a ret r ieva l s ys t em or t ra ns mit t ed in a ny form or b y a nymeans electronic mechanical photocopying or otherwise without the prior writtenpermission of the publisher SAS Institute Inc

US Government Restricted Rights Notice Use du plicat ion or disclosure of the

software by the government is subject to restrictions as set forth in FAR 52227ndash19Commercial Computer Softw ar e-Restricted Rights (J une 1987)

SAS Institute Inc SAS Campus Drive Cary North Carolina 27513

1st print ing October 1999

S AS reg a nd a ll ot her SA S I ns t it u t e I nc p rodu ct or s ervice na m es a re regis t ered t ra dema rkst d k f SA S I t it t I i t h U S A d t h t i reg i di t U SA

Page 22: Statespace Model

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Part 2 General Information

BY Statement

BY variable

A BY statement can be used with the STATESPACE procedure to obtain separateanalyses on observations in groups defined by the BY variables

FORM Statement

FORM variable value

The FORM statement specifies the number of times a variable is included in the state

vector Values can be specified for any variable listed in the VAR statement If a

value is specified for each variable in the VAR statement the state vector for the state

space model is entirely specified and automatic selection of the state space model is

not performed

The FORM statement forces the state vectorz

t

to contain a specific variable a given

number of times For example if Y is one of the variables inx

t

then the statement

form y 3

forces the state vector to containY

t

Y

t + 1 j t

andY

t + 2 j t

possibly along with other

variables

The following statements illustrate the use of the FORM statement

proc statespace data=in

var x y

f o r m x 3 y 2

run

These statements fit a state space model with the following state vector

z

t

=

2

6

6

6

6

4

x

t j t

y

t j t

x

t + 1 j t

y

t + 1 j t

x

t + 2 j t

3

7

7

7

7

5

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Chapter 18 Syntax

ID Statement

ID variable

The ID statement specifies a variable that identifies observations in the input data setThe variable specified in the ID statement is included in the OUT= data set The

values of the ID variable are extrapolated for the forecast observations based on the

values of the INTERVAL= and INTPER= options

INITIAL Statement

INITIAL F (rowcolumn)= value G(row column)= value

The INITIAL statement gives initial values to the specified elements of theF

andG

matrices These initial values are used as starting values for the iterative estimation

Parts of theF

andG

matrices represent fixed structural identities If an element

specified is a fixed structural element instead of a free parameter the corresponding

initialization is ignored

The following is an example of an INITIAL statement

initial f(32)=0 g(41)=0 g(51)=0

RESTRICT Statement

RESTRICT F(rowcolumn)= value G(rowcolumn)= value

The RESTRICT statement restricts the specified elements of theF

andG

matrices

to the specified values

To use the restrict statement you need to know the form of the model Either specify

the form of the model with the FORM statement or do a preliminary run perhaps

with the NOEST option to find the form of the model that PROC STATESPACE

selects for the data

The following is an example of a RESTRICT statement

restrict f(32)=0 g(41)=0 g(51)=0

Parts of theF

andG

matrices represent fixed structural identities If a restriction is

specified for an element that is a fixed structural element instead of a free parameter

the restriction is ignored

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Part 2 General Information

VAR Statement

VAR variable (difference difference )

The VAR statement specifies the variables in the input data set to model and fore-cast The VAR statement also specifies differencing of the input variables The VAR

statement is required

Differencing is specified by following the variable name with a list of difference

periods separated by commas See the section Stationarity and Differencing for

more information on differencing of input variables

The order in which variables are listed in the VAR statement controls the order in

which variables are included in the state vector Usually potential inputs should be

listed before potential outputs

For example assuming the input data are monthly the following VAR statement

specifies modeling and forecasting of the one period and seasonal second differenceof X and Y

var x(112) y(112)

In this example the vector time series analyzed is

x

t

=

1 B 1 B

1 2

X

t

x

1 B 1 B

1 2

Y

t

y

where B represents the back shift operator and x and y represent the means of thedifferenced series If the NOCENTER option is specified the mean differences are

not subtracted

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Chapter 18 Details

Details

Missing Values

The STATESPACE procedure does not support missing values The procedure uses

the first contiguous group of observations with no missing values for any of the VARstatement variables Observations at the beginning of the data set with missing values

for any VAR statement variable are not used or included in the output data set

Stationarity and Differencing

The state space model used by the STATESPACE procedure assumes that the time

series are stationary Hence the data should be checked for stationarity One way

to check for stationarity is to plot the series A graph of series over time can show a

time trend or variability changes

You can also check stationarity by using the sample autocorrelation functions dis-

played by the ARIMA procedure The autocorrelation functions of nonstationaryseries tend to decay slowly See Chapter 7 ldquoThe ARIMA Procedurerdquo for more in-

formation

Another alternative is to use the STATIONARITY= option on the IDENTIFY state-

ment in PROC ARIMA to apply Dickey-Fuller tests for unit roots in the time series

See Chapter 7 ldquoThe ARIMA Procedurerdquo for more information on Dickey-Fuller unit

root tests

The most popular way to transform a nonstationary series to stationarity is by dif-

ferencing Differencing of the time series is specified in the VAR statement For

example to take a simple first difference of the series X use this statement

var x(1)

In this example the change in X from one period to the next is analyzed When the se-

ries has a seasonal pattern differencing at a period equal to the length of the seasonal

cycle may be desirable For example suppose the variable X is measured quarterly

and shows a seasonal cycle over the year You can use the following statement to

analyze the series of changes from the same quarter in the previous year

var x(4)

To difference twice add another differencing period to the list For ex-

ample the following statement analyzes the series of second differences

X

t

X

t 1

X

t 1

X

t 2

= X

t

2 X

t 1

+ X

t 2

var x(11)

The following statement analyzes the seasonal second difference series

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Part 2 General Information

var x(14)

The series modeled is the 1-period difference of the 4-period difference

X

t

X

t 4

X

t 1

X

t 5

= X

t

X

t 1

X

t 4

+ X

t 5

Another way to obtain stationary series is to use a regression on time to de-trend the

data If the time series has a deterministic linear trend regressing the series on time

produces residuals that should be stationary The following statements write residuals

of X and Y to the variable RX and RY in the output data set DETREND

data a

set a

t=_n_

run

proc reg data=a

model x y = t

output out=detrend r=rx ry

run

You then use PROC STATESPACE to forecast the de-trended series RX and RY A

disadvantage of this method is that you need to add the trend back to the forecast

series in an additional step A more serious disadvantage of the de-trending method

is that it assumes a deterministic trend In practice most time series appear to have a

stochastic rather than a deterministic trend Differencing is a more flexible and often

more appropriate method

There are several other methods to handle nonstationary time series For more infor-

mation and examples refer to Brockwell and Davis (1991)

Preliminary Autoregressive Models

After computing the sample autocovariance matrices PROC STATESPACE fits a

sequence of vector autoregressive models These preliminary autoregressive models

are used to estimate the autoregressive order of the process and limit the order of the

autocovariances considered in the state vector selection process

Yule-Walker Equations for Forward and Backward Models

Unlike a univariate autoregressive model a multivariate autoregressive model has

different forms depending on whether the present observation is being predicted from

the past observations or from the future observations

Letx

t

be the r -component stationary time series given by the VAR statement after

differencing and subtracting the vector of sample means (If the NOCENTER option

is specified the mean is not subtracted) Let n be the number of observations of x

t

from the input data set

Lete

t

be a vector white noise sequence with mean vector 0 and variance matrix

p

and letn

t

be a vector white noise sequence with mean vector 0 and variance matrix

p

Let p be the order of the vector autoregressive model forx

t

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The forward autoregressive form based on the past observations is written as follows

x

t

=

p

X

i = 1

p

i

x

t i

+ e

t

The backward autoregressive form based on the future observations is written as fol-

lows

x

t

=

p

X

i = 1

p

i

x

t + i

+ n

t

LettingE

denote the expected value operator the autocovariance sequence for thex

t

series

i

is

i

= E x

t

x

0

t i

The Yule-Walker equations for the autoregressive model that matches the first p ele-

ments of the autocovariance sequence are

2

6

6

4

0

1

p 1

0

1

0

p 2

0

p 1

0

p 2

0

3

7

7

5

2

6

6

4

p

1

p

2

p

p

3

7

7

5

=

2

6

6

4

1

2

p

3

7

7

5

and

2

6

6

6

4

0

0

1

0

p 1

1

0

0

p 2

p 1

p 2

0

3

7

7

7

5

2

6

6

4

p

1

p

2

p

p

3

7

7

5

=

2

6

6

4

0

1

0

2

0

p

3

7

7

5

Here

p

i

are the coefficient matrices for the past observation form of the vector au-toregressive model and

p

i

are the coefficient matrices for the future observation

form More information on the Yule-Walker equations in the multivariate setting can

be found in Whittle (1963) and Ansley and Newbold (1979)

The innovation variance matrices for the two forms can be written as follows

p

=

0

p

X

i = 1

p

i

0

i

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p

=

0

p

X

i = 1

p

i

i

The autoregressive models are fit to the data using the preceding Yule-Walker equa-

tions with

i

replaced by the sample covariance sequenceC

i

The covariance matri-

ces are calculated as

C

i

=

1

N 1

N

X

t = i + 1

x

t

x

0

t i

Let b

p

b

p

b

p

and b

p

represent the Yule-Walker estimates of

p

p

p

and

p

respectively These matrices are written to an output data set when the OUTAR=

option is specified

When the PRINTOUT=LONG option is specified the sequence of matrices b

p

and

the corresponding correlation matrices are printed The sequence of matricesb

p isused to compute Akaike information criteria for selection of the autoregressive order

of the process

Akaike Information Criterion

The Akaike information criterion or AIC is defined as -2( maximum of log likeli-

hood )+2(number of parameters) Since the vector autoregressive models are esti-

mates from the Yule-Walker equations not by maximum likelihood the exact likeli-

hood values are not available for computing the AIC However for the vector autore-

gressive model the maximum of the log likelihood can be approximated as

l n L

n

2

l n j

b

p

j

Thus the AIC for the order p model is computed as

A I C

p

= n l n j

b

p

j + 2 p r

2

You can use the printed AIC array to compute a likelihood ratio test of the autoregres-

sive order The log-likelihood ratio test statistic for testing the order p model against

the order p 1

model is

n l n j

b

p

j + n l n j

b

p 1

j

This quantity is asymptotically distributed as a

2 withr

2 degrees of freedom if the

series is autoregressive of order p 1

It can be computed from the AIC array as

A I C

p 1

A I C

p

+ 2 r

2

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You can evaluate the significance of these test statistics with the PROBCHI function

in a SAS DATA step or with a

2 table

Determining the Autoregressive Order

Although the autoregressive models can be used for prediction their primary value is

to aid in the selection of a suitable portion of the sample covariance matrix for use in

computing canonical correlations If the multivariate time series x

t is of autoregres-sive order p then the vector of past values to lag p is considered to contain essentially

all the information relevant for prediction of future values of the time series

By default PROC STATESPACE selects the order p producing the autoregressive

model with the smallestA I C

p

If the value p for the minimumA I C

p

is less than the

value of the PASTMIN= option then p is set to the PASTMIN= value Alternatively

you can use the ARMAX= and PASTMIN= options to force PROC STATESPACE to

use an order you select

Significance Limits for Partial Autocorrelations

The STATESPACE procedure prints a schematic representation of the partial autocor-

relation matrices indicating which partial autocorrelations are significantly greater orsignificantly less than 0 Figure 1810 shows an example of this table

The STATESPACE Procedure

Schematic Representation of Partial Autocorrelations

NameLag 1 2 3 4 5 6 7 8 9 10

x ++ +

y ++

+ is gt 2std error - is lt -2std error is between

Figure 1810 Significant Partial Autocorrelations

The partial autocorrelations are from the sample partial autoregressive matrices b

p

p

The standard errors used for the significance limits of the partial autocorrelations are

computed from the sequence of matrices

p

and

p

Under the assumption that the observed series arises from an autoregressive process

of order p 1

the pth sample partial autoregressive matrix b

p

p

has an asymptotic

variance matrix 1

n

1

p

p

The significance limits for b

p

p

used in the schematic plot of the sample partial autore-

gressive sequence are derived by replacing

p

and

p

with their sample estimators

to produce the variance estimate as follows

d

V a r

b

p

p

=

1

n r p

b

1

p

b

p

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Canonical Correlation Analysis

Given the order p letp

t

be the vector of current and past values relevant to prediction

of x

t + 1

p

t

= x

0

t

x

0

t 1

x

0

t p

0

Letf

t

be the vector of current and future values

f

t

= x

0

t

x

0

t + 1

x

0

t + p

0

In the canonical correlation analysis consider submatrices of the sample covariance

matrix of p

t

andf

t

This covariance matrixV

has a block Hankel form

V =

2

6

6

6

4

C

0

C

0

1

C

0

2

C

0

p

C

0

1

C

0

2

C

0

3

C

0

p + 1

C

0

p

C

0

p + 1

C

0

p + 2

C

0

2 p

3

7

7

7

5

State Vector Selection Process

The canonical correlation analysis forms a sequence of potential state vectorsz

j

t

Examine a sequencef

j

t

of subvectors of f

t

and form the submatrixV

j consisting

of the rows and columns of V

corresponding to the components of f

j

t

and compute

its canonical correlations

The smallest canonical correlation of V

j is then used in the selection of the com-

ponents of the state vector The selection process is described in the following For

more details about this process refer to Akaike (1976)

In the following discussion the notationx

t + k j t

denotes the wide sense conditional

expectation (best linear predictor) of x

t + k

given allx

s

with s less than or equal to t

In the notationx

i t + 1

the first subscript denotes the ith component of x

t + 1

The initial state vectorz

1

t

is set tox

t

The sequencef

j

t

is initialized by setting

f

1

t

= z

1

0

t

x

1 t + 1 j t

0

= x

0

t

x

1 t + 1 j t

0

That is start by considering whether to addx

1 t + 1 j t

to the initial state vectorz

1

t

The procedure forms the submatrixV

1 corresponding tof

1

t

and computes its canon-

ical correlations Denote the smallest canonical correlation of V

1 as

m i n

If

m i n

is

significantly greater than 0x

1 t + 1 j t

is added to the state vector

If the smallest canonical correlation of V

1 is not significantly greater than 0 then a

linear combination of f

1

t

is uncorrelated with the pastp

t

Assuming that the determi-

nant of C

0

is not 0 (that is no input series is a constant) you can take the coefficient

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Chapter 18 Details

of x

1 t + 1 j t

in this linear combination to be 1 Denote the coefficients of z

1

t

in this

linear combination as

This gives the relationship

x

1 t + 1 j t

=

0

x

t

Therefore the current state vector already contains all the past information useful forpredicting

x

1 t + 1

and any greater leads of x

1 t

The variablex

1 t + 1 j t

is not added to

the state vector nor are any termsx

1 t + k j t

considered as possible components of the

state vector The variablex

1

is no longer active for state vector selection

The process described forx

1 t + 1 j t

is repeated for the remaining elements of f

t

The

next candidate for inclusion in the state vector is the next component of f

t

corre-

sponding to an active variable Components of f

t

corresponding to inactive variables

that produced a zero

m i n

in a previous step are skipped

Denote the next candidate asx

l t + k j t

The vectorf

j

t

is formed from the current state

vector andx

l t + k j t

as follows

f

j

t

= z

j

0

t

x

l t + k j t

0

The matrixV

j is formed fromf

j

t

and its canonical correlations are computed The

smallest canonical correlation of V

j is judged to be either greater than or equal to 0

If it is judged to be greater than 0x

l t + k j t

is added to the state vector If it is judged

to be 0 then a linear combination of f

j

t

is uncorrelated with thep

t

and the variable

x

l

is now inactive

The state vector selection process continues until no active variables remain

Testing Significance of Canonical Correlations For each step in the canonical correlation sequence the significance of the small-

est canonical correlation

m i n

is judged by an information criterion from Akaike

(1976) This information criterion is

n l n 1

2

m i n

r p + 1 q + 1

where q is the dimension of f

j

t

at the current step r is the order of the state vector p

is the order of the vector autoregressive process and

is the value of the SIGCORR=

option The default is SIGCORR=2 If this information criterion is less than or equal

to 0

m i n

is taken to be 0 otherwise it is taken to be significantly greater than 0

(Do not confuse this information criterion with the AIC)

Variables inx

t + p j t

are not added in the model even with positive information crite-

rion because of the singularity of V

You can force the consideration of more candi-

date state variables by increasing the size of theV

matrix by specifying a PASTMIN=

option value larger than p

Printing the Canonical Correlations

To print the details of the canonical correlation analysis process specify the CAN-

CORR option in the PROC STATESPACE statement The CANCORR option prints

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the candidate state vectors the canonical correlations and the information criteria for

testing the significance of the smallest canonical correlation

Bartlettrsquos

2 and its degrees of freedom are also printed when the CANCORR option

is specified The formula used for Bartlettrsquos

2 is

2

= n 5 r p + 1 q + 1 l n 1

2

m i n

withr p + 1 q + 1

degrees of freedom

Figure 1811 shows the output of the CANCORR option for the introductory example

shown in the Getting Started section of this chapter

The STATESPACE Procedure

Canonical Correlations Analysis

Information Chi

x(TT) y(TT) x(T+1T) Criterion Square DF

1 1 0237045 3566167 114505 4

Information Chi

x(TT) y(TT) x(T+1T) y(T+1T) Criterion Square DF

1 1 0238244 0056565 -535906 0636134 3

Information Chi

x(TT) y(TT) x(T+1T) x(T+2T) Criterion Square DF

1 1 0237602 0087493 -446312 1525353 3

Figure 1811 Canonical Correlations Analysis

New variables are added to the state vector if the information criteria are positive Inthis example

y

t + 1 j t

andx

t + 2 j t

are not added to the state space vector because the

information criteria for these models are negative

If the information criterion is nearly 0 then you may want to investigate models

that arise if the opposite decision is made regarding

m i n

This investigation can be

accomplished by using a FORM statement to specify part or all of the state vector

Preliminary Estimates of F

When a candidate variablex

l t + k j t

yields a zero

m i n

and is not added to the state

vector a linear combination of f

j

t

is uncorrelated with thep

t

Because of the method

used to construct thef

j

t

sequence the coefficient of x

l t + k j t

inl

can be taken as 1

Denote the coefficients of z

j

t

in this linear combination as l

This gives the relationship

x

l t + k j t

= l

0

z

j

t

The vectorl

is used as a preliminary estimate of the first r columns of the row of the

transition matrixF

corresponding tox

l t + k 1 j t

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Parameter Estimation

The model isz

t + 1

= F z

t

+ G e

t + 1

wheree

t

is a sequence of independent multivari-

ate normal innovations with mean vector 0 and variance

e e

The observed sequence

x

t

composes the first r components of z

t

and thusx

t

= H z

t

where H is ther s

matrix I

r

0

LetE

be ther n

matrix of innovations

E = e

1

e

n

If the number of observations n is reasonably large the log likelihood L can be

approximated up to an additive constant as follows

L =

n

2

l n j

e e

j

1

2

t r a c e

1

e e

E E

0

The elements of

e e

are taken as free parameters and are estimated as follows

S

0

=

1

n

E E

0

Replacing

e e

byS

0

in the likelihood equation the log likelihood up to an additive

constant is

L =

n

2

l n j S

0

j

Letting B be the backshift operator the formal relation betweenx

t

ande

t

is

x

t

= H I B F

1

G e

t

e

t

= H I B F

1

G

1

x

t

=

1

X

i = 0

i

x

t i

LettingC

i

be the ith lagged sample covariance of x

t

and neglecting end effects the

matrixS

0

is

S

0

=

1

X

i j = 0

i

C

i + j

0

j

For the computation of S

0

the infinite sum is truncated at the value of the KLAG=

option The value of the KLAG= option should be large enough that the sequence

i

is approximately 0 beyond that point

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Let

be the vector of free parameters in theF

andG

matrices The derivative of the

log likelihood with respect to the parameter

is

L

=

n

2

t r a c e

S

1

0

S

0

The second derivative is

2

L

0

=

n

2

t r a c e

S

1

0

S

0

0

S

1

0

S

0

t r a c e

S

1

0

2

S

0

0

Near the maximum the first term is unimportant and the second term can be approx-

imated to give the following second derivative approximation

2

L

0

=

n t r a c e

S

1

0

E

E

0

0

The first derivative matrix and this second derivative matrix approximation are com-

puted from the sample covariance matrixC

0

and the truncated sequence

i

The

approximate likelihood function is maximized by a modified Newton-Raphson algo-

rithm employing these derivative matrices

The matrixS

0

is used as the estimate of the innovation covariance matrix

e e

The

negative of the inverse of the second derivative matrix at the maximum is used as an

approximate covariance matrix for the parameter estimates The standard errors of

the parameter estimates printed in the parameter estimates tables are taken from the

diagonal of this covariance matrix The parameter covariance matrix is printed when

the COVB option is specified

If the data are nearly nonstationary a better estimate of

e e

and the other parameters

can sometimes be obtained by specifying the RESIDEST option The RESIDEST

option estimates the parameters using conditional least squares instead of maximum

likelihood

The residuals are computed using the state space equation and the sample mean val-

ues of the variables in the model as start-up values The estimate of S

0

is then com-

puted using the residuals from the ith observation on where i is the maximum number

of times any variable occurs in the state vector A multivariate Gauss-Marquardt al-

gorithm is used to minimizej S

0

j

Refer to Harvey (1981a) for a further description

of this method

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Forecasting

Given estimates of F

G

and

e e

forecasts of x

t

are computed from the conditional

expectation of z

t

In forecasting the parameters F G and

e e

are replaced with the estimates or by val-

ues specified in the RESTRICT statement One-step-ahead forecasting is performedfor the observation

x

t

wheret n b

Heren

is the number of observations and

b is the value of the BACK= option For the observationx

t

wheret n b

m-

step-ahead forecasting is performed form = t n + b

The forecasts are generated

recursively with the initial conditionz

0

= 0

The m-step-ahead forecast of z

t + m

isz

t + m j t

wherez

t + m j t

denotes the conditional

expectation of z

t + m

given the information available at time t The m-step-ahead

forecast of x

t + m

isx

t + m j t

= H z

t + m j t

where the matrixH = I

r

0

Let

i

= F

i

G

Note that the lasts r

elements of z

t

consist of the elements of x

u j t

foru t

The state vector z

t + m

can be represented as

z

t + m

= F

m

z

t

+

m 1

X

i = 0

i

e

t + m i

Sincee

t + i j t

= 0

fori 0

the m-step-ahead forecastz

t + m j t

is

z

t + m j t

= F

m

z

t

= F z

t + m 1 j t

Therefore the m-step-ahead forecast of x

t + m

is

x

t + m j t

= H z

t + m j t

The m-step-ahead forecast error is

z

t + m

z

t + m j t

=

m 1

X

i = 0

i

e

t + m i

The variance of the m-step-ahead forecast error is

V

z m

=

m 1

X

i = 0

i

e e

0

i

LettingV

z 0

= 0

the variance of the m-step-ahead forecast error of z

t + m

V

z m

can

be computed recursively as follows

V

z m

= V

z m 1

+

m 1

e e

0

m 1

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The variance of the m-step-ahead forecast error of x

t + m

is ther r

left upper sub-

matrix of V

z m

that is

V

x m

= H V

z m

H

0

Unless the NOCENTER option is specified the sample mean vector is added to the

forecast When differencing is specified the forecasts xt + m j t

plus the sample mean

vector are integrated back to produce forecasts for the original series

Lety

t

be the original series specified by the VAR statement with some 0 values

appended corresponding to the unobserved past observations Let B be the backshift

operator and let B

be thes s

matrix polynomial in the backshift operator

corresponding to the differencing specified by the VAR statement The off-diagonal

elements of

i

are 0 Note that

0

= I

s

whereI

s

is thes s

identity matrix Then

z

t

= B y

t

This gives the relationship

y

t

=

1

B z

t

=

1

X

i = 0

i

z

t i

where

1

B =

P

1

i = 0

i

B

i and

0

= I

s

The m-step-ahead forecast of y

t + m

is

y

t + m j t

=

m 1

X

i = 0

i

z

t + m i j t

+

1

X

i = m

i

z

t + m i

The m-step-ahead forecast error of y

t + m

is

m 1

X

i = 0

i

z

t + m i

z

t + m i j t

=

m 1

X

i = 0

i

X

u = 0

u

i u

e

t + m i

LettingV

y 0

= 0

the variance of the m-step-ahead forecast error of y

t + m

V

y m

is

V

y m

=

m 1

X

i = 0

i

X

u = 0

u

i u

e e

i

X

u = 0

u

i u

0

= V

y m 1

+

m 1

X

u = 0

u

m 1 u

e e

m 1

X

u = 0

u

m 1 u

0

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Chapter 18 Details

Relation of ARMA and State Space Forms

Every state space model has an ARMA representation and conversely every ARMA

model has a state space representation This section discusses this equivalence The

following material is adapted from Akaike (1974) where there is a more complete

discussion Pham-Dinh-Tuan (1978) also contains a discussion of this material

Suppose you are given the following ARMA model

B x

t

= B e

t

or in more detail

x

t

1

x

t 1

p

x

t p

= e

t

+

1

e

t 1

+ +

q

e

t q

(1)

wheree

t

is a sequence of independent multivariate normal random vectors with mean

0 and variance matrix

e e

B is the backshift operator (B x

t

= x

t 1

) B

and

B

are matrix polynomials in B andx

t

is the observed process

If the roots of the determinantial equationj B j = 0

are outside the unit circle in

the complex plane the model can also be written as

x

t

=

1

B B e

t

=

1

X

i = 0

i

e

t i

The

i

matrices are known as the impulse response matrices and can be computed

as

1

B B

You can assume p q

since if this is not initially true you can add more terms

i

that are identically 0 without changing the model

To write this set of equations in a state space form proceed as follows Letx

t + i j t

be

the conditional expectation of x

t + i

givenx

w

forw t

The following relations hold

x

t + i j t

=

1

X

j = i

j

e

t + i j

x

t + i j t + 1

= x

t + i j t

+

i 1

e

t + 1

However from equation (1) you can derive the following relationship

x

t + p j t

=

1

x

t + p 1 j t

+ +

p

x

t

(2)

Hence wheni = p

you can substitute forx

t + p j t

in the right-hand side of equation

(2) and close the system of equations

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This substitution results in the following model in the state space form

z

t + 1

= F z

t

+ G e

t + 1

2

6

6

6

4

x

t + 1

x

t + 2 j t + 1

x

t + p j t + 1

3

7

7

7

5

=

2

6

6

4

0 I 0 0

0 0 I 0

p

p 1

1

3

7

7

5

2

6

6

6

4

x

t

x

t + 1 j t

x

t + p 1 j t

3

7

7

7

5

+

2

6

6

4

I

1

p 1

3

7

7

5

e

t + 1

Note that the state vectorz

t

is composed of conditional expectations of x

t

and the

first r components of z

t

are equal tox

t

The state space form can be cast into an ARMA form by solving the system of dif-

ference equations for the first r components

When converting from an ARMA form to a state space form you can generate a

state vector larger than needed that is the state space model may not be a minimal

representation When going from a state space form to an ARMA form you can have

nontrivial common factors in the autoregressive and moving average operators that

yield an ARMA model larger than necessary

If the state space form used is not a minimal representation some but not all compo-

nents of x

t + i j t

may be linearly dependent This situation corresponds to

p

p 1

being of less than full rank when B

and B

have no common nontrivial

left factors In this casez

t

consists of a subset of the possible components of

x

t + i j t

i = 1 2 p 1

However once a component of x

t + i j t

(for example the

jth one) is linearly dependent on the previous conditional expectations then all sub-

sequent jth components of x

t + k j t

fork i

must also be linearly dependent Note

that in this case equivalent but seemingly different structures can arise if the order of

the components withinx

t

is changed

OUT= Data Set

The forecasts are contained in the output data set specified by the OUT= option on the

PROC STATESPACE statement The OUT= data set contains the following variables

the BY variables

the ID variable

the VAR statement variables These variables contain the actual values from

the input data set

FORi numeric variables containing the forecasts The variable FORi contains

the forecasts for the ith variable in the VAR statement list Forecasts are one-

step-ahead predictions until the end of the data or until the observation specified

by the BACK= option

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Chapter 18 Details

RESi numeric variables containing the residual for the forecast of the ith vari-

able in the VAR statement list For forecast observations the actual values are

missing and the RESi variables contain missing values

STDi numeric variables containing the standard deviation for the forecast of

the ith variable in the VAR statement list The values of the STDi variables can

be used to construct univariate confidence limits for the corresponding fore-casts However such confidence limits do not take into account the covariance

of the forecasts

OUTAR= Data Set

The OUTAR= data set contains the estimates of the preliminary autoregressive mod-

els The OUTAR= data set contains the following variables

ORDER a numeric variable containing the order p of the autoregressive model

that the observation represents

AIC a numeric variable containing the value of the information criterionA I C

p

SIGFl numeric variables containing the estimate of the innovation covariance

matrices for the forward autoregressive models The variable SIGFl contains

the lth column of b

p

in the observations with ORDER= p

SIGBl numeric variables containing the estimate of the innovation covariance

matrices for the backward autoregressive models The variable SIGBl contains

the lth column of b

p

in the observations with ORDER= p

FORk ndashl numeric variables containing the estimates of the autoregressive pa-

rameter matrices for the forward models The variable FORk ndashl contains the

lth column of the lag k autoregressive parameter matrix b

p

k

in the observations

with ORDER= p

BACk ndashl numeric variables containing the estimates of the autoregressive pa-

rameter matrices for the backward models The variable BACk ndashl contains the

lth column of the lag k autoregressive parameter matrix b

p

k

in the observations

with ORDER= p

The estimates for the order p autoregressive model can be selected as those observa-

tions with ORDER= p Within these observations the k lth element of

p

i

is given by

the value of the FORindashl variable in the k th observation The k lth element of

p

i

is

given by the value of BACindashl variable in the k th observation The k lth element of

p

is given by SIGFl in the k th observation The k lth element of

p

is given by SIGBlin the k th observation

Table 181 shows an example of the OUTAR= data set with ARMAX=3 andx

t

of

dimension 2 In Table 181 i j

indicate the ijth element of the matrix

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Part 2 General Information

Table 181 Values in the OUTAR= Data Set

Obs ORDER AIC SIGF1 SIGF2 SIGB1 SIGB2 FOR1ndash 1 FOR1ndash 2 FOR2ndash 1 FOR2ndash 2 FOR3ndash 1

1 0 AIC0

0 1 1

0 1 2

0 1 1

0 1 2

2 0 AIC0

0 2 1

0 2 2

0 2 1

0 2 2

3 1 AIC1

1 1 1

1 1 2

1 1 1

1 1 2

1

1

1 1

1

1

1 2

4 1 AIC1

1 2 1

1 1 2

1 2 1

1 1 2

1

1

2 1

1

1

2 2

5 2 AIC2

2 1 1

2 1 2

2 1 1

2 1 2

2

1

1 1

2

1

1 2

2

2

1 1

2

2

1 2

6 2 AIC2

2 2 1

2 1 2

2 2 1

2 1 2

2

1

2 1

2

1

2 2

2

2

2 1

2

2

2 2

7 3 AIC3

3 1 1

3 1 2

3 1 1

3 1 2

3

1

1 1

3

1

1 2

3

2

1 1

3

2

1 2

3

3

1 1

8 3 AIC3

3 2 1

3 1 2

3 2 1

3 1 2

3

1

2 1

3

1

2 2

3

2

2 1

3

2

2 2

3

3

2 1

Obs FOR3ndash 2 BACK1ndash 1 BACK1ndash 2 BACK2ndash 1 BACK2ndash 2 BACK3ndash 1 BACK3ndash 2

1

2

3

1

1

1 1

1

1

1 2

4

1

1

2 1

1

1

2 2

5

2

1

1 1

2

1

1 2

2

2

1 1

2

2

1 2

6

2

1

2 1

2

1

2 2

2

2

2 1

2

2

2 2

7

3

3

1 2

3

1

1 1

3

1

1 2

3

2

1 1

3

2

1 2

3

3

1 1

3

3

1 2

8

3

3

2 2

3

1

2 1

3

1

2 2

3

2

2 1

3

2

2 2

3

3

2 1

3

3

2 2

The estimated autoregressive parameters can be used in the IML procedure to obtain

autoregressive estimates of the spectral density function or forecasts based on the

autoregressive models

OUTMODEL= Data Set

The OUTMODEL= data set contains the estimates of theF

andG

matrices and their

standard errors the names of the components of the state vector and the estimates of

the innovation covariance matrix The variables contained in the OUTMODEL= data

set are as follows

the BY variables

STATEVEC a character variable containing the name of the component of the

state vector corresponding to the observation The STATEVEC variable has

the value STD for standard deviations observations which contain the standarderrors for the estimates given in the preceding observation

Fndash j numeric variables containing the columns of theF

matrix The variable

Fndash j contains the jth column of F

The number of Fndash j variables is equal to

the value of the DIMMAX= option If the model is of smaller dimension the

extraneous variables are set to missing

Gndash j numeric variables containing the columns of theG

matrix The variable

Gndash j contains the jth column of G

The number of Gndash j variables is equal to r

the dimension of x

t

given by the number of variables in the VAR statement

SIGndash j numeric variables containing the columns of the innovation covariance

matrix The variable SIGndash j contains the jth column of

e e

There are r vari-ables SIGndash j

Table 182 shows an example of the OUTMODEL= data set withx

t

= x

t

y

t

0

z

t

= x

t

y

t

x

t + 1 j t

0 and DIMMAX=4 In Table 182F

i j

andG

i j

are the ijth

elements of F

andG

respectively Note that all elements for Fndash4 are missing because

F

is a3 3

matrix

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Chapter 18 Details

Table 182 Value in the OUTMODEL= Data Set

Obs STATEVEC Fndash 1 Fndash 2 Fndash 3 Fndash 4 Gndash 1 Gndash 2 SIGndash 1 SIGndash 2

1 X(TT) 0 0 1 1 0

1 1

1 2

2 STD

3 Y(TT) F2 1

F2 2

F2 3

0 1

2 1

2 2

4 STD std F2 1

std F2 2

std F2 3

5 X(T+1T) F3 1

F3 2

F3 3

G3 1

G3 2

6 STD std F3 1

std F3 2

std F3 3

std G3 1

std G3 2

Printed Output

The printed output produced by the STATESPACE procedure is described in the fol-

lowing

1 descriptive statistics which include the number of observations used the

names of the variables their means and standard deviations (Std) and the dif-

ferencing operations used

2 the Akaike information criteria for the sequence of preliminary autoregressive

models

3 if the PRINTOUT=LONG option is specified the sample autocovariance ma-

trices of the input series at various lags

4 if the PRINTOUT=LONG option is specified the sample autocorrelation ma-

trices of the input series

5 a schematic representation of the autocorrelation matrices showing the signif-

icant autocorrelations

6 if the PRINTOUT=LONG option is specified the partial autoregressive ma-

trices (These are

p

p

as described in Preliminary Autoregressive Modelsearlier in this chapter)

7 a schematic representation of the partial autocorrelation matrices showing the

significant partial autocorrelations

8 the Yule-Walker estimates of the autoregressive parameters for the autoregres-

sive model with the minimum AIC

9 if the PRINTOUT=LONG option is specified the autocovariance matrices of

the residuals of the minimum AIC model This is the sequence of estimated

innovation variance matrices for the solutions of the Yule-Walker equations

10 if the PRINTOUT=LONG option is specified the autocorrelation matrices of the residuals of the minimum AIC model

11 If the CANCORR option is specified the canonical correlations analysis for

each potential state vector considered in the state vector selection process

This includes the potential state vector the canonical correlations the informa-

tion criterion for the smallest canonical correlation Bartlettrsquos

2 statistic (ldquoChi

Squarerdquo) for the smallest canonical correlation and the degrees of freedom of

Bartlettrsquos

2

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Part 2 General Information

12 the components of the chosen state vector

13 the preliminary estimate of the transition matrixF

the input matrixG

and

the variance matrix for the innovations

e e

14 if the ITPRINT option is specified the iteration history of the likelihood max-

imization For each iteration this shows the iteration number the number of step halvings the determinant of the innovation variance matrix the damping

factor Lambda and the values of the parameters

15 the state vector printed again to aid interpretation of the following listing of F

andG

16 the final estimate of the transition matrixF

17 the final estimate of the input matrixG

18 the final estimate of the variance matrix for the innovations

e e

19 a table listing the estimates of the free parameters inF

andG

and their standarderrors and t statistics

20 if the COVB option is specified the covariance matrix of the parameter esti-

mates

21 if the COVB option is specified the correlation matrix of the parameter esti-

mates

22 if the PRINT option is specified the forecasts and their standard errors

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Chapter 18 Details

ODS Table Names

PROC STATESPACE assigns a name to each table it creates You can use these

names to reference the table when using the Output Delivery System (ODS) to select

tables and create output data sets These names are listed in the following table For

more information on ODS see Chapter 6 ldquoUsing the Output Delivery Systemrdquo

Table 183 ODS Tables Produced in PROC STATESPACE

ODS Table Name Description Option

NObs Number of observations default

Summary Simple summary statistics table default

InfoCriterion Information criterion table default

CovLags Covariance Matrices of Input Series PRINTOUT=LONG

CorrLags Correlation Matrices of Input Series PRINTOUT=LONG

PartialAR Partial Autoregressive Matrices PRINTOUT=LONG

YWEstimates Yule-Walker Estimates for Minimum AIC default

CovResiduals Covariance of Residuals PRINTOUT=LONGCorrResiduals Residual Correlations from AR Models PRINTOUT=LONG

StateVector State vector table default

CorrGraph Schematic Representation of Correlations default

TransitionMatrix Transition Matrix default

InputMatrix Input Matrix default

VarInnov Variance Matrix for the Innovation default

CovB Covariance of Parameter Estimates COVB

CorrB Correlation of Parameter Estimates COVB

CanCorr Canonical Correlation Analysis CANCORR

IterHistory Iterative Fitting table ITPRINT

ParameterEstimates Parameter Estimates Table default

Forecasts Forecasts Table PRINT

ConvergenceStatus Convergence Status Table default

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Part 2 General Information

Example

Example 181 Series J from Box and Jenkins

This example analyzes the gas furnace data (series J) from Box and Jenkins (The

data are not shown Refer to Box and Jenkins (1976) for the data)

First a model is selected and fit automatically using the following statements

title1 rsquoGas Furnace Datarsquo

title2 rsquoBox amp Jenkins Series Jrsquo

title3 rsquoAutomatically Selected Modelrsquo

proc statespace data=seriesj cancorr

var x y

run

The results for the automatically selected model are shown in Output 1811

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Chapter 18 Example

Output 1811 Results for Automatically Selected Model

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Number of Observations 296

Standard

Variable Mean Error

x -005683 1072766

y 5350912 3202121

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Information Criterion for Autoregressive Models

Lag=0 Lag=1 Lag=2 Lag=3 Lag=4 Lag=5 Lag=6 Lag=7 Lag=8

6513862 -103357 -163296 -164512 -165152 -164891 -164934 -164315 -163856

Information

Criterion for

Autoregressive

Models

Lag=9 Lag=10

-16348 -163359

Schematic Representation of Correlations

NameLag 0 1 2 3 4 5 6 7 8 9 10

x +- +- +- +- +- +- +- +- +- +- +-

y -+ -+ -+ -+ -+ -+ -+ -+ -+ -+ -+

+ is gt 2std error - is lt -2std error is between

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Part 2 General Information

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Schematic Representation of Partial Autocorrelations

NameLag 1 2 3 4 5 6 7 8 9 10

x + - + -

y -+ -- - + +

+ is gt 2std error - is lt -2std error is between

Yule-Walker Estimates for Minimum AIC

------Lag=1------ ------Lag=2------ ------Lag=3------ ------Lag=4------

x y x y x y x y

x 1925887 -000124 -120166 0004224 0116918 -000867 0104236 0003268

y 0050496 1299793 -002046 -03277 -071182 -025701 0195411 0133417

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Canonical Correlations Analysis

Information Chi

x(TT) y(TT) x(T+1T) Criterion Square DF

1 1 0804883 2929228 3047481 8

Information Chix(TT) y(TT) x(T+1T) y(T+1T) Criterion Square DF

1 1 0906681 0607529 1223358 1347237 7

Information Chi

x(TT) y(TT) x (T+1T) y (T+1T) x (T+2T) Criterion Square DF

1 1 0909434 0610278 0186274 -154701 1034705 6

Information Chi

x(TT) y(TT) x (T+1T) y (T+1T) y (T+2T) Criterion Square DF

1 1 091014 0618937 0206823 0940392 1280924 6

Information Chi

x(TT) y(TT) x(T+1T) y(T+1T) y(T+2T) y(T+3T) Criterion Square DF

1 1 0912963 0628785 0226598 0083258 -794103 2041584 5

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Chapter 18 Example

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Selected Statespace Form and Preliminary Estimates

State Vector

x(TT) y(TT) x(T+1T) y(T+1T) y(T+2T)

Estimate of Transition Matrix

0 0 1 0 0

0 0 0 1 0

-084718 0026794 1711715 -005019 0

0 0 0 0 1

-019785 0334274 -018174 -123557 1787475

Input Matrix for Innovation

1 0

0 11925887 -000124

0050496 1299793

0142421 1361696

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Selected Statespace Form and Preliminary Estimates

Variance Matrix for Innovation

0035274 -000734

-000734 0097569

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Part 2 General Information

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Selected Statespace Form and Fitted Model

State Vector

x(TT) y(TT) x(T+1T) y(T+1T) y(T+2T)

Estimate of Transition Matrix

0 0 1 0 0

0 0 0 1 0

-086192 0030609 1724235 -005483 0

0 0 0 0 1

-034839 0292124 -009435 -109823 1671418

Input Matrix for Innovation

1 0

0 1192442 -000416

0015621 1258495

008058 1353204

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Selected Statespace Form and Fitted Model

Variance Matrix for Innovation

0035579 -000728

-000728 0095577

Parameter Estimates

Standard

Parameter Estimate Error t Value

F(31) -086192 0072961 -1181

F(32) 0030609 0026167 117

F(33) 1724235 0061599 2799

F(34) -005483 0030169 -182

F(51) -034839 0135253 -258

F(52) 0292124 0046299 631

F(53) -009435 0096527 -098F(54) -109823 0109525 -1003

F(55) 1671418 0083737 1996

G(31) 1924420 0058162 3309

G(32) -000416 0035255 -012

G(41) 0015621 0095771 016

G(42) 1258495 0055742 2258

G(51) 0080580 0151622 053

G(52) 1353204 0091388 1481

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Chapter 18 Example

The two series are believed to have a transfer function relation with the gas rate

(variable X) as the input and the CO2

concentration (variable Y) as the output Since

the parameter estimates shown in Output 1811 support this kind of model the model

is reestimated with the feedback parameters restricted to 0 The following statements

fit the transfer function (no feedback) model

title3 rsquoTransfer Function Modelrsquo

proc statespace data=seriesj printout=none

var x y

restrict f(32)=0 f(34)=0

g(32)=0 g(41)=0 g(51)=0

run

The last 2 pages of the output are shown in Output 1812

Output 1812 STATESPACE Output for Transfer Function Model

Gas Furnace Data

Box amp Jenkins Series J

Transfer Function Model

The STATESPACE Procedure

Selected Statespace Form and Fitted Model

State Vector

x(TT) y(TT) x(T+1T) y(T+1T) y(T+2T)

Estimate of Transition Matrix

0 0 1 0 0

0 0 0 1 0

-068882 0 1598717 0 0

0 0 0 0 1

-035944 0284179 -00963 -107313 1650047

Input Matrix for Innovation

1 0

0 1

1923446 0

0 1260856

0 1346332

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Part 2 General Information

Gas Furnace Data

Box amp Jenkins Series J

Transfer Function Model

The STATESPACE Procedure

Selected Statespace Form and Fitted Model

Variance Matrix for Innovation

0036995 -00072

-00072 0095712

Parameter Estimates

Standard

Parameter Estimate Error t Value

F(31) -068882 0050549 -1363

F(33) 1598717 0050924 3139

F(51) -035944 0229044 -157

F(52) 0284179 0096944 293

F(53) -009630 0140876 -068

F(54) -107313 0250385 -429

F(55) 1650047 0188533 875G(31) 1923446 0056328 3415

G(42) 1260856 0056464 2233

G(52) 1346332 0091086 1478

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Chapter 18 References

References

Akaike H (1974) Markovian Representation of Stochastic Processes and Its Ap-

plication to the Analysis of Autoregressive Moving Average Processes Annals

of the Institute of Statistical Mathematics 26 363-387

Akaike H (1976) Canonical Correlations Analysis of Time Series and the Use of

an Information Criterion in Advances and Case Studies in System Identification

eds R Mehra and DG Lainiotis New York Academic Press

Anderson TW (1971) The Statistical Analysis of Time Series New York John

Wiley amp Sons

Ansley CF and Newbold P (1979) Multivariate Partial Autocorrelations Pro-

ceedings of the Business and Economic Statistics Section American Statistical

Association 349-353

Box GEP and Jenkins G (1976) Time Series Analysis Forecasting and Control

San Francisco Holden-Day

Brockwell PJ and Davis RA (1991) Time Series Theory and Methods 2nd Edi-

tion Springer-Verlag

Hannan EJ (1970) Multiple Time Series New York John Wiley amp Sons

Hannan EJ (1976) The Identification and Parameterization of ARMAX and State

Space Forms Econometrica 44 713-722

Harvey AC (1981a) The Econometric Analysis of Time Series New York John

Wiley amp Sons

Harvey AC (1981b) Time Series Models New York John Wiley amp Sons

Jones RH (1974) Identification and Autoregressive Spectrum Estimation IEEE Transactions on Automatic Control AC-19 894-897

Pham-Dinh-Tuan (1978) On the Fitting of Multivariate Processes of the

Autoregressive-Moving Average Type Biometrika 65 99-107

Priestley MB (1980) System Identification Kalman Filtering and Stochastic Con-

trol in Directions in Time Series eds DR Brillinger and GC Tiao Institute

of Mathematical Statistics

Whittle P (1963) On the Fitting of Multivariate Autoregressions and the Approxi-

mate Canonical Factorization of a Spectral Density Matrix Biometrika 50 129-

134

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The correct bibliographic citation for this manual is as follows SAS Institute Inc SAS

ET S U serrsquos Gui de Version 8 Cary NC SAS Institute Inc 1999 1546 pp

SASET S Userrsquos Guide Version 8

Copyright copy 1999 by S AS I ns t it u t e I nc Ca ry N C U S A

ISBN 1ndash58025ndash489ndash6

All r ight s res erved P r int ed in t he U n it ed St a t es of A merica N o pa rt of t his p u blica t ionma y b e reprodu ced s t ored in a ret r ieva l s ys t em or t ra ns mit t ed in a ny form or b y a nymeans electronic mechanical photocopying or otherwise without the prior writtenpermission of the publisher SAS Institute Inc

US Government Restricted Rights Notice Use du plicat ion or disclosure of the

software by the government is subject to restrictions as set forth in FAR 52227ndash19Commercial Computer Softw ar e-Restricted Rights (J une 1987)

SAS Institute Inc SAS Campus Drive Cary North Carolina 27513

1st print ing October 1999

S AS reg a nd a ll ot her SA S I ns t it u t e I nc p rodu ct or s ervice na m es a re regis t ered t ra dema rkst d k f SA S I t it t I i t h U S A d t h t i reg i di t U SA

Page 23: Statespace Model

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Chapter 18 Syntax

ID Statement

ID variable

The ID statement specifies a variable that identifies observations in the input data setThe variable specified in the ID statement is included in the OUT= data set The

values of the ID variable are extrapolated for the forecast observations based on the

values of the INTERVAL= and INTPER= options

INITIAL Statement

INITIAL F (rowcolumn)= value G(row column)= value

The INITIAL statement gives initial values to the specified elements of theF

andG

matrices These initial values are used as starting values for the iterative estimation

Parts of theF

andG

matrices represent fixed structural identities If an element

specified is a fixed structural element instead of a free parameter the corresponding

initialization is ignored

The following is an example of an INITIAL statement

initial f(32)=0 g(41)=0 g(51)=0

RESTRICT Statement

RESTRICT F(rowcolumn)= value G(rowcolumn)= value

The RESTRICT statement restricts the specified elements of theF

andG

matrices

to the specified values

To use the restrict statement you need to know the form of the model Either specify

the form of the model with the FORM statement or do a preliminary run perhaps

with the NOEST option to find the form of the model that PROC STATESPACE

selects for the data

The following is an example of a RESTRICT statement

restrict f(32)=0 g(41)=0 g(51)=0

Parts of theF

andG

matrices represent fixed structural identities If a restriction is

specified for an element that is a fixed structural element instead of a free parameter

the restriction is ignored

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Part 2 General Information

VAR Statement

VAR variable (difference difference )

The VAR statement specifies the variables in the input data set to model and fore-cast The VAR statement also specifies differencing of the input variables The VAR

statement is required

Differencing is specified by following the variable name with a list of difference

periods separated by commas See the section Stationarity and Differencing for

more information on differencing of input variables

The order in which variables are listed in the VAR statement controls the order in

which variables are included in the state vector Usually potential inputs should be

listed before potential outputs

For example assuming the input data are monthly the following VAR statement

specifies modeling and forecasting of the one period and seasonal second differenceof X and Y

var x(112) y(112)

In this example the vector time series analyzed is

x

t

=

1 B 1 B

1 2

X

t

x

1 B 1 B

1 2

Y

t

y

where B represents the back shift operator and x and y represent the means of thedifferenced series If the NOCENTER option is specified the mean differences are

not subtracted

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Chapter 18 Details

Details

Missing Values

The STATESPACE procedure does not support missing values The procedure uses

the first contiguous group of observations with no missing values for any of the VARstatement variables Observations at the beginning of the data set with missing values

for any VAR statement variable are not used or included in the output data set

Stationarity and Differencing

The state space model used by the STATESPACE procedure assumes that the time

series are stationary Hence the data should be checked for stationarity One way

to check for stationarity is to plot the series A graph of series over time can show a

time trend or variability changes

You can also check stationarity by using the sample autocorrelation functions dis-

played by the ARIMA procedure The autocorrelation functions of nonstationaryseries tend to decay slowly See Chapter 7 ldquoThe ARIMA Procedurerdquo for more in-

formation

Another alternative is to use the STATIONARITY= option on the IDENTIFY state-

ment in PROC ARIMA to apply Dickey-Fuller tests for unit roots in the time series

See Chapter 7 ldquoThe ARIMA Procedurerdquo for more information on Dickey-Fuller unit

root tests

The most popular way to transform a nonstationary series to stationarity is by dif-

ferencing Differencing of the time series is specified in the VAR statement For

example to take a simple first difference of the series X use this statement

var x(1)

In this example the change in X from one period to the next is analyzed When the se-

ries has a seasonal pattern differencing at a period equal to the length of the seasonal

cycle may be desirable For example suppose the variable X is measured quarterly

and shows a seasonal cycle over the year You can use the following statement to

analyze the series of changes from the same quarter in the previous year

var x(4)

To difference twice add another differencing period to the list For ex-

ample the following statement analyzes the series of second differences

X

t

X

t 1

X

t 1

X

t 2

= X

t

2 X

t 1

+ X

t 2

var x(11)

The following statement analyzes the seasonal second difference series

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var x(14)

The series modeled is the 1-period difference of the 4-period difference

X

t

X

t 4

X

t 1

X

t 5

= X

t

X

t 1

X

t 4

+ X

t 5

Another way to obtain stationary series is to use a regression on time to de-trend the

data If the time series has a deterministic linear trend regressing the series on time

produces residuals that should be stationary The following statements write residuals

of X and Y to the variable RX and RY in the output data set DETREND

data a

set a

t=_n_

run

proc reg data=a

model x y = t

output out=detrend r=rx ry

run

You then use PROC STATESPACE to forecast the de-trended series RX and RY A

disadvantage of this method is that you need to add the trend back to the forecast

series in an additional step A more serious disadvantage of the de-trending method

is that it assumes a deterministic trend In practice most time series appear to have a

stochastic rather than a deterministic trend Differencing is a more flexible and often

more appropriate method

There are several other methods to handle nonstationary time series For more infor-

mation and examples refer to Brockwell and Davis (1991)

Preliminary Autoregressive Models

After computing the sample autocovariance matrices PROC STATESPACE fits a

sequence of vector autoregressive models These preliminary autoregressive models

are used to estimate the autoregressive order of the process and limit the order of the

autocovariances considered in the state vector selection process

Yule-Walker Equations for Forward and Backward Models

Unlike a univariate autoregressive model a multivariate autoregressive model has

different forms depending on whether the present observation is being predicted from

the past observations or from the future observations

Letx

t

be the r -component stationary time series given by the VAR statement after

differencing and subtracting the vector of sample means (If the NOCENTER option

is specified the mean is not subtracted) Let n be the number of observations of x

t

from the input data set

Lete

t

be a vector white noise sequence with mean vector 0 and variance matrix

p

and letn

t

be a vector white noise sequence with mean vector 0 and variance matrix

p

Let p be the order of the vector autoregressive model forx

t

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The forward autoregressive form based on the past observations is written as follows

x

t

=

p

X

i = 1

p

i

x

t i

+ e

t

The backward autoregressive form based on the future observations is written as fol-

lows

x

t

=

p

X

i = 1

p

i

x

t + i

+ n

t

LettingE

denote the expected value operator the autocovariance sequence for thex

t

series

i

is

i

= E x

t

x

0

t i

The Yule-Walker equations for the autoregressive model that matches the first p ele-

ments of the autocovariance sequence are

2

6

6

4

0

1

p 1

0

1

0

p 2

0

p 1

0

p 2

0

3

7

7

5

2

6

6

4

p

1

p

2

p

p

3

7

7

5

=

2

6

6

4

1

2

p

3

7

7

5

and

2

6

6

6

4

0

0

1

0

p 1

1

0

0

p 2

p 1

p 2

0

3

7

7

7

5

2

6

6

4

p

1

p

2

p

p

3

7

7

5

=

2

6

6

4

0

1

0

2

0

p

3

7

7

5

Here

p

i

are the coefficient matrices for the past observation form of the vector au-toregressive model and

p

i

are the coefficient matrices for the future observation

form More information on the Yule-Walker equations in the multivariate setting can

be found in Whittle (1963) and Ansley and Newbold (1979)

The innovation variance matrices for the two forms can be written as follows

p

=

0

p

X

i = 1

p

i

0

i

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p

=

0

p

X

i = 1

p

i

i

The autoregressive models are fit to the data using the preceding Yule-Walker equa-

tions with

i

replaced by the sample covariance sequenceC

i

The covariance matri-

ces are calculated as

C

i

=

1

N 1

N

X

t = i + 1

x

t

x

0

t i

Let b

p

b

p

b

p

and b

p

represent the Yule-Walker estimates of

p

p

p

and

p

respectively These matrices are written to an output data set when the OUTAR=

option is specified

When the PRINTOUT=LONG option is specified the sequence of matrices b

p

and

the corresponding correlation matrices are printed The sequence of matricesb

p isused to compute Akaike information criteria for selection of the autoregressive order

of the process

Akaike Information Criterion

The Akaike information criterion or AIC is defined as -2( maximum of log likeli-

hood )+2(number of parameters) Since the vector autoregressive models are esti-

mates from the Yule-Walker equations not by maximum likelihood the exact likeli-

hood values are not available for computing the AIC However for the vector autore-

gressive model the maximum of the log likelihood can be approximated as

l n L

n

2

l n j

b

p

j

Thus the AIC for the order p model is computed as

A I C

p

= n l n j

b

p

j + 2 p r

2

You can use the printed AIC array to compute a likelihood ratio test of the autoregres-

sive order The log-likelihood ratio test statistic for testing the order p model against

the order p 1

model is

n l n j

b

p

j + n l n j

b

p 1

j

This quantity is asymptotically distributed as a

2 withr

2 degrees of freedom if the

series is autoregressive of order p 1

It can be computed from the AIC array as

A I C

p 1

A I C

p

+ 2 r

2

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You can evaluate the significance of these test statistics with the PROBCHI function

in a SAS DATA step or with a

2 table

Determining the Autoregressive Order

Although the autoregressive models can be used for prediction their primary value is

to aid in the selection of a suitable portion of the sample covariance matrix for use in

computing canonical correlations If the multivariate time series x

t is of autoregres-sive order p then the vector of past values to lag p is considered to contain essentially

all the information relevant for prediction of future values of the time series

By default PROC STATESPACE selects the order p producing the autoregressive

model with the smallestA I C

p

If the value p for the minimumA I C

p

is less than the

value of the PASTMIN= option then p is set to the PASTMIN= value Alternatively

you can use the ARMAX= and PASTMIN= options to force PROC STATESPACE to

use an order you select

Significance Limits for Partial Autocorrelations

The STATESPACE procedure prints a schematic representation of the partial autocor-

relation matrices indicating which partial autocorrelations are significantly greater orsignificantly less than 0 Figure 1810 shows an example of this table

The STATESPACE Procedure

Schematic Representation of Partial Autocorrelations

NameLag 1 2 3 4 5 6 7 8 9 10

x ++ +

y ++

+ is gt 2std error - is lt -2std error is between

Figure 1810 Significant Partial Autocorrelations

The partial autocorrelations are from the sample partial autoregressive matrices b

p

p

The standard errors used for the significance limits of the partial autocorrelations are

computed from the sequence of matrices

p

and

p

Under the assumption that the observed series arises from an autoregressive process

of order p 1

the pth sample partial autoregressive matrix b

p

p

has an asymptotic

variance matrix 1

n

1

p

p

The significance limits for b

p

p

used in the schematic plot of the sample partial autore-

gressive sequence are derived by replacing

p

and

p

with their sample estimators

to produce the variance estimate as follows

d

V a r

b

p

p

=

1

n r p

b

1

p

b

p

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Canonical Correlation Analysis

Given the order p letp

t

be the vector of current and past values relevant to prediction

of x

t + 1

p

t

= x

0

t

x

0

t 1

x

0

t p

0

Letf

t

be the vector of current and future values

f

t

= x

0

t

x

0

t + 1

x

0

t + p

0

In the canonical correlation analysis consider submatrices of the sample covariance

matrix of p

t

andf

t

This covariance matrixV

has a block Hankel form

V =

2

6

6

6

4

C

0

C

0

1

C

0

2

C

0

p

C

0

1

C

0

2

C

0

3

C

0

p + 1

C

0

p

C

0

p + 1

C

0

p + 2

C

0

2 p

3

7

7

7

5

State Vector Selection Process

The canonical correlation analysis forms a sequence of potential state vectorsz

j

t

Examine a sequencef

j

t

of subvectors of f

t

and form the submatrixV

j consisting

of the rows and columns of V

corresponding to the components of f

j

t

and compute

its canonical correlations

The smallest canonical correlation of V

j is then used in the selection of the com-

ponents of the state vector The selection process is described in the following For

more details about this process refer to Akaike (1976)

In the following discussion the notationx

t + k j t

denotes the wide sense conditional

expectation (best linear predictor) of x

t + k

given allx

s

with s less than or equal to t

In the notationx

i t + 1

the first subscript denotes the ith component of x

t + 1

The initial state vectorz

1

t

is set tox

t

The sequencef

j

t

is initialized by setting

f

1

t

= z

1

0

t

x

1 t + 1 j t

0

= x

0

t

x

1 t + 1 j t

0

That is start by considering whether to addx

1 t + 1 j t

to the initial state vectorz

1

t

The procedure forms the submatrixV

1 corresponding tof

1

t

and computes its canon-

ical correlations Denote the smallest canonical correlation of V

1 as

m i n

If

m i n

is

significantly greater than 0x

1 t + 1 j t

is added to the state vector

If the smallest canonical correlation of V

1 is not significantly greater than 0 then a

linear combination of f

1

t

is uncorrelated with the pastp

t

Assuming that the determi-

nant of C

0

is not 0 (that is no input series is a constant) you can take the coefficient

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Chapter 18 Details

of x

1 t + 1 j t

in this linear combination to be 1 Denote the coefficients of z

1

t

in this

linear combination as

This gives the relationship

x

1 t + 1 j t

=

0

x

t

Therefore the current state vector already contains all the past information useful forpredicting

x

1 t + 1

and any greater leads of x

1 t

The variablex

1 t + 1 j t

is not added to

the state vector nor are any termsx

1 t + k j t

considered as possible components of the

state vector The variablex

1

is no longer active for state vector selection

The process described forx

1 t + 1 j t

is repeated for the remaining elements of f

t

The

next candidate for inclusion in the state vector is the next component of f

t

corre-

sponding to an active variable Components of f

t

corresponding to inactive variables

that produced a zero

m i n

in a previous step are skipped

Denote the next candidate asx

l t + k j t

The vectorf

j

t

is formed from the current state

vector andx

l t + k j t

as follows

f

j

t

= z

j

0

t

x

l t + k j t

0

The matrixV

j is formed fromf

j

t

and its canonical correlations are computed The

smallest canonical correlation of V

j is judged to be either greater than or equal to 0

If it is judged to be greater than 0x

l t + k j t

is added to the state vector If it is judged

to be 0 then a linear combination of f

j

t

is uncorrelated with thep

t

and the variable

x

l

is now inactive

The state vector selection process continues until no active variables remain

Testing Significance of Canonical Correlations For each step in the canonical correlation sequence the significance of the small-

est canonical correlation

m i n

is judged by an information criterion from Akaike

(1976) This information criterion is

n l n 1

2

m i n

r p + 1 q + 1

where q is the dimension of f

j

t

at the current step r is the order of the state vector p

is the order of the vector autoregressive process and

is the value of the SIGCORR=

option The default is SIGCORR=2 If this information criterion is less than or equal

to 0

m i n

is taken to be 0 otherwise it is taken to be significantly greater than 0

(Do not confuse this information criterion with the AIC)

Variables inx

t + p j t

are not added in the model even with positive information crite-

rion because of the singularity of V

You can force the consideration of more candi-

date state variables by increasing the size of theV

matrix by specifying a PASTMIN=

option value larger than p

Printing the Canonical Correlations

To print the details of the canonical correlation analysis process specify the CAN-

CORR option in the PROC STATESPACE statement The CANCORR option prints

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the candidate state vectors the canonical correlations and the information criteria for

testing the significance of the smallest canonical correlation

Bartlettrsquos

2 and its degrees of freedom are also printed when the CANCORR option

is specified The formula used for Bartlettrsquos

2 is

2

= n 5 r p + 1 q + 1 l n 1

2

m i n

withr p + 1 q + 1

degrees of freedom

Figure 1811 shows the output of the CANCORR option for the introductory example

shown in the Getting Started section of this chapter

The STATESPACE Procedure

Canonical Correlations Analysis

Information Chi

x(TT) y(TT) x(T+1T) Criterion Square DF

1 1 0237045 3566167 114505 4

Information Chi

x(TT) y(TT) x(T+1T) y(T+1T) Criterion Square DF

1 1 0238244 0056565 -535906 0636134 3

Information Chi

x(TT) y(TT) x(T+1T) x(T+2T) Criterion Square DF

1 1 0237602 0087493 -446312 1525353 3

Figure 1811 Canonical Correlations Analysis

New variables are added to the state vector if the information criteria are positive Inthis example

y

t + 1 j t

andx

t + 2 j t

are not added to the state space vector because the

information criteria for these models are negative

If the information criterion is nearly 0 then you may want to investigate models

that arise if the opposite decision is made regarding

m i n

This investigation can be

accomplished by using a FORM statement to specify part or all of the state vector

Preliminary Estimates of F

When a candidate variablex

l t + k j t

yields a zero

m i n

and is not added to the state

vector a linear combination of f

j

t

is uncorrelated with thep

t

Because of the method

used to construct thef

j

t

sequence the coefficient of x

l t + k j t

inl

can be taken as 1

Denote the coefficients of z

j

t

in this linear combination as l

This gives the relationship

x

l t + k j t

= l

0

z

j

t

The vectorl

is used as a preliminary estimate of the first r columns of the row of the

transition matrixF

corresponding tox

l t + k 1 j t

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Parameter Estimation

The model isz

t + 1

= F z

t

+ G e

t + 1

wheree

t

is a sequence of independent multivari-

ate normal innovations with mean vector 0 and variance

e e

The observed sequence

x

t

composes the first r components of z

t

and thusx

t

= H z

t

where H is ther s

matrix I

r

0

LetE

be ther n

matrix of innovations

E = e

1

e

n

If the number of observations n is reasonably large the log likelihood L can be

approximated up to an additive constant as follows

L =

n

2

l n j

e e

j

1

2

t r a c e

1

e e

E E

0

The elements of

e e

are taken as free parameters and are estimated as follows

S

0

=

1

n

E E

0

Replacing

e e

byS

0

in the likelihood equation the log likelihood up to an additive

constant is

L =

n

2

l n j S

0

j

Letting B be the backshift operator the formal relation betweenx

t

ande

t

is

x

t

= H I B F

1

G e

t

e

t

= H I B F

1

G

1

x

t

=

1

X

i = 0

i

x

t i

LettingC

i

be the ith lagged sample covariance of x

t

and neglecting end effects the

matrixS

0

is

S

0

=

1

X

i j = 0

i

C

i + j

0

j

For the computation of S

0

the infinite sum is truncated at the value of the KLAG=

option The value of the KLAG= option should be large enough that the sequence

i

is approximately 0 beyond that point

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Let

be the vector of free parameters in theF

andG

matrices The derivative of the

log likelihood with respect to the parameter

is

L

=

n

2

t r a c e

S

1

0

S

0

The second derivative is

2

L

0

=

n

2

t r a c e

S

1

0

S

0

0

S

1

0

S

0

t r a c e

S

1

0

2

S

0

0

Near the maximum the first term is unimportant and the second term can be approx-

imated to give the following second derivative approximation

2

L

0

=

n t r a c e

S

1

0

E

E

0

0

The first derivative matrix and this second derivative matrix approximation are com-

puted from the sample covariance matrixC

0

and the truncated sequence

i

The

approximate likelihood function is maximized by a modified Newton-Raphson algo-

rithm employing these derivative matrices

The matrixS

0

is used as the estimate of the innovation covariance matrix

e e

The

negative of the inverse of the second derivative matrix at the maximum is used as an

approximate covariance matrix for the parameter estimates The standard errors of

the parameter estimates printed in the parameter estimates tables are taken from the

diagonal of this covariance matrix The parameter covariance matrix is printed when

the COVB option is specified

If the data are nearly nonstationary a better estimate of

e e

and the other parameters

can sometimes be obtained by specifying the RESIDEST option The RESIDEST

option estimates the parameters using conditional least squares instead of maximum

likelihood

The residuals are computed using the state space equation and the sample mean val-

ues of the variables in the model as start-up values The estimate of S

0

is then com-

puted using the residuals from the ith observation on where i is the maximum number

of times any variable occurs in the state vector A multivariate Gauss-Marquardt al-

gorithm is used to minimizej S

0

j

Refer to Harvey (1981a) for a further description

of this method

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Forecasting

Given estimates of F

G

and

e e

forecasts of x

t

are computed from the conditional

expectation of z

t

In forecasting the parameters F G and

e e

are replaced with the estimates or by val-

ues specified in the RESTRICT statement One-step-ahead forecasting is performedfor the observation

x

t

wheret n b

Heren

is the number of observations and

b is the value of the BACK= option For the observationx

t

wheret n b

m-

step-ahead forecasting is performed form = t n + b

The forecasts are generated

recursively with the initial conditionz

0

= 0

The m-step-ahead forecast of z

t + m

isz

t + m j t

wherez

t + m j t

denotes the conditional

expectation of z

t + m

given the information available at time t The m-step-ahead

forecast of x

t + m

isx

t + m j t

= H z

t + m j t

where the matrixH = I

r

0

Let

i

= F

i

G

Note that the lasts r

elements of z

t

consist of the elements of x

u j t

foru t

The state vector z

t + m

can be represented as

z

t + m

= F

m

z

t

+

m 1

X

i = 0

i

e

t + m i

Sincee

t + i j t

= 0

fori 0

the m-step-ahead forecastz

t + m j t

is

z

t + m j t

= F

m

z

t

= F z

t + m 1 j t

Therefore the m-step-ahead forecast of x

t + m

is

x

t + m j t

= H z

t + m j t

The m-step-ahead forecast error is

z

t + m

z

t + m j t

=

m 1

X

i = 0

i

e

t + m i

The variance of the m-step-ahead forecast error is

V

z m

=

m 1

X

i = 0

i

e e

0

i

LettingV

z 0

= 0

the variance of the m-step-ahead forecast error of z

t + m

V

z m

can

be computed recursively as follows

V

z m

= V

z m 1

+

m 1

e e

0

m 1

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The variance of the m-step-ahead forecast error of x

t + m

is ther r

left upper sub-

matrix of V

z m

that is

V

x m

= H V

z m

H

0

Unless the NOCENTER option is specified the sample mean vector is added to the

forecast When differencing is specified the forecasts xt + m j t

plus the sample mean

vector are integrated back to produce forecasts for the original series

Lety

t

be the original series specified by the VAR statement with some 0 values

appended corresponding to the unobserved past observations Let B be the backshift

operator and let B

be thes s

matrix polynomial in the backshift operator

corresponding to the differencing specified by the VAR statement The off-diagonal

elements of

i

are 0 Note that

0

= I

s

whereI

s

is thes s

identity matrix Then

z

t

= B y

t

This gives the relationship

y

t

=

1

B z

t

=

1

X

i = 0

i

z

t i

where

1

B =

P

1

i = 0

i

B

i and

0

= I

s

The m-step-ahead forecast of y

t + m

is

y

t + m j t

=

m 1

X

i = 0

i

z

t + m i j t

+

1

X

i = m

i

z

t + m i

The m-step-ahead forecast error of y

t + m

is

m 1

X

i = 0

i

z

t + m i

z

t + m i j t

=

m 1

X

i = 0

i

X

u = 0

u

i u

e

t + m i

LettingV

y 0

= 0

the variance of the m-step-ahead forecast error of y

t + m

V

y m

is

V

y m

=

m 1

X

i = 0

i

X

u = 0

u

i u

e e

i

X

u = 0

u

i u

0

= V

y m 1

+

m 1

X

u = 0

u

m 1 u

e e

m 1

X

u = 0

u

m 1 u

0

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Relation of ARMA and State Space Forms

Every state space model has an ARMA representation and conversely every ARMA

model has a state space representation This section discusses this equivalence The

following material is adapted from Akaike (1974) where there is a more complete

discussion Pham-Dinh-Tuan (1978) also contains a discussion of this material

Suppose you are given the following ARMA model

B x

t

= B e

t

or in more detail

x

t

1

x

t 1

p

x

t p

= e

t

+

1

e

t 1

+ +

q

e

t q

(1)

wheree

t

is a sequence of independent multivariate normal random vectors with mean

0 and variance matrix

e e

B is the backshift operator (B x

t

= x

t 1

) B

and

B

are matrix polynomials in B andx

t

is the observed process

If the roots of the determinantial equationj B j = 0

are outside the unit circle in

the complex plane the model can also be written as

x

t

=

1

B B e

t

=

1

X

i = 0

i

e

t i

The

i

matrices are known as the impulse response matrices and can be computed

as

1

B B

You can assume p q

since if this is not initially true you can add more terms

i

that are identically 0 without changing the model

To write this set of equations in a state space form proceed as follows Letx

t + i j t

be

the conditional expectation of x

t + i

givenx

w

forw t

The following relations hold

x

t + i j t

=

1

X

j = i

j

e

t + i j

x

t + i j t + 1

= x

t + i j t

+

i 1

e

t + 1

However from equation (1) you can derive the following relationship

x

t + p j t

=

1

x

t + p 1 j t

+ +

p

x

t

(2)

Hence wheni = p

you can substitute forx

t + p j t

in the right-hand side of equation

(2) and close the system of equations

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Part 2 General Information

This substitution results in the following model in the state space form

z

t + 1

= F z

t

+ G e

t + 1

2

6

6

6

4

x

t + 1

x

t + 2 j t + 1

x

t + p j t + 1

3

7

7

7

5

=

2

6

6

4

0 I 0 0

0 0 I 0

p

p 1

1

3

7

7

5

2

6

6

6

4

x

t

x

t + 1 j t

x

t + p 1 j t

3

7

7

7

5

+

2

6

6

4

I

1

p 1

3

7

7

5

e

t + 1

Note that the state vectorz

t

is composed of conditional expectations of x

t

and the

first r components of z

t

are equal tox

t

The state space form can be cast into an ARMA form by solving the system of dif-

ference equations for the first r components

When converting from an ARMA form to a state space form you can generate a

state vector larger than needed that is the state space model may not be a minimal

representation When going from a state space form to an ARMA form you can have

nontrivial common factors in the autoregressive and moving average operators that

yield an ARMA model larger than necessary

If the state space form used is not a minimal representation some but not all compo-

nents of x

t + i j t

may be linearly dependent This situation corresponds to

p

p 1

being of less than full rank when B

and B

have no common nontrivial

left factors In this casez

t

consists of a subset of the possible components of

x

t + i j t

i = 1 2 p 1

However once a component of x

t + i j t

(for example the

jth one) is linearly dependent on the previous conditional expectations then all sub-

sequent jth components of x

t + k j t

fork i

must also be linearly dependent Note

that in this case equivalent but seemingly different structures can arise if the order of

the components withinx

t

is changed

OUT= Data Set

The forecasts are contained in the output data set specified by the OUT= option on the

PROC STATESPACE statement The OUT= data set contains the following variables

the BY variables

the ID variable

the VAR statement variables These variables contain the actual values from

the input data set

FORi numeric variables containing the forecasts The variable FORi contains

the forecasts for the ith variable in the VAR statement list Forecasts are one-

step-ahead predictions until the end of the data or until the observation specified

by the BACK= option

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Chapter 18 Details

RESi numeric variables containing the residual for the forecast of the ith vari-

able in the VAR statement list For forecast observations the actual values are

missing and the RESi variables contain missing values

STDi numeric variables containing the standard deviation for the forecast of

the ith variable in the VAR statement list The values of the STDi variables can

be used to construct univariate confidence limits for the corresponding fore-casts However such confidence limits do not take into account the covariance

of the forecasts

OUTAR= Data Set

The OUTAR= data set contains the estimates of the preliminary autoregressive mod-

els The OUTAR= data set contains the following variables

ORDER a numeric variable containing the order p of the autoregressive model

that the observation represents

AIC a numeric variable containing the value of the information criterionA I C

p

SIGFl numeric variables containing the estimate of the innovation covariance

matrices for the forward autoregressive models The variable SIGFl contains

the lth column of b

p

in the observations with ORDER= p

SIGBl numeric variables containing the estimate of the innovation covariance

matrices for the backward autoregressive models The variable SIGBl contains

the lth column of b

p

in the observations with ORDER= p

FORk ndashl numeric variables containing the estimates of the autoregressive pa-

rameter matrices for the forward models The variable FORk ndashl contains the

lth column of the lag k autoregressive parameter matrix b

p

k

in the observations

with ORDER= p

BACk ndashl numeric variables containing the estimates of the autoregressive pa-

rameter matrices for the backward models The variable BACk ndashl contains the

lth column of the lag k autoregressive parameter matrix b

p

k

in the observations

with ORDER= p

The estimates for the order p autoregressive model can be selected as those observa-

tions with ORDER= p Within these observations the k lth element of

p

i

is given by

the value of the FORindashl variable in the k th observation The k lth element of

p

i

is

given by the value of BACindashl variable in the k th observation The k lth element of

p

is given by SIGFl in the k th observation The k lth element of

p

is given by SIGBlin the k th observation

Table 181 shows an example of the OUTAR= data set with ARMAX=3 andx

t

of

dimension 2 In Table 181 i j

indicate the ijth element of the matrix

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Part 2 General Information

Table 181 Values in the OUTAR= Data Set

Obs ORDER AIC SIGF1 SIGF2 SIGB1 SIGB2 FOR1ndash 1 FOR1ndash 2 FOR2ndash 1 FOR2ndash 2 FOR3ndash 1

1 0 AIC0

0 1 1

0 1 2

0 1 1

0 1 2

2 0 AIC0

0 2 1

0 2 2

0 2 1

0 2 2

3 1 AIC1

1 1 1

1 1 2

1 1 1

1 1 2

1

1

1 1

1

1

1 2

4 1 AIC1

1 2 1

1 1 2

1 2 1

1 1 2

1

1

2 1

1

1

2 2

5 2 AIC2

2 1 1

2 1 2

2 1 1

2 1 2

2

1

1 1

2

1

1 2

2

2

1 1

2

2

1 2

6 2 AIC2

2 2 1

2 1 2

2 2 1

2 1 2

2

1

2 1

2

1

2 2

2

2

2 1

2

2

2 2

7 3 AIC3

3 1 1

3 1 2

3 1 1

3 1 2

3

1

1 1

3

1

1 2

3

2

1 1

3

2

1 2

3

3

1 1

8 3 AIC3

3 2 1

3 1 2

3 2 1

3 1 2

3

1

2 1

3

1

2 2

3

2

2 1

3

2

2 2

3

3

2 1

Obs FOR3ndash 2 BACK1ndash 1 BACK1ndash 2 BACK2ndash 1 BACK2ndash 2 BACK3ndash 1 BACK3ndash 2

1

2

3

1

1

1 1

1

1

1 2

4

1

1

2 1

1

1

2 2

5

2

1

1 1

2

1

1 2

2

2

1 1

2

2

1 2

6

2

1

2 1

2

1

2 2

2

2

2 1

2

2

2 2

7

3

3

1 2

3

1

1 1

3

1

1 2

3

2

1 1

3

2

1 2

3

3

1 1

3

3

1 2

8

3

3

2 2

3

1

2 1

3

1

2 2

3

2

2 1

3

2

2 2

3

3

2 1

3

3

2 2

The estimated autoregressive parameters can be used in the IML procedure to obtain

autoregressive estimates of the spectral density function or forecasts based on the

autoregressive models

OUTMODEL= Data Set

The OUTMODEL= data set contains the estimates of theF

andG

matrices and their

standard errors the names of the components of the state vector and the estimates of

the innovation covariance matrix The variables contained in the OUTMODEL= data

set are as follows

the BY variables

STATEVEC a character variable containing the name of the component of the

state vector corresponding to the observation The STATEVEC variable has

the value STD for standard deviations observations which contain the standarderrors for the estimates given in the preceding observation

Fndash j numeric variables containing the columns of theF

matrix The variable

Fndash j contains the jth column of F

The number of Fndash j variables is equal to

the value of the DIMMAX= option If the model is of smaller dimension the

extraneous variables are set to missing

Gndash j numeric variables containing the columns of theG

matrix The variable

Gndash j contains the jth column of G

The number of Gndash j variables is equal to r

the dimension of x

t

given by the number of variables in the VAR statement

SIGndash j numeric variables containing the columns of the innovation covariance

matrix The variable SIGndash j contains the jth column of

e e

There are r vari-ables SIGndash j

Table 182 shows an example of the OUTMODEL= data set withx

t

= x

t

y

t

0

z

t

= x

t

y

t

x

t + 1 j t

0 and DIMMAX=4 In Table 182F

i j

andG

i j

are the ijth

elements of F

andG

respectively Note that all elements for Fndash4 are missing because

F

is a3 3

matrix

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Chapter 18 Details

Table 182 Value in the OUTMODEL= Data Set

Obs STATEVEC Fndash 1 Fndash 2 Fndash 3 Fndash 4 Gndash 1 Gndash 2 SIGndash 1 SIGndash 2

1 X(TT) 0 0 1 1 0

1 1

1 2

2 STD

3 Y(TT) F2 1

F2 2

F2 3

0 1

2 1

2 2

4 STD std F2 1

std F2 2

std F2 3

5 X(T+1T) F3 1

F3 2

F3 3

G3 1

G3 2

6 STD std F3 1

std F3 2

std F3 3

std G3 1

std G3 2

Printed Output

The printed output produced by the STATESPACE procedure is described in the fol-

lowing

1 descriptive statistics which include the number of observations used the

names of the variables their means and standard deviations (Std) and the dif-

ferencing operations used

2 the Akaike information criteria for the sequence of preliminary autoregressive

models

3 if the PRINTOUT=LONG option is specified the sample autocovariance ma-

trices of the input series at various lags

4 if the PRINTOUT=LONG option is specified the sample autocorrelation ma-

trices of the input series

5 a schematic representation of the autocorrelation matrices showing the signif-

icant autocorrelations

6 if the PRINTOUT=LONG option is specified the partial autoregressive ma-

trices (These are

p

p

as described in Preliminary Autoregressive Modelsearlier in this chapter)

7 a schematic representation of the partial autocorrelation matrices showing the

significant partial autocorrelations

8 the Yule-Walker estimates of the autoregressive parameters for the autoregres-

sive model with the minimum AIC

9 if the PRINTOUT=LONG option is specified the autocovariance matrices of

the residuals of the minimum AIC model This is the sequence of estimated

innovation variance matrices for the solutions of the Yule-Walker equations

10 if the PRINTOUT=LONG option is specified the autocorrelation matrices of the residuals of the minimum AIC model

11 If the CANCORR option is specified the canonical correlations analysis for

each potential state vector considered in the state vector selection process

This includes the potential state vector the canonical correlations the informa-

tion criterion for the smallest canonical correlation Bartlettrsquos

2 statistic (ldquoChi

Squarerdquo) for the smallest canonical correlation and the degrees of freedom of

Bartlettrsquos

2

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Part 2 General Information

12 the components of the chosen state vector

13 the preliminary estimate of the transition matrixF

the input matrixG

and

the variance matrix for the innovations

e e

14 if the ITPRINT option is specified the iteration history of the likelihood max-

imization For each iteration this shows the iteration number the number of step halvings the determinant of the innovation variance matrix the damping

factor Lambda and the values of the parameters

15 the state vector printed again to aid interpretation of the following listing of F

andG

16 the final estimate of the transition matrixF

17 the final estimate of the input matrixG

18 the final estimate of the variance matrix for the innovations

e e

19 a table listing the estimates of the free parameters inF

andG

and their standarderrors and t statistics

20 if the COVB option is specified the covariance matrix of the parameter esti-

mates

21 if the COVB option is specified the correlation matrix of the parameter esti-

mates

22 if the PRINT option is specified the forecasts and their standard errors

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Chapter 18 Details

ODS Table Names

PROC STATESPACE assigns a name to each table it creates You can use these

names to reference the table when using the Output Delivery System (ODS) to select

tables and create output data sets These names are listed in the following table For

more information on ODS see Chapter 6 ldquoUsing the Output Delivery Systemrdquo

Table 183 ODS Tables Produced in PROC STATESPACE

ODS Table Name Description Option

NObs Number of observations default

Summary Simple summary statistics table default

InfoCriterion Information criterion table default

CovLags Covariance Matrices of Input Series PRINTOUT=LONG

CorrLags Correlation Matrices of Input Series PRINTOUT=LONG

PartialAR Partial Autoregressive Matrices PRINTOUT=LONG

YWEstimates Yule-Walker Estimates for Minimum AIC default

CovResiduals Covariance of Residuals PRINTOUT=LONGCorrResiduals Residual Correlations from AR Models PRINTOUT=LONG

StateVector State vector table default

CorrGraph Schematic Representation of Correlations default

TransitionMatrix Transition Matrix default

InputMatrix Input Matrix default

VarInnov Variance Matrix for the Innovation default

CovB Covariance of Parameter Estimates COVB

CorrB Correlation of Parameter Estimates COVB

CanCorr Canonical Correlation Analysis CANCORR

IterHistory Iterative Fitting table ITPRINT

ParameterEstimates Parameter Estimates Table default

Forecasts Forecasts Table PRINT

ConvergenceStatus Convergence Status Table default

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Part 2 General Information

Example

Example 181 Series J from Box and Jenkins

This example analyzes the gas furnace data (series J) from Box and Jenkins (The

data are not shown Refer to Box and Jenkins (1976) for the data)

First a model is selected and fit automatically using the following statements

title1 rsquoGas Furnace Datarsquo

title2 rsquoBox amp Jenkins Series Jrsquo

title3 rsquoAutomatically Selected Modelrsquo

proc statespace data=seriesj cancorr

var x y

run

The results for the automatically selected model are shown in Output 1811

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Chapter 18 Example

Output 1811 Results for Automatically Selected Model

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Number of Observations 296

Standard

Variable Mean Error

x -005683 1072766

y 5350912 3202121

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Information Criterion for Autoregressive Models

Lag=0 Lag=1 Lag=2 Lag=3 Lag=4 Lag=5 Lag=6 Lag=7 Lag=8

6513862 -103357 -163296 -164512 -165152 -164891 -164934 -164315 -163856

Information

Criterion for

Autoregressive

Models

Lag=9 Lag=10

-16348 -163359

Schematic Representation of Correlations

NameLag 0 1 2 3 4 5 6 7 8 9 10

x +- +- +- +- +- +- +- +- +- +- +-

y -+ -+ -+ -+ -+ -+ -+ -+ -+ -+ -+

+ is gt 2std error - is lt -2std error is between

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Part 2 General Information

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Schematic Representation of Partial Autocorrelations

NameLag 1 2 3 4 5 6 7 8 9 10

x + - + -

y -+ -- - + +

+ is gt 2std error - is lt -2std error is between

Yule-Walker Estimates for Minimum AIC

------Lag=1------ ------Lag=2------ ------Lag=3------ ------Lag=4------

x y x y x y x y

x 1925887 -000124 -120166 0004224 0116918 -000867 0104236 0003268

y 0050496 1299793 -002046 -03277 -071182 -025701 0195411 0133417

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Canonical Correlations Analysis

Information Chi

x(TT) y(TT) x(T+1T) Criterion Square DF

1 1 0804883 2929228 3047481 8

Information Chix(TT) y(TT) x(T+1T) y(T+1T) Criterion Square DF

1 1 0906681 0607529 1223358 1347237 7

Information Chi

x(TT) y(TT) x (T+1T) y (T+1T) x (T+2T) Criterion Square DF

1 1 0909434 0610278 0186274 -154701 1034705 6

Information Chi

x(TT) y(TT) x (T+1T) y (T+1T) y (T+2T) Criterion Square DF

1 1 091014 0618937 0206823 0940392 1280924 6

Information Chi

x(TT) y(TT) x(T+1T) y(T+1T) y(T+2T) y(T+3T) Criterion Square DF

1 1 0912963 0628785 0226598 0083258 -794103 2041584 5

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Chapter 18 Example

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Selected Statespace Form and Preliminary Estimates

State Vector

x(TT) y(TT) x(T+1T) y(T+1T) y(T+2T)

Estimate of Transition Matrix

0 0 1 0 0

0 0 0 1 0

-084718 0026794 1711715 -005019 0

0 0 0 0 1

-019785 0334274 -018174 -123557 1787475

Input Matrix for Innovation

1 0

0 11925887 -000124

0050496 1299793

0142421 1361696

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Selected Statespace Form and Preliminary Estimates

Variance Matrix for Innovation

0035274 -000734

-000734 0097569

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Part 2 General Information

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Selected Statespace Form and Fitted Model

State Vector

x(TT) y(TT) x(T+1T) y(T+1T) y(T+2T)

Estimate of Transition Matrix

0 0 1 0 0

0 0 0 1 0

-086192 0030609 1724235 -005483 0

0 0 0 0 1

-034839 0292124 -009435 -109823 1671418

Input Matrix for Innovation

1 0

0 1192442 -000416

0015621 1258495

008058 1353204

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Selected Statespace Form and Fitted Model

Variance Matrix for Innovation

0035579 -000728

-000728 0095577

Parameter Estimates

Standard

Parameter Estimate Error t Value

F(31) -086192 0072961 -1181

F(32) 0030609 0026167 117

F(33) 1724235 0061599 2799

F(34) -005483 0030169 -182

F(51) -034839 0135253 -258

F(52) 0292124 0046299 631

F(53) -009435 0096527 -098F(54) -109823 0109525 -1003

F(55) 1671418 0083737 1996

G(31) 1924420 0058162 3309

G(32) -000416 0035255 -012

G(41) 0015621 0095771 016

G(42) 1258495 0055742 2258

G(51) 0080580 0151622 053

G(52) 1353204 0091388 1481

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Chapter 18 Example

The two series are believed to have a transfer function relation with the gas rate

(variable X) as the input and the CO2

concentration (variable Y) as the output Since

the parameter estimates shown in Output 1811 support this kind of model the model

is reestimated with the feedback parameters restricted to 0 The following statements

fit the transfer function (no feedback) model

title3 rsquoTransfer Function Modelrsquo

proc statespace data=seriesj printout=none

var x y

restrict f(32)=0 f(34)=0

g(32)=0 g(41)=0 g(51)=0

run

The last 2 pages of the output are shown in Output 1812

Output 1812 STATESPACE Output for Transfer Function Model

Gas Furnace Data

Box amp Jenkins Series J

Transfer Function Model

The STATESPACE Procedure

Selected Statespace Form and Fitted Model

State Vector

x(TT) y(TT) x(T+1T) y(T+1T) y(T+2T)

Estimate of Transition Matrix

0 0 1 0 0

0 0 0 1 0

-068882 0 1598717 0 0

0 0 0 0 1

-035944 0284179 -00963 -107313 1650047

Input Matrix for Innovation

1 0

0 1

1923446 0

0 1260856

0 1346332

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Part 2 General Information

Gas Furnace Data

Box amp Jenkins Series J

Transfer Function Model

The STATESPACE Procedure

Selected Statespace Form and Fitted Model

Variance Matrix for Innovation

0036995 -00072

-00072 0095712

Parameter Estimates

Standard

Parameter Estimate Error t Value

F(31) -068882 0050549 -1363

F(33) 1598717 0050924 3139

F(51) -035944 0229044 -157

F(52) 0284179 0096944 293

F(53) -009630 0140876 -068

F(54) -107313 0250385 -429

F(55) 1650047 0188533 875G(31) 1923446 0056328 3415

G(42) 1260856 0056464 2233

G(52) 1346332 0091086 1478

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Chapter 18 References

References

Akaike H (1974) Markovian Representation of Stochastic Processes and Its Ap-

plication to the Analysis of Autoregressive Moving Average Processes Annals

of the Institute of Statistical Mathematics 26 363-387

Akaike H (1976) Canonical Correlations Analysis of Time Series and the Use of

an Information Criterion in Advances and Case Studies in System Identification

eds R Mehra and DG Lainiotis New York Academic Press

Anderson TW (1971) The Statistical Analysis of Time Series New York John

Wiley amp Sons

Ansley CF and Newbold P (1979) Multivariate Partial Autocorrelations Pro-

ceedings of the Business and Economic Statistics Section American Statistical

Association 349-353

Box GEP and Jenkins G (1976) Time Series Analysis Forecasting and Control

San Francisco Holden-Day

Brockwell PJ and Davis RA (1991) Time Series Theory and Methods 2nd Edi-

tion Springer-Verlag

Hannan EJ (1970) Multiple Time Series New York John Wiley amp Sons

Hannan EJ (1976) The Identification and Parameterization of ARMAX and State

Space Forms Econometrica 44 713-722

Harvey AC (1981a) The Econometric Analysis of Time Series New York John

Wiley amp Sons

Harvey AC (1981b) Time Series Models New York John Wiley amp Sons

Jones RH (1974) Identification and Autoregressive Spectrum Estimation IEEE Transactions on Automatic Control AC-19 894-897

Pham-Dinh-Tuan (1978) On the Fitting of Multivariate Processes of the

Autoregressive-Moving Average Type Biometrika 65 99-107

Priestley MB (1980) System Identification Kalman Filtering and Stochastic Con-

trol in Directions in Time Series eds DR Brillinger and GC Tiao Institute

of Mathematical Statistics

Whittle P (1963) On the Fitting of Multivariate Autoregressions and the Approxi-

mate Canonical Factorization of a Spectral Density Matrix Biometrika 50 129-

134

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The correct bibliographic citation for this manual is as follows SAS Institute Inc SAS

ET S U serrsquos Gui de Version 8 Cary NC SAS Institute Inc 1999 1546 pp

SASET S Userrsquos Guide Version 8

Copyright copy 1999 by S AS I ns t it u t e I nc Ca ry N C U S A

ISBN 1ndash58025ndash489ndash6

All r ight s res erved P r int ed in t he U n it ed St a t es of A merica N o pa rt of t his p u blica t ionma y b e reprodu ced s t ored in a ret r ieva l s ys t em or t ra ns mit t ed in a ny form or b y a nymeans electronic mechanical photocopying or otherwise without the prior writtenpermission of the publisher SAS Institute Inc

US Government Restricted Rights Notice Use du plicat ion or disclosure of the

software by the government is subject to restrictions as set forth in FAR 52227ndash19Commercial Computer Softw ar e-Restricted Rights (J une 1987)

SAS Institute Inc SAS Campus Drive Cary North Carolina 27513

1st print ing October 1999

S AS reg a nd a ll ot her SA S I ns t it u t e I nc p rodu ct or s ervice na m es a re regis t ered t ra dema rkst d k f SA S I t it t I i t h U S A d t h t i reg i di t U SA

Page 24: Statespace Model

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Part 2 General Information

VAR Statement

VAR variable (difference difference )

The VAR statement specifies the variables in the input data set to model and fore-cast The VAR statement also specifies differencing of the input variables The VAR

statement is required

Differencing is specified by following the variable name with a list of difference

periods separated by commas See the section Stationarity and Differencing for

more information on differencing of input variables

The order in which variables are listed in the VAR statement controls the order in

which variables are included in the state vector Usually potential inputs should be

listed before potential outputs

For example assuming the input data are monthly the following VAR statement

specifies modeling and forecasting of the one period and seasonal second differenceof X and Y

var x(112) y(112)

In this example the vector time series analyzed is

x

t

=

1 B 1 B

1 2

X

t

x

1 B 1 B

1 2

Y

t

y

where B represents the back shift operator and x and y represent the means of thedifferenced series If the NOCENTER option is specified the mean differences are

not subtracted

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Chapter 18 Details

Details

Missing Values

The STATESPACE procedure does not support missing values The procedure uses

the first contiguous group of observations with no missing values for any of the VARstatement variables Observations at the beginning of the data set with missing values

for any VAR statement variable are not used or included in the output data set

Stationarity and Differencing

The state space model used by the STATESPACE procedure assumes that the time

series are stationary Hence the data should be checked for stationarity One way

to check for stationarity is to plot the series A graph of series over time can show a

time trend or variability changes

You can also check stationarity by using the sample autocorrelation functions dis-

played by the ARIMA procedure The autocorrelation functions of nonstationaryseries tend to decay slowly See Chapter 7 ldquoThe ARIMA Procedurerdquo for more in-

formation

Another alternative is to use the STATIONARITY= option on the IDENTIFY state-

ment in PROC ARIMA to apply Dickey-Fuller tests for unit roots in the time series

See Chapter 7 ldquoThe ARIMA Procedurerdquo for more information on Dickey-Fuller unit

root tests

The most popular way to transform a nonstationary series to stationarity is by dif-

ferencing Differencing of the time series is specified in the VAR statement For

example to take a simple first difference of the series X use this statement

var x(1)

In this example the change in X from one period to the next is analyzed When the se-

ries has a seasonal pattern differencing at a period equal to the length of the seasonal

cycle may be desirable For example suppose the variable X is measured quarterly

and shows a seasonal cycle over the year You can use the following statement to

analyze the series of changes from the same quarter in the previous year

var x(4)

To difference twice add another differencing period to the list For ex-

ample the following statement analyzes the series of second differences

X

t

X

t 1

X

t 1

X

t 2

= X

t

2 X

t 1

+ X

t 2

var x(11)

The following statement analyzes the seasonal second difference series

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Part 2 General Information

var x(14)

The series modeled is the 1-period difference of the 4-period difference

X

t

X

t 4

X

t 1

X

t 5

= X

t

X

t 1

X

t 4

+ X

t 5

Another way to obtain stationary series is to use a regression on time to de-trend the

data If the time series has a deterministic linear trend regressing the series on time

produces residuals that should be stationary The following statements write residuals

of X and Y to the variable RX and RY in the output data set DETREND

data a

set a

t=_n_

run

proc reg data=a

model x y = t

output out=detrend r=rx ry

run

You then use PROC STATESPACE to forecast the de-trended series RX and RY A

disadvantage of this method is that you need to add the trend back to the forecast

series in an additional step A more serious disadvantage of the de-trending method

is that it assumes a deterministic trend In practice most time series appear to have a

stochastic rather than a deterministic trend Differencing is a more flexible and often

more appropriate method

There are several other methods to handle nonstationary time series For more infor-

mation and examples refer to Brockwell and Davis (1991)

Preliminary Autoregressive Models

After computing the sample autocovariance matrices PROC STATESPACE fits a

sequence of vector autoregressive models These preliminary autoregressive models

are used to estimate the autoregressive order of the process and limit the order of the

autocovariances considered in the state vector selection process

Yule-Walker Equations for Forward and Backward Models

Unlike a univariate autoregressive model a multivariate autoregressive model has

different forms depending on whether the present observation is being predicted from

the past observations or from the future observations

Letx

t

be the r -component stationary time series given by the VAR statement after

differencing and subtracting the vector of sample means (If the NOCENTER option

is specified the mean is not subtracted) Let n be the number of observations of x

t

from the input data set

Lete

t

be a vector white noise sequence with mean vector 0 and variance matrix

p

and letn

t

be a vector white noise sequence with mean vector 0 and variance matrix

p

Let p be the order of the vector autoregressive model forx

t

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Chapter 18 Details

The forward autoregressive form based on the past observations is written as follows

x

t

=

p

X

i = 1

p

i

x

t i

+ e

t

The backward autoregressive form based on the future observations is written as fol-

lows

x

t

=

p

X

i = 1

p

i

x

t + i

+ n

t

LettingE

denote the expected value operator the autocovariance sequence for thex

t

series

i

is

i

= E x

t

x

0

t i

The Yule-Walker equations for the autoregressive model that matches the first p ele-

ments of the autocovariance sequence are

2

6

6

4

0

1

p 1

0

1

0

p 2

0

p 1

0

p 2

0

3

7

7

5

2

6

6

4

p

1

p

2

p

p

3

7

7

5

=

2

6

6

4

1

2

p

3

7

7

5

and

2

6

6

6

4

0

0

1

0

p 1

1

0

0

p 2

p 1

p 2

0

3

7

7

7

5

2

6

6

4

p

1

p

2

p

p

3

7

7

5

=

2

6

6

4

0

1

0

2

0

p

3

7

7

5

Here

p

i

are the coefficient matrices for the past observation form of the vector au-toregressive model and

p

i

are the coefficient matrices for the future observation

form More information on the Yule-Walker equations in the multivariate setting can

be found in Whittle (1963) and Ansley and Newbold (1979)

The innovation variance matrices for the two forms can be written as follows

p

=

0

p

X

i = 1

p

i

0

i

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Part 2 General Information

p

=

0

p

X

i = 1

p

i

i

The autoregressive models are fit to the data using the preceding Yule-Walker equa-

tions with

i

replaced by the sample covariance sequenceC

i

The covariance matri-

ces are calculated as

C

i

=

1

N 1

N

X

t = i + 1

x

t

x

0

t i

Let b

p

b

p

b

p

and b

p

represent the Yule-Walker estimates of

p

p

p

and

p

respectively These matrices are written to an output data set when the OUTAR=

option is specified

When the PRINTOUT=LONG option is specified the sequence of matrices b

p

and

the corresponding correlation matrices are printed The sequence of matricesb

p isused to compute Akaike information criteria for selection of the autoregressive order

of the process

Akaike Information Criterion

The Akaike information criterion or AIC is defined as -2( maximum of log likeli-

hood )+2(number of parameters) Since the vector autoregressive models are esti-

mates from the Yule-Walker equations not by maximum likelihood the exact likeli-

hood values are not available for computing the AIC However for the vector autore-

gressive model the maximum of the log likelihood can be approximated as

l n L

n

2

l n j

b

p

j

Thus the AIC for the order p model is computed as

A I C

p

= n l n j

b

p

j + 2 p r

2

You can use the printed AIC array to compute a likelihood ratio test of the autoregres-

sive order The log-likelihood ratio test statistic for testing the order p model against

the order p 1

model is

n l n j

b

p

j + n l n j

b

p 1

j

This quantity is asymptotically distributed as a

2 withr

2 degrees of freedom if the

series is autoregressive of order p 1

It can be computed from the AIC array as

A I C

p 1

A I C

p

+ 2 r

2

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Chapter 18 Details

You can evaluate the significance of these test statistics with the PROBCHI function

in a SAS DATA step or with a

2 table

Determining the Autoregressive Order

Although the autoregressive models can be used for prediction their primary value is

to aid in the selection of a suitable portion of the sample covariance matrix for use in

computing canonical correlations If the multivariate time series x

t is of autoregres-sive order p then the vector of past values to lag p is considered to contain essentially

all the information relevant for prediction of future values of the time series

By default PROC STATESPACE selects the order p producing the autoregressive

model with the smallestA I C

p

If the value p for the minimumA I C

p

is less than the

value of the PASTMIN= option then p is set to the PASTMIN= value Alternatively

you can use the ARMAX= and PASTMIN= options to force PROC STATESPACE to

use an order you select

Significance Limits for Partial Autocorrelations

The STATESPACE procedure prints a schematic representation of the partial autocor-

relation matrices indicating which partial autocorrelations are significantly greater orsignificantly less than 0 Figure 1810 shows an example of this table

The STATESPACE Procedure

Schematic Representation of Partial Autocorrelations

NameLag 1 2 3 4 5 6 7 8 9 10

x ++ +

y ++

+ is gt 2std error - is lt -2std error is between

Figure 1810 Significant Partial Autocorrelations

The partial autocorrelations are from the sample partial autoregressive matrices b

p

p

The standard errors used for the significance limits of the partial autocorrelations are

computed from the sequence of matrices

p

and

p

Under the assumption that the observed series arises from an autoregressive process

of order p 1

the pth sample partial autoregressive matrix b

p

p

has an asymptotic

variance matrix 1

n

1

p

p

The significance limits for b

p

p

used in the schematic plot of the sample partial autore-

gressive sequence are derived by replacing

p

and

p

with their sample estimators

to produce the variance estimate as follows

d

V a r

b

p

p

=

1

n r p

b

1

p

b

p

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Part 2 General Information

Canonical Correlation Analysis

Given the order p letp

t

be the vector of current and past values relevant to prediction

of x

t + 1

p

t

= x

0

t

x

0

t 1

x

0

t p

0

Letf

t

be the vector of current and future values

f

t

= x

0

t

x

0

t + 1

x

0

t + p

0

In the canonical correlation analysis consider submatrices of the sample covariance

matrix of p

t

andf

t

This covariance matrixV

has a block Hankel form

V =

2

6

6

6

4

C

0

C

0

1

C

0

2

C

0

p

C

0

1

C

0

2

C

0

3

C

0

p + 1

C

0

p

C

0

p + 1

C

0

p + 2

C

0

2 p

3

7

7

7

5

State Vector Selection Process

The canonical correlation analysis forms a sequence of potential state vectorsz

j

t

Examine a sequencef

j

t

of subvectors of f

t

and form the submatrixV

j consisting

of the rows and columns of V

corresponding to the components of f

j

t

and compute

its canonical correlations

The smallest canonical correlation of V

j is then used in the selection of the com-

ponents of the state vector The selection process is described in the following For

more details about this process refer to Akaike (1976)

In the following discussion the notationx

t + k j t

denotes the wide sense conditional

expectation (best linear predictor) of x

t + k

given allx

s

with s less than or equal to t

In the notationx

i t + 1

the first subscript denotes the ith component of x

t + 1

The initial state vectorz

1

t

is set tox

t

The sequencef

j

t

is initialized by setting

f

1

t

= z

1

0

t

x

1 t + 1 j t

0

= x

0

t

x

1 t + 1 j t

0

That is start by considering whether to addx

1 t + 1 j t

to the initial state vectorz

1

t

The procedure forms the submatrixV

1 corresponding tof

1

t

and computes its canon-

ical correlations Denote the smallest canonical correlation of V

1 as

m i n

If

m i n

is

significantly greater than 0x

1 t + 1 j t

is added to the state vector

If the smallest canonical correlation of V

1 is not significantly greater than 0 then a

linear combination of f

1

t

is uncorrelated with the pastp

t

Assuming that the determi-

nant of C

0

is not 0 (that is no input series is a constant) you can take the coefficient

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Chapter 18 Details

of x

1 t + 1 j t

in this linear combination to be 1 Denote the coefficients of z

1

t

in this

linear combination as

This gives the relationship

x

1 t + 1 j t

=

0

x

t

Therefore the current state vector already contains all the past information useful forpredicting

x

1 t + 1

and any greater leads of x

1 t

The variablex

1 t + 1 j t

is not added to

the state vector nor are any termsx

1 t + k j t

considered as possible components of the

state vector The variablex

1

is no longer active for state vector selection

The process described forx

1 t + 1 j t

is repeated for the remaining elements of f

t

The

next candidate for inclusion in the state vector is the next component of f

t

corre-

sponding to an active variable Components of f

t

corresponding to inactive variables

that produced a zero

m i n

in a previous step are skipped

Denote the next candidate asx

l t + k j t

The vectorf

j

t

is formed from the current state

vector andx

l t + k j t

as follows

f

j

t

= z

j

0

t

x

l t + k j t

0

The matrixV

j is formed fromf

j

t

and its canonical correlations are computed The

smallest canonical correlation of V

j is judged to be either greater than or equal to 0

If it is judged to be greater than 0x

l t + k j t

is added to the state vector If it is judged

to be 0 then a linear combination of f

j

t

is uncorrelated with thep

t

and the variable

x

l

is now inactive

The state vector selection process continues until no active variables remain

Testing Significance of Canonical Correlations For each step in the canonical correlation sequence the significance of the small-

est canonical correlation

m i n

is judged by an information criterion from Akaike

(1976) This information criterion is

n l n 1

2

m i n

r p + 1 q + 1

where q is the dimension of f

j

t

at the current step r is the order of the state vector p

is the order of the vector autoregressive process and

is the value of the SIGCORR=

option The default is SIGCORR=2 If this information criterion is less than or equal

to 0

m i n

is taken to be 0 otherwise it is taken to be significantly greater than 0

(Do not confuse this information criterion with the AIC)

Variables inx

t + p j t

are not added in the model even with positive information crite-

rion because of the singularity of V

You can force the consideration of more candi-

date state variables by increasing the size of theV

matrix by specifying a PASTMIN=

option value larger than p

Printing the Canonical Correlations

To print the details of the canonical correlation analysis process specify the CAN-

CORR option in the PROC STATESPACE statement The CANCORR option prints

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Part 2 General Information

the candidate state vectors the canonical correlations and the information criteria for

testing the significance of the smallest canonical correlation

Bartlettrsquos

2 and its degrees of freedom are also printed when the CANCORR option

is specified The formula used for Bartlettrsquos

2 is

2

= n 5 r p + 1 q + 1 l n 1

2

m i n

withr p + 1 q + 1

degrees of freedom

Figure 1811 shows the output of the CANCORR option for the introductory example

shown in the Getting Started section of this chapter

The STATESPACE Procedure

Canonical Correlations Analysis

Information Chi

x(TT) y(TT) x(T+1T) Criterion Square DF

1 1 0237045 3566167 114505 4

Information Chi

x(TT) y(TT) x(T+1T) y(T+1T) Criterion Square DF

1 1 0238244 0056565 -535906 0636134 3

Information Chi

x(TT) y(TT) x(T+1T) x(T+2T) Criterion Square DF

1 1 0237602 0087493 -446312 1525353 3

Figure 1811 Canonical Correlations Analysis

New variables are added to the state vector if the information criteria are positive Inthis example

y

t + 1 j t

andx

t + 2 j t

are not added to the state space vector because the

information criteria for these models are negative

If the information criterion is nearly 0 then you may want to investigate models

that arise if the opposite decision is made regarding

m i n

This investigation can be

accomplished by using a FORM statement to specify part or all of the state vector

Preliminary Estimates of F

When a candidate variablex

l t + k j t

yields a zero

m i n

and is not added to the state

vector a linear combination of f

j

t

is uncorrelated with thep

t

Because of the method

used to construct thef

j

t

sequence the coefficient of x

l t + k j t

inl

can be taken as 1

Denote the coefficients of z

j

t

in this linear combination as l

This gives the relationship

x

l t + k j t

= l

0

z

j

t

The vectorl

is used as a preliminary estimate of the first r columns of the row of the

transition matrixF

corresponding tox

l t + k 1 j t

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Chapter 18 Details

Parameter Estimation

The model isz

t + 1

= F z

t

+ G e

t + 1

wheree

t

is a sequence of independent multivari-

ate normal innovations with mean vector 0 and variance

e e

The observed sequence

x

t

composes the first r components of z

t

and thusx

t

= H z

t

where H is ther s

matrix I

r

0

LetE

be ther n

matrix of innovations

E = e

1

e

n

If the number of observations n is reasonably large the log likelihood L can be

approximated up to an additive constant as follows

L =

n

2

l n j

e e

j

1

2

t r a c e

1

e e

E E

0

The elements of

e e

are taken as free parameters and are estimated as follows

S

0

=

1

n

E E

0

Replacing

e e

byS

0

in the likelihood equation the log likelihood up to an additive

constant is

L =

n

2

l n j S

0

j

Letting B be the backshift operator the formal relation betweenx

t

ande

t

is

x

t

= H I B F

1

G e

t

e

t

= H I B F

1

G

1

x

t

=

1

X

i = 0

i

x

t i

LettingC

i

be the ith lagged sample covariance of x

t

and neglecting end effects the

matrixS

0

is

S

0

=

1

X

i j = 0

i

C

i + j

0

j

For the computation of S

0

the infinite sum is truncated at the value of the KLAG=

option The value of the KLAG= option should be large enough that the sequence

i

is approximately 0 beyond that point

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Part 2 General Information

Let

be the vector of free parameters in theF

andG

matrices The derivative of the

log likelihood with respect to the parameter

is

L

=

n

2

t r a c e

S

1

0

S

0

The second derivative is

2

L

0

=

n

2

t r a c e

S

1

0

S

0

0

S

1

0

S

0

t r a c e

S

1

0

2

S

0

0

Near the maximum the first term is unimportant and the second term can be approx-

imated to give the following second derivative approximation

2

L

0

=

n t r a c e

S

1

0

E

E

0

0

The first derivative matrix and this second derivative matrix approximation are com-

puted from the sample covariance matrixC

0

and the truncated sequence

i

The

approximate likelihood function is maximized by a modified Newton-Raphson algo-

rithm employing these derivative matrices

The matrixS

0

is used as the estimate of the innovation covariance matrix

e e

The

negative of the inverse of the second derivative matrix at the maximum is used as an

approximate covariance matrix for the parameter estimates The standard errors of

the parameter estimates printed in the parameter estimates tables are taken from the

diagonal of this covariance matrix The parameter covariance matrix is printed when

the COVB option is specified

If the data are nearly nonstationary a better estimate of

e e

and the other parameters

can sometimes be obtained by specifying the RESIDEST option The RESIDEST

option estimates the parameters using conditional least squares instead of maximum

likelihood

The residuals are computed using the state space equation and the sample mean val-

ues of the variables in the model as start-up values The estimate of S

0

is then com-

puted using the residuals from the ith observation on where i is the maximum number

of times any variable occurs in the state vector A multivariate Gauss-Marquardt al-

gorithm is used to minimizej S

0

j

Refer to Harvey (1981a) for a further description

of this method

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Chapter 18 Details

Forecasting

Given estimates of F

G

and

e e

forecasts of x

t

are computed from the conditional

expectation of z

t

In forecasting the parameters F G and

e e

are replaced with the estimates or by val-

ues specified in the RESTRICT statement One-step-ahead forecasting is performedfor the observation

x

t

wheret n b

Heren

is the number of observations and

b is the value of the BACK= option For the observationx

t

wheret n b

m-

step-ahead forecasting is performed form = t n + b

The forecasts are generated

recursively with the initial conditionz

0

= 0

The m-step-ahead forecast of z

t + m

isz

t + m j t

wherez

t + m j t

denotes the conditional

expectation of z

t + m

given the information available at time t The m-step-ahead

forecast of x

t + m

isx

t + m j t

= H z

t + m j t

where the matrixH = I

r

0

Let

i

= F

i

G

Note that the lasts r

elements of z

t

consist of the elements of x

u j t

foru t

The state vector z

t + m

can be represented as

z

t + m

= F

m

z

t

+

m 1

X

i = 0

i

e

t + m i

Sincee

t + i j t

= 0

fori 0

the m-step-ahead forecastz

t + m j t

is

z

t + m j t

= F

m

z

t

= F z

t + m 1 j t

Therefore the m-step-ahead forecast of x

t + m

is

x

t + m j t

= H z

t + m j t

The m-step-ahead forecast error is

z

t + m

z

t + m j t

=

m 1

X

i = 0

i

e

t + m i

The variance of the m-step-ahead forecast error is

V

z m

=

m 1

X

i = 0

i

e e

0

i

LettingV

z 0

= 0

the variance of the m-step-ahead forecast error of z

t + m

V

z m

can

be computed recursively as follows

V

z m

= V

z m 1

+

m 1

e e

0

m 1

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The variance of the m-step-ahead forecast error of x

t + m

is ther r

left upper sub-

matrix of V

z m

that is

V

x m

= H V

z m

H

0

Unless the NOCENTER option is specified the sample mean vector is added to the

forecast When differencing is specified the forecasts xt + m j t

plus the sample mean

vector are integrated back to produce forecasts for the original series

Lety

t

be the original series specified by the VAR statement with some 0 values

appended corresponding to the unobserved past observations Let B be the backshift

operator and let B

be thes s

matrix polynomial in the backshift operator

corresponding to the differencing specified by the VAR statement The off-diagonal

elements of

i

are 0 Note that

0

= I

s

whereI

s

is thes s

identity matrix Then

z

t

= B y

t

This gives the relationship

y

t

=

1

B z

t

=

1

X

i = 0

i

z

t i

where

1

B =

P

1

i = 0

i

B

i and

0

= I

s

The m-step-ahead forecast of y

t + m

is

y

t + m j t

=

m 1

X

i = 0

i

z

t + m i j t

+

1

X

i = m

i

z

t + m i

The m-step-ahead forecast error of y

t + m

is

m 1

X

i = 0

i

z

t + m i

z

t + m i j t

=

m 1

X

i = 0

i

X

u = 0

u

i u

e

t + m i

LettingV

y 0

= 0

the variance of the m-step-ahead forecast error of y

t + m

V

y m

is

V

y m

=

m 1

X

i = 0

i

X

u = 0

u

i u

e e

i

X

u = 0

u

i u

0

= V

y m 1

+

m 1

X

u = 0

u

m 1 u

e e

m 1

X

u = 0

u

m 1 u

0

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Chapter 18 Details

Relation of ARMA and State Space Forms

Every state space model has an ARMA representation and conversely every ARMA

model has a state space representation This section discusses this equivalence The

following material is adapted from Akaike (1974) where there is a more complete

discussion Pham-Dinh-Tuan (1978) also contains a discussion of this material

Suppose you are given the following ARMA model

B x

t

= B e

t

or in more detail

x

t

1

x

t 1

p

x

t p

= e

t

+

1

e

t 1

+ +

q

e

t q

(1)

wheree

t

is a sequence of independent multivariate normal random vectors with mean

0 and variance matrix

e e

B is the backshift operator (B x

t

= x

t 1

) B

and

B

are matrix polynomials in B andx

t

is the observed process

If the roots of the determinantial equationj B j = 0

are outside the unit circle in

the complex plane the model can also be written as

x

t

=

1

B B e

t

=

1

X

i = 0

i

e

t i

The

i

matrices are known as the impulse response matrices and can be computed

as

1

B B

You can assume p q

since if this is not initially true you can add more terms

i

that are identically 0 without changing the model

To write this set of equations in a state space form proceed as follows Letx

t + i j t

be

the conditional expectation of x

t + i

givenx

w

forw t

The following relations hold

x

t + i j t

=

1

X

j = i

j

e

t + i j

x

t + i j t + 1

= x

t + i j t

+

i 1

e

t + 1

However from equation (1) you can derive the following relationship

x

t + p j t

=

1

x

t + p 1 j t

+ +

p

x

t

(2)

Hence wheni = p

you can substitute forx

t + p j t

in the right-hand side of equation

(2) and close the system of equations

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Part 2 General Information

This substitution results in the following model in the state space form

z

t + 1

= F z

t

+ G e

t + 1

2

6

6

6

4

x

t + 1

x

t + 2 j t + 1

x

t + p j t + 1

3

7

7

7

5

=

2

6

6

4

0 I 0 0

0 0 I 0

p

p 1

1

3

7

7

5

2

6

6

6

4

x

t

x

t + 1 j t

x

t + p 1 j t

3

7

7

7

5

+

2

6

6

4

I

1

p 1

3

7

7

5

e

t + 1

Note that the state vectorz

t

is composed of conditional expectations of x

t

and the

first r components of z

t

are equal tox

t

The state space form can be cast into an ARMA form by solving the system of dif-

ference equations for the first r components

When converting from an ARMA form to a state space form you can generate a

state vector larger than needed that is the state space model may not be a minimal

representation When going from a state space form to an ARMA form you can have

nontrivial common factors in the autoregressive and moving average operators that

yield an ARMA model larger than necessary

If the state space form used is not a minimal representation some but not all compo-

nents of x

t + i j t

may be linearly dependent This situation corresponds to

p

p 1

being of less than full rank when B

and B

have no common nontrivial

left factors In this casez

t

consists of a subset of the possible components of

x

t + i j t

i = 1 2 p 1

However once a component of x

t + i j t

(for example the

jth one) is linearly dependent on the previous conditional expectations then all sub-

sequent jth components of x

t + k j t

fork i

must also be linearly dependent Note

that in this case equivalent but seemingly different structures can arise if the order of

the components withinx

t

is changed

OUT= Data Set

The forecasts are contained in the output data set specified by the OUT= option on the

PROC STATESPACE statement The OUT= data set contains the following variables

the BY variables

the ID variable

the VAR statement variables These variables contain the actual values from

the input data set

FORi numeric variables containing the forecasts The variable FORi contains

the forecasts for the ith variable in the VAR statement list Forecasts are one-

step-ahead predictions until the end of the data or until the observation specified

by the BACK= option

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Chapter 18 Details

RESi numeric variables containing the residual for the forecast of the ith vari-

able in the VAR statement list For forecast observations the actual values are

missing and the RESi variables contain missing values

STDi numeric variables containing the standard deviation for the forecast of

the ith variable in the VAR statement list The values of the STDi variables can

be used to construct univariate confidence limits for the corresponding fore-casts However such confidence limits do not take into account the covariance

of the forecasts

OUTAR= Data Set

The OUTAR= data set contains the estimates of the preliminary autoregressive mod-

els The OUTAR= data set contains the following variables

ORDER a numeric variable containing the order p of the autoregressive model

that the observation represents

AIC a numeric variable containing the value of the information criterionA I C

p

SIGFl numeric variables containing the estimate of the innovation covariance

matrices for the forward autoregressive models The variable SIGFl contains

the lth column of b

p

in the observations with ORDER= p

SIGBl numeric variables containing the estimate of the innovation covariance

matrices for the backward autoregressive models The variable SIGBl contains

the lth column of b

p

in the observations with ORDER= p

FORk ndashl numeric variables containing the estimates of the autoregressive pa-

rameter matrices for the forward models The variable FORk ndashl contains the

lth column of the lag k autoregressive parameter matrix b

p

k

in the observations

with ORDER= p

BACk ndashl numeric variables containing the estimates of the autoregressive pa-

rameter matrices for the backward models The variable BACk ndashl contains the

lth column of the lag k autoregressive parameter matrix b

p

k

in the observations

with ORDER= p

The estimates for the order p autoregressive model can be selected as those observa-

tions with ORDER= p Within these observations the k lth element of

p

i

is given by

the value of the FORindashl variable in the k th observation The k lth element of

p

i

is

given by the value of BACindashl variable in the k th observation The k lth element of

p

is given by SIGFl in the k th observation The k lth element of

p

is given by SIGBlin the k th observation

Table 181 shows an example of the OUTAR= data set with ARMAX=3 andx

t

of

dimension 2 In Table 181 i j

indicate the ijth element of the matrix

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Part 2 General Information

Table 181 Values in the OUTAR= Data Set

Obs ORDER AIC SIGF1 SIGF2 SIGB1 SIGB2 FOR1ndash 1 FOR1ndash 2 FOR2ndash 1 FOR2ndash 2 FOR3ndash 1

1 0 AIC0

0 1 1

0 1 2

0 1 1

0 1 2

2 0 AIC0

0 2 1

0 2 2

0 2 1

0 2 2

3 1 AIC1

1 1 1

1 1 2

1 1 1

1 1 2

1

1

1 1

1

1

1 2

4 1 AIC1

1 2 1

1 1 2

1 2 1

1 1 2

1

1

2 1

1

1

2 2

5 2 AIC2

2 1 1

2 1 2

2 1 1

2 1 2

2

1

1 1

2

1

1 2

2

2

1 1

2

2

1 2

6 2 AIC2

2 2 1

2 1 2

2 2 1

2 1 2

2

1

2 1

2

1

2 2

2

2

2 1

2

2

2 2

7 3 AIC3

3 1 1

3 1 2

3 1 1

3 1 2

3

1

1 1

3

1

1 2

3

2

1 1

3

2

1 2

3

3

1 1

8 3 AIC3

3 2 1

3 1 2

3 2 1

3 1 2

3

1

2 1

3

1

2 2

3

2

2 1

3

2

2 2

3

3

2 1

Obs FOR3ndash 2 BACK1ndash 1 BACK1ndash 2 BACK2ndash 1 BACK2ndash 2 BACK3ndash 1 BACK3ndash 2

1

2

3

1

1

1 1

1

1

1 2

4

1

1

2 1

1

1

2 2

5

2

1

1 1

2

1

1 2

2

2

1 1

2

2

1 2

6

2

1

2 1

2

1

2 2

2

2

2 1

2

2

2 2

7

3

3

1 2

3

1

1 1

3

1

1 2

3

2

1 1

3

2

1 2

3

3

1 1

3

3

1 2

8

3

3

2 2

3

1

2 1

3

1

2 2

3

2

2 1

3

2

2 2

3

3

2 1

3

3

2 2

The estimated autoregressive parameters can be used in the IML procedure to obtain

autoregressive estimates of the spectral density function or forecasts based on the

autoregressive models

OUTMODEL= Data Set

The OUTMODEL= data set contains the estimates of theF

andG

matrices and their

standard errors the names of the components of the state vector and the estimates of

the innovation covariance matrix The variables contained in the OUTMODEL= data

set are as follows

the BY variables

STATEVEC a character variable containing the name of the component of the

state vector corresponding to the observation The STATEVEC variable has

the value STD for standard deviations observations which contain the standarderrors for the estimates given in the preceding observation

Fndash j numeric variables containing the columns of theF

matrix The variable

Fndash j contains the jth column of F

The number of Fndash j variables is equal to

the value of the DIMMAX= option If the model is of smaller dimension the

extraneous variables are set to missing

Gndash j numeric variables containing the columns of theG

matrix The variable

Gndash j contains the jth column of G

The number of Gndash j variables is equal to r

the dimension of x

t

given by the number of variables in the VAR statement

SIGndash j numeric variables containing the columns of the innovation covariance

matrix The variable SIGndash j contains the jth column of

e e

There are r vari-ables SIGndash j

Table 182 shows an example of the OUTMODEL= data set withx

t

= x

t

y

t

0

z

t

= x

t

y

t

x

t + 1 j t

0 and DIMMAX=4 In Table 182F

i j

andG

i j

are the ijth

elements of F

andG

respectively Note that all elements for Fndash4 are missing because

F

is a3 3

matrix

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Chapter 18 Details

Table 182 Value in the OUTMODEL= Data Set

Obs STATEVEC Fndash 1 Fndash 2 Fndash 3 Fndash 4 Gndash 1 Gndash 2 SIGndash 1 SIGndash 2

1 X(TT) 0 0 1 1 0

1 1

1 2

2 STD

3 Y(TT) F2 1

F2 2

F2 3

0 1

2 1

2 2

4 STD std F2 1

std F2 2

std F2 3

5 X(T+1T) F3 1

F3 2

F3 3

G3 1

G3 2

6 STD std F3 1

std F3 2

std F3 3

std G3 1

std G3 2

Printed Output

The printed output produced by the STATESPACE procedure is described in the fol-

lowing

1 descriptive statistics which include the number of observations used the

names of the variables their means and standard deviations (Std) and the dif-

ferencing operations used

2 the Akaike information criteria for the sequence of preliminary autoregressive

models

3 if the PRINTOUT=LONG option is specified the sample autocovariance ma-

trices of the input series at various lags

4 if the PRINTOUT=LONG option is specified the sample autocorrelation ma-

trices of the input series

5 a schematic representation of the autocorrelation matrices showing the signif-

icant autocorrelations

6 if the PRINTOUT=LONG option is specified the partial autoregressive ma-

trices (These are

p

p

as described in Preliminary Autoregressive Modelsearlier in this chapter)

7 a schematic representation of the partial autocorrelation matrices showing the

significant partial autocorrelations

8 the Yule-Walker estimates of the autoregressive parameters for the autoregres-

sive model with the minimum AIC

9 if the PRINTOUT=LONG option is specified the autocovariance matrices of

the residuals of the minimum AIC model This is the sequence of estimated

innovation variance matrices for the solutions of the Yule-Walker equations

10 if the PRINTOUT=LONG option is specified the autocorrelation matrices of the residuals of the minimum AIC model

11 If the CANCORR option is specified the canonical correlations analysis for

each potential state vector considered in the state vector selection process

This includes the potential state vector the canonical correlations the informa-

tion criterion for the smallest canonical correlation Bartlettrsquos

2 statistic (ldquoChi

Squarerdquo) for the smallest canonical correlation and the degrees of freedom of

Bartlettrsquos

2

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Part 2 General Information

12 the components of the chosen state vector

13 the preliminary estimate of the transition matrixF

the input matrixG

and

the variance matrix for the innovations

e e

14 if the ITPRINT option is specified the iteration history of the likelihood max-

imization For each iteration this shows the iteration number the number of step halvings the determinant of the innovation variance matrix the damping

factor Lambda and the values of the parameters

15 the state vector printed again to aid interpretation of the following listing of F

andG

16 the final estimate of the transition matrixF

17 the final estimate of the input matrixG

18 the final estimate of the variance matrix for the innovations

e e

19 a table listing the estimates of the free parameters inF

andG

and their standarderrors and t statistics

20 if the COVB option is specified the covariance matrix of the parameter esti-

mates

21 if the COVB option is specified the correlation matrix of the parameter esti-

mates

22 if the PRINT option is specified the forecasts and their standard errors

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Chapter 18 Details

ODS Table Names

PROC STATESPACE assigns a name to each table it creates You can use these

names to reference the table when using the Output Delivery System (ODS) to select

tables and create output data sets These names are listed in the following table For

more information on ODS see Chapter 6 ldquoUsing the Output Delivery Systemrdquo

Table 183 ODS Tables Produced in PROC STATESPACE

ODS Table Name Description Option

NObs Number of observations default

Summary Simple summary statistics table default

InfoCriterion Information criterion table default

CovLags Covariance Matrices of Input Series PRINTOUT=LONG

CorrLags Correlation Matrices of Input Series PRINTOUT=LONG

PartialAR Partial Autoregressive Matrices PRINTOUT=LONG

YWEstimates Yule-Walker Estimates for Minimum AIC default

CovResiduals Covariance of Residuals PRINTOUT=LONGCorrResiduals Residual Correlations from AR Models PRINTOUT=LONG

StateVector State vector table default

CorrGraph Schematic Representation of Correlations default

TransitionMatrix Transition Matrix default

InputMatrix Input Matrix default

VarInnov Variance Matrix for the Innovation default

CovB Covariance of Parameter Estimates COVB

CorrB Correlation of Parameter Estimates COVB

CanCorr Canonical Correlation Analysis CANCORR

IterHistory Iterative Fitting table ITPRINT

ParameterEstimates Parameter Estimates Table default

Forecasts Forecasts Table PRINT

ConvergenceStatus Convergence Status Table default

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Part 2 General Information

Example

Example 181 Series J from Box and Jenkins

This example analyzes the gas furnace data (series J) from Box and Jenkins (The

data are not shown Refer to Box and Jenkins (1976) for the data)

First a model is selected and fit automatically using the following statements

title1 rsquoGas Furnace Datarsquo

title2 rsquoBox amp Jenkins Series Jrsquo

title3 rsquoAutomatically Selected Modelrsquo

proc statespace data=seriesj cancorr

var x y

run

The results for the automatically selected model are shown in Output 1811

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Chapter 18 Example

Output 1811 Results for Automatically Selected Model

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Number of Observations 296

Standard

Variable Mean Error

x -005683 1072766

y 5350912 3202121

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Information Criterion for Autoregressive Models

Lag=0 Lag=1 Lag=2 Lag=3 Lag=4 Lag=5 Lag=6 Lag=7 Lag=8

6513862 -103357 -163296 -164512 -165152 -164891 -164934 -164315 -163856

Information

Criterion for

Autoregressive

Models

Lag=9 Lag=10

-16348 -163359

Schematic Representation of Correlations

NameLag 0 1 2 3 4 5 6 7 8 9 10

x +- +- +- +- +- +- +- +- +- +- +-

y -+ -+ -+ -+ -+ -+ -+ -+ -+ -+ -+

+ is gt 2std error - is lt -2std error is between

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Part 2 General Information

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Schematic Representation of Partial Autocorrelations

NameLag 1 2 3 4 5 6 7 8 9 10

x + - + -

y -+ -- - + +

+ is gt 2std error - is lt -2std error is between

Yule-Walker Estimates for Minimum AIC

------Lag=1------ ------Lag=2------ ------Lag=3------ ------Lag=4------

x y x y x y x y

x 1925887 -000124 -120166 0004224 0116918 -000867 0104236 0003268

y 0050496 1299793 -002046 -03277 -071182 -025701 0195411 0133417

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Canonical Correlations Analysis

Information Chi

x(TT) y(TT) x(T+1T) Criterion Square DF

1 1 0804883 2929228 3047481 8

Information Chix(TT) y(TT) x(T+1T) y(T+1T) Criterion Square DF

1 1 0906681 0607529 1223358 1347237 7

Information Chi

x(TT) y(TT) x (T+1T) y (T+1T) x (T+2T) Criterion Square DF

1 1 0909434 0610278 0186274 -154701 1034705 6

Information Chi

x(TT) y(TT) x (T+1T) y (T+1T) y (T+2T) Criterion Square DF

1 1 091014 0618937 0206823 0940392 1280924 6

Information Chi

x(TT) y(TT) x(T+1T) y(T+1T) y(T+2T) y(T+3T) Criterion Square DF

1 1 0912963 0628785 0226598 0083258 -794103 2041584 5

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Chapter 18 Example

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Selected Statespace Form and Preliminary Estimates

State Vector

x(TT) y(TT) x(T+1T) y(T+1T) y(T+2T)

Estimate of Transition Matrix

0 0 1 0 0

0 0 0 1 0

-084718 0026794 1711715 -005019 0

0 0 0 0 1

-019785 0334274 -018174 -123557 1787475

Input Matrix for Innovation

1 0

0 11925887 -000124

0050496 1299793

0142421 1361696

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Selected Statespace Form and Preliminary Estimates

Variance Matrix for Innovation

0035274 -000734

-000734 0097569

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Part 2 General Information

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Selected Statespace Form and Fitted Model

State Vector

x(TT) y(TT) x(T+1T) y(T+1T) y(T+2T)

Estimate of Transition Matrix

0 0 1 0 0

0 0 0 1 0

-086192 0030609 1724235 -005483 0

0 0 0 0 1

-034839 0292124 -009435 -109823 1671418

Input Matrix for Innovation

1 0

0 1192442 -000416

0015621 1258495

008058 1353204

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Selected Statespace Form and Fitted Model

Variance Matrix for Innovation

0035579 -000728

-000728 0095577

Parameter Estimates

Standard

Parameter Estimate Error t Value

F(31) -086192 0072961 -1181

F(32) 0030609 0026167 117

F(33) 1724235 0061599 2799

F(34) -005483 0030169 -182

F(51) -034839 0135253 -258

F(52) 0292124 0046299 631

F(53) -009435 0096527 -098F(54) -109823 0109525 -1003

F(55) 1671418 0083737 1996

G(31) 1924420 0058162 3309

G(32) -000416 0035255 -012

G(41) 0015621 0095771 016

G(42) 1258495 0055742 2258

G(51) 0080580 0151622 053

G(52) 1353204 0091388 1481

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Chapter 18 Example

The two series are believed to have a transfer function relation with the gas rate

(variable X) as the input and the CO2

concentration (variable Y) as the output Since

the parameter estimates shown in Output 1811 support this kind of model the model

is reestimated with the feedback parameters restricted to 0 The following statements

fit the transfer function (no feedback) model

title3 rsquoTransfer Function Modelrsquo

proc statespace data=seriesj printout=none

var x y

restrict f(32)=0 f(34)=0

g(32)=0 g(41)=0 g(51)=0

run

The last 2 pages of the output are shown in Output 1812

Output 1812 STATESPACE Output for Transfer Function Model

Gas Furnace Data

Box amp Jenkins Series J

Transfer Function Model

The STATESPACE Procedure

Selected Statespace Form and Fitted Model

State Vector

x(TT) y(TT) x(T+1T) y(T+1T) y(T+2T)

Estimate of Transition Matrix

0 0 1 0 0

0 0 0 1 0

-068882 0 1598717 0 0

0 0 0 0 1

-035944 0284179 -00963 -107313 1650047

Input Matrix for Innovation

1 0

0 1

1923446 0

0 1260856

0 1346332

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Part 2 General Information

Gas Furnace Data

Box amp Jenkins Series J

Transfer Function Model

The STATESPACE Procedure

Selected Statespace Form and Fitted Model

Variance Matrix for Innovation

0036995 -00072

-00072 0095712

Parameter Estimates

Standard

Parameter Estimate Error t Value

F(31) -068882 0050549 -1363

F(33) 1598717 0050924 3139

F(51) -035944 0229044 -157

F(52) 0284179 0096944 293

F(53) -009630 0140876 -068

F(54) -107313 0250385 -429

F(55) 1650047 0188533 875G(31) 1923446 0056328 3415

G(42) 1260856 0056464 2233

G(52) 1346332 0091086 1478

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Chapter 18 References

References

Akaike H (1974) Markovian Representation of Stochastic Processes and Its Ap-

plication to the Analysis of Autoregressive Moving Average Processes Annals

of the Institute of Statistical Mathematics 26 363-387

Akaike H (1976) Canonical Correlations Analysis of Time Series and the Use of

an Information Criterion in Advances and Case Studies in System Identification

eds R Mehra and DG Lainiotis New York Academic Press

Anderson TW (1971) The Statistical Analysis of Time Series New York John

Wiley amp Sons

Ansley CF and Newbold P (1979) Multivariate Partial Autocorrelations Pro-

ceedings of the Business and Economic Statistics Section American Statistical

Association 349-353

Box GEP and Jenkins G (1976) Time Series Analysis Forecasting and Control

San Francisco Holden-Day

Brockwell PJ and Davis RA (1991) Time Series Theory and Methods 2nd Edi-

tion Springer-Verlag

Hannan EJ (1970) Multiple Time Series New York John Wiley amp Sons

Hannan EJ (1976) The Identification and Parameterization of ARMAX and State

Space Forms Econometrica 44 713-722

Harvey AC (1981a) The Econometric Analysis of Time Series New York John

Wiley amp Sons

Harvey AC (1981b) Time Series Models New York John Wiley amp Sons

Jones RH (1974) Identification and Autoregressive Spectrum Estimation IEEE Transactions on Automatic Control AC-19 894-897

Pham-Dinh-Tuan (1978) On the Fitting of Multivariate Processes of the

Autoregressive-Moving Average Type Biometrika 65 99-107

Priestley MB (1980) System Identification Kalman Filtering and Stochastic Con-

trol in Directions in Time Series eds DR Brillinger and GC Tiao Institute

of Mathematical Statistics

Whittle P (1963) On the Fitting of Multivariate Autoregressions and the Approxi-

mate Canonical Factorization of a Spectral Density Matrix Biometrika 50 129-

134

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The correct bibliographic citation for this manual is as follows SAS Institute Inc SAS

ET S U serrsquos Gui de Version 8 Cary NC SAS Institute Inc 1999 1546 pp

SASET S Userrsquos Guide Version 8

Copyright copy 1999 by S AS I ns t it u t e I nc Ca ry N C U S A

ISBN 1ndash58025ndash489ndash6

All r ight s res erved P r int ed in t he U n it ed St a t es of A merica N o pa rt of t his p u blica t ionma y b e reprodu ced s t ored in a ret r ieva l s ys t em or t ra ns mit t ed in a ny form or b y a nymeans electronic mechanical photocopying or otherwise without the prior writtenpermission of the publisher SAS Institute Inc

US Government Restricted Rights Notice Use du plicat ion or disclosure of the

software by the government is subject to restrictions as set forth in FAR 52227ndash19Commercial Computer Softw ar e-Restricted Rights (J une 1987)

SAS Institute Inc SAS Campus Drive Cary North Carolina 27513

1st print ing October 1999

S AS reg a nd a ll ot her SA S I ns t it u t e I nc p rodu ct or s ervice na m es a re regis t ered t ra dema rkst d k f SA S I t it t I i t h U S A d t h t i reg i di t U SA

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Chapter 18 Details

Details

Missing Values

The STATESPACE procedure does not support missing values The procedure uses

the first contiguous group of observations with no missing values for any of the VARstatement variables Observations at the beginning of the data set with missing values

for any VAR statement variable are not used or included in the output data set

Stationarity and Differencing

The state space model used by the STATESPACE procedure assumes that the time

series are stationary Hence the data should be checked for stationarity One way

to check for stationarity is to plot the series A graph of series over time can show a

time trend or variability changes

You can also check stationarity by using the sample autocorrelation functions dis-

played by the ARIMA procedure The autocorrelation functions of nonstationaryseries tend to decay slowly See Chapter 7 ldquoThe ARIMA Procedurerdquo for more in-

formation

Another alternative is to use the STATIONARITY= option on the IDENTIFY state-

ment in PROC ARIMA to apply Dickey-Fuller tests for unit roots in the time series

See Chapter 7 ldquoThe ARIMA Procedurerdquo for more information on Dickey-Fuller unit

root tests

The most popular way to transform a nonstationary series to stationarity is by dif-

ferencing Differencing of the time series is specified in the VAR statement For

example to take a simple first difference of the series X use this statement

var x(1)

In this example the change in X from one period to the next is analyzed When the se-

ries has a seasonal pattern differencing at a period equal to the length of the seasonal

cycle may be desirable For example suppose the variable X is measured quarterly

and shows a seasonal cycle over the year You can use the following statement to

analyze the series of changes from the same quarter in the previous year

var x(4)

To difference twice add another differencing period to the list For ex-

ample the following statement analyzes the series of second differences

X

t

X

t 1

X

t 1

X

t 2

= X

t

2 X

t 1

+ X

t 2

var x(11)

The following statement analyzes the seasonal second difference series

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Part 2 General Information

var x(14)

The series modeled is the 1-period difference of the 4-period difference

X

t

X

t 4

X

t 1

X

t 5

= X

t

X

t 1

X

t 4

+ X

t 5

Another way to obtain stationary series is to use a regression on time to de-trend the

data If the time series has a deterministic linear trend regressing the series on time

produces residuals that should be stationary The following statements write residuals

of X and Y to the variable RX and RY in the output data set DETREND

data a

set a

t=_n_

run

proc reg data=a

model x y = t

output out=detrend r=rx ry

run

You then use PROC STATESPACE to forecast the de-trended series RX and RY A

disadvantage of this method is that you need to add the trend back to the forecast

series in an additional step A more serious disadvantage of the de-trending method

is that it assumes a deterministic trend In practice most time series appear to have a

stochastic rather than a deterministic trend Differencing is a more flexible and often

more appropriate method

There are several other methods to handle nonstationary time series For more infor-

mation and examples refer to Brockwell and Davis (1991)

Preliminary Autoregressive Models

After computing the sample autocovariance matrices PROC STATESPACE fits a

sequence of vector autoregressive models These preliminary autoregressive models

are used to estimate the autoregressive order of the process and limit the order of the

autocovariances considered in the state vector selection process

Yule-Walker Equations for Forward and Backward Models

Unlike a univariate autoregressive model a multivariate autoregressive model has

different forms depending on whether the present observation is being predicted from

the past observations or from the future observations

Letx

t

be the r -component stationary time series given by the VAR statement after

differencing and subtracting the vector of sample means (If the NOCENTER option

is specified the mean is not subtracted) Let n be the number of observations of x

t

from the input data set

Lete

t

be a vector white noise sequence with mean vector 0 and variance matrix

p

and letn

t

be a vector white noise sequence with mean vector 0 and variance matrix

p

Let p be the order of the vector autoregressive model forx

t

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Chapter 18 Details

The forward autoregressive form based on the past observations is written as follows

x

t

=

p

X

i = 1

p

i

x

t i

+ e

t

The backward autoregressive form based on the future observations is written as fol-

lows

x

t

=

p

X

i = 1

p

i

x

t + i

+ n

t

LettingE

denote the expected value operator the autocovariance sequence for thex

t

series

i

is

i

= E x

t

x

0

t i

The Yule-Walker equations for the autoregressive model that matches the first p ele-

ments of the autocovariance sequence are

2

6

6

4

0

1

p 1

0

1

0

p 2

0

p 1

0

p 2

0

3

7

7

5

2

6

6

4

p

1

p

2

p

p

3

7

7

5

=

2

6

6

4

1

2

p

3

7

7

5

and

2

6

6

6

4

0

0

1

0

p 1

1

0

0

p 2

p 1

p 2

0

3

7

7

7

5

2

6

6

4

p

1

p

2

p

p

3

7

7

5

=

2

6

6

4

0

1

0

2

0

p

3

7

7

5

Here

p

i

are the coefficient matrices for the past observation form of the vector au-toregressive model and

p

i

are the coefficient matrices for the future observation

form More information on the Yule-Walker equations in the multivariate setting can

be found in Whittle (1963) and Ansley and Newbold (1979)

The innovation variance matrices for the two forms can be written as follows

p

=

0

p

X

i = 1

p

i

0

i

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Part 2 General Information

p

=

0

p

X

i = 1

p

i

i

The autoregressive models are fit to the data using the preceding Yule-Walker equa-

tions with

i

replaced by the sample covariance sequenceC

i

The covariance matri-

ces are calculated as

C

i

=

1

N 1

N

X

t = i + 1

x

t

x

0

t i

Let b

p

b

p

b

p

and b

p

represent the Yule-Walker estimates of

p

p

p

and

p

respectively These matrices are written to an output data set when the OUTAR=

option is specified

When the PRINTOUT=LONG option is specified the sequence of matrices b

p

and

the corresponding correlation matrices are printed The sequence of matricesb

p isused to compute Akaike information criteria for selection of the autoregressive order

of the process

Akaike Information Criterion

The Akaike information criterion or AIC is defined as -2( maximum of log likeli-

hood )+2(number of parameters) Since the vector autoregressive models are esti-

mates from the Yule-Walker equations not by maximum likelihood the exact likeli-

hood values are not available for computing the AIC However for the vector autore-

gressive model the maximum of the log likelihood can be approximated as

l n L

n

2

l n j

b

p

j

Thus the AIC for the order p model is computed as

A I C

p

= n l n j

b

p

j + 2 p r

2

You can use the printed AIC array to compute a likelihood ratio test of the autoregres-

sive order The log-likelihood ratio test statistic for testing the order p model against

the order p 1

model is

n l n j

b

p

j + n l n j

b

p 1

j

This quantity is asymptotically distributed as a

2 withr

2 degrees of freedom if the

series is autoregressive of order p 1

It can be computed from the AIC array as

A I C

p 1

A I C

p

+ 2 r

2

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Chapter 18 Details

You can evaluate the significance of these test statistics with the PROBCHI function

in a SAS DATA step or with a

2 table

Determining the Autoregressive Order

Although the autoregressive models can be used for prediction their primary value is

to aid in the selection of a suitable portion of the sample covariance matrix for use in

computing canonical correlations If the multivariate time series x

t is of autoregres-sive order p then the vector of past values to lag p is considered to contain essentially

all the information relevant for prediction of future values of the time series

By default PROC STATESPACE selects the order p producing the autoregressive

model with the smallestA I C

p

If the value p for the minimumA I C

p

is less than the

value of the PASTMIN= option then p is set to the PASTMIN= value Alternatively

you can use the ARMAX= and PASTMIN= options to force PROC STATESPACE to

use an order you select

Significance Limits for Partial Autocorrelations

The STATESPACE procedure prints a schematic representation of the partial autocor-

relation matrices indicating which partial autocorrelations are significantly greater orsignificantly less than 0 Figure 1810 shows an example of this table

The STATESPACE Procedure

Schematic Representation of Partial Autocorrelations

NameLag 1 2 3 4 5 6 7 8 9 10

x ++ +

y ++

+ is gt 2std error - is lt -2std error is between

Figure 1810 Significant Partial Autocorrelations

The partial autocorrelations are from the sample partial autoregressive matrices b

p

p

The standard errors used for the significance limits of the partial autocorrelations are

computed from the sequence of matrices

p

and

p

Under the assumption that the observed series arises from an autoregressive process

of order p 1

the pth sample partial autoregressive matrix b

p

p

has an asymptotic

variance matrix 1

n

1

p

p

The significance limits for b

p

p

used in the schematic plot of the sample partial autore-

gressive sequence are derived by replacing

p

and

p

with their sample estimators

to produce the variance estimate as follows

d

V a r

b

p

p

=

1

n r p

b

1

p

b

p

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Part 2 General Information

Canonical Correlation Analysis

Given the order p letp

t

be the vector of current and past values relevant to prediction

of x

t + 1

p

t

= x

0

t

x

0

t 1

x

0

t p

0

Letf

t

be the vector of current and future values

f

t

= x

0

t

x

0

t + 1

x

0

t + p

0

In the canonical correlation analysis consider submatrices of the sample covariance

matrix of p

t

andf

t

This covariance matrixV

has a block Hankel form

V =

2

6

6

6

4

C

0

C

0

1

C

0

2

C

0

p

C

0

1

C

0

2

C

0

3

C

0

p + 1

C

0

p

C

0

p + 1

C

0

p + 2

C

0

2 p

3

7

7

7

5

State Vector Selection Process

The canonical correlation analysis forms a sequence of potential state vectorsz

j

t

Examine a sequencef

j

t

of subvectors of f

t

and form the submatrixV

j consisting

of the rows and columns of V

corresponding to the components of f

j

t

and compute

its canonical correlations

The smallest canonical correlation of V

j is then used in the selection of the com-

ponents of the state vector The selection process is described in the following For

more details about this process refer to Akaike (1976)

In the following discussion the notationx

t + k j t

denotes the wide sense conditional

expectation (best linear predictor) of x

t + k

given allx

s

with s less than or equal to t

In the notationx

i t + 1

the first subscript denotes the ith component of x

t + 1

The initial state vectorz

1

t

is set tox

t

The sequencef

j

t

is initialized by setting

f

1

t

= z

1

0

t

x

1 t + 1 j t

0

= x

0

t

x

1 t + 1 j t

0

That is start by considering whether to addx

1 t + 1 j t

to the initial state vectorz

1

t

The procedure forms the submatrixV

1 corresponding tof

1

t

and computes its canon-

ical correlations Denote the smallest canonical correlation of V

1 as

m i n

If

m i n

is

significantly greater than 0x

1 t + 1 j t

is added to the state vector

If the smallest canonical correlation of V

1 is not significantly greater than 0 then a

linear combination of f

1

t

is uncorrelated with the pastp

t

Assuming that the determi-

nant of C

0

is not 0 (that is no input series is a constant) you can take the coefficient

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Chapter 18 Details

of x

1 t + 1 j t

in this linear combination to be 1 Denote the coefficients of z

1

t

in this

linear combination as

This gives the relationship

x

1 t + 1 j t

=

0

x

t

Therefore the current state vector already contains all the past information useful forpredicting

x

1 t + 1

and any greater leads of x

1 t

The variablex

1 t + 1 j t

is not added to

the state vector nor are any termsx

1 t + k j t

considered as possible components of the

state vector The variablex

1

is no longer active for state vector selection

The process described forx

1 t + 1 j t

is repeated for the remaining elements of f

t

The

next candidate for inclusion in the state vector is the next component of f

t

corre-

sponding to an active variable Components of f

t

corresponding to inactive variables

that produced a zero

m i n

in a previous step are skipped

Denote the next candidate asx

l t + k j t

The vectorf

j

t

is formed from the current state

vector andx

l t + k j t

as follows

f

j

t

= z

j

0

t

x

l t + k j t

0

The matrixV

j is formed fromf

j

t

and its canonical correlations are computed The

smallest canonical correlation of V

j is judged to be either greater than or equal to 0

If it is judged to be greater than 0x

l t + k j t

is added to the state vector If it is judged

to be 0 then a linear combination of f

j

t

is uncorrelated with thep

t

and the variable

x

l

is now inactive

The state vector selection process continues until no active variables remain

Testing Significance of Canonical Correlations For each step in the canonical correlation sequence the significance of the small-

est canonical correlation

m i n

is judged by an information criterion from Akaike

(1976) This information criterion is

n l n 1

2

m i n

r p + 1 q + 1

where q is the dimension of f

j

t

at the current step r is the order of the state vector p

is the order of the vector autoregressive process and

is the value of the SIGCORR=

option The default is SIGCORR=2 If this information criterion is less than or equal

to 0

m i n

is taken to be 0 otherwise it is taken to be significantly greater than 0

(Do not confuse this information criterion with the AIC)

Variables inx

t + p j t

are not added in the model even with positive information crite-

rion because of the singularity of V

You can force the consideration of more candi-

date state variables by increasing the size of theV

matrix by specifying a PASTMIN=

option value larger than p

Printing the Canonical Correlations

To print the details of the canonical correlation analysis process specify the CAN-

CORR option in the PROC STATESPACE statement The CANCORR option prints

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Part 2 General Information

the candidate state vectors the canonical correlations and the information criteria for

testing the significance of the smallest canonical correlation

Bartlettrsquos

2 and its degrees of freedom are also printed when the CANCORR option

is specified The formula used for Bartlettrsquos

2 is

2

= n 5 r p + 1 q + 1 l n 1

2

m i n

withr p + 1 q + 1

degrees of freedom

Figure 1811 shows the output of the CANCORR option for the introductory example

shown in the Getting Started section of this chapter

The STATESPACE Procedure

Canonical Correlations Analysis

Information Chi

x(TT) y(TT) x(T+1T) Criterion Square DF

1 1 0237045 3566167 114505 4

Information Chi

x(TT) y(TT) x(T+1T) y(T+1T) Criterion Square DF

1 1 0238244 0056565 -535906 0636134 3

Information Chi

x(TT) y(TT) x(T+1T) x(T+2T) Criterion Square DF

1 1 0237602 0087493 -446312 1525353 3

Figure 1811 Canonical Correlations Analysis

New variables are added to the state vector if the information criteria are positive Inthis example

y

t + 1 j t

andx

t + 2 j t

are not added to the state space vector because the

information criteria for these models are negative

If the information criterion is nearly 0 then you may want to investigate models

that arise if the opposite decision is made regarding

m i n

This investigation can be

accomplished by using a FORM statement to specify part or all of the state vector

Preliminary Estimates of F

When a candidate variablex

l t + k j t

yields a zero

m i n

and is not added to the state

vector a linear combination of f

j

t

is uncorrelated with thep

t

Because of the method

used to construct thef

j

t

sequence the coefficient of x

l t + k j t

inl

can be taken as 1

Denote the coefficients of z

j

t

in this linear combination as l

This gives the relationship

x

l t + k j t

= l

0

z

j

t

The vectorl

is used as a preliminary estimate of the first r columns of the row of the

transition matrixF

corresponding tox

l t + k 1 j t

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Chapter 18 Details

Parameter Estimation

The model isz

t + 1

= F z

t

+ G e

t + 1

wheree

t

is a sequence of independent multivari-

ate normal innovations with mean vector 0 and variance

e e

The observed sequence

x

t

composes the first r components of z

t

and thusx

t

= H z

t

where H is ther s

matrix I

r

0

LetE

be ther n

matrix of innovations

E = e

1

e

n

If the number of observations n is reasonably large the log likelihood L can be

approximated up to an additive constant as follows

L =

n

2

l n j

e e

j

1

2

t r a c e

1

e e

E E

0

The elements of

e e

are taken as free parameters and are estimated as follows

S

0

=

1

n

E E

0

Replacing

e e

byS

0

in the likelihood equation the log likelihood up to an additive

constant is

L =

n

2

l n j S

0

j

Letting B be the backshift operator the formal relation betweenx

t

ande

t

is

x

t

= H I B F

1

G e

t

e

t

= H I B F

1

G

1

x

t

=

1

X

i = 0

i

x

t i

LettingC

i

be the ith lagged sample covariance of x

t

and neglecting end effects the

matrixS

0

is

S

0

=

1

X

i j = 0

i

C

i + j

0

j

For the computation of S

0

the infinite sum is truncated at the value of the KLAG=

option The value of the KLAG= option should be large enough that the sequence

i

is approximately 0 beyond that point

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Part 2 General Information

Let

be the vector of free parameters in theF

andG

matrices The derivative of the

log likelihood with respect to the parameter

is

L

=

n

2

t r a c e

S

1

0

S

0

The second derivative is

2

L

0

=

n

2

t r a c e

S

1

0

S

0

0

S

1

0

S

0

t r a c e

S

1

0

2

S

0

0

Near the maximum the first term is unimportant and the second term can be approx-

imated to give the following second derivative approximation

2

L

0

=

n t r a c e

S

1

0

E

E

0

0

The first derivative matrix and this second derivative matrix approximation are com-

puted from the sample covariance matrixC

0

and the truncated sequence

i

The

approximate likelihood function is maximized by a modified Newton-Raphson algo-

rithm employing these derivative matrices

The matrixS

0

is used as the estimate of the innovation covariance matrix

e e

The

negative of the inverse of the second derivative matrix at the maximum is used as an

approximate covariance matrix for the parameter estimates The standard errors of

the parameter estimates printed in the parameter estimates tables are taken from the

diagonal of this covariance matrix The parameter covariance matrix is printed when

the COVB option is specified

If the data are nearly nonstationary a better estimate of

e e

and the other parameters

can sometimes be obtained by specifying the RESIDEST option The RESIDEST

option estimates the parameters using conditional least squares instead of maximum

likelihood

The residuals are computed using the state space equation and the sample mean val-

ues of the variables in the model as start-up values The estimate of S

0

is then com-

puted using the residuals from the ith observation on where i is the maximum number

of times any variable occurs in the state vector A multivariate Gauss-Marquardt al-

gorithm is used to minimizej S

0

j

Refer to Harvey (1981a) for a further description

of this method

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Chapter 18 Details

Forecasting

Given estimates of F

G

and

e e

forecasts of x

t

are computed from the conditional

expectation of z

t

In forecasting the parameters F G and

e e

are replaced with the estimates or by val-

ues specified in the RESTRICT statement One-step-ahead forecasting is performedfor the observation

x

t

wheret n b

Heren

is the number of observations and

b is the value of the BACK= option For the observationx

t

wheret n b

m-

step-ahead forecasting is performed form = t n + b

The forecasts are generated

recursively with the initial conditionz

0

= 0

The m-step-ahead forecast of z

t + m

isz

t + m j t

wherez

t + m j t

denotes the conditional

expectation of z

t + m

given the information available at time t The m-step-ahead

forecast of x

t + m

isx

t + m j t

= H z

t + m j t

where the matrixH = I

r

0

Let

i

= F

i

G

Note that the lasts r

elements of z

t

consist of the elements of x

u j t

foru t

The state vector z

t + m

can be represented as

z

t + m

= F

m

z

t

+

m 1

X

i = 0

i

e

t + m i

Sincee

t + i j t

= 0

fori 0

the m-step-ahead forecastz

t + m j t

is

z

t + m j t

= F

m

z

t

= F z

t + m 1 j t

Therefore the m-step-ahead forecast of x

t + m

is

x

t + m j t

= H z

t + m j t

The m-step-ahead forecast error is

z

t + m

z

t + m j t

=

m 1

X

i = 0

i

e

t + m i

The variance of the m-step-ahead forecast error is

V

z m

=

m 1

X

i = 0

i

e e

0

i

LettingV

z 0

= 0

the variance of the m-step-ahead forecast error of z

t + m

V

z m

can

be computed recursively as follows

V

z m

= V

z m 1

+

m 1

e e

0

m 1

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Part 2 General Information

The variance of the m-step-ahead forecast error of x

t + m

is ther r

left upper sub-

matrix of V

z m

that is

V

x m

= H V

z m

H

0

Unless the NOCENTER option is specified the sample mean vector is added to the

forecast When differencing is specified the forecasts xt + m j t

plus the sample mean

vector are integrated back to produce forecasts for the original series

Lety

t

be the original series specified by the VAR statement with some 0 values

appended corresponding to the unobserved past observations Let B be the backshift

operator and let B

be thes s

matrix polynomial in the backshift operator

corresponding to the differencing specified by the VAR statement The off-diagonal

elements of

i

are 0 Note that

0

= I

s

whereI

s

is thes s

identity matrix Then

z

t

= B y

t

This gives the relationship

y

t

=

1

B z

t

=

1

X

i = 0

i

z

t i

where

1

B =

P

1

i = 0

i

B

i and

0

= I

s

The m-step-ahead forecast of y

t + m

is

y

t + m j t

=

m 1

X

i = 0

i

z

t + m i j t

+

1

X

i = m

i

z

t + m i

The m-step-ahead forecast error of y

t + m

is

m 1

X

i = 0

i

z

t + m i

z

t + m i j t

=

m 1

X

i = 0

i

X

u = 0

u

i u

e

t + m i

LettingV

y 0

= 0

the variance of the m-step-ahead forecast error of y

t + m

V

y m

is

V

y m

=

m 1

X

i = 0

i

X

u = 0

u

i u

e e

i

X

u = 0

u

i u

0

= V

y m 1

+

m 1

X

u = 0

u

m 1 u

e e

m 1

X

u = 0

u

m 1 u

0

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Chapter 18 Details

Relation of ARMA and State Space Forms

Every state space model has an ARMA representation and conversely every ARMA

model has a state space representation This section discusses this equivalence The

following material is adapted from Akaike (1974) where there is a more complete

discussion Pham-Dinh-Tuan (1978) also contains a discussion of this material

Suppose you are given the following ARMA model

B x

t

= B e

t

or in more detail

x

t

1

x

t 1

p

x

t p

= e

t

+

1

e

t 1

+ +

q

e

t q

(1)

wheree

t

is a sequence of independent multivariate normal random vectors with mean

0 and variance matrix

e e

B is the backshift operator (B x

t

= x

t 1

) B

and

B

are matrix polynomials in B andx

t

is the observed process

If the roots of the determinantial equationj B j = 0

are outside the unit circle in

the complex plane the model can also be written as

x

t

=

1

B B e

t

=

1

X

i = 0

i

e

t i

The

i

matrices are known as the impulse response matrices and can be computed

as

1

B B

You can assume p q

since if this is not initially true you can add more terms

i

that are identically 0 without changing the model

To write this set of equations in a state space form proceed as follows Letx

t + i j t

be

the conditional expectation of x

t + i

givenx

w

forw t

The following relations hold

x

t + i j t

=

1

X

j = i

j

e

t + i j

x

t + i j t + 1

= x

t + i j t

+

i 1

e

t + 1

However from equation (1) you can derive the following relationship

x

t + p j t

=

1

x

t + p 1 j t

+ +

p

x

t

(2)

Hence wheni = p

you can substitute forx

t + p j t

in the right-hand side of equation

(2) and close the system of equations

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Part 2 General Information

This substitution results in the following model in the state space form

z

t + 1

= F z

t

+ G e

t + 1

2

6

6

6

4

x

t + 1

x

t + 2 j t + 1

x

t + p j t + 1

3

7

7

7

5

=

2

6

6

4

0 I 0 0

0 0 I 0

p

p 1

1

3

7

7

5

2

6

6

6

4

x

t

x

t + 1 j t

x

t + p 1 j t

3

7

7

7

5

+

2

6

6

4

I

1

p 1

3

7

7

5

e

t + 1

Note that the state vectorz

t

is composed of conditional expectations of x

t

and the

first r components of z

t

are equal tox

t

The state space form can be cast into an ARMA form by solving the system of dif-

ference equations for the first r components

When converting from an ARMA form to a state space form you can generate a

state vector larger than needed that is the state space model may not be a minimal

representation When going from a state space form to an ARMA form you can have

nontrivial common factors in the autoregressive and moving average operators that

yield an ARMA model larger than necessary

If the state space form used is not a minimal representation some but not all compo-

nents of x

t + i j t

may be linearly dependent This situation corresponds to

p

p 1

being of less than full rank when B

and B

have no common nontrivial

left factors In this casez

t

consists of a subset of the possible components of

x

t + i j t

i = 1 2 p 1

However once a component of x

t + i j t

(for example the

jth one) is linearly dependent on the previous conditional expectations then all sub-

sequent jth components of x

t + k j t

fork i

must also be linearly dependent Note

that in this case equivalent but seemingly different structures can arise if the order of

the components withinx

t

is changed

OUT= Data Set

The forecasts are contained in the output data set specified by the OUT= option on the

PROC STATESPACE statement The OUT= data set contains the following variables

the BY variables

the ID variable

the VAR statement variables These variables contain the actual values from

the input data set

FORi numeric variables containing the forecasts The variable FORi contains

the forecasts for the ith variable in the VAR statement list Forecasts are one-

step-ahead predictions until the end of the data or until the observation specified

by the BACK= option

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Chapter 18 Details

RESi numeric variables containing the residual for the forecast of the ith vari-

able in the VAR statement list For forecast observations the actual values are

missing and the RESi variables contain missing values

STDi numeric variables containing the standard deviation for the forecast of

the ith variable in the VAR statement list The values of the STDi variables can

be used to construct univariate confidence limits for the corresponding fore-casts However such confidence limits do not take into account the covariance

of the forecasts

OUTAR= Data Set

The OUTAR= data set contains the estimates of the preliminary autoregressive mod-

els The OUTAR= data set contains the following variables

ORDER a numeric variable containing the order p of the autoregressive model

that the observation represents

AIC a numeric variable containing the value of the information criterionA I C

p

SIGFl numeric variables containing the estimate of the innovation covariance

matrices for the forward autoregressive models The variable SIGFl contains

the lth column of b

p

in the observations with ORDER= p

SIGBl numeric variables containing the estimate of the innovation covariance

matrices for the backward autoregressive models The variable SIGBl contains

the lth column of b

p

in the observations with ORDER= p

FORk ndashl numeric variables containing the estimates of the autoregressive pa-

rameter matrices for the forward models The variable FORk ndashl contains the

lth column of the lag k autoregressive parameter matrix b

p

k

in the observations

with ORDER= p

BACk ndashl numeric variables containing the estimates of the autoregressive pa-

rameter matrices for the backward models The variable BACk ndashl contains the

lth column of the lag k autoregressive parameter matrix b

p

k

in the observations

with ORDER= p

The estimates for the order p autoregressive model can be selected as those observa-

tions with ORDER= p Within these observations the k lth element of

p

i

is given by

the value of the FORindashl variable in the k th observation The k lth element of

p

i

is

given by the value of BACindashl variable in the k th observation The k lth element of

p

is given by SIGFl in the k th observation The k lth element of

p

is given by SIGBlin the k th observation

Table 181 shows an example of the OUTAR= data set with ARMAX=3 andx

t

of

dimension 2 In Table 181 i j

indicate the ijth element of the matrix

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Part 2 General Information

Table 181 Values in the OUTAR= Data Set

Obs ORDER AIC SIGF1 SIGF2 SIGB1 SIGB2 FOR1ndash 1 FOR1ndash 2 FOR2ndash 1 FOR2ndash 2 FOR3ndash 1

1 0 AIC0

0 1 1

0 1 2

0 1 1

0 1 2

2 0 AIC0

0 2 1

0 2 2

0 2 1

0 2 2

3 1 AIC1

1 1 1

1 1 2

1 1 1

1 1 2

1

1

1 1

1

1

1 2

4 1 AIC1

1 2 1

1 1 2

1 2 1

1 1 2

1

1

2 1

1

1

2 2

5 2 AIC2

2 1 1

2 1 2

2 1 1

2 1 2

2

1

1 1

2

1

1 2

2

2

1 1

2

2

1 2

6 2 AIC2

2 2 1

2 1 2

2 2 1

2 1 2

2

1

2 1

2

1

2 2

2

2

2 1

2

2

2 2

7 3 AIC3

3 1 1

3 1 2

3 1 1

3 1 2

3

1

1 1

3

1

1 2

3

2

1 1

3

2

1 2

3

3

1 1

8 3 AIC3

3 2 1

3 1 2

3 2 1

3 1 2

3

1

2 1

3

1

2 2

3

2

2 1

3

2

2 2

3

3

2 1

Obs FOR3ndash 2 BACK1ndash 1 BACK1ndash 2 BACK2ndash 1 BACK2ndash 2 BACK3ndash 1 BACK3ndash 2

1

2

3

1

1

1 1

1

1

1 2

4

1

1

2 1

1

1

2 2

5

2

1

1 1

2

1

1 2

2

2

1 1

2

2

1 2

6

2

1

2 1

2

1

2 2

2

2

2 1

2

2

2 2

7

3

3

1 2

3

1

1 1

3

1

1 2

3

2

1 1

3

2

1 2

3

3

1 1

3

3

1 2

8

3

3

2 2

3

1

2 1

3

1

2 2

3

2

2 1

3

2

2 2

3

3

2 1

3

3

2 2

The estimated autoregressive parameters can be used in the IML procedure to obtain

autoregressive estimates of the spectral density function or forecasts based on the

autoregressive models

OUTMODEL= Data Set

The OUTMODEL= data set contains the estimates of theF

andG

matrices and their

standard errors the names of the components of the state vector and the estimates of

the innovation covariance matrix The variables contained in the OUTMODEL= data

set are as follows

the BY variables

STATEVEC a character variable containing the name of the component of the

state vector corresponding to the observation The STATEVEC variable has

the value STD for standard deviations observations which contain the standarderrors for the estimates given in the preceding observation

Fndash j numeric variables containing the columns of theF

matrix The variable

Fndash j contains the jth column of F

The number of Fndash j variables is equal to

the value of the DIMMAX= option If the model is of smaller dimension the

extraneous variables are set to missing

Gndash j numeric variables containing the columns of theG

matrix The variable

Gndash j contains the jth column of G

The number of Gndash j variables is equal to r

the dimension of x

t

given by the number of variables in the VAR statement

SIGndash j numeric variables containing the columns of the innovation covariance

matrix The variable SIGndash j contains the jth column of

e e

There are r vari-ables SIGndash j

Table 182 shows an example of the OUTMODEL= data set withx

t

= x

t

y

t

0

z

t

= x

t

y

t

x

t + 1 j t

0 and DIMMAX=4 In Table 182F

i j

andG

i j

are the ijth

elements of F

andG

respectively Note that all elements for Fndash4 are missing because

F

is a3 3

matrix

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Chapter 18 Details

Table 182 Value in the OUTMODEL= Data Set

Obs STATEVEC Fndash 1 Fndash 2 Fndash 3 Fndash 4 Gndash 1 Gndash 2 SIGndash 1 SIGndash 2

1 X(TT) 0 0 1 1 0

1 1

1 2

2 STD

3 Y(TT) F2 1

F2 2

F2 3

0 1

2 1

2 2

4 STD std F2 1

std F2 2

std F2 3

5 X(T+1T) F3 1

F3 2

F3 3

G3 1

G3 2

6 STD std F3 1

std F3 2

std F3 3

std G3 1

std G3 2

Printed Output

The printed output produced by the STATESPACE procedure is described in the fol-

lowing

1 descriptive statistics which include the number of observations used the

names of the variables their means and standard deviations (Std) and the dif-

ferencing operations used

2 the Akaike information criteria for the sequence of preliminary autoregressive

models

3 if the PRINTOUT=LONG option is specified the sample autocovariance ma-

trices of the input series at various lags

4 if the PRINTOUT=LONG option is specified the sample autocorrelation ma-

trices of the input series

5 a schematic representation of the autocorrelation matrices showing the signif-

icant autocorrelations

6 if the PRINTOUT=LONG option is specified the partial autoregressive ma-

trices (These are

p

p

as described in Preliminary Autoregressive Modelsearlier in this chapter)

7 a schematic representation of the partial autocorrelation matrices showing the

significant partial autocorrelations

8 the Yule-Walker estimates of the autoregressive parameters for the autoregres-

sive model with the minimum AIC

9 if the PRINTOUT=LONG option is specified the autocovariance matrices of

the residuals of the minimum AIC model This is the sequence of estimated

innovation variance matrices for the solutions of the Yule-Walker equations

10 if the PRINTOUT=LONG option is specified the autocorrelation matrices of the residuals of the minimum AIC model

11 If the CANCORR option is specified the canonical correlations analysis for

each potential state vector considered in the state vector selection process

This includes the potential state vector the canonical correlations the informa-

tion criterion for the smallest canonical correlation Bartlettrsquos

2 statistic (ldquoChi

Squarerdquo) for the smallest canonical correlation and the degrees of freedom of

Bartlettrsquos

2

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Part 2 General Information

12 the components of the chosen state vector

13 the preliminary estimate of the transition matrixF

the input matrixG

and

the variance matrix for the innovations

e e

14 if the ITPRINT option is specified the iteration history of the likelihood max-

imization For each iteration this shows the iteration number the number of step halvings the determinant of the innovation variance matrix the damping

factor Lambda and the values of the parameters

15 the state vector printed again to aid interpretation of the following listing of F

andG

16 the final estimate of the transition matrixF

17 the final estimate of the input matrixG

18 the final estimate of the variance matrix for the innovations

e e

19 a table listing the estimates of the free parameters inF

andG

and their standarderrors and t statistics

20 if the COVB option is specified the covariance matrix of the parameter esti-

mates

21 if the COVB option is specified the correlation matrix of the parameter esti-

mates

22 if the PRINT option is specified the forecasts and their standard errors

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Chapter 18 Details

ODS Table Names

PROC STATESPACE assigns a name to each table it creates You can use these

names to reference the table when using the Output Delivery System (ODS) to select

tables and create output data sets These names are listed in the following table For

more information on ODS see Chapter 6 ldquoUsing the Output Delivery Systemrdquo

Table 183 ODS Tables Produced in PROC STATESPACE

ODS Table Name Description Option

NObs Number of observations default

Summary Simple summary statistics table default

InfoCriterion Information criterion table default

CovLags Covariance Matrices of Input Series PRINTOUT=LONG

CorrLags Correlation Matrices of Input Series PRINTOUT=LONG

PartialAR Partial Autoregressive Matrices PRINTOUT=LONG

YWEstimates Yule-Walker Estimates for Minimum AIC default

CovResiduals Covariance of Residuals PRINTOUT=LONGCorrResiduals Residual Correlations from AR Models PRINTOUT=LONG

StateVector State vector table default

CorrGraph Schematic Representation of Correlations default

TransitionMatrix Transition Matrix default

InputMatrix Input Matrix default

VarInnov Variance Matrix for the Innovation default

CovB Covariance of Parameter Estimates COVB

CorrB Correlation of Parameter Estimates COVB

CanCorr Canonical Correlation Analysis CANCORR

IterHistory Iterative Fitting table ITPRINT

ParameterEstimates Parameter Estimates Table default

Forecasts Forecasts Table PRINT

ConvergenceStatus Convergence Status Table default

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Part 2 General Information

Example

Example 181 Series J from Box and Jenkins

This example analyzes the gas furnace data (series J) from Box and Jenkins (The

data are not shown Refer to Box and Jenkins (1976) for the data)

First a model is selected and fit automatically using the following statements

title1 rsquoGas Furnace Datarsquo

title2 rsquoBox amp Jenkins Series Jrsquo

title3 rsquoAutomatically Selected Modelrsquo

proc statespace data=seriesj cancorr

var x y

run

The results for the automatically selected model are shown in Output 1811

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Chapter 18 Example

Output 1811 Results for Automatically Selected Model

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Number of Observations 296

Standard

Variable Mean Error

x -005683 1072766

y 5350912 3202121

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Information Criterion for Autoregressive Models

Lag=0 Lag=1 Lag=2 Lag=3 Lag=4 Lag=5 Lag=6 Lag=7 Lag=8

6513862 -103357 -163296 -164512 -165152 -164891 -164934 -164315 -163856

Information

Criterion for

Autoregressive

Models

Lag=9 Lag=10

-16348 -163359

Schematic Representation of Correlations

NameLag 0 1 2 3 4 5 6 7 8 9 10

x +- +- +- +- +- +- +- +- +- +- +-

y -+ -+ -+ -+ -+ -+ -+ -+ -+ -+ -+

+ is gt 2std error - is lt -2std error is between

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Part 2 General Information

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Schematic Representation of Partial Autocorrelations

NameLag 1 2 3 4 5 6 7 8 9 10

x + - + -

y -+ -- - + +

+ is gt 2std error - is lt -2std error is between

Yule-Walker Estimates for Minimum AIC

------Lag=1------ ------Lag=2------ ------Lag=3------ ------Lag=4------

x y x y x y x y

x 1925887 -000124 -120166 0004224 0116918 -000867 0104236 0003268

y 0050496 1299793 -002046 -03277 -071182 -025701 0195411 0133417

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Canonical Correlations Analysis

Information Chi

x(TT) y(TT) x(T+1T) Criterion Square DF

1 1 0804883 2929228 3047481 8

Information Chix(TT) y(TT) x(T+1T) y(T+1T) Criterion Square DF

1 1 0906681 0607529 1223358 1347237 7

Information Chi

x(TT) y(TT) x (T+1T) y (T+1T) x (T+2T) Criterion Square DF

1 1 0909434 0610278 0186274 -154701 1034705 6

Information Chi

x(TT) y(TT) x (T+1T) y (T+1T) y (T+2T) Criterion Square DF

1 1 091014 0618937 0206823 0940392 1280924 6

Information Chi

x(TT) y(TT) x(T+1T) y(T+1T) y(T+2T) y(T+3T) Criterion Square DF

1 1 0912963 0628785 0226598 0083258 -794103 2041584 5

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Chapter 18 Example

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Selected Statespace Form and Preliminary Estimates

State Vector

x(TT) y(TT) x(T+1T) y(T+1T) y(T+2T)

Estimate of Transition Matrix

0 0 1 0 0

0 0 0 1 0

-084718 0026794 1711715 -005019 0

0 0 0 0 1

-019785 0334274 -018174 -123557 1787475

Input Matrix for Innovation

1 0

0 11925887 -000124

0050496 1299793

0142421 1361696

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Selected Statespace Form and Preliminary Estimates

Variance Matrix for Innovation

0035274 -000734

-000734 0097569

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Part 2 General Information

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Selected Statespace Form and Fitted Model

State Vector

x(TT) y(TT) x(T+1T) y(T+1T) y(T+2T)

Estimate of Transition Matrix

0 0 1 0 0

0 0 0 1 0

-086192 0030609 1724235 -005483 0

0 0 0 0 1

-034839 0292124 -009435 -109823 1671418

Input Matrix for Innovation

1 0

0 1192442 -000416

0015621 1258495

008058 1353204

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Selected Statespace Form and Fitted Model

Variance Matrix for Innovation

0035579 -000728

-000728 0095577

Parameter Estimates

Standard

Parameter Estimate Error t Value

F(31) -086192 0072961 -1181

F(32) 0030609 0026167 117

F(33) 1724235 0061599 2799

F(34) -005483 0030169 -182

F(51) -034839 0135253 -258

F(52) 0292124 0046299 631

F(53) -009435 0096527 -098F(54) -109823 0109525 -1003

F(55) 1671418 0083737 1996

G(31) 1924420 0058162 3309

G(32) -000416 0035255 -012

G(41) 0015621 0095771 016

G(42) 1258495 0055742 2258

G(51) 0080580 0151622 053

G(52) 1353204 0091388 1481

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Chapter 18 Example

The two series are believed to have a transfer function relation with the gas rate

(variable X) as the input and the CO2

concentration (variable Y) as the output Since

the parameter estimates shown in Output 1811 support this kind of model the model

is reestimated with the feedback parameters restricted to 0 The following statements

fit the transfer function (no feedback) model

title3 rsquoTransfer Function Modelrsquo

proc statespace data=seriesj printout=none

var x y

restrict f(32)=0 f(34)=0

g(32)=0 g(41)=0 g(51)=0

run

The last 2 pages of the output are shown in Output 1812

Output 1812 STATESPACE Output for Transfer Function Model

Gas Furnace Data

Box amp Jenkins Series J

Transfer Function Model

The STATESPACE Procedure

Selected Statespace Form and Fitted Model

State Vector

x(TT) y(TT) x(T+1T) y(T+1T) y(T+2T)

Estimate of Transition Matrix

0 0 1 0 0

0 0 0 1 0

-068882 0 1598717 0 0

0 0 0 0 1

-035944 0284179 -00963 -107313 1650047

Input Matrix for Innovation

1 0

0 1

1923446 0

0 1260856

0 1346332

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Part 2 General Information

Gas Furnace Data

Box amp Jenkins Series J

Transfer Function Model

The STATESPACE Procedure

Selected Statespace Form and Fitted Model

Variance Matrix for Innovation

0036995 -00072

-00072 0095712

Parameter Estimates

Standard

Parameter Estimate Error t Value

F(31) -068882 0050549 -1363

F(33) 1598717 0050924 3139

F(51) -035944 0229044 -157

F(52) 0284179 0096944 293

F(53) -009630 0140876 -068

F(54) -107313 0250385 -429

F(55) 1650047 0188533 875G(31) 1923446 0056328 3415

G(42) 1260856 0056464 2233

G(52) 1346332 0091086 1478

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Chapter 18 References

References

Akaike H (1974) Markovian Representation of Stochastic Processes and Its Ap-

plication to the Analysis of Autoregressive Moving Average Processes Annals

of the Institute of Statistical Mathematics 26 363-387

Akaike H (1976) Canonical Correlations Analysis of Time Series and the Use of

an Information Criterion in Advances and Case Studies in System Identification

eds R Mehra and DG Lainiotis New York Academic Press

Anderson TW (1971) The Statistical Analysis of Time Series New York John

Wiley amp Sons

Ansley CF and Newbold P (1979) Multivariate Partial Autocorrelations Pro-

ceedings of the Business and Economic Statistics Section American Statistical

Association 349-353

Box GEP and Jenkins G (1976) Time Series Analysis Forecasting and Control

San Francisco Holden-Day

Brockwell PJ and Davis RA (1991) Time Series Theory and Methods 2nd Edi-

tion Springer-Verlag

Hannan EJ (1970) Multiple Time Series New York John Wiley amp Sons

Hannan EJ (1976) The Identification and Parameterization of ARMAX and State

Space Forms Econometrica 44 713-722

Harvey AC (1981a) The Econometric Analysis of Time Series New York John

Wiley amp Sons

Harvey AC (1981b) Time Series Models New York John Wiley amp Sons

Jones RH (1974) Identification and Autoregressive Spectrum Estimation IEEE Transactions on Automatic Control AC-19 894-897

Pham-Dinh-Tuan (1978) On the Fitting of Multivariate Processes of the

Autoregressive-Moving Average Type Biometrika 65 99-107

Priestley MB (1980) System Identification Kalman Filtering and Stochastic Con-

trol in Directions in Time Series eds DR Brillinger and GC Tiao Institute

of Mathematical Statistics

Whittle P (1963) On the Fitting of Multivariate Autoregressions and the Approxi-

mate Canonical Factorization of a Spectral Density Matrix Biometrika 50 129-

134

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The correct bibliographic citation for this manual is as follows SAS Institute Inc SAS

ET S U serrsquos Gui de Version 8 Cary NC SAS Institute Inc 1999 1546 pp

SASET S Userrsquos Guide Version 8

Copyright copy 1999 by S AS I ns t it u t e I nc Ca ry N C U S A

ISBN 1ndash58025ndash489ndash6

All r ight s res erved P r int ed in t he U n it ed St a t es of A merica N o pa rt of t his p u blica t ionma y b e reprodu ced s t ored in a ret r ieva l s ys t em or t ra ns mit t ed in a ny form or b y a nymeans electronic mechanical photocopying or otherwise without the prior writtenpermission of the publisher SAS Institute Inc

US Government Restricted Rights Notice Use du plicat ion or disclosure of the

software by the government is subject to restrictions as set forth in FAR 52227ndash19Commercial Computer Softw ar e-Restricted Rights (J une 1987)

SAS Institute Inc SAS Campus Drive Cary North Carolina 27513

1st print ing October 1999

S AS reg a nd a ll ot her SA S I ns t it u t e I nc p rodu ct or s ervice na m es a re regis t ered t ra dema rkst d k f SA S I t it t I i t h U S A d t h t i reg i di t U SA

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Part 2 General Information

var x(14)

The series modeled is the 1-period difference of the 4-period difference

X

t

X

t 4

X

t 1

X

t 5

= X

t

X

t 1

X

t 4

+ X

t 5

Another way to obtain stationary series is to use a regression on time to de-trend the

data If the time series has a deterministic linear trend regressing the series on time

produces residuals that should be stationary The following statements write residuals

of X and Y to the variable RX and RY in the output data set DETREND

data a

set a

t=_n_

run

proc reg data=a

model x y = t

output out=detrend r=rx ry

run

You then use PROC STATESPACE to forecast the de-trended series RX and RY A

disadvantage of this method is that you need to add the trend back to the forecast

series in an additional step A more serious disadvantage of the de-trending method

is that it assumes a deterministic trend In practice most time series appear to have a

stochastic rather than a deterministic trend Differencing is a more flexible and often

more appropriate method

There are several other methods to handle nonstationary time series For more infor-

mation and examples refer to Brockwell and Davis (1991)

Preliminary Autoregressive Models

After computing the sample autocovariance matrices PROC STATESPACE fits a

sequence of vector autoregressive models These preliminary autoregressive models

are used to estimate the autoregressive order of the process and limit the order of the

autocovariances considered in the state vector selection process

Yule-Walker Equations for Forward and Backward Models

Unlike a univariate autoregressive model a multivariate autoregressive model has

different forms depending on whether the present observation is being predicted from

the past observations or from the future observations

Letx

t

be the r -component stationary time series given by the VAR statement after

differencing and subtracting the vector of sample means (If the NOCENTER option

is specified the mean is not subtracted) Let n be the number of observations of x

t

from the input data set

Lete

t

be a vector white noise sequence with mean vector 0 and variance matrix

p

and letn

t

be a vector white noise sequence with mean vector 0 and variance matrix

p

Let p be the order of the vector autoregressive model forx

t

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The forward autoregressive form based on the past observations is written as follows

x

t

=

p

X

i = 1

p

i

x

t i

+ e

t

The backward autoregressive form based on the future observations is written as fol-

lows

x

t

=

p

X

i = 1

p

i

x

t + i

+ n

t

LettingE

denote the expected value operator the autocovariance sequence for thex

t

series

i

is

i

= E x

t

x

0

t i

The Yule-Walker equations for the autoregressive model that matches the first p ele-

ments of the autocovariance sequence are

2

6

6

4

0

1

p 1

0

1

0

p 2

0

p 1

0

p 2

0

3

7

7

5

2

6

6

4

p

1

p

2

p

p

3

7

7

5

=

2

6

6

4

1

2

p

3

7

7

5

and

2

6

6

6

4

0

0

1

0

p 1

1

0

0

p 2

p 1

p 2

0

3

7

7

7

5

2

6

6

4

p

1

p

2

p

p

3

7

7

5

=

2

6

6

4

0

1

0

2

0

p

3

7

7

5

Here

p

i

are the coefficient matrices for the past observation form of the vector au-toregressive model and

p

i

are the coefficient matrices for the future observation

form More information on the Yule-Walker equations in the multivariate setting can

be found in Whittle (1963) and Ansley and Newbold (1979)

The innovation variance matrices for the two forms can be written as follows

p

=

0

p

X

i = 1

p

i

0

i

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Part 2 General Information

p

=

0

p

X

i = 1

p

i

i

The autoregressive models are fit to the data using the preceding Yule-Walker equa-

tions with

i

replaced by the sample covariance sequenceC

i

The covariance matri-

ces are calculated as

C

i

=

1

N 1

N

X

t = i + 1

x

t

x

0

t i

Let b

p

b

p

b

p

and b

p

represent the Yule-Walker estimates of

p

p

p

and

p

respectively These matrices are written to an output data set when the OUTAR=

option is specified

When the PRINTOUT=LONG option is specified the sequence of matrices b

p

and

the corresponding correlation matrices are printed The sequence of matricesb

p isused to compute Akaike information criteria for selection of the autoregressive order

of the process

Akaike Information Criterion

The Akaike information criterion or AIC is defined as -2( maximum of log likeli-

hood )+2(number of parameters) Since the vector autoregressive models are esti-

mates from the Yule-Walker equations not by maximum likelihood the exact likeli-

hood values are not available for computing the AIC However for the vector autore-

gressive model the maximum of the log likelihood can be approximated as

l n L

n

2

l n j

b

p

j

Thus the AIC for the order p model is computed as

A I C

p

= n l n j

b

p

j + 2 p r

2

You can use the printed AIC array to compute a likelihood ratio test of the autoregres-

sive order The log-likelihood ratio test statistic for testing the order p model against

the order p 1

model is

n l n j

b

p

j + n l n j

b

p 1

j

This quantity is asymptotically distributed as a

2 withr

2 degrees of freedom if the

series is autoregressive of order p 1

It can be computed from the AIC array as

A I C

p 1

A I C

p

+ 2 r

2

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You can evaluate the significance of these test statistics with the PROBCHI function

in a SAS DATA step or with a

2 table

Determining the Autoregressive Order

Although the autoregressive models can be used for prediction their primary value is

to aid in the selection of a suitable portion of the sample covariance matrix for use in

computing canonical correlations If the multivariate time series x

t is of autoregres-sive order p then the vector of past values to lag p is considered to contain essentially

all the information relevant for prediction of future values of the time series

By default PROC STATESPACE selects the order p producing the autoregressive

model with the smallestA I C

p

If the value p for the minimumA I C

p

is less than the

value of the PASTMIN= option then p is set to the PASTMIN= value Alternatively

you can use the ARMAX= and PASTMIN= options to force PROC STATESPACE to

use an order you select

Significance Limits for Partial Autocorrelations

The STATESPACE procedure prints a schematic representation of the partial autocor-

relation matrices indicating which partial autocorrelations are significantly greater orsignificantly less than 0 Figure 1810 shows an example of this table

The STATESPACE Procedure

Schematic Representation of Partial Autocorrelations

NameLag 1 2 3 4 5 6 7 8 9 10

x ++ +

y ++

+ is gt 2std error - is lt -2std error is between

Figure 1810 Significant Partial Autocorrelations

The partial autocorrelations are from the sample partial autoregressive matrices b

p

p

The standard errors used for the significance limits of the partial autocorrelations are

computed from the sequence of matrices

p

and

p

Under the assumption that the observed series arises from an autoregressive process

of order p 1

the pth sample partial autoregressive matrix b

p

p

has an asymptotic

variance matrix 1

n

1

p

p

The significance limits for b

p

p

used in the schematic plot of the sample partial autore-

gressive sequence are derived by replacing

p

and

p

with their sample estimators

to produce the variance estimate as follows

d

V a r

b

p

p

=

1

n r p

b

1

p

b

p

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Canonical Correlation Analysis

Given the order p letp

t

be the vector of current and past values relevant to prediction

of x

t + 1

p

t

= x

0

t

x

0

t 1

x

0

t p

0

Letf

t

be the vector of current and future values

f

t

= x

0

t

x

0

t + 1

x

0

t + p

0

In the canonical correlation analysis consider submatrices of the sample covariance

matrix of p

t

andf

t

This covariance matrixV

has a block Hankel form

V =

2

6

6

6

4

C

0

C

0

1

C

0

2

C

0

p

C

0

1

C

0

2

C

0

3

C

0

p + 1

C

0

p

C

0

p + 1

C

0

p + 2

C

0

2 p

3

7

7

7

5

State Vector Selection Process

The canonical correlation analysis forms a sequence of potential state vectorsz

j

t

Examine a sequencef

j

t

of subvectors of f

t

and form the submatrixV

j consisting

of the rows and columns of V

corresponding to the components of f

j

t

and compute

its canonical correlations

The smallest canonical correlation of V

j is then used in the selection of the com-

ponents of the state vector The selection process is described in the following For

more details about this process refer to Akaike (1976)

In the following discussion the notationx

t + k j t

denotes the wide sense conditional

expectation (best linear predictor) of x

t + k

given allx

s

with s less than or equal to t

In the notationx

i t + 1

the first subscript denotes the ith component of x

t + 1

The initial state vectorz

1

t

is set tox

t

The sequencef

j

t

is initialized by setting

f

1

t

= z

1

0

t

x

1 t + 1 j t

0

= x

0

t

x

1 t + 1 j t

0

That is start by considering whether to addx

1 t + 1 j t

to the initial state vectorz

1

t

The procedure forms the submatrixV

1 corresponding tof

1

t

and computes its canon-

ical correlations Denote the smallest canonical correlation of V

1 as

m i n

If

m i n

is

significantly greater than 0x

1 t + 1 j t

is added to the state vector

If the smallest canonical correlation of V

1 is not significantly greater than 0 then a

linear combination of f

1

t

is uncorrelated with the pastp

t

Assuming that the determi-

nant of C

0

is not 0 (that is no input series is a constant) you can take the coefficient

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Chapter 18 Details

of x

1 t + 1 j t

in this linear combination to be 1 Denote the coefficients of z

1

t

in this

linear combination as

This gives the relationship

x

1 t + 1 j t

=

0

x

t

Therefore the current state vector already contains all the past information useful forpredicting

x

1 t + 1

and any greater leads of x

1 t

The variablex

1 t + 1 j t

is not added to

the state vector nor are any termsx

1 t + k j t

considered as possible components of the

state vector The variablex

1

is no longer active for state vector selection

The process described forx

1 t + 1 j t

is repeated for the remaining elements of f

t

The

next candidate for inclusion in the state vector is the next component of f

t

corre-

sponding to an active variable Components of f

t

corresponding to inactive variables

that produced a zero

m i n

in a previous step are skipped

Denote the next candidate asx

l t + k j t

The vectorf

j

t

is formed from the current state

vector andx

l t + k j t

as follows

f

j

t

= z

j

0

t

x

l t + k j t

0

The matrixV

j is formed fromf

j

t

and its canonical correlations are computed The

smallest canonical correlation of V

j is judged to be either greater than or equal to 0

If it is judged to be greater than 0x

l t + k j t

is added to the state vector If it is judged

to be 0 then a linear combination of f

j

t

is uncorrelated with thep

t

and the variable

x

l

is now inactive

The state vector selection process continues until no active variables remain

Testing Significance of Canonical Correlations For each step in the canonical correlation sequence the significance of the small-

est canonical correlation

m i n

is judged by an information criterion from Akaike

(1976) This information criterion is

n l n 1

2

m i n

r p + 1 q + 1

where q is the dimension of f

j

t

at the current step r is the order of the state vector p

is the order of the vector autoregressive process and

is the value of the SIGCORR=

option The default is SIGCORR=2 If this information criterion is less than or equal

to 0

m i n

is taken to be 0 otherwise it is taken to be significantly greater than 0

(Do not confuse this information criterion with the AIC)

Variables inx

t + p j t

are not added in the model even with positive information crite-

rion because of the singularity of V

You can force the consideration of more candi-

date state variables by increasing the size of theV

matrix by specifying a PASTMIN=

option value larger than p

Printing the Canonical Correlations

To print the details of the canonical correlation analysis process specify the CAN-

CORR option in the PROC STATESPACE statement The CANCORR option prints

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the candidate state vectors the canonical correlations and the information criteria for

testing the significance of the smallest canonical correlation

Bartlettrsquos

2 and its degrees of freedom are also printed when the CANCORR option

is specified The formula used for Bartlettrsquos

2 is

2

= n 5 r p + 1 q + 1 l n 1

2

m i n

withr p + 1 q + 1

degrees of freedom

Figure 1811 shows the output of the CANCORR option for the introductory example

shown in the Getting Started section of this chapter

The STATESPACE Procedure

Canonical Correlations Analysis

Information Chi

x(TT) y(TT) x(T+1T) Criterion Square DF

1 1 0237045 3566167 114505 4

Information Chi

x(TT) y(TT) x(T+1T) y(T+1T) Criterion Square DF

1 1 0238244 0056565 -535906 0636134 3

Information Chi

x(TT) y(TT) x(T+1T) x(T+2T) Criterion Square DF

1 1 0237602 0087493 -446312 1525353 3

Figure 1811 Canonical Correlations Analysis

New variables are added to the state vector if the information criteria are positive Inthis example

y

t + 1 j t

andx

t + 2 j t

are not added to the state space vector because the

information criteria for these models are negative

If the information criterion is nearly 0 then you may want to investigate models

that arise if the opposite decision is made regarding

m i n

This investigation can be

accomplished by using a FORM statement to specify part or all of the state vector

Preliminary Estimates of F

When a candidate variablex

l t + k j t

yields a zero

m i n

and is not added to the state

vector a linear combination of f

j

t

is uncorrelated with thep

t

Because of the method

used to construct thef

j

t

sequence the coefficient of x

l t + k j t

inl

can be taken as 1

Denote the coefficients of z

j

t

in this linear combination as l

This gives the relationship

x

l t + k j t

= l

0

z

j

t

The vectorl

is used as a preliminary estimate of the first r columns of the row of the

transition matrixF

corresponding tox

l t + k 1 j t

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Chapter 18 Details

Parameter Estimation

The model isz

t + 1

= F z

t

+ G e

t + 1

wheree

t

is a sequence of independent multivari-

ate normal innovations with mean vector 0 and variance

e e

The observed sequence

x

t

composes the first r components of z

t

and thusx

t

= H z

t

where H is ther s

matrix I

r

0

LetE

be ther n

matrix of innovations

E = e

1

e

n

If the number of observations n is reasonably large the log likelihood L can be

approximated up to an additive constant as follows

L =

n

2

l n j

e e

j

1

2

t r a c e

1

e e

E E

0

The elements of

e e

are taken as free parameters and are estimated as follows

S

0

=

1

n

E E

0

Replacing

e e

byS

0

in the likelihood equation the log likelihood up to an additive

constant is

L =

n

2

l n j S

0

j

Letting B be the backshift operator the formal relation betweenx

t

ande

t

is

x

t

= H I B F

1

G e

t

e

t

= H I B F

1

G

1

x

t

=

1

X

i = 0

i

x

t i

LettingC

i

be the ith lagged sample covariance of x

t

and neglecting end effects the

matrixS

0

is

S

0

=

1

X

i j = 0

i

C

i + j

0

j

For the computation of S

0

the infinite sum is truncated at the value of the KLAG=

option The value of the KLAG= option should be large enough that the sequence

i

is approximately 0 beyond that point

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Let

be the vector of free parameters in theF

andG

matrices The derivative of the

log likelihood with respect to the parameter

is

L

=

n

2

t r a c e

S

1

0

S

0

The second derivative is

2

L

0

=

n

2

t r a c e

S

1

0

S

0

0

S

1

0

S

0

t r a c e

S

1

0

2

S

0

0

Near the maximum the first term is unimportant and the second term can be approx-

imated to give the following second derivative approximation

2

L

0

=

n t r a c e

S

1

0

E

E

0

0

The first derivative matrix and this second derivative matrix approximation are com-

puted from the sample covariance matrixC

0

and the truncated sequence

i

The

approximate likelihood function is maximized by a modified Newton-Raphson algo-

rithm employing these derivative matrices

The matrixS

0

is used as the estimate of the innovation covariance matrix

e e

The

negative of the inverse of the second derivative matrix at the maximum is used as an

approximate covariance matrix for the parameter estimates The standard errors of

the parameter estimates printed in the parameter estimates tables are taken from the

diagonal of this covariance matrix The parameter covariance matrix is printed when

the COVB option is specified

If the data are nearly nonstationary a better estimate of

e e

and the other parameters

can sometimes be obtained by specifying the RESIDEST option The RESIDEST

option estimates the parameters using conditional least squares instead of maximum

likelihood

The residuals are computed using the state space equation and the sample mean val-

ues of the variables in the model as start-up values The estimate of S

0

is then com-

puted using the residuals from the ith observation on where i is the maximum number

of times any variable occurs in the state vector A multivariate Gauss-Marquardt al-

gorithm is used to minimizej S

0

j

Refer to Harvey (1981a) for a further description

of this method

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Chapter 18 Details

Forecasting

Given estimates of F

G

and

e e

forecasts of x

t

are computed from the conditional

expectation of z

t

In forecasting the parameters F G and

e e

are replaced with the estimates or by val-

ues specified in the RESTRICT statement One-step-ahead forecasting is performedfor the observation

x

t

wheret n b

Heren

is the number of observations and

b is the value of the BACK= option For the observationx

t

wheret n b

m-

step-ahead forecasting is performed form = t n + b

The forecasts are generated

recursively with the initial conditionz

0

= 0

The m-step-ahead forecast of z

t + m

isz

t + m j t

wherez

t + m j t

denotes the conditional

expectation of z

t + m

given the information available at time t The m-step-ahead

forecast of x

t + m

isx

t + m j t

= H z

t + m j t

where the matrixH = I

r

0

Let

i

= F

i

G

Note that the lasts r

elements of z

t

consist of the elements of x

u j t

foru t

The state vector z

t + m

can be represented as

z

t + m

= F

m

z

t

+

m 1

X

i = 0

i

e

t + m i

Sincee

t + i j t

= 0

fori 0

the m-step-ahead forecastz

t + m j t

is

z

t + m j t

= F

m

z

t

= F z

t + m 1 j t

Therefore the m-step-ahead forecast of x

t + m

is

x

t + m j t

= H z

t + m j t

The m-step-ahead forecast error is

z

t + m

z

t + m j t

=

m 1

X

i = 0

i

e

t + m i

The variance of the m-step-ahead forecast error is

V

z m

=

m 1

X

i = 0

i

e e

0

i

LettingV

z 0

= 0

the variance of the m-step-ahead forecast error of z

t + m

V

z m

can

be computed recursively as follows

V

z m

= V

z m 1

+

m 1

e e

0

m 1

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The variance of the m-step-ahead forecast error of x

t + m

is ther r

left upper sub-

matrix of V

z m

that is

V

x m

= H V

z m

H

0

Unless the NOCENTER option is specified the sample mean vector is added to the

forecast When differencing is specified the forecasts xt + m j t

plus the sample mean

vector are integrated back to produce forecasts for the original series

Lety

t

be the original series specified by the VAR statement with some 0 values

appended corresponding to the unobserved past observations Let B be the backshift

operator and let B

be thes s

matrix polynomial in the backshift operator

corresponding to the differencing specified by the VAR statement The off-diagonal

elements of

i

are 0 Note that

0

= I

s

whereI

s

is thes s

identity matrix Then

z

t

= B y

t

This gives the relationship

y

t

=

1

B z

t

=

1

X

i = 0

i

z

t i

where

1

B =

P

1

i = 0

i

B

i and

0

= I

s

The m-step-ahead forecast of y

t + m

is

y

t + m j t

=

m 1

X

i = 0

i

z

t + m i j t

+

1

X

i = m

i

z

t + m i

The m-step-ahead forecast error of y

t + m

is

m 1

X

i = 0

i

z

t + m i

z

t + m i j t

=

m 1

X

i = 0

i

X

u = 0

u

i u

e

t + m i

LettingV

y 0

= 0

the variance of the m-step-ahead forecast error of y

t + m

V

y m

is

V

y m

=

m 1

X

i = 0

i

X

u = 0

u

i u

e e

i

X

u = 0

u

i u

0

= V

y m 1

+

m 1

X

u = 0

u

m 1 u

e e

m 1

X

u = 0

u

m 1 u

0

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Chapter 18 Details

Relation of ARMA and State Space Forms

Every state space model has an ARMA representation and conversely every ARMA

model has a state space representation This section discusses this equivalence The

following material is adapted from Akaike (1974) where there is a more complete

discussion Pham-Dinh-Tuan (1978) also contains a discussion of this material

Suppose you are given the following ARMA model

B x

t

= B e

t

or in more detail

x

t

1

x

t 1

p

x

t p

= e

t

+

1

e

t 1

+ +

q

e

t q

(1)

wheree

t

is a sequence of independent multivariate normal random vectors with mean

0 and variance matrix

e e

B is the backshift operator (B x

t

= x

t 1

) B

and

B

are matrix polynomials in B andx

t

is the observed process

If the roots of the determinantial equationj B j = 0

are outside the unit circle in

the complex plane the model can also be written as

x

t

=

1

B B e

t

=

1

X

i = 0

i

e

t i

The

i

matrices are known as the impulse response matrices and can be computed

as

1

B B

You can assume p q

since if this is not initially true you can add more terms

i

that are identically 0 without changing the model

To write this set of equations in a state space form proceed as follows Letx

t + i j t

be

the conditional expectation of x

t + i

givenx

w

forw t

The following relations hold

x

t + i j t

=

1

X

j = i

j

e

t + i j

x

t + i j t + 1

= x

t + i j t

+

i 1

e

t + 1

However from equation (1) you can derive the following relationship

x

t + p j t

=

1

x

t + p 1 j t

+ +

p

x

t

(2)

Hence wheni = p

you can substitute forx

t + p j t

in the right-hand side of equation

(2) and close the system of equations

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Part 2 General Information

This substitution results in the following model in the state space form

z

t + 1

= F z

t

+ G e

t + 1

2

6

6

6

4

x

t + 1

x

t + 2 j t + 1

x

t + p j t + 1

3

7

7

7

5

=

2

6

6

4

0 I 0 0

0 0 I 0

p

p 1

1

3

7

7

5

2

6

6

6

4

x

t

x

t + 1 j t

x

t + p 1 j t

3

7

7

7

5

+

2

6

6

4

I

1

p 1

3

7

7

5

e

t + 1

Note that the state vectorz

t

is composed of conditional expectations of x

t

and the

first r components of z

t

are equal tox

t

The state space form can be cast into an ARMA form by solving the system of dif-

ference equations for the first r components

When converting from an ARMA form to a state space form you can generate a

state vector larger than needed that is the state space model may not be a minimal

representation When going from a state space form to an ARMA form you can have

nontrivial common factors in the autoregressive and moving average operators that

yield an ARMA model larger than necessary

If the state space form used is not a minimal representation some but not all compo-

nents of x

t + i j t

may be linearly dependent This situation corresponds to

p

p 1

being of less than full rank when B

and B

have no common nontrivial

left factors In this casez

t

consists of a subset of the possible components of

x

t + i j t

i = 1 2 p 1

However once a component of x

t + i j t

(for example the

jth one) is linearly dependent on the previous conditional expectations then all sub-

sequent jth components of x

t + k j t

fork i

must also be linearly dependent Note

that in this case equivalent but seemingly different structures can arise if the order of

the components withinx

t

is changed

OUT= Data Set

The forecasts are contained in the output data set specified by the OUT= option on the

PROC STATESPACE statement The OUT= data set contains the following variables

the BY variables

the ID variable

the VAR statement variables These variables contain the actual values from

the input data set

FORi numeric variables containing the forecasts The variable FORi contains

the forecasts for the ith variable in the VAR statement list Forecasts are one-

step-ahead predictions until the end of the data or until the observation specified

by the BACK= option

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Chapter 18 Details

RESi numeric variables containing the residual for the forecast of the ith vari-

able in the VAR statement list For forecast observations the actual values are

missing and the RESi variables contain missing values

STDi numeric variables containing the standard deviation for the forecast of

the ith variable in the VAR statement list The values of the STDi variables can

be used to construct univariate confidence limits for the corresponding fore-casts However such confidence limits do not take into account the covariance

of the forecasts

OUTAR= Data Set

The OUTAR= data set contains the estimates of the preliminary autoregressive mod-

els The OUTAR= data set contains the following variables

ORDER a numeric variable containing the order p of the autoregressive model

that the observation represents

AIC a numeric variable containing the value of the information criterionA I C

p

SIGFl numeric variables containing the estimate of the innovation covariance

matrices for the forward autoregressive models The variable SIGFl contains

the lth column of b

p

in the observations with ORDER= p

SIGBl numeric variables containing the estimate of the innovation covariance

matrices for the backward autoregressive models The variable SIGBl contains

the lth column of b

p

in the observations with ORDER= p

FORk ndashl numeric variables containing the estimates of the autoregressive pa-

rameter matrices for the forward models The variable FORk ndashl contains the

lth column of the lag k autoregressive parameter matrix b

p

k

in the observations

with ORDER= p

BACk ndashl numeric variables containing the estimates of the autoregressive pa-

rameter matrices for the backward models The variable BACk ndashl contains the

lth column of the lag k autoregressive parameter matrix b

p

k

in the observations

with ORDER= p

The estimates for the order p autoregressive model can be selected as those observa-

tions with ORDER= p Within these observations the k lth element of

p

i

is given by

the value of the FORindashl variable in the k th observation The k lth element of

p

i

is

given by the value of BACindashl variable in the k th observation The k lth element of

p

is given by SIGFl in the k th observation The k lth element of

p

is given by SIGBlin the k th observation

Table 181 shows an example of the OUTAR= data set with ARMAX=3 andx

t

of

dimension 2 In Table 181 i j

indicate the ijth element of the matrix

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Part 2 General Information

Table 181 Values in the OUTAR= Data Set

Obs ORDER AIC SIGF1 SIGF2 SIGB1 SIGB2 FOR1ndash 1 FOR1ndash 2 FOR2ndash 1 FOR2ndash 2 FOR3ndash 1

1 0 AIC0

0 1 1

0 1 2

0 1 1

0 1 2

2 0 AIC0

0 2 1

0 2 2

0 2 1

0 2 2

3 1 AIC1

1 1 1

1 1 2

1 1 1

1 1 2

1

1

1 1

1

1

1 2

4 1 AIC1

1 2 1

1 1 2

1 2 1

1 1 2

1

1

2 1

1

1

2 2

5 2 AIC2

2 1 1

2 1 2

2 1 1

2 1 2

2

1

1 1

2

1

1 2

2

2

1 1

2

2

1 2

6 2 AIC2

2 2 1

2 1 2

2 2 1

2 1 2

2

1

2 1

2

1

2 2

2

2

2 1

2

2

2 2

7 3 AIC3

3 1 1

3 1 2

3 1 1

3 1 2

3

1

1 1

3

1

1 2

3

2

1 1

3

2

1 2

3

3

1 1

8 3 AIC3

3 2 1

3 1 2

3 2 1

3 1 2

3

1

2 1

3

1

2 2

3

2

2 1

3

2

2 2

3

3

2 1

Obs FOR3ndash 2 BACK1ndash 1 BACK1ndash 2 BACK2ndash 1 BACK2ndash 2 BACK3ndash 1 BACK3ndash 2

1

2

3

1

1

1 1

1

1

1 2

4

1

1

2 1

1

1

2 2

5

2

1

1 1

2

1

1 2

2

2

1 1

2

2

1 2

6

2

1

2 1

2

1

2 2

2

2

2 1

2

2

2 2

7

3

3

1 2

3

1

1 1

3

1

1 2

3

2

1 1

3

2

1 2

3

3

1 1

3

3

1 2

8

3

3

2 2

3

1

2 1

3

1

2 2

3

2

2 1

3

2

2 2

3

3

2 1

3

3

2 2

The estimated autoregressive parameters can be used in the IML procedure to obtain

autoregressive estimates of the spectral density function or forecasts based on the

autoregressive models

OUTMODEL= Data Set

The OUTMODEL= data set contains the estimates of theF

andG

matrices and their

standard errors the names of the components of the state vector and the estimates of

the innovation covariance matrix The variables contained in the OUTMODEL= data

set are as follows

the BY variables

STATEVEC a character variable containing the name of the component of the

state vector corresponding to the observation The STATEVEC variable has

the value STD for standard deviations observations which contain the standarderrors for the estimates given in the preceding observation

Fndash j numeric variables containing the columns of theF

matrix The variable

Fndash j contains the jth column of F

The number of Fndash j variables is equal to

the value of the DIMMAX= option If the model is of smaller dimension the

extraneous variables are set to missing

Gndash j numeric variables containing the columns of theG

matrix The variable

Gndash j contains the jth column of G

The number of Gndash j variables is equal to r

the dimension of x

t

given by the number of variables in the VAR statement

SIGndash j numeric variables containing the columns of the innovation covariance

matrix The variable SIGndash j contains the jth column of

e e

There are r vari-ables SIGndash j

Table 182 shows an example of the OUTMODEL= data set withx

t

= x

t

y

t

0

z

t

= x

t

y

t

x

t + 1 j t

0 and DIMMAX=4 In Table 182F

i j

andG

i j

are the ijth

elements of F

andG

respectively Note that all elements for Fndash4 are missing because

F

is a3 3

matrix

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Chapter 18 Details

Table 182 Value in the OUTMODEL= Data Set

Obs STATEVEC Fndash 1 Fndash 2 Fndash 3 Fndash 4 Gndash 1 Gndash 2 SIGndash 1 SIGndash 2

1 X(TT) 0 0 1 1 0

1 1

1 2

2 STD

3 Y(TT) F2 1

F2 2

F2 3

0 1

2 1

2 2

4 STD std F2 1

std F2 2

std F2 3

5 X(T+1T) F3 1

F3 2

F3 3

G3 1

G3 2

6 STD std F3 1

std F3 2

std F3 3

std G3 1

std G3 2

Printed Output

The printed output produced by the STATESPACE procedure is described in the fol-

lowing

1 descriptive statistics which include the number of observations used the

names of the variables their means and standard deviations (Std) and the dif-

ferencing operations used

2 the Akaike information criteria for the sequence of preliminary autoregressive

models

3 if the PRINTOUT=LONG option is specified the sample autocovariance ma-

trices of the input series at various lags

4 if the PRINTOUT=LONG option is specified the sample autocorrelation ma-

trices of the input series

5 a schematic representation of the autocorrelation matrices showing the signif-

icant autocorrelations

6 if the PRINTOUT=LONG option is specified the partial autoregressive ma-

trices (These are

p

p

as described in Preliminary Autoregressive Modelsearlier in this chapter)

7 a schematic representation of the partial autocorrelation matrices showing the

significant partial autocorrelations

8 the Yule-Walker estimates of the autoregressive parameters for the autoregres-

sive model with the minimum AIC

9 if the PRINTOUT=LONG option is specified the autocovariance matrices of

the residuals of the minimum AIC model This is the sequence of estimated

innovation variance matrices for the solutions of the Yule-Walker equations

10 if the PRINTOUT=LONG option is specified the autocorrelation matrices of the residuals of the minimum AIC model

11 If the CANCORR option is specified the canonical correlations analysis for

each potential state vector considered in the state vector selection process

This includes the potential state vector the canonical correlations the informa-

tion criterion for the smallest canonical correlation Bartlettrsquos

2 statistic (ldquoChi

Squarerdquo) for the smallest canonical correlation and the degrees of freedom of

Bartlettrsquos

2

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Part 2 General Information

12 the components of the chosen state vector

13 the preliminary estimate of the transition matrixF

the input matrixG

and

the variance matrix for the innovations

e e

14 if the ITPRINT option is specified the iteration history of the likelihood max-

imization For each iteration this shows the iteration number the number of step halvings the determinant of the innovation variance matrix the damping

factor Lambda and the values of the parameters

15 the state vector printed again to aid interpretation of the following listing of F

andG

16 the final estimate of the transition matrixF

17 the final estimate of the input matrixG

18 the final estimate of the variance matrix for the innovations

e e

19 a table listing the estimates of the free parameters inF

andG

and their standarderrors and t statistics

20 if the COVB option is specified the covariance matrix of the parameter esti-

mates

21 if the COVB option is specified the correlation matrix of the parameter esti-

mates

22 if the PRINT option is specified the forecasts and their standard errors

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Chapter 18 Details

ODS Table Names

PROC STATESPACE assigns a name to each table it creates You can use these

names to reference the table when using the Output Delivery System (ODS) to select

tables and create output data sets These names are listed in the following table For

more information on ODS see Chapter 6 ldquoUsing the Output Delivery Systemrdquo

Table 183 ODS Tables Produced in PROC STATESPACE

ODS Table Name Description Option

NObs Number of observations default

Summary Simple summary statistics table default

InfoCriterion Information criterion table default

CovLags Covariance Matrices of Input Series PRINTOUT=LONG

CorrLags Correlation Matrices of Input Series PRINTOUT=LONG

PartialAR Partial Autoregressive Matrices PRINTOUT=LONG

YWEstimates Yule-Walker Estimates for Minimum AIC default

CovResiduals Covariance of Residuals PRINTOUT=LONGCorrResiduals Residual Correlations from AR Models PRINTOUT=LONG

StateVector State vector table default

CorrGraph Schematic Representation of Correlations default

TransitionMatrix Transition Matrix default

InputMatrix Input Matrix default

VarInnov Variance Matrix for the Innovation default

CovB Covariance of Parameter Estimates COVB

CorrB Correlation of Parameter Estimates COVB

CanCorr Canonical Correlation Analysis CANCORR

IterHistory Iterative Fitting table ITPRINT

ParameterEstimates Parameter Estimates Table default

Forecasts Forecasts Table PRINT

ConvergenceStatus Convergence Status Table default

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Part 2 General Information

Example

Example 181 Series J from Box and Jenkins

This example analyzes the gas furnace data (series J) from Box and Jenkins (The

data are not shown Refer to Box and Jenkins (1976) for the data)

First a model is selected and fit automatically using the following statements

title1 rsquoGas Furnace Datarsquo

title2 rsquoBox amp Jenkins Series Jrsquo

title3 rsquoAutomatically Selected Modelrsquo

proc statespace data=seriesj cancorr

var x y

run

The results for the automatically selected model are shown in Output 1811

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Chapter 18 Example

Output 1811 Results for Automatically Selected Model

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Number of Observations 296

Standard

Variable Mean Error

x -005683 1072766

y 5350912 3202121

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Information Criterion for Autoregressive Models

Lag=0 Lag=1 Lag=2 Lag=3 Lag=4 Lag=5 Lag=6 Lag=7 Lag=8

6513862 -103357 -163296 -164512 -165152 -164891 -164934 -164315 -163856

Information

Criterion for

Autoregressive

Models

Lag=9 Lag=10

-16348 -163359

Schematic Representation of Correlations

NameLag 0 1 2 3 4 5 6 7 8 9 10

x +- +- +- +- +- +- +- +- +- +- +-

y -+ -+ -+ -+ -+ -+ -+ -+ -+ -+ -+

+ is gt 2std error - is lt -2std error is between

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Part 2 General Information

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Schematic Representation of Partial Autocorrelations

NameLag 1 2 3 4 5 6 7 8 9 10

x + - + -

y -+ -- - + +

+ is gt 2std error - is lt -2std error is between

Yule-Walker Estimates for Minimum AIC

------Lag=1------ ------Lag=2------ ------Lag=3------ ------Lag=4------

x y x y x y x y

x 1925887 -000124 -120166 0004224 0116918 -000867 0104236 0003268

y 0050496 1299793 -002046 -03277 -071182 -025701 0195411 0133417

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Canonical Correlations Analysis

Information Chi

x(TT) y(TT) x(T+1T) Criterion Square DF

1 1 0804883 2929228 3047481 8

Information Chix(TT) y(TT) x(T+1T) y(T+1T) Criterion Square DF

1 1 0906681 0607529 1223358 1347237 7

Information Chi

x(TT) y(TT) x (T+1T) y (T+1T) x (T+2T) Criterion Square DF

1 1 0909434 0610278 0186274 -154701 1034705 6

Information Chi

x(TT) y(TT) x (T+1T) y (T+1T) y (T+2T) Criterion Square DF

1 1 091014 0618937 0206823 0940392 1280924 6

Information Chi

x(TT) y(TT) x(T+1T) y(T+1T) y(T+2T) y(T+3T) Criterion Square DF

1 1 0912963 0628785 0226598 0083258 -794103 2041584 5

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Chapter 18 Example

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Selected Statespace Form and Preliminary Estimates

State Vector

x(TT) y(TT) x(T+1T) y(T+1T) y(T+2T)

Estimate of Transition Matrix

0 0 1 0 0

0 0 0 1 0

-084718 0026794 1711715 -005019 0

0 0 0 0 1

-019785 0334274 -018174 -123557 1787475

Input Matrix for Innovation

1 0

0 11925887 -000124

0050496 1299793

0142421 1361696

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Selected Statespace Form and Preliminary Estimates

Variance Matrix for Innovation

0035274 -000734

-000734 0097569

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Part 2 General Information

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Selected Statespace Form and Fitted Model

State Vector

x(TT) y(TT) x(T+1T) y(T+1T) y(T+2T)

Estimate of Transition Matrix

0 0 1 0 0

0 0 0 1 0

-086192 0030609 1724235 -005483 0

0 0 0 0 1

-034839 0292124 -009435 -109823 1671418

Input Matrix for Innovation

1 0

0 1192442 -000416

0015621 1258495

008058 1353204

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Selected Statespace Form and Fitted Model

Variance Matrix for Innovation

0035579 -000728

-000728 0095577

Parameter Estimates

Standard

Parameter Estimate Error t Value

F(31) -086192 0072961 -1181

F(32) 0030609 0026167 117

F(33) 1724235 0061599 2799

F(34) -005483 0030169 -182

F(51) -034839 0135253 -258

F(52) 0292124 0046299 631

F(53) -009435 0096527 -098F(54) -109823 0109525 -1003

F(55) 1671418 0083737 1996

G(31) 1924420 0058162 3309

G(32) -000416 0035255 -012

G(41) 0015621 0095771 016

G(42) 1258495 0055742 2258

G(51) 0080580 0151622 053

G(52) 1353204 0091388 1481

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Chapter 18 Example

The two series are believed to have a transfer function relation with the gas rate

(variable X) as the input and the CO2

concentration (variable Y) as the output Since

the parameter estimates shown in Output 1811 support this kind of model the model

is reestimated with the feedback parameters restricted to 0 The following statements

fit the transfer function (no feedback) model

title3 rsquoTransfer Function Modelrsquo

proc statespace data=seriesj printout=none

var x y

restrict f(32)=0 f(34)=0

g(32)=0 g(41)=0 g(51)=0

run

The last 2 pages of the output are shown in Output 1812

Output 1812 STATESPACE Output for Transfer Function Model

Gas Furnace Data

Box amp Jenkins Series J

Transfer Function Model

The STATESPACE Procedure

Selected Statespace Form and Fitted Model

State Vector

x(TT) y(TT) x(T+1T) y(T+1T) y(T+2T)

Estimate of Transition Matrix

0 0 1 0 0

0 0 0 1 0

-068882 0 1598717 0 0

0 0 0 0 1

-035944 0284179 -00963 -107313 1650047

Input Matrix for Innovation

1 0

0 1

1923446 0

0 1260856

0 1346332

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Part 2 General Information

Gas Furnace Data

Box amp Jenkins Series J

Transfer Function Model

The STATESPACE Procedure

Selected Statespace Form and Fitted Model

Variance Matrix for Innovation

0036995 -00072

-00072 0095712

Parameter Estimates

Standard

Parameter Estimate Error t Value

F(31) -068882 0050549 -1363

F(33) 1598717 0050924 3139

F(51) -035944 0229044 -157

F(52) 0284179 0096944 293

F(53) -009630 0140876 -068

F(54) -107313 0250385 -429

F(55) 1650047 0188533 875G(31) 1923446 0056328 3415

G(42) 1260856 0056464 2233

G(52) 1346332 0091086 1478

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Chapter 18 References

References

Akaike H (1974) Markovian Representation of Stochastic Processes and Its Ap-

plication to the Analysis of Autoregressive Moving Average Processes Annals

of the Institute of Statistical Mathematics 26 363-387

Akaike H (1976) Canonical Correlations Analysis of Time Series and the Use of

an Information Criterion in Advances and Case Studies in System Identification

eds R Mehra and DG Lainiotis New York Academic Press

Anderson TW (1971) The Statistical Analysis of Time Series New York John

Wiley amp Sons

Ansley CF and Newbold P (1979) Multivariate Partial Autocorrelations Pro-

ceedings of the Business and Economic Statistics Section American Statistical

Association 349-353

Box GEP and Jenkins G (1976) Time Series Analysis Forecasting and Control

San Francisco Holden-Day

Brockwell PJ and Davis RA (1991) Time Series Theory and Methods 2nd Edi-

tion Springer-Verlag

Hannan EJ (1970) Multiple Time Series New York John Wiley amp Sons

Hannan EJ (1976) The Identification and Parameterization of ARMAX and State

Space Forms Econometrica 44 713-722

Harvey AC (1981a) The Econometric Analysis of Time Series New York John

Wiley amp Sons

Harvey AC (1981b) Time Series Models New York John Wiley amp Sons

Jones RH (1974) Identification and Autoregressive Spectrum Estimation IEEE Transactions on Automatic Control AC-19 894-897

Pham-Dinh-Tuan (1978) On the Fitting of Multivariate Processes of the

Autoregressive-Moving Average Type Biometrika 65 99-107

Priestley MB (1980) System Identification Kalman Filtering and Stochastic Con-

trol in Directions in Time Series eds DR Brillinger and GC Tiao Institute

of Mathematical Statistics

Whittle P (1963) On the Fitting of Multivariate Autoregressions and the Approxi-

mate Canonical Factorization of a Spectral Density Matrix Biometrika 50 129-

134

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The correct bibliographic citation for this manual is as follows SAS Institute Inc SAS

ET S U serrsquos Gui de Version 8 Cary NC SAS Institute Inc 1999 1546 pp

SASET S Userrsquos Guide Version 8

Copyright copy 1999 by S AS I ns t it u t e I nc Ca ry N C U S A

ISBN 1ndash58025ndash489ndash6

All r ight s res erved P r int ed in t he U n it ed St a t es of A merica N o pa rt of t his p u blica t ionma y b e reprodu ced s t ored in a ret r ieva l s ys t em or t ra ns mit t ed in a ny form or b y a nymeans electronic mechanical photocopying or otherwise without the prior writtenpermission of the publisher SAS Institute Inc

US Government Restricted Rights Notice Use du plicat ion or disclosure of the

software by the government is subject to restrictions as set forth in FAR 52227ndash19Commercial Computer Softw ar e-Restricted Rights (J une 1987)

SAS Institute Inc SAS Campus Drive Cary North Carolina 27513

1st print ing October 1999

S AS reg a nd a ll ot her SA S I ns t it u t e I nc p rodu ct or s ervice na m es a re regis t ered t ra dema rkst d k f SA S I t it t I i t h U S A d t h t i reg i di t U SA

Page 27: Statespace Model

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Chapter 18 Details

The forward autoregressive form based on the past observations is written as follows

x

t

=

p

X

i = 1

p

i

x

t i

+ e

t

The backward autoregressive form based on the future observations is written as fol-

lows

x

t

=

p

X

i = 1

p

i

x

t + i

+ n

t

LettingE

denote the expected value operator the autocovariance sequence for thex

t

series

i

is

i

= E x

t

x

0

t i

The Yule-Walker equations for the autoregressive model that matches the first p ele-

ments of the autocovariance sequence are

2

6

6

4

0

1

p 1

0

1

0

p 2

0

p 1

0

p 2

0

3

7

7

5

2

6

6

4

p

1

p

2

p

p

3

7

7

5

=

2

6

6

4

1

2

p

3

7

7

5

and

2

6

6

6

4

0

0

1

0

p 1

1

0

0

p 2

p 1

p 2

0

3

7

7

7

5

2

6

6

4

p

1

p

2

p

p

3

7

7

5

=

2

6

6

4

0

1

0

2

0

p

3

7

7

5

Here

p

i

are the coefficient matrices for the past observation form of the vector au-toregressive model and

p

i

are the coefficient matrices for the future observation

form More information on the Yule-Walker equations in the multivariate setting can

be found in Whittle (1963) and Ansley and Newbold (1979)

The innovation variance matrices for the two forms can be written as follows

p

=

0

p

X

i = 1

p

i

0

i

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p

=

0

p

X

i = 1

p

i

i

The autoregressive models are fit to the data using the preceding Yule-Walker equa-

tions with

i

replaced by the sample covariance sequenceC

i

The covariance matri-

ces are calculated as

C

i

=

1

N 1

N

X

t = i + 1

x

t

x

0

t i

Let b

p

b

p

b

p

and b

p

represent the Yule-Walker estimates of

p

p

p

and

p

respectively These matrices are written to an output data set when the OUTAR=

option is specified

When the PRINTOUT=LONG option is specified the sequence of matrices b

p

and

the corresponding correlation matrices are printed The sequence of matricesb

p isused to compute Akaike information criteria for selection of the autoregressive order

of the process

Akaike Information Criterion

The Akaike information criterion or AIC is defined as -2( maximum of log likeli-

hood )+2(number of parameters) Since the vector autoregressive models are esti-

mates from the Yule-Walker equations not by maximum likelihood the exact likeli-

hood values are not available for computing the AIC However for the vector autore-

gressive model the maximum of the log likelihood can be approximated as

l n L

n

2

l n j

b

p

j

Thus the AIC for the order p model is computed as

A I C

p

= n l n j

b

p

j + 2 p r

2

You can use the printed AIC array to compute a likelihood ratio test of the autoregres-

sive order The log-likelihood ratio test statistic for testing the order p model against

the order p 1

model is

n l n j

b

p

j + n l n j

b

p 1

j

This quantity is asymptotically distributed as a

2 withr

2 degrees of freedom if the

series is autoregressive of order p 1

It can be computed from the AIC array as

A I C

p 1

A I C

p

+ 2 r

2

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You can evaluate the significance of these test statistics with the PROBCHI function

in a SAS DATA step or with a

2 table

Determining the Autoregressive Order

Although the autoregressive models can be used for prediction their primary value is

to aid in the selection of a suitable portion of the sample covariance matrix for use in

computing canonical correlations If the multivariate time series x

t is of autoregres-sive order p then the vector of past values to lag p is considered to contain essentially

all the information relevant for prediction of future values of the time series

By default PROC STATESPACE selects the order p producing the autoregressive

model with the smallestA I C

p

If the value p for the minimumA I C

p

is less than the

value of the PASTMIN= option then p is set to the PASTMIN= value Alternatively

you can use the ARMAX= and PASTMIN= options to force PROC STATESPACE to

use an order you select

Significance Limits for Partial Autocorrelations

The STATESPACE procedure prints a schematic representation of the partial autocor-

relation matrices indicating which partial autocorrelations are significantly greater orsignificantly less than 0 Figure 1810 shows an example of this table

The STATESPACE Procedure

Schematic Representation of Partial Autocorrelations

NameLag 1 2 3 4 5 6 7 8 9 10

x ++ +

y ++

+ is gt 2std error - is lt -2std error is between

Figure 1810 Significant Partial Autocorrelations

The partial autocorrelations are from the sample partial autoregressive matrices b

p

p

The standard errors used for the significance limits of the partial autocorrelations are

computed from the sequence of matrices

p

and

p

Under the assumption that the observed series arises from an autoregressive process

of order p 1

the pth sample partial autoregressive matrix b

p

p

has an asymptotic

variance matrix 1

n

1

p

p

The significance limits for b

p

p

used in the schematic plot of the sample partial autore-

gressive sequence are derived by replacing

p

and

p

with their sample estimators

to produce the variance estimate as follows

d

V a r

b

p

p

=

1

n r p

b

1

p

b

p

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Canonical Correlation Analysis

Given the order p letp

t

be the vector of current and past values relevant to prediction

of x

t + 1

p

t

= x

0

t

x

0

t 1

x

0

t p

0

Letf

t

be the vector of current and future values

f

t

= x

0

t

x

0

t + 1

x

0

t + p

0

In the canonical correlation analysis consider submatrices of the sample covariance

matrix of p

t

andf

t

This covariance matrixV

has a block Hankel form

V =

2

6

6

6

4

C

0

C

0

1

C

0

2

C

0

p

C

0

1

C

0

2

C

0

3

C

0

p + 1

C

0

p

C

0

p + 1

C

0

p + 2

C

0

2 p

3

7

7

7

5

State Vector Selection Process

The canonical correlation analysis forms a sequence of potential state vectorsz

j

t

Examine a sequencef

j

t

of subvectors of f

t

and form the submatrixV

j consisting

of the rows and columns of V

corresponding to the components of f

j

t

and compute

its canonical correlations

The smallest canonical correlation of V

j is then used in the selection of the com-

ponents of the state vector The selection process is described in the following For

more details about this process refer to Akaike (1976)

In the following discussion the notationx

t + k j t

denotes the wide sense conditional

expectation (best linear predictor) of x

t + k

given allx

s

with s less than or equal to t

In the notationx

i t + 1

the first subscript denotes the ith component of x

t + 1

The initial state vectorz

1

t

is set tox

t

The sequencef

j

t

is initialized by setting

f

1

t

= z

1

0

t

x

1 t + 1 j t

0

= x

0

t

x

1 t + 1 j t

0

That is start by considering whether to addx

1 t + 1 j t

to the initial state vectorz

1

t

The procedure forms the submatrixV

1 corresponding tof

1

t

and computes its canon-

ical correlations Denote the smallest canonical correlation of V

1 as

m i n

If

m i n

is

significantly greater than 0x

1 t + 1 j t

is added to the state vector

If the smallest canonical correlation of V

1 is not significantly greater than 0 then a

linear combination of f

1

t

is uncorrelated with the pastp

t

Assuming that the determi-

nant of C

0

is not 0 (that is no input series is a constant) you can take the coefficient

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Chapter 18 Details

of x

1 t + 1 j t

in this linear combination to be 1 Denote the coefficients of z

1

t

in this

linear combination as

This gives the relationship

x

1 t + 1 j t

=

0

x

t

Therefore the current state vector already contains all the past information useful forpredicting

x

1 t + 1

and any greater leads of x

1 t

The variablex

1 t + 1 j t

is not added to

the state vector nor are any termsx

1 t + k j t

considered as possible components of the

state vector The variablex

1

is no longer active for state vector selection

The process described forx

1 t + 1 j t

is repeated for the remaining elements of f

t

The

next candidate for inclusion in the state vector is the next component of f

t

corre-

sponding to an active variable Components of f

t

corresponding to inactive variables

that produced a zero

m i n

in a previous step are skipped

Denote the next candidate asx

l t + k j t

The vectorf

j

t

is formed from the current state

vector andx

l t + k j t

as follows

f

j

t

= z

j

0

t

x

l t + k j t

0

The matrixV

j is formed fromf

j

t

and its canonical correlations are computed The

smallest canonical correlation of V

j is judged to be either greater than or equal to 0

If it is judged to be greater than 0x

l t + k j t

is added to the state vector If it is judged

to be 0 then a linear combination of f

j

t

is uncorrelated with thep

t

and the variable

x

l

is now inactive

The state vector selection process continues until no active variables remain

Testing Significance of Canonical Correlations For each step in the canonical correlation sequence the significance of the small-

est canonical correlation

m i n

is judged by an information criterion from Akaike

(1976) This information criterion is

n l n 1

2

m i n

r p + 1 q + 1

where q is the dimension of f

j

t

at the current step r is the order of the state vector p

is the order of the vector autoregressive process and

is the value of the SIGCORR=

option The default is SIGCORR=2 If this information criterion is less than or equal

to 0

m i n

is taken to be 0 otherwise it is taken to be significantly greater than 0

(Do not confuse this information criterion with the AIC)

Variables inx

t + p j t

are not added in the model even with positive information crite-

rion because of the singularity of V

You can force the consideration of more candi-

date state variables by increasing the size of theV

matrix by specifying a PASTMIN=

option value larger than p

Printing the Canonical Correlations

To print the details of the canonical correlation analysis process specify the CAN-

CORR option in the PROC STATESPACE statement The CANCORR option prints

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the candidate state vectors the canonical correlations and the information criteria for

testing the significance of the smallest canonical correlation

Bartlettrsquos

2 and its degrees of freedom are also printed when the CANCORR option

is specified The formula used for Bartlettrsquos

2 is

2

= n 5 r p + 1 q + 1 l n 1

2

m i n

withr p + 1 q + 1

degrees of freedom

Figure 1811 shows the output of the CANCORR option for the introductory example

shown in the Getting Started section of this chapter

The STATESPACE Procedure

Canonical Correlations Analysis

Information Chi

x(TT) y(TT) x(T+1T) Criterion Square DF

1 1 0237045 3566167 114505 4

Information Chi

x(TT) y(TT) x(T+1T) y(T+1T) Criterion Square DF

1 1 0238244 0056565 -535906 0636134 3

Information Chi

x(TT) y(TT) x(T+1T) x(T+2T) Criterion Square DF

1 1 0237602 0087493 -446312 1525353 3

Figure 1811 Canonical Correlations Analysis

New variables are added to the state vector if the information criteria are positive Inthis example

y

t + 1 j t

andx

t + 2 j t

are not added to the state space vector because the

information criteria for these models are negative

If the information criterion is nearly 0 then you may want to investigate models

that arise if the opposite decision is made regarding

m i n

This investigation can be

accomplished by using a FORM statement to specify part or all of the state vector

Preliminary Estimates of F

When a candidate variablex

l t + k j t

yields a zero

m i n

and is not added to the state

vector a linear combination of f

j

t

is uncorrelated with thep

t

Because of the method

used to construct thef

j

t

sequence the coefficient of x

l t + k j t

inl

can be taken as 1

Denote the coefficients of z

j

t

in this linear combination as l

This gives the relationship

x

l t + k j t

= l

0

z

j

t

The vectorl

is used as a preliminary estimate of the first r columns of the row of the

transition matrixF

corresponding tox

l t + k 1 j t

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Chapter 18 Details

Parameter Estimation

The model isz

t + 1

= F z

t

+ G e

t + 1

wheree

t

is a sequence of independent multivari-

ate normal innovations with mean vector 0 and variance

e e

The observed sequence

x

t

composes the first r components of z

t

and thusx

t

= H z

t

where H is ther s

matrix I

r

0

LetE

be ther n

matrix of innovations

E = e

1

e

n

If the number of observations n is reasonably large the log likelihood L can be

approximated up to an additive constant as follows

L =

n

2

l n j

e e

j

1

2

t r a c e

1

e e

E E

0

The elements of

e e

are taken as free parameters and are estimated as follows

S

0

=

1

n

E E

0

Replacing

e e

byS

0

in the likelihood equation the log likelihood up to an additive

constant is

L =

n

2

l n j S

0

j

Letting B be the backshift operator the formal relation betweenx

t

ande

t

is

x

t

= H I B F

1

G e

t

e

t

= H I B F

1

G

1

x

t

=

1

X

i = 0

i

x

t i

LettingC

i

be the ith lagged sample covariance of x

t

and neglecting end effects the

matrixS

0

is

S

0

=

1

X

i j = 0

i

C

i + j

0

j

For the computation of S

0

the infinite sum is truncated at the value of the KLAG=

option The value of the KLAG= option should be large enough that the sequence

i

is approximately 0 beyond that point

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Let

be the vector of free parameters in theF

andG

matrices The derivative of the

log likelihood with respect to the parameter

is

L

=

n

2

t r a c e

S

1

0

S

0

The second derivative is

2

L

0

=

n

2

t r a c e

S

1

0

S

0

0

S

1

0

S

0

t r a c e

S

1

0

2

S

0

0

Near the maximum the first term is unimportant and the second term can be approx-

imated to give the following second derivative approximation

2

L

0

=

n t r a c e

S

1

0

E

E

0

0

The first derivative matrix and this second derivative matrix approximation are com-

puted from the sample covariance matrixC

0

and the truncated sequence

i

The

approximate likelihood function is maximized by a modified Newton-Raphson algo-

rithm employing these derivative matrices

The matrixS

0

is used as the estimate of the innovation covariance matrix

e e

The

negative of the inverse of the second derivative matrix at the maximum is used as an

approximate covariance matrix for the parameter estimates The standard errors of

the parameter estimates printed in the parameter estimates tables are taken from the

diagonal of this covariance matrix The parameter covariance matrix is printed when

the COVB option is specified

If the data are nearly nonstationary a better estimate of

e e

and the other parameters

can sometimes be obtained by specifying the RESIDEST option The RESIDEST

option estimates the parameters using conditional least squares instead of maximum

likelihood

The residuals are computed using the state space equation and the sample mean val-

ues of the variables in the model as start-up values The estimate of S

0

is then com-

puted using the residuals from the ith observation on where i is the maximum number

of times any variable occurs in the state vector A multivariate Gauss-Marquardt al-

gorithm is used to minimizej S

0

j

Refer to Harvey (1981a) for a further description

of this method

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Forecasting

Given estimates of F

G

and

e e

forecasts of x

t

are computed from the conditional

expectation of z

t

In forecasting the parameters F G and

e e

are replaced with the estimates or by val-

ues specified in the RESTRICT statement One-step-ahead forecasting is performedfor the observation

x

t

wheret n b

Heren

is the number of observations and

b is the value of the BACK= option For the observationx

t

wheret n b

m-

step-ahead forecasting is performed form = t n + b

The forecasts are generated

recursively with the initial conditionz

0

= 0

The m-step-ahead forecast of z

t + m

isz

t + m j t

wherez

t + m j t

denotes the conditional

expectation of z

t + m

given the information available at time t The m-step-ahead

forecast of x

t + m

isx

t + m j t

= H z

t + m j t

where the matrixH = I

r

0

Let

i

= F

i

G

Note that the lasts r

elements of z

t

consist of the elements of x

u j t

foru t

The state vector z

t + m

can be represented as

z

t + m

= F

m

z

t

+

m 1

X

i = 0

i

e

t + m i

Sincee

t + i j t

= 0

fori 0

the m-step-ahead forecastz

t + m j t

is

z

t + m j t

= F

m

z

t

= F z

t + m 1 j t

Therefore the m-step-ahead forecast of x

t + m

is

x

t + m j t

= H z

t + m j t

The m-step-ahead forecast error is

z

t + m

z

t + m j t

=

m 1

X

i = 0

i

e

t + m i

The variance of the m-step-ahead forecast error is

V

z m

=

m 1

X

i = 0

i

e e

0

i

LettingV

z 0

= 0

the variance of the m-step-ahead forecast error of z

t + m

V

z m

can

be computed recursively as follows

V

z m

= V

z m 1

+

m 1

e e

0

m 1

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The variance of the m-step-ahead forecast error of x

t + m

is ther r

left upper sub-

matrix of V

z m

that is

V

x m

= H V

z m

H

0

Unless the NOCENTER option is specified the sample mean vector is added to the

forecast When differencing is specified the forecasts xt + m j t

plus the sample mean

vector are integrated back to produce forecasts for the original series

Lety

t

be the original series specified by the VAR statement with some 0 values

appended corresponding to the unobserved past observations Let B be the backshift

operator and let B

be thes s

matrix polynomial in the backshift operator

corresponding to the differencing specified by the VAR statement The off-diagonal

elements of

i

are 0 Note that

0

= I

s

whereI

s

is thes s

identity matrix Then

z

t

= B y

t

This gives the relationship

y

t

=

1

B z

t

=

1

X

i = 0

i

z

t i

where

1

B =

P

1

i = 0

i

B

i and

0

= I

s

The m-step-ahead forecast of y

t + m

is

y

t + m j t

=

m 1

X

i = 0

i

z

t + m i j t

+

1

X

i = m

i

z

t + m i

The m-step-ahead forecast error of y

t + m

is

m 1

X

i = 0

i

z

t + m i

z

t + m i j t

=

m 1

X

i = 0

i

X

u = 0

u

i u

e

t + m i

LettingV

y 0

= 0

the variance of the m-step-ahead forecast error of y

t + m

V

y m

is

V

y m

=

m 1

X

i = 0

i

X

u = 0

u

i u

e e

i

X

u = 0

u

i u

0

= V

y m 1

+

m 1

X

u = 0

u

m 1 u

e e

m 1

X

u = 0

u

m 1 u

0

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Chapter 18 Details

Relation of ARMA and State Space Forms

Every state space model has an ARMA representation and conversely every ARMA

model has a state space representation This section discusses this equivalence The

following material is adapted from Akaike (1974) where there is a more complete

discussion Pham-Dinh-Tuan (1978) also contains a discussion of this material

Suppose you are given the following ARMA model

B x

t

= B e

t

or in more detail

x

t

1

x

t 1

p

x

t p

= e

t

+

1

e

t 1

+ +

q

e

t q

(1)

wheree

t

is a sequence of independent multivariate normal random vectors with mean

0 and variance matrix

e e

B is the backshift operator (B x

t

= x

t 1

) B

and

B

are matrix polynomials in B andx

t

is the observed process

If the roots of the determinantial equationj B j = 0

are outside the unit circle in

the complex plane the model can also be written as

x

t

=

1

B B e

t

=

1

X

i = 0

i

e

t i

The

i

matrices are known as the impulse response matrices and can be computed

as

1

B B

You can assume p q

since if this is not initially true you can add more terms

i

that are identically 0 without changing the model

To write this set of equations in a state space form proceed as follows Letx

t + i j t

be

the conditional expectation of x

t + i

givenx

w

forw t

The following relations hold

x

t + i j t

=

1

X

j = i

j

e

t + i j

x

t + i j t + 1

= x

t + i j t

+

i 1

e

t + 1

However from equation (1) you can derive the following relationship

x

t + p j t

=

1

x

t + p 1 j t

+ +

p

x

t

(2)

Hence wheni = p

you can substitute forx

t + p j t

in the right-hand side of equation

(2) and close the system of equations

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This substitution results in the following model in the state space form

z

t + 1

= F z

t

+ G e

t + 1

2

6

6

6

4

x

t + 1

x

t + 2 j t + 1

x

t + p j t + 1

3

7

7

7

5

=

2

6

6

4

0 I 0 0

0 0 I 0

p

p 1

1

3

7

7

5

2

6

6

6

4

x

t

x

t + 1 j t

x

t + p 1 j t

3

7

7

7

5

+

2

6

6

4

I

1

p 1

3

7

7

5

e

t + 1

Note that the state vectorz

t

is composed of conditional expectations of x

t

and the

first r components of z

t

are equal tox

t

The state space form can be cast into an ARMA form by solving the system of dif-

ference equations for the first r components

When converting from an ARMA form to a state space form you can generate a

state vector larger than needed that is the state space model may not be a minimal

representation When going from a state space form to an ARMA form you can have

nontrivial common factors in the autoregressive and moving average operators that

yield an ARMA model larger than necessary

If the state space form used is not a minimal representation some but not all compo-

nents of x

t + i j t

may be linearly dependent This situation corresponds to

p

p 1

being of less than full rank when B

and B

have no common nontrivial

left factors In this casez

t

consists of a subset of the possible components of

x

t + i j t

i = 1 2 p 1

However once a component of x

t + i j t

(for example the

jth one) is linearly dependent on the previous conditional expectations then all sub-

sequent jth components of x

t + k j t

fork i

must also be linearly dependent Note

that in this case equivalent but seemingly different structures can arise if the order of

the components withinx

t

is changed

OUT= Data Set

The forecasts are contained in the output data set specified by the OUT= option on the

PROC STATESPACE statement The OUT= data set contains the following variables

the BY variables

the ID variable

the VAR statement variables These variables contain the actual values from

the input data set

FORi numeric variables containing the forecasts The variable FORi contains

the forecasts for the ith variable in the VAR statement list Forecasts are one-

step-ahead predictions until the end of the data or until the observation specified

by the BACK= option

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Chapter 18 Details

RESi numeric variables containing the residual for the forecast of the ith vari-

able in the VAR statement list For forecast observations the actual values are

missing and the RESi variables contain missing values

STDi numeric variables containing the standard deviation for the forecast of

the ith variable in the VAR statement list The values of the STDi variables can

be used to construct univariate confidence limits for the corresponding fore-casts However such confidence limits do not take into account the covariance

of the forecasts

OUTAR= Data Set

The OUTAR= data set contains the estimates of the preliminary autoregressive mod-

els The OUTAR= data set contains the following variables

ORDER a numeric variable containing the order p of the autoregressive model

that the observation represents

AIC a numeric variable containing the value of the information criterionA I C

p

SIGFl numeric variables containing the estimate of the innovation covariance

matrices for the forward autoregressive models The variable SIGFl contains

the lth column of b

p

in the observations with ORDER= p

SIGBl numeric variables containing the estimate of the innovation covariance

matrices for the backward autoregressive models The variable SIGBl contains

the lth column of b

p

in the observations with ORDER= p

FORk ndashl numeric variables containing the estimates of the autoregressive pa-

rameter matrices for the forward models The variable FORk ndashl contains the

lth column of the lag k autoregressive parameter matrix b

p

k

in the observations

with ORDER= p

BACk ndashl numeric variables containing the estimates of the autoregressive pa-

rameter matrices for the backward models The variable BACk ndashl contains the

lth column of the lag k autoregressive parameter matrix b

p

k

in the observations

with ORDER= p

The estimates for the order p autoregressive model can be selected as those observa-

tions with ORDER= p Within these observations the k lth element of

p

i

is given by

the value of the FORindashl variable in the k th observation The k lth element of

p

i

is

given by the value of BACindashl variable in the k th observation The k lth element of

p

is given by SIGFl in the k th observation The k lth element of

p

is given by SIGBlin the k th observation

Table 181 shows an example of the OUTAR= data set with ARMAX=3 andx

t

of

dimension 2 In Table 181 i j

indicate the ijth element of the matrix

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Part 2 General Information

Table 181 Values in the OUTAR= Data Set

Obs ORDER AIC SIGF1 SIGF2 SIGB1 SIGB2 FOR1ndash 1 FOR1ndash 2 FOR2ndash 1 FOR2ndash 2 FOR3ndash 1

1 0 AIC0

0 1 1

0 1 2

0 1 1

0 1 2

2 0 AIC0

0 2 1

0 2 2

0 2 1

0 2 2

3 1 AIC1

1 1 1

1 1 2

1 1 1

1 1 2

1

1

1 1

1

1

1 2

4 1 AIC1

1 2 1

1 1 2

1 2 1

1 1 2

1

1

2 1

1

1

2 2

5 2 AIC2

2 1 1

2 1 2

2 1 1

2 1 2

2

1

1 1

2

1

1 2

2

2

1 1

2

2

1 2

6 2 AIC2

2 2 1

2 1 2

2 2 1

2 1 2

2

1

2 1

2

1

2 2

2

2

2 1

2

2

2 2

7 3 AIC3

3 1 1

3 1 2

3 1 1

3 1 2

3

1

1 1

3

1

1 2

3

2

1 1

3

2

1 2

3

3

1 1

8 3 AIC3

3 2 1

3 1 2

3 2 1

3 1 2

3

1

2 1

3

1

2 2

3

2

2 1

3

2

2 2

3

3

2 1

Obs FOR3ndash 2 BACK1ndash 1 BACK1ndash 2 BACK2ndash 1 BACK2ndash 2 BACK3ndash 1 BACK3ndash 2

1

2

3

1

1

1 1

1

1

1 2

4

1

1

2 1

1

1

2 2

5

2

1

1 1

2

1

1 2

2

2

1 1

2

2

1 2

6

2

1

2 1

2

1

2 2

2

2

2 1

2

2

2 2

7

3

3

1 2

3

1

1 1

3

1

1 2

3

2

1 1

3

2

1 2

3

3

1 1

3

3

1 2

8

3

3

2 2

3

1

2 1

3

1

2 2

3

2

2 1

3

2

2 2

3

3

2 1

3

3

2 2

The estimated autoregressive parameters can be used in the IML procedure to obtain

autoregressive estimates of the spectral density function or forecasts based on the

autoregressive models

OUTMODEL= Data Set

The OUTMODEL= data set contains the estimates of theF

andG

matrices and their

standard errors the names of the components of the state vector and the estimates of

the innovation covariance matrix The variables contained in the OUTMODEL= data

set are as follows

the BY variables

STATEVEC a character variable containing the name of the component of the

state vector corresponding to the observation The STATEVEC variable has

the value STD for standard deviations observations which contain the standarderrors for the estimates given in the preceding observation

Fndash j numeric variables containing the columns of theF

matrix The variable

Fndash j contains the jth column of F

The number of Fndash j variables is equal to

the value of the DIMMAX= option If the model is of smaller dimension the

extraneous variables are set to missing

Gndash j numeric variables containing the columns of theG

matrix The variable

Gndash j contains the jth column of G

The number of Gndash j variables is equal to r

the dimension of x

t

given by the number of variables in the VAR statement

SIGndash j numeric variables containing the columns of the innovation covariance

matrix The variable SIGndash j contains the jth column of

e e

There are r vari-ables SIGndash j

Table 182 shows an example of the OUTMODEL= data set withx

t

= x

t

y

t

0

z

t

= x

t

y

t

x

t + 1 j t

0 and DIMMAX=4 In Table 182F

i j

andG

i j

are the ijth

elements of F

andG

respectively Note that all elements for Fndash4 are missing because

F

is a3 3

matrix

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Chapter 18 Details

Table 182 Value in the OUTMODEL= Data Set

Obs STATEVEC Fndash 1 Fndash 2 Fndash 3 Fndash 4 Gndash 1 Gndash 2 SIGndash 1 SIGndash 2

1 X(TT) 0 0 1 1 0

1 1

1 2

2 STD

3 Y(TT) F2 1

F2 2

F2 3

0 1

2 1

2 2

4 STD std F2 1

std F2 2

std F2 3

5 X(T+1T) F3 1

F3 2

F3 3

G3 1

G3 2

6 STD std F3 1

std F3 2

std F3 3

std G3 1

std G3 2

Printed Output

The printed output produced by the STATESPACE procedure is described in the fol-

lowing

1 descriptive statistics which include the number of observations used the

names of the variables their means and standard deviations (Std) and the dif-

ferencing operations used

2 the Akaike information criteria for the sequence of preliminary autoregressive

models

3 if the PRINTOUT=LONG option is specified the sample autocovariance ma-

trices of the input series at various lags

4 if the PRINTOUT=LONG option is specified the sample autocorrelation ma-

trices of the input series

5 a schematic representation of the autocorrelation matrices showing the signif-

icant autocorrelations

6 if the PRINTOUT=LONG option is specified the partial autoregressive ma-

trices (These are

p

p

as described in Preliminary Autoregressive Modelsearlier in this chapter)

7 a schematic representation of the partial autocorrelation matrices showing the

significant partial autocorrelations

8 the Yule-Walker estimates of the autoregressive parameters for the autoregres-

sive model with the minimum AIC

9 if the PRINTOUT=LONG option is specified the autocovariance matrices of

the residuals of the minimum AIC model This is the sequence of estimated

innovation variance matrices for the solutions of the Yule-Walker equations

10 if the PRINTOUT=LONG option is specified the autocorrelation matrices of the residuals of the minimum AIC model

11 If the CANCORR option is specified the canonical correlations analysis for

each potential state vector considered in the state vector selection process

This includes the potential state vector the canonical correlations the informa-

tion criterion for the smallest canonical correlation Bartlettrsquos

2 statistic (ldquoChi

Squarerdquo) for the smallest canonical correlation and the degrees of freedom of

Bartlettrsquos

2

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Part 2 General Information

12 the components of the chosen state vector

13 the preliminary estimate of the transition matrixF

the input matrixG

and

the variance matrix for the innovations

e e

14 if the ITPRINT option is specified the iteration history of the likelihood max-

imization For each iteration this shows the iteration number the number of step halvings the determinant of the innovation variance matrix the damping

factor Lambda and the values of the parameters

15 the state vector printed again to aid interpretation of the following listing of F

andG

16 the final estimate of the transition matrixF

17 the final estimate of the input matrixG

18 the final estimate of the variance matrix for the innovations

e e

19 a table listing the estimates of the free parameters inF

andG

and their standarderrors and t statistics

20 if the COVB option is specified the covariance matrix of the parameter esti-

mates

21 if the COVB option is specified the correlation matrix of the parameter esti-

mates

22 if the PRINT option is specified the forecasts and their standard errors

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Chapter 18 Details

ODS Table Names

PROC STATESPACE assigns a name to each table it creates You can use these

names to reference the table when using the Output Delivery System (ODS) to select

tables and create output data sets These names are listed in the following table For

more information on ODS see Chapter 6 ldquoUsing the Output Delivery Systemrdquo

Table 183 ODS Tables Produced in PROC STATESPACE

ODS Table Name Description Option

NObs Number of observations default

Summary Simple summary statistics table default

InfoCriterion Information criterion table default

CovLags Covariance Matrices of Input Series PRINTOUT=LONG

CorrLags Correlation Matrices of Input Series PRINTOUT=LONG

PartialAR Partial Autoregressive Matrices PRINTOUT=LONG

YWEstimates Yule-Walker Estimates for Minimum AIC default

CovResiduals Covariance of Residuals PRINTOUT=LONGCorrResiduals Residual Correlations from AR Models PRINTOUT=LONG

StateVector State vector table default

CorrGraph Schematic Representation of Correlations default

TransitionMatrix Transition Matrix default

InputMatrix Input Matrix default

VarInnov Variance Matrix for the Innovation default

CovB Covariance of Parameter Estimates COVB

CorrB Correlation of Parameter Estimates COVB

CanCorr Canonical Correlation Analysis CANCORR

IterHistory Iterative Fitting table ITPRINT

ParameterEstimates Parameter Estimates Table default

Forecasts Forecasts Table PRINT

ConvergenceStatus Convergence Status Table default

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Part 2 General Information

Example

Example 181 Series J from Box and Jenkins

This example analyzes the gas furnace data (series J) from Box and Jenkins (The

data are not shown Refer to Box and Jenkins (1976) for the data)

First a model is selected and fit automatically using the following statements

title1 rsquoGas Furnace Datarsquo

title2 rsquoBox amp Jenkins Series Jrsquo

title3 rsquoAutomatically Selected Modelrsquo

proc statespace data=seriesj cancorr

var x y

run

The results for the automatically selected model are shown in Output 1811

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Chapter 18 Example

Output 1811 Results for Automatically Selected Model

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Number of Observations 296

Standard

Variable Mean Error

x -005683 1072766

y 5350912 3202121

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Information Criterion for Autoregressive Models

Lag=0 Lag=1 Lag=2 Lag=3 Lag=4 Lag=5 Lag=6 Lag=7 Lag=8

6513862 -103357 -163296 -164512 -165152 -164891 -164934 -164315 -163856

Information

Criterion for

Autoregressive

Models

Lag=9 Lag=10

-16348 -163359

Schematic Representation of Correlations

NameLag 0 1 2 3 4 5 6 7 8 9 10

x +- +- +- +- +- +- +- +- +- +- +-

y -+ -+ -+ -+ -+ -+ -+ -+ -+ -+ -+

+ is gt 2std error - is lt -2std error is between

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Part 2 General Information

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Schematic Representation of Partial Autocorrelations

NameLag 1 2 3 4 5 6 7 8 9 10

x + - + -

y -+ -- - + +

+ is gt 2std error - is lt -2std error is between

Yule-Walker Estimates for Minimum AIC

------Lag=1------ ------Lag=2------ ------Lag=3------ ------Lag=4------

x y x y x y x y

x 1925887 -000124 -120166 0004224 0116918 -000867 0104236 0003268

y 0050496 1299793 -002046 -03277 -071182 -025701 0195411 0133417

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Canonical Correlations Analysis

Information Chi

x(TT) y(TT) x(T+1T) Criterion Square DF

1 1 0804883 2929228 3047481 8

Information Chix(TT) y(TT) x(T+1T) y(T+1T) Criterion Square DF

1 1 0906681 0607529 1223358 1347237 7

Information Chi

x(TT) y(TT) x (T+1T) y (T+1T) x (T+2T) Criterion Square DF

1 1 0909434 0610278 0186274 -154701 1034705 6

Information Chi

x(TT) y(TT) x (T+1T) y (T+1T) y (T+2T) Criterion Square DF

1 1 091014 0618937 0206823 0940392 1280924 6

Information Chi

x(TT) y(TT) x(T+1T) y(T+1T) y(T+2T) y(T+3T) Criterion Square DF

1 1 0912963 0628785 0226598 0083258 -794103 2041584 5

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Chapter 18 Example

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Selected Statespace Form and Preliminary Estimates

State Vector

x(TT) y(TT) x(T+1T) y(T+1T) y(T+2T)

Estimate of Transition Matrix

0 0 1 0 0

0 0 0 1 0

-084718 0026794 1711715 -005019 0

0 0 0 0 1

-019785 0334274 -018174 -123557 1787475

Input Matrix for Innovation

1 0

0 11925887 -000124

0050496 1299793

0142421 1361696

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Selected Statespace Form and Preliminary Estimates

Variance Matrix for Innovation

0035274 -000734

-000734 0097569

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Part 2 General Information

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Selected Statespace Form and Fitted Model

State Vector

x(TT) y(TT) x(T+1T) y(T+1T) y(T+2T)

Estimate of Transition Matrix

0 0 1 0 0

0 0 0 1 0

-086192 0030609 1724235 -005483 0

0 0 0 0 1

-034839 0292124 -009435 -109823 1671418

Input Matrix for Innovation

1 0

0 1192442 -000416

0015621 1258495

008058 1353204

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Selected Statespace Form and Fitted Model

Variance Matrix for Innovation

0035579 -000728

-000728 0095577

Parameter Estimates

Standard

Parameter Estimate Error t Value

F(31) -086192 0072961 -1181

F(32) 0030609 0026167 117

F(33) 1724235 0061599 2799

F(34) -005483 0030169 -182

F(51) -034839 0135253 -258

F(52) 0292124 0046299 631

F(53) -009435 0096527 -098F(54) -109823 0109525 -1003

F(55) 1671418 0083737 1996

G(31) 1924420 0058162 3309

G(32) -000416 0035255 -012

G(41) 0015621 0095771 016

G(42) 1258495 0055742 2258

G(51) 0080580 0151622 053

G(52) 1353204 0091388 1481

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Chapter 18 Example

The two series are believed to have a transfer function relation with the gas rate

(variable X) as the input and the CO2

concentration (variable Y) as the output Since

the parameter estimates shown in Output 1811 support this kind of model the model

is reestimated with the feedback parameters restricted to 0 The following statements

fit the transfer function (no feedback) model

title3 rsquoTransfer Function Modelrsquo

proc statespace data=seriesj printout=none

var x y

restrict f(32)=0 f(34)=0

g(32)=0 g(41)=0 g(51)=0

run

The last 2 pages of the output are shown in Output 1812

Output 1812 STATESPACE Output for Transfer Function Model

Gas Furnace Data

Box amp Jenkins Series J

Transfer Function Model

The STATESPACE Procedure

Selected Statespace Form and Fitted Model

State Vector

x(TT) y(TT) x(T+1T) y(T+1T) y(T+2T)

Estimate of Transition Matrix

0 0 1 0 0

0 0 0 1 0

-068882 0 1598717 0 0

0 0 0 0 1

-035944 0284179 -00963 -107313 1650047

Input Matrix for Innovation

1 0

0 1

1923446 0

0 1260856

0 1346332

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Part 2 General Information

Gas Furnace Data

Box amp Jenkins Series J

Transfer Function Model

The STATESPACE Procedure

Selected Statespace Form and Fitted Model

Variance Matrix for Innovation

0036995 -00072

-00072 0095712

Parameter Estimates

Standard

Parameter Estimate Error t Value

F(31) -068882 0050549 -1363

F(33) 1598717 0050924 3139

F(51) -035944 0229044 -157

F(52) 0284179 0096944 293

F(53) -009630 0140876 -068

F(54) -107313 0250385 -429

F(55) 1650047 0188533 875G(31) 1923446 0056328 3415

G(42) 1260856 0056464 2233

G(52) 1346332 0091086 1478

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Chapter 18 References

References

Akaike H (1974) Markovian Representation of Stochastic Processes and Its Ap-

plication to the Analysis of Autoregressive Moving Average Processes Annals

of the Institute of Statistical Mathematics 26 363-387

Akaike H (1976) Canonical Correlations Analysis of Time Series and the Use of

an Information Criterion in Advances and Case Studies in System Identification

eds R Mehra and DG Lainiotis New York Academic Press

Anderson TW (1971) The Statistical Analysis of Time Series New York John

Wiley amp Sons

Ansley CF and Newbold P (1979) Multivariate Partial Autocorrelations Pro-

ceedings of the Business and Economic Statistics Section American Statistical

Association 349-353

Box GEP and Jenkins G (1976) Time Series Analysis Forecasting and Control

San Francisco Holden-Day

Brockwell PJ and Davis RA (1991) Time Series Theory and Methods 2nd Edi-

tion Springer-Verlag

Hannan EJ (1970) Multiple Time Series New York John Wiley amp Sons

Hannan EJ (1976) The Identification and Parameterization of ARMAX and State

Space Forms Econometrica 44 713-722

Harvey AC (1981a) The Econometric Analysis of Time Series New York John

Wiley amp Sons

Harvey AC (1981b) Time Series Models New York John Wiley amp Sons

Jones RH (1974) Identification and Autoregressive Spectrum Estimation IEEE Transactions on Automatic Control AC-19 894-897

Pham-Dinh-Tuan (1978) On the Fitting of Multivariate Processes of the

Autoregressive-Moving Average Type Biometrika 65 99-107

Priestley MB (1980) System Identification Kalman Filtering and Stochastic Con-

trol in Directions in Time Series eds DR Brillinger and GC Tiao Institute

of Mathematical Statistics

Whittle P (1963) On the Fitting of Multivariate Autoregressions and the Approxi-

mate Canonical Factorization of a Spectral Density Matrix Biometrika 50 129-

134

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The correct bibliographic citation for this manual is as follows SAS Institute Inc SAS

ET S U serrsquos Gui de Version 8 Cary NC SAS Institute Inc 1999 1546 pp

SASET S Userrsquos Guide Version 8

Copyright copy 1999 by S AS I ns t it u t e I nc Ca ry N C U S A

ISBN 1ndash58025ndash489ndash6

All r ight s res erved P r int ed in t he U n it ed St a t es of A merica N o pa rt of t his p u blica t ionma y b e reprodu ced s t ored in a ret r ieva l s ys t em or t ra ns mit t ed in a ny form or b y a nymeans electronic mechanical photocopying or otherwise without the prior writtenpermission of the publisher SAS Institute Inc

US Government Restricted Rights Notice Use du plicat ion or disclosure of the

software by the government is subject to restrictions as set forth in FAR 52227ndash19Commercial Computer Softw ar e-Restricted Rights (J une 1987)

SAS Institute Inc SAS Campus Drive Cary North Carolina 27513

1st print ing October 1999

S AS reg a nd a ll ot her SA S I ns t it u t e I nc p rodu ct or s ervice na m es a re regis t ered t ra dema rkst d k f SA S I t it t I i t h U S A d t h t i reg i di t U SA

Page 28: Statespace Model

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Part 2 General Information

p

=

0

p

X

i = 1

p

i

i

The autoregressive models are fit to the data using the preceding Yule-Walker equa-

tions with

i

replaced by the sample covariance sequenceC

i

The covariance matri-

ces are calculated as

C

i

=

1

N 1

N

X

t = i + 1

x

t

x

0

t i

Let b

p

b

p

b

p

and b

p

represent the Yule-Walker estimates of

p

p

p

and

p

respectively These matrices are written to an output data set when the OUTAR=

option is specified

When the PRINTOUT=LONG option is specified the sequence of matrices b

p

and

the corresponding correlation matrices are printed The sequence of matricesb

p isused to compute Akaike information criteria for selection of the autoregressive order

of the process

Akaike Information Criterion

The Akaike information criterion or AIC is defined as -2( maximum of log likeli-

hood )+2(number of parameters) Since the vector autoregressive models are esti-

mates from the Yule-Walker equations not by maximum likelihood the exact likeli-

hood values are not available for computing the AIC However for the vector autore-

gressive model the maximum of the log likelihood can be approximated as

l n L

n

2

l n j

b

p

j

Thus the AIC for the order p model is computed as

A I C

p

= n l n j

b

p

j + 2 p r

2

You can use the printed AIC array to compute a likelihood ratio test of the autoregres-

sive order The log-likelihood ratio test statistic for testing the order p model against

the order p 1

model is

n l n j

b

p

j + n l n j

b

p 1

j

This quantity is asymptotically distributed as a

2 withr

2 degrees of freedom if the

series is autoregressive of order p 1

It can be computed from the AIC array as

A I C

p 1

A I C

p

+ 2 r

2

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Chapter 18 Details

You can evaluate the significance of these test statistics with the PROBCHI function

in a SAS DATA step or with a

2 table

Determining the Autoregressive Order

Although the autoregressive models can be used for prediction their primary value is

to aid in the selection of a suitable portion of the sample covariance matrix for use in

computing canonical correlations If the multivariate time series x

t is of autoregres-sive order p then the vector of past values to lag p is considered to contain essentially

all the information relevant for prediction of future values of the time series

By default PROC STATESPACE selects the order p producing the autoregressive

model with the smallestA I C

p

If the value p for the minimumA I C

p

is less than the

value of the PASTMIN= option then p is set to the PASTMIN= value Alternatively

you can use the ARMAX= and PASTMIN= options to force PROC STATESPACE to

use an order you select

Significance Limits for Partial Autocorrelations

The STATESPACE procedure prints a schematic representation of the partial autocor-

relation matrices indicating which partial autocorrelations are significantly greater orsignificantly less than 0 Figure 1810 shows an example of this table

The STATESPACE Procedure

Schematic Representation of Partial Autocorrelations

NameLag 1 2 3 4 5 6 7 8 9 10

x ++ +

y ++

+ is gt 2std error - is lt -2std error is between

Figure 1810 Significant Partial Autocorrelations

The partial autocorrelations are from the sample partial autoregressive matrices b

p

p

The standard errors used for the significance limits of the partial autocorrelations are

computed from the sequence of matrices

p

and

p

Under the assumption that the observed series arises from an autoregressive process

of order p 1

the pth sample partial autoregressive matrix b

p

p

has an asymptotic

variance matrix 1

n

1

p

p

The significance limits for b

p

p

used in the schematic plot of the sample partial autore-

gressive sequence are derived by replacing

p

and

p

with their sample estimators

to produce the variance estimate as follows

d

V a r

b

p

p

=

1

n r p

b

1

p

b

p

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Part 2 General Information

Canonical Correlation Analysis

Given the order p letp

t

be the vector of current and past values relevant to prediction

of x

t + 1

p

t

= x

0

t

x

0

t 1

x

0

t p

0

Letf

t

be the vector of current and future values

f

t

= x

0

t

x

0

t + 1

x

0

t + p

0

In the canonical correlation analysis consider submatrices of the sample covariance

matrix of p

t

andf

t

This covariance matrixV

has a block Hankel form

V =

2

6

6

6

4

C

0

C

0

1

C

0

2

C

0

p

C

0

1

C

0

2

C

0

3

C

0

p + 1

C

0

p

C

0

p + 1

C

0

p + 2

C

0

2 p

3

7

7

7

5

State Vector Selection Process

The canonical correlation analysis forms a sequence of potential state vectorsz

j

t

Examine a sequencef

j

t

of subvectors of f

t

and form the submatrixV

j consisting

of the rows and columns of V

corresponding to the components of f

j

t

and compute

its canonical correlations

The smallest canonical correlation of V

j is then used in the selection of the com-

ponents of the state vector The selection process is described in the following For

more details about this process refer to Akaike (1976)

In the following discussion the notationx

t + k j t

denotes the wide sense conditional

expectation (best linear predictor) of x

t + k

given allx

s

with s less than or equal to t

In the notationx

i t + 1

the first subscript denotes the ith component of x

t + 1

The initial state vectorz

1

t

is set tox

t

The sequencef

j

t

is initialized by setting

f

1

t

= z

1

0

t

x

1 t + 1 j t

0

= x

0

t

x

1 t + 1 j t

0

That is start by considering whether to addx

1 t + 1 j t

to the initial state vectorz

1

t

The procedure forms the submatrixV

1 corresponding tof

1

t

and computes its canon-

ical correlations Denote the smallest canonical correlation of V

1 as

m i n

If

m i n

is

significantly greater than 0x

1 t + 1 j t

is added to the state vector

If the smallest canonical correlation of V

1 is not significantly greater than 0 then a

linear combination of f

1

t

is uncorrelated with the pastp

t

Assuming that the determi-

nant of C

0

is not 0 (that is no input series is a constant) you can take the coefficient

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Chapter 18 Details

of x

1 t + 1 j t

in this linear combination to be 1 Denote the coefficients of z

1

t

in this

linear combination as

This gives the relationship

x

1 t + 1 j t

=

0

x

t

Therefore the current state vector already contains all the past information useful forpredicting

x

1 t + 1

and any greater leads of x

1 t

The variablex

1 t + 1 j t

is not added to

the state vector nor are any termsx

1 t + k j t

considered as possible components of the

state vector The variablex

1

is no longer active for state vector selection

The process described forx

1 t + 1 j t

is repeated for the remaining elements of f

t

The

next candidate for inclusion in the state vector is the next component of f

t

corre-

sponding to an active variable Components of f

t

corresponding to inactive variables

that produced a zero

m i n

in a previous step are skipped

Denote the next candidate asx

l t + k j t

The vectorf

j

t

is formed from the current state

vector andx

l t + k j t

as follows

f

j

t

= z

j

0

t

x

l t + k j t

0

The matrixV

j is formed fromf

j

t

and its canonical correlations are computed The

smallest canonical correlation of V

j is judged to be either greater than or equal to 0

If it is judged to be greater than 0x

l t + k j t

is added to the state vector If it is judged

to be 0 then a linear combination of f

j

t

is uncorrelated with thep

t

and the variable

x

l

is now inactive

The state vector selection process continues until no active variables remain

Testing Significance of Canonical Correlations For each step in the canonical correlation sequence the significance of the small-

est canonical correlation

m i n

is judged by an information criterion from Akaike

(1976) This information criterion is

n l n 1

2

m i n

r p + 1 q + 1

where q is the dimension of f

j

t

at the current step r is the order of the state vector p

is the order of the vector autoregressive process and

is the value of the SIGCORR=

option The default is SIGCORR=2 If this information criterion is less than or equal

to 0

m i n

is taken to be 0 otherwise it is taken to be significantly greater than 0

(Do not confuse this information criterion with the AIC)

Variables inx

t + p j t

are not added in the model even with positive information crite-

rion because of the singularity of V

You can force the consideration of more candi-

date state variables by increasing the size of theV

matrix by specifying a PASTMIN=

option value larger than p

Printing the Canonical Correlations

To print the details of the canonical correlation analysis process specify the CAN-

CORR option in the PROC STATESPACE statement The CANCORR option prints

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the candidate state vectors the canonical correlations and the information criteria for

testing the significance of the smallest canonical correlation

Bartlettrsquos

2 and its degrees of freedom are also printed when the CANCORR option

is specified The formula used for Bartlettrsquos

2 is

2

= n 5 r p + 1 q + 1 l n 1

2

m i n

withr p + 1 q + 1

degrees of freedom

Figure 1811 shows the output of the CANCORR option for the introductory example

shown in the Getting Started section of this chapter

The STATESPACE Procedure

Canonical Correlations Analysis

Information Chi

x(TT) y(TT) x(T+1T) Criterion Square DF

1 1 0237045 3566167 114505 4

Information Chi

x(TT) y(TT) x(T+1T) y(T+1T) Criterion Square DF

1 1 0238244 0056565 -535906 0636134 3

Information Chi

x(TT) y(TT) x(T+1T) x(T+2T) Criterion Square DF

1 1 0237602 0087493 -446312 1525353 3

Figure 1811 Canonical Correlations Analysis

New variables are added to the state vector if the information criteria are positive Inthis example

y

t + 1 j t

andx

t + 2 j t

are not added to the state space vector because the

information criteria for these models are negative

If the information criterion is nearly 0 then you may want to investigate models

that arise if the opposite decision is made regarding

m i n

This investigation can be

accomplished by using a FORM statement to specify part or all of the state vector

Preliminary Estimates of F

When a candidate variablex

l t + k j t

yields a zero

m i n

and is not added to the state

vector a linear combination of f

j

t

is uncorrelated with thep

t

Because of the method

used to construct thef

j

t

sequence the coefficient of x

l t + k j t

inl

can be taken as 1

Denote the coefficients of z

j

t

in this linear combination as l

This gives the relationship

x

l t + k j t

= l

0

z

j

t

The vectorl

is used as a preliminary estimate of the first r columns of the row of the

transition matrixF

corresponding tox

l t + k 1 j t

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Parameter Estimation

The model isz

t + 1

= F z

t

+ G e

t + 1

wheree

t

is a sequence of independent multivari-

ate normal innovations with mean vector 0 and variance

e e

The observed sequence

x

t

composes the first r components of z

t

and thusx

t

= H z

t

where H is ther s

matrix I

r

0

LetE

be ther n

matrix of innovations

E = e

1

e

n

If the number of observations n is reasonably large the log likelihood L can be

approximated up to an additive constant as follows

L =

n

2

l n j

e e

j

1

2

t r a c e

1

e e

E E

0

The elements of

e e

are taken as free parameters and are estimated as follows

S

0

=

1

n

E E

0

Replacing

e e

byS

0

in the likelihood equation the log likelihood up to an additive

constant is

L =

n

2

l n j S

0

j

Letting B be the backshift operator the formal relation betweenx

t

ande

t

is

x

t

= H I B F

1

G e

t

e

t

= H I B F

1

G

1

x

t

=

1

X

i = 0

i

x

t i

LettingC

i

be the ith lagged sample covariance of x

t

and neglecting end effects the

matrixS

0

is

S

0

=

1

X

i j = 0

i

C

i + j

0

j

For the computation of S

0

the infinite sum is truncated at the value of the KLAG=

option The value of the KLAG= option should be large enough that the sequence

i

is approximately 0 beyond that point

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Let

be the vector of free parameters in theF

andG

matrices The derivative of the

log likelihood with respect to the parameter

is

L

=

n

2

t r a c e

S

1

0

S

0

The second derivative is

2

L

0

=

n

2

t r a c e

S

1

0

S

0

0

S

1

0

S

0

t r a c e

S

1

0

2

S

0

0

Near the maximum the first term is unimportant and the second term can be approx-

imated to give the following second derivative approximation

2

L

0

=

n t r a c e

S

1

0

E

E

0

0

The first derivative matrix and this second derivative matrix approximation are com-

puted from the sample covariance matrixC

0

and the truncated sequence

i

The

approximate likelihood function is maximized by a modified Newton-Raphson algo-

rithm employing these derivative matrices

The matrixS

0

is used as the estimate of the innovation covariance matrix

e e

The

negative of the inverse of the second derivative matrix at the maximum is used as an

approximate covariance matrix for the parameter estimates The standard errors of

the parameter estimates printed in the parameter estimates tables are taken from the

diagonal of this covariance matrix The parameter covariance matrix is printed when

the COVB option is specified

If the data are nearly nonstationary a better estimate of

e e

and the other parameters

can sometimes be obtained by specifying the RESIDEST option The RESIDEST

option estimates the parameters using conditional least squares instead of maximum

likelihood

The residuals are computed using the state space equation and the sample mean val-

ues of the variables in the model as start-up values The estimate of S

0

is then com-

puted using the residuals from the ith observation on where i is the maximum number

of times any variable occurs in the state vector A multivariate Gauss-Marquardt al-

gorithm is used to minimizej S

0

j

Refer to Harvey (1981a) for a further description

of this method

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Forecasting

Given estimates of F

G

and

e e

forecasts of x

t

are computed from the conditional

expectation of z

t

In forecasting the parameters F G and

e e

are replaced with the estimates or by val-

ues specified in the RESTRICT statement One-step-ahead forecasting is performedfor the observation

x

t

wheret n b

Heren

is the number of observations and

b is the value of the BACK= option For the observationx

t

wheret n b

m-

step-ahead forecasting is performed form = t n + b

The forecasts are generated

recursively with the initial conditionz

0

= 0

The m-step-ahead forecast of z

t + m

isz

t + m j t

wherez

t + m j t

denotes the conditional

expectation of z

t + m

given the information available at time t The m-step-ahead

forecast of x

t + m

isx

t + m j t

= H z

t + m j t

where the matrixH = I

r

0

Let

i

= F

i

G

Note that the lasts r

elements of z

t

consist of the elements of x

u j t

foru t

The state vector z

t + m

can be represented as

z

t + m

= F

m

z

t

+

m 1

X

i = 0

i

e

t + m i

Sincee

t + i j t

= 0

fori 0

the m-step-ahead forecastz

t + m j t

is

z

t + m j t

= F

m

z

t

= F z

t + m 1 j t

Therefore the m-step-ahead forecast of x

t + m

is

x

t + m j t

= H z

t + m j t

The m-step-ahead forecast error is

z

t + m

z

t + m j t

=

m 1

X

i = 0

i

e

t + m i

The variance of the m-step-ahead forecast error is

V

z m

=

m 1

X

i = 0

i

e e

0

i

LettingV

z 0

= 0

the variance of the m-step-ahead forecast error of z

t + m

V

z m

can

be computed recursively as follows

V

z m

= V

z m 1

+

m 1

e e

0

m 1

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The variance of the m-step-ahead forecast error of x

t + m

is ther r

left upper sub-

matrix of V

z m

that is

V

x m

= H V

z m

H

0

Unless the NOCENTER option is specified the sample mean vector is added to the

forecast When differencing is specified the forecasts xt + m j t

plus the sample mean

vector are integrated back to produce forecasts for the original series

Lety

t

be the original series specified by the VAR statement with some 0 values

appended corresponding to the unobserved past observations Let B be the backshift

operator and let B

be thes s

matrix polynomial in the backshift operator

corresponding to the differencing specified by the VAR statement The off-diagonal

elements of

i

are 0 Note that

0

= I

s

whereI

s

is thes s

identity matrix Then

z

t

= B y

t

This gives the relationship

y

t

=

1

B z

t

=

1

X

i = 0

i

z

t i

where

1

B =

P

1

i = 0

i

B

i and

0

= I

s

The m-step-ahead forecast of y

t + m

is

y

t + m j t

=

m 1

X

i = 0

i

z

t + m i j t

+

1

X

i = m

i

z

t + m i

The m-step-ahead forecast error of y

t + m

is

m 1

X

i = 0

i

z

t + m i

z

t + m i j t

=

m 1

X

i = 0

i

X

u = 0

u

i u

e

t + m i

LettingV

y 0

= 0

the variance of the m-step-ahead forecast error of y

t + m

V

y m

is

V

y m

=

m 1

X

i = 0

i

X

u = 0

u

i u

e e

i

X

u = 0

u

i u

0

= V

y m 1

+

m 1

X

u = 0

u

m 1 u

e e

m 1

X

u = 0

u

m 1 u

0

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Chapter 18 Details

Relation of ARMA and State Space Forms

Every state space model has an ARMA representation and conversely every ARMA

model has a state space representation This section discusses this equivalence The

following material is adapted from Akaike (1974) where there is a more complete

discussion Pham-Dinh-Tuan (1978) also contains a discussion of this material

Suppose you are given the following ARMA model

B x

t

= B e

t

or in more detail

x

t

1

x

t 1

p

x

t p

= e

t

+

1

e

t 1

+ +

q

e

t q

(1)

wheree

t

is a sequence of independent multivariate normal random vectors with mean

0 and variance matrix

e e

B is the backshift operator (B x

t

= x

t 1

) B

and

B

are matrix polynomials in B andx

t

is the observed process

If the roots of the determinantial equationj B j = 0

are outside the unit circle in

the complex plane the model can also be written as

x

t

=

1

B B e

t

=

1

X

i = 0

i

e

t i

The

i

matrices are known as the impulse response matrices and can be computed

as

1

B B

You can assume p q

since if this is not initially true you can add more terms

i

that are identically 0 without changing the model

To write this set of equations in a state space form proceed as follows Letx

t + i j t

be

the conditional expectation of x

t + i

givenx

w

forw t

The following relations hold

x

t + i j t

=

1

X

j = i

j

e

t + i j

x

t + i j t + 1

= x

t + i j t

+

i 1

e

t + 1

However from equation (1) you can derive the following relationship

x

t + p j t

=

1

x

t + p 1 j t

+ +

p

x

t

(2)

Hence wheni = p

you can substitute forx

t + p j t

in the right-hand side of equation

(2) and close the system of equations

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This substitution results in the following model in the state space form

z

t + 1

= F z

t

+ G e

t + 1

2

6

6

6

4

x

t + 1

x

t + 2 j t + 1

x

t + p j t + 1

3

7

7

7

5

=

2

6

6

4

0 I 0 0

0 0 I 0

p

p 1

1

3

7

7

5

2

6

6

6

4

x

t

x

t + 1 j t

x

t + p 1 j t

3

7

7

7

5

+

2

6

6

4

I

1

p 1

3

7

7

5

e

t + 1

Note that the state vectorz

t

is composed of conditional expectations of x

t

and the

first r components of z

t

are equal tox

t

The state space form can be cast into an ARMA form by solving the system of dif-

ference equations for the first r components

When converting from an ARMA form to a state space form you can generate a

state vector larger than needed that is the state space model may not be a minimal

representation When going from a state space form to an ARMA form you can have

nontrivial common factors in the autoregressive and moving average operators that

yield an ARMA model larger than necessary

If the state space form used is not a minimal representation some but not all compo-

nents of x

t + i j t

may be linearly dependent This situation corresponds to

p

p 1

being of less than full rank when B

and B

have no common nontrivial

left factors In this casez

t

consists of a subset of the possible components of

x

t + i j t

i = 1 2 p 1

However once a component of x

t + i j t

(for example the

jth one) is linearly dependent on the previous conditional expectations then all sub-

sequent jth components of x

t + k j t

fork i

must also be linearly dependent Note

that in this case equivalent but seemingly different structures can arise if the order of

the components withinx

t

is changed

OUT= Data Set

The forecasts are contained in the output data set specified by the OUT= option on the

PROC STATESPACE statement The OUT= data set contains the following variables

the BY variables

the ID variable

the VAR statement variables These variables contain the actual values from

the input data set

FORi numeric variables containing the forecasts The variable FORi contains

the forecasts for the ith variable in the VAR statement list Forecasts are one-

step-ahead predictions until the end of the data or until the observation specified

by the BACK= option

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RESi numeric variables containing the residual for the forecast of the ith vari-

able in the VAR statement list For forecast observations the actual values are

missing and the RESi variables contain missing values

STDi numeric variables containing the standard deviation for the forecast of

the ith variable in the VAR statement list The values of the STDi variables can

be used to construct univariate confidence limits for the corresponding fore-casts However such confidence limits do not take into account the covariance

of the forecasts

OUTAR= Data Set

The OUTAR= data set contains the estimates of the preliminary autoregressive mod-

els The OUTAR= data set contains the following variables

ORDER a numeric variable containing the order p of the autoregressive model

that the observation represents

AIC a numeric variable containing the value of the information criterionA I C

p

SIGFl numeric variables containing the estimate of the innovation covariance

matrices for the forward autoregressive models The variable SIGFl contains

the lth column of b

p

in the observations with ORDER= p

SIGBl numeric variables containing the estimate of the innovation covariance

matrices for the backward autoregressive models The variable SIGBl contains

the lth column of b

p

in the observations with ORDER= p

FORk ndashl numeric variables containing the estimates of the autoregressive pa-

rameter matrices for the forward models The variable FORk ndashl contains the

lth column of the lag k autoregressive parameter matrix b

p

k

in the observations

with ORDER= p

BACk ndashl numeric variables containing the estimates of the autoregressive pa-

rameter matrices for the backward models The variable BACk ndashl contains the

lth column of the lag k autoregressive parameter matrix b

p

k

in the observations

with ORDER= p

The estimates for the order p autoregressive model can be selected as those observa-

tions with ORDER= p Within these observations the k lth element of

p

i

is given by

the value of the FORindashl variable in the k th observation The k lth element of

p

i

is

given by the value of BACindashl variable in the k th observation The k lth element of

p

is given by SIGFl in the k th observation The k lth element of

p

is given by SIGBlin the k th observation

Table 181 shows an example of the OUTAR= data set with ARMAX=3 andx

t

of

dimension 2 In Table 181 i j

indicate the ijth element of the matrix

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Table 181 Values in the OUTAR= Data Set

Obs ORDER AIC SIGF1 SIGF2 SIGB1 SIGB2 FOR1ndash 1 FOR1ndash 2 FOR2ndash 1 FOR2ndash 2 FOR3ndash 1

1 0 AIC0

0 1 1

0 1 2

0 1 1

0 1 2

2 0 AIC0

0 2 1

0 2 2

0 2 1

0 2 2

3 1 AIC1

1 1 1

1 1 2

1 1 1

1 1 2

1

1

1 1

1

1

1 2

4 1 AIC1

1 2 1

1 1 2

1 2 1

1 1 2

1

1

2 1

1

1

2 2

5 2 AIC2

2 1 1

2 1 2

2 1 1

2 1 2

2

1

1 1

2

1

1 2

2

2

1 1

2

2

1 2

6 2 AIC2

2 2 1

2 1 2

2 2 1

2 1 2

2

1

2 1

2

1

2 2

2

2

2 1

2

2

2 2

7 3 AIC3

3 1 1

3 1 2

3 1 1

3 1 2

3

1

1 1

3

1

1 2

3

2

1 1

3

2

1 2

3

3

1 1

8 3 AIC3

3 2 1

3 1 2

3 2 1

3 1 2

3

1

2 1

3

1

2 2

3

2

2 1

3

2

2 2

3

3

2 1

Obs FOR3ndash 2 BACK1ndash 1 BACK1ndash 2 BACK2ndash 1 BACK2ndash 2 BACK3ndash 1 BACK3ndash 2

1

2

3

1

1

1 1

1

1

1 2

4

1

1

2 1

1

1

2 2

5

2

1

1 1

2

1

1 2

2

2

1 1

2

2

1 2

6

2

1

2 1

2

1

2 2

2

2

2 1

2

2

2 2

7

3

3

1 2

3

1

1 1

3

1

1 2

3

2

1 1

3

2

1 2

3

3

1 1

3

3

1 2

8

3

3

2 2

3

1

2 1

3

1

2 2

3

2

2 1

3

2

2 2

3

3

2 1

3

3

2 2

The estimated autoregressive parameters can be used in the IML procedure to obtain

autoregressive estimates of the spectral density function or forecasts based on the

autoregressive models

OUTMODEL= Data Set

The OUTMODEL= data set contains the estimates of theF

andG

matrices and their

standard errors the names of the components of the state vector and the estimates of

the innovation covariance matrix The variables contained in the OUTMODEL= data

set are as follows

the BY variables

STATEVEC a character variable containing the name of the component of the

state vector corresponding to the observation The STATEVEC variable has

the value STD for standard deviations observations which contain the standarderrors for the estimates given in the preceding observation

Fndash j numeric variables containing the columns of theF

matrix The variable

Fndash j contains the jth column of F

The number of Fndash j variables is equal to

the value of the DIMMAX= option If the model is of smaller dimension the

extraneous variables are set to missing

Gndash j numeric variables containing the columns of theG

matrix The variable

Gndash j contains the jth column of G

The number of Gndash j variables is equal to r

the dimension of x

t

given by the number of variables in the VAR statement

SIGndash j numeric variables containing the columns of the innovation covariance

matrix The variable SIGndash j contains the jth column of

e e

There are r vari-ables SIGndash j

Table 182 shows an example of the OUTMODEL= data set withx

t

= x

t

y

t

0

z

t

= x

t

y

t

x

t + 1 j t

0 and DIMMAX=4 In Table 182F

i j

andG

i j

are the ijth

elements of F

andG

respectively Note that all elements for Fndash4 are missing because

F

is a3 3

matrix

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Chapter 18 Details

Table 182 Value in the OUTMODEL= Data Set

Obs STATEVEC Fndash 1 Fndash 2 Fndash 3 Fndash 4 Gndash 1 Gndash 2 SIGndash 1 SIGndash 2

1 X(TT) 0 0 1 1 0

1 1

1 2

2 STD

3 Y(TT) F2 1

F2 2

F2 3

0 1

2 1

2 2

4 STD std F2 1

std F2 2

std F2 3

5 X(T+1T) F3 1

F3 2

F3 3

G3 1

G3 2

6 STD std F3 1

std F3 2

std F3 3

std G3 1

std G3 2

Printed Output

The printed output produced by the STATESPACE procedure is described in the fol-

lowing

1 descriptive statistics which include the number of observations used the

names of the variables their means and standard deviations (Std) and the dif-

ferencing operations used

2 the Akaike information criteria for the sequence of preliminary autoregressive

models

3 if the PRINTOUT=LONG option is specified the sample autocovariance ma-

trices of the input series at various lags

4 if the PRINTOUT=LONG option is specified the sample autocorrelation ma-

trices of the input series

5 a schematic representation of the autocorrelation matrices showing the signif-

icant autocorrelations

6 if the PRINTOUT=LONG option is specified the partial autoregressive ma-

trices (These are

p

p

as described in Preliminary Autoregressive Modelsearlier in this chapter)

7 a schematic representation of the partial autocorrelation matrices showing the

significant partial autocorrelations

8 the Yule-Walker estimates of the autoregressive parameters for the autoregres-

sive model with the minimum AIC

9 if the PRINTOUT=LONG option is specified the autocovariance matrices of

the residuals of the minimum AIC model This is the sequence of estimated

innovation variance matrices for the solutions of the Yule-Walker equations

10 if the PRINTOUT=LONG option is specified the autocorrelation matrices of the residuals of the minimum AIC model

11 If the CANCORR option is specified the canonical correlations analysis for

each potential state vector considered in the state vector selection process

This includes the potential state vector the canonical correlations the informa-

tion criterion for the smallest canonical correlation Bartlettrsquos

2 statistic (ldquoChi

Squarerdquo) for the smallest canonical correlation and the degrees of freedom of

Bartlettrsquos

2

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Part 2 General Information

12 the components of the chosen state vector

13 the preliminary estimate of the transition matrixF

the input matrixG

and

the variance matrix for the innovations

e e

14 if the ITPRINT option is specified the iteration history of the likelihood max-

imization For each iteration this shows the iteration number the number of step halvings the determinant of the innovation variance matrix the damping

factor Lambda and the values of the parameters

15 the state vector printed again to aid interpretation of the following listing of F

andG

16 the final estimate of the transition matrixF

17 the final estimate of the input matrixG

18 the final estimate of the variance matrix for the innovations

e e

19 a table listing the estimates of the free parameters inF

andG

and their standarderrors and t statistics

20 if the COVB option is specified the covariance matrix of the parameter esti-

mates

21 if the COVB option is specified the correlation matrix of the parameter esti-

mates

22 if the PRINT option is specified the forecasts and their standard errors

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Chapter 18 Details

ODS Table Names

PROC STATESPACE assigns a name to each table it creates You can use these

names to reference the table when using the Output Delivery System (ODS) to select

tables and create output data sets These names are listed in the following table For

more information on ODS see Chapter 6 ldquoUsing the Output Delivery Systemrdquo

Table 183 ODS Tables Produced in PROC STATESPACE

ODS Table Name Description Option

NObs Number of observations default

Summary Simple summary statistics table default

InfoCriterion Information criterion table default

CovLags Covariance Matrices of Input Series PRINTOUT=LONG

CorrLags Correlation Matrices of Input Series PRINTOUT=LONG

PartialAR Partial Autoregressive Matrices PRINTOUT=LONG

YWEstimates Yule-Walker Estimates for Minimum AIC default

CovResiduals Covariance of Residuals PRINTOUT=LONGCorrResiduals Residual Correlations from AR Models PRINTOUT=LONG

StateVector State vector table default

CorrGraph Schematic Representation of Correlations default

TransitionMatrix Transition Matrix default

InputMatrix Input Matrix default

VarInnov Variance Matrix for the Innovation default

CovB Covariance of Parameter Estimates COVB

CorrB Correlation of Parameter Estimates COVB

CanCorr Canonical Correlation Analysis CANCORR

IterHistory Iterative Fitting table ITPRINT

ParameterEstimates Parameter Estimates Table default

Forecasts Forecasts Table PRINT

ConvergenceStatus Convergence Status Table default

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Part 2 General Information

Example

Example 181 Series J from Box and Jenkins

This example analyzes the gas furnace data (series J) from Box and Jenkins (The

data are not shown Refer to Box and Jenkins (1976) for the data)

First a model is selected and fit automatically using the following statements

title1 rsquoGas Furnace Datarsquo

title2 rsquoBox amp Jenkins Series Jrsquo

title3 rsquoAutomatically Selected Modelrsquo

proc statespace data=seriesj cancorr

var x y

run

The results for the automatically selected model are shown in Output 1811

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Chapter 18 Example

Output 1811 Results for Automatically Selected Model

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Number of Observations 296

Standard

Variable Mean Error

x -005683 1072766

y 5350912 3202121

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Information Criterion for Autoregressive Models

Lag=0 Lag=1 Lag=2 Lag=3 Lag=4 Lag=5 Lag=6 Lag=7 Lag=8

6513862 -103357 -163296 -164512 -165152 -164891 -164934 -164315 -163856

Information

Criterion for

Autoregressive

Models

Lag=9 Lag=10

-16348 -163359

Schematic Representation of Correlations

NameLag 0 1 2 3 4 5 6 7 8 9 10

x +- +- +- +- +- +- +- +- +- +- +-

y -+ -+ -+ -+ -+ -+ -+ -+ -+ -+ -+

+ is gt 2std error - is lt -2std error is between

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Part 2 General Information

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Schematic Representation of Partial Autocorrelations

NameLag 1 2 3 4 5 6 7 8 9 10

x + - + -

y -+ -- - + +

+ is gt 2std error - is lt -2std error is between

Yule-Walker Estimates for Minimum AIC

------Lag=1------ ------Lag=2------ ------Lag=3------ ------Lag=4------

x y x y x y x y

x 1925887 -000124 -120166 0004224 0116918 -000867 0104236 0003268

y 0050496 1299793 -002046 -03277 -071182 -025701 0195411 0133417

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Canonical Correlations Analysis

Information Chi

x(TT) y(TT) x(T+1T) Criterion Square DF

1 1 0804883 2929228 3047481 8

Information Chix(TT) y(TT) x(T+1T) y(T+1T) Criterion Square DF

1 1 0906681 0607529 1223358 1347237 7

Information Chi

x(TT) y(TT) x (T+1T) y (T+1T) x (T+2T) Criterion Square DF

1 1 0909434 0610278 0186274 -154701 1034705 6

Information Chi

x(TT) y(TT) x (T+1T) y (T+1T) y (T+2T) Criterion Square DF

1 1 091014 0618937 0206823 0940392 1280924 6

Information Chi

x(TT) y(TT) x(T+1T) y(T+1T) y(T+2T) y(T+3T) Criterion Square DF

1 1 0912963 0628785 0226598 0083258 -794103 2041584 5

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Chapter 18 Example

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Selected Statespace Form and Preliminary Estimates

State Vector

x(TT) y(TT) x(T+1T) y(T+1T) y(T+2T)

Estimate of Transition Matrix

0 0 1 0 0

0 0 0 1 0

-084718 0026794 1711715 -005019 0

0 0 0 0 1

-019785 0334274 -018174 -123557 1787475

Input Matrix for Innovation

1 0

0 11925887 -000124

0050496 1299793

0142421 1361696

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Selected Statespace Form and Preliminary Estimates

Variance Matrix for Innovation

0035274 -000734

-000734 0097569

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Part 2 General Information

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Selected Statespace Form and Fitted Model

State Vector

x(TT) y(TT) x(T+1T) y(T+1T) y(T+2T)

Estimate of Transition Matrix

0 0 1 0 0

0 0 0 1 0

-086192 0030609 1724235 -005483 0

0 0 0 0 1

-034839 0292124 -009435 -109823 1671418

Input Matrix for Innovation

1 0

0 1192442 -000416

0015621 1258495

008058 1353204

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Selected Statespace Form and Fitted Model

Variance Matrix for Innovation

0035579 -000728

-000728 0095577

Parameter Estimates

Standard

Parameter Estimate Error t Value

F(31) -086192 0072961 -1181

F(32) 0030609 0026167 117

F(33) 1724235 0061599 2799

F(34) -005483 0030169 -182

F(51) -034839 0135253 -258

F(52) 0292124 0046299 631

F(53) -009435 0096527 -098F(54) -109823 0109525 -1003

F(55) 1671418 0083737 1996

G(31) 1924420 0058162 3309

G(32) -000416 0035255 -012

G(41) 0015621 0095771 016

G(42) 1258495 0055742 2258

G(51) 0080580 0151622 053

G(52) 1353204 0091388 1481

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Chapter 18 Example

The two series are believed to have a transfer function relation with the gas rate

(variable X) as the input and the CO2

concentration (variable Y) as the output Since

the parameter estimates shown in Output 1811 support this kind of model the model

is reestimated with the feedback parameters restricted to 0 The following statements

fit the transfer function (no feedback) model

title3 rsquoTransfer Function Modelrsquo

proc statespace data=seriesj printout=none

var x y

restrict f(32)=0 f(34)=0

g(32)=0 g(41)=0 g(51)=0

run

The last 2 pages of the output are shown in Output 1812

Output 1812 STATESPACE Output for Transfer Function Model

Gas Furnace Data

Box amp Jenkins Series J

Transfer Function Model

The STATESPACE Procedure

Selected Statespace Form and Fitted Model

State Vector

x(TT) y(TT) x(T+1T) y(T+1T) y(T+2T)

Estimate of Transition Matrix

0 0 1 0 0

0 0 0 1 0

-068882 0 1598717 0 0

0 0 0 0 1

-035944 0284179 -00963 -107313 1650047

Input Matrix for Innovation

1 0

0 1

1923446 0

0 1260856

0 1346332

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Part 2 General Information

Gas Furnace Data

Box amp Jenkins Series J

Transfer Function Model

The STATESPACE Procedure

Selected Statespace Form and Fitted Model

Variance Matrix for Innovation

0036995 -00072

-00072 0095712

Parameter Estimates

Standard

Parameter Estimate Error t Value

F(31) -068882 0050549 -1363

F(33) 1598717 0050924 3139

F(51) -035944 0229044 -157

F(52) 0284179 0096944 293

F(53) -009630 0140876 -068

F(54) -107313 0250385 -429

F(55) 1650047 0188533 875G(31) 1923446 0056328 3415

G(42) 1260856 0056464 2233

G(52) 1346332 0091086 1478

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Chapter 18 References

References

Akaike H (1974) Markovian Representation of Stochastic Processes and Its Ap-

plication to the Analysis of Autoregressive Moving Average Processes Annals

of the Institute of Statistical Mathematics 26 363-387

Akaike H (1976) Canonical Correlations Analysis of Time Series and the Use of

an Information Criterion in Advances and Case Studies in System Identification

eds R Mehra and DG Lainiotis New York Academic Press

Anderson TW (1971) The Statistical Analysis of Time Series New York John

Wiley amp Sons

Ansley CF and Newbold P (1979) Multivariate Partial Autocorrelations Pro-

ceedings of the Business and Economic Statistics Section American Statistical

Association 349-353

Box GEP and Jenkins G (1976) Time Series Analysis Forecasting and Control

San Francisco Holden-Day

Brockwell PJ and Davis RA (1991) Time Series Theory and Methods 2nd Edi-

tion Springer-Verlag

Hannan EJ (1970) Multiple Time Series New York John Wiley amp Sons

Hannan EJ (1976) The Identification and Parameterization of ARMAX and State

Space Forms Econometrica 44 713-722

Harvey AC (1981a) The Econometric Analysis of Time Series New York John

Wiley amp Sons

Harvey AC (1981b) Time Series Models New York John Wiley amp Sons

Jones RH (1974) Identification and Autoregressive Spectrum Estimation IEEE Transactions on Automatic Control AC-19 894-897

Pham-Dinh-Tuan (1978) On the Fitting of Multivariate Processes of the

Autoregressive-Moving Average Type Biometrika 65 99-107

Priestley MB (1980) System Identification Kalman Filtering and Stochastic Con-

trol in Directions in Time Series eds DR Brillinger and GC Tiao Institute

of Mathematical Statistics

Whittle P (1963) On the Fitting of Multivariate Autoregressions and the Approxi-

mate Canonical Factorization of a Spectral Density Matrix Biometrika 50 129-

134

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The correct bibliographic citation for this manual is as follows SAS Institute Inc SAS

ET S U serrsquos Gui de Version 8 Cary NC SAS Institute Inc 1999 1546 pp

SASET S Userrsquos Guide Version 8

Copyright copy 1999 by S AS I ns t it u t e I nc Ca ry N C U S A

ISBN 1ndash58025ndash489ndash6

All r ight s res erved P r int ed in t he U n it ed St a t es of A merica N o pa rt of t his p u blica t ionma y b e reprodu ced s t ored in a ret r ieva l s ys t em or t ra ns mit t ed in a ny form or b y a nymeans electronic mechanical photocopying or otherwise without the prior writtenpermission of the publisher SAS Institute Inc

US Government Restricted Rights Notice Use du plicat ion or disclosure of the

software by the government is subject to restrictions as set forth in FAR 52227ndash19Commercial Computer Softw ar e-Restricted Rights (J une 1987)

SAS Institute Inc SAS Campus Drive Cary North Carolina 27513

1st print ing October 1999

S AS reg a nd a ll ot her SA S I ns t it u t e I nc p rodu ct or s ervice na m es a re regis t ered t ra dema rkst d k f SA S I t it t I i t h U S A d t h t i reg i di t U SA

Page 29: Statespace Model

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Chapter 18 Details

You can evaluate the significance of these test statistics with the PROBCHI function

in a SAS DATA step or with a

2 table

Determining the Autoregressive Order

Although the autoregressive models can be used for prediction their primary value is

to aid in the selection of a suitable portion of the sample covariance matrix for use in

computing canonical correlations If the multivariate time series x

t is of autoregres-sive order p then the vector of past values to lag p is considered to contain essentially

all the information relevant for prediction of future values of the time series

By default PROC STATESPACE selects the order p producing the autoregressive

model with the smallestA I C

p

If the value p for the minimumA I C

p

is less than the

value of the PASTMIN= option then p is set to the PASTMIN= value Alternatively

you can use the ARMAX= and PASTMIN= options to force PROC STATESPACE to

use an order you select

Significance Limits for Partial Autocorrelations

The STATESPACE procedure prints a schematic representation of the partial autocor-

relation matrices indicating which partial autocorrelations are significantly greater orsignificantly less than 0 Figure 1810 shows an example of this table

The STATESPACE Procedure

Schematic Representation of Partial Autocorrelations

NameLag 1 2 3 4 5 6 7 8 9 10

x ++ +

y ++

+ is gt 2std error - is lt -2std error is between

Figure 1810 Significant Partial Autocorrelations

The partial autocorrelations are from the sample partial autoregressive matrices b

p

p

The standard errors used for the significance limits of the partial autocorrelations are

computed from the sequence of matrices

p

and

p

Under the assumption that the observed series arises from an autoregressive process

of order p 1

the pth sample partial autoregressive matrix b

p

p

has an asymptotic

variance matrix 1

n

1

p

p

The significance limits for b

p

p

used in the schematic plot of the sample partial autore-

gressive sequence are derived by replacing

p

and

p

with their sample estimators

to produce the variance estimate as follows

d

V a r

b

p

p

=

1

n r p

b

1

p

b

p

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Part 2 General Information

Canonical Correlation Analysis

Given the order p letp

t

be the vector of current and past values relevant to prediction

of x

t + 1

p

t

= x

0

t

x

0

t 1

x

0

t p

0

Letf

t

be the vector of current and future values

f

t

= x

0

t

x

0

t + 1

x

0

t + p

0

In the canonical correlation analysis consider submatrices of the sample covariance

matrix of p

t

andf

t

This covariance matrixV

has a block Hankel form

V =

2

6

6

6

4

C

0

C

0

1

C

0

2

C

0

p

C

0

1

C

0

2

C

0

3

C

0

p + 1

C

0

p

C

0

p + 1

C

0

p + 2

C

0

2 p

3

7

7

7

5

State Vector Selection Process

The canonical correlation analysis forms a sequence of potential state vectorsz

j

t

Examine a sequencef

j

t

of subvectors of f

t

and form the submatrixV

j consisting

of the rows and columns of V

corresponding to the components of f

j

t

and compute

its canonical correlations

The smallest canonical correlation of V

j is then used in the selection of the com-

ponents of the state vector The selection process is described in the following For

more details about this process refer to Akaike (1976)

In the following discussion the notationx

t + k j t

denotes the wide sense conditional

expectation (best linear predictor) of x

t + k

given allx

s

with s less than or equal to t

In the notationx

i t + 1

the first subscript denotes the ith component of x

t + 1

The initial state vectorz

1

t

is set tox

t

The sequencef

j

t

is initialized by setting

f

1

t

= z

1

0

t

x

1 t + 1 j t

0

= x

0

t

x

1 t + 1 j t

0

That is start by considering whether to addx

1 t + 1 j t

to the initial state vectorz

1

t

The procedure forms the submatrixV

1 corresponding tof

1

t

and computes its canon-

ical correlations Denote the smallest canonical correlation of V

1 as

m i n

If

m i n

is

significantly greater than 0x

1 t + 1 j t

is added to the state vector

If the smallest canonical correlation of V

1 is not significantly greater than 0 then a

linear combination of f

1

t

is uncorrelated with the pastp

t

Assuming that the determi-

nant of C

0

is not 0 (that is no input series is a constant) you can take the coefficient

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Chapter 18 Details

of x

1 t + 1 j t

in this linear combination to be 1 Denote the coefficients of z

1

t

in this

linear combination as

This gives the relationship

x

1 t + 1 j t

=

0

x

t

Therefore the current state vector already contains all the past information useful forpredicting

x

1 t + 1

and any greater leads of x

1 t

The variablex

1 t + 1 j t

is not added to

the state vector nor are any termsx

1 t + k j t

considered as possible components of the

state vector The variablex

1

is no longer active for state vector selection

The process described forx

1 t + 1 j t

is repeated for the remaining elements of f

t

The

next candidate for inclusion in the state vector is the next component of f

t

corre-

sponding to an active variable Components of f

t

corresponding to inactive variables

that produced a zero

m i n

in a previous step are skipped

Denote the next candidate asx

l t + k j t

The vectorf

j

t

is formed from the current state

vector andx

l t + k j t

as follows

f

j

t

= z

j

0

t

x

l t + k j t

0

The matrixV

j is formed fromf

j

t

and its canonical correlations are computed The

smallest canonical correlation of V

j is judged to be either greater than or equal to 0

If it is judged to be greater than 0x

l t + k j t

is added to the state vector If it is judged

to be 0 then a linear combination of f

j

t

is uncorrelated with thep

t

and the variable

x

l

is now inactive

The state vector selection process continues until no active variables remain

Testing Significance of Canonical Correlations For each step in the canonical correlation sequence the significance of the small-

est canonical correlation

m i n

is judged by an information criterion from Akaike

(1976) This information criterion is

n l n 1

2

m i n

r p + 1 q + 1

where q is the dimension of f

j

t

at the current step r is the order of the state vector p

is the order of the vector autoregressive process and

is the value of the SIGCORR=

option The default is SIGCORR=2 If this information criterion is less than or equal

to 0

m i n

is taken to be 0 otherwise it is taken to be significantly greater than 0

(Do not confuse this information criterion with the AIC)

Variables inx

t + p j t

are not added in the model even with positive information crite-

rion because of the singularity of V

You can force the consideration of more candi-

date state variables by increasing the size of theV

matrix by specifying a PASTMIN=

option value larger than p

Printing the Canonical Correlations

To print the details of the canonical correlation analysis process specify the CAN-

CORR option in the PROC STATESPACE statement The CANCORR option prints

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Part 2 General Information

the candidate state vectors the canonical correlations and the information criteria for

testing the significance of the smallest canonical correlation

Bartlettrsquos

2 and its degrees of freedom are also printed when the CANCORR option

is specified The formula used for Bartlettrsquos

2 is

2

= n 5 r p + 1 q + 1 l n 1

2

m i n

withr p + 1 q + 1

degrees of freedom

Figure 1811 shows the output of the CANCORR option for the introductory example

shown in the Getting Started section of this chapter

The STATESPACE Procedure

Canonical Correlations Analysis

Information Chi

x(TT) y(TT) x(T+1T) Criterion Square DF

1 1 0237045 3566167 114505 4

Information Chi

x(TT) y(TT) x(T+1T) y(T+1T) Criterion Square DF

1 1 0238244 0056565 -535906 0636134 3

Information Chi

x(TT) y(TT) x(T+1T) x(T+2T) Criterion Square DF

1 1 0237602 0087493 -446312 1525353 3

Figure 1811 Canonical Correlations Analysis

New variables are added to the state vector if the information criteria are positive Inthis example

y

t + 1 j t

andx

t + 2 j t

are not added to the state space vector because the

information criteria for these models are negative

If the information criterion is nearly 0 then you may want to investigate models

that arise if the opposite decision is made regarding

m i n

This investigation can be

accomplished by using a FORM statement to specify part or all of the state vector

Preliminary Estimates of F

When a candidate variablex

l t + k j t

yields a zero

m i n

and is not added to the state

vector a linear combination of f

j

t

is uncorrelated with thep

t

Because of the method

used to construct thef

j

t

sequence the coefficient of x

l t + k j t

inl

can be taken as 1

Denote the coefficients of z

j

t

in this linear combination as l

This gives the relationship

x

l t + k j t

= l

0

z

j

t

The vectorl

is used as a preliminary estimate of the first r columns of the row of the

transition matrixF

corresponding tox

l t + k 1 j t

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Chapter 18 Details

Parameter Estimation

The model isz

t + 1

= F z

t

+ G e

t + 1

wheree

t

is a sequence of independent multivari-

ate normal innovations with mean vector 0 and variance

e e

The observed sequence

x

t

composes the first r components of z

t

and thusx

t

= H z

t

where H is ther s

matrix I

r

0

LetE

be ther n

matrix of innovations

E = e

1

e

n

If the number of observations n is reasonably large the log likelihood L can be

approximated up to an additive constant as follows

L =

n

2

l n j

e e

j

1

2

t r a c e

1

e e

E E

0

The elements of

e e

are taken as free parameters and are estimated as follows

S

0

=

1

n

E E

0

Replacing

e e

byS

0

in the likelihood equation the log likelihood up to an additive

constant is

L =

n

2

l n j S

0

j

Letting B be the backshift operator the formal relation betweenx

t

ande

t

is

x

t

= H I B F

1

G e

t

e

t

= H I B F

1

G

1

x

t

=

1

X

i = 0

i

x

t i

LettingC

i

be the ith lagged sample covariance of x

t

and neglecting end effects the

matrixS

0

is

S

0

=

1

X

i j = 0

i

C

i + j

0

j

For the computation of S

0

the infinite sum is truncated at the value of the KLAG=

option The value of the KLAG= option should be large enough that the sequence

i

is approximately 0 beyond that point

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Part 2 General Information

Let

be the vector of free parameters in theF

andG

matrices The derivative of the

log likelihood with respect to the parameter

is

L

=

n

2

t r a c e

S

1

0

S

0

The second derivative is

2

L

0

=

n

2

t r a c e

S

1

0

S

0

0

S

1

0

S

0

t r a c e

S

1

0

2

S

0

0

Near the maximum the first term is unimportant and the second term can be approx-

imated to give the following second derivative approximation

2

L

0

=

n t r a c e

S

1

0

E

E

0

0

The first derivative matrix and this second derivative matrix approximation are com-

puted from the sample covariance matrixC

0

and the truncated sequence

i

The

approximate likelihood function is maximized by a modified Newton-Raphson algo-

rithm employing these derivative matrices

The matrixS

0

is used as the estimate of the innovation covariance matrix

e e

The

negative of the inverse of the second derivative matrix at the maximum is used as an

approximate covariance matrix for the parameter estimates The standard errors of

the parameter estimates printed in the parameter estimates tables are taken from the

diagonal of this covariance matrix The parameter covariance matrix is printed when

the COVB option is specified

If the data are nearly nonstationary a better estimate of

e e

and the other parameters

can sometimes be obtained by specifying the RESIDEST option The RESIDEST

option estimates the parameters using conditional least squares instead of maximum

likelihood

The residuals are computed using the state space equation and the sample mean val-

ues of the variables in the model as start-up values The estimate of S

0

is then com-

puted using the residuals from the ith observation on where i is the maximum number

of times any variable occurs in the state vector A multivariate Gauss-Marquardt al-

gorithm is used to minimizej S

0

j

Refer to Harvey (1981a) for a further description

of this method

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Chapter 18 Details

Forecasting

Given estimates of F

G

and

e e

forecasts of x

t

are computed from the conditional

expectation of z

t

In forecasting the parameters F G and

e e

are replaced with the estimates or by val-

ues specified in the RESTRICT statement One-step-ahead forecasting is performedfor the observation

x

t

wheret n b

Heren

is the number of observations and

b is the value of the BACK= option For the observationx

t

wheret n b

m-

step-ahead forecasting is performed form = t n + b

The forecasts are generated

recursively with the initial conditionz

0

= 0

The m-step-ahead forecast of z

t + m

isz

t + m j t

wherez

t + m j t

denotes the conditional

expectation of z

t + m

given the information available at time t The m-step-ahead

forecast of x

t + m

isx

t + m j t

= H z

t + m j t

where the matrixH = I

r

0

Let

i

= F

i

G

Note that the lasts r

elements of z

t

consist of the elements of x

u j t

foru t

The state vector z

t + m

can be represented as

z

t + m

= F

m

z

t

+

m 1

X

i = 0

i

e

t + m i

Sincee

t + i j t

= 0

fori 0

the m-step-ahead forecastz

t + m j t

is

z

t + m j t

= F

m

z

t

= F z

t + m 1 j t

Therefore the m-step-ahead forecast of x

t + m

is

x

t + m j t

= H z

t + m j t

The m-step-ahead forecast error is

z

t + m

z

t + m j t

=

m 1

X

i = 0

i

e

t + m i

The variance of the m-step-ahead forecast error is

V

z m

=

m 1

X

i = 0

i

e e

0

i

LettingV

z 0

= 0

the variance of the m-step-ahead forecast error of z

t + m

V

z m

can

be computed recursively as follows

V

z m

= V

z m 1

+

m 1

e e

0

m 1

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Part 2 General Information

The variance of the m-step-ahead forecast error of x

t + m

is ther r

left upper sub-

matrix of V

z m

that is

V

x m

= H V

z m

H

0

Unless the NOCENTER option is specified the sample mean vector is added to the

forecast When differencing is specified the forecasts xt + m j t

plus the sample mean

vector are integrated back to produce forecasts for the original series

Lety

t

be the original series specified by the VAR statement with some 0 values

appended corresponding to the unobserved past observations Let B be the backshift

operator and let B

be thes s

matrix polynomial in the backshift operator

corresponding to the differencing specified by the VAR statement The off-diagonal

elements of

i

are 0 Note that

0

= I

s

whereI

s

is thes s

identity matrix Then

z

t

= B y

t

This gives the relationship

y

t

=

1

B z

t

=

1

X

i = 0

i

z

t i

where

1

B =

P

1

i = 0

i

B

i and

0

= I

s

The m-step-ahead forecast of y

t + m

is

y

t + m j t

=

m 1

X

i = 0

i

z

t + m i j t

+

1

X

i = m

i

z

t + m i

The m-step-ahead forecast error of y

t + m

is

m 1

X

i = 0

i

z

t + m i

z

t + m i j t

=

m 1

X

i = 0

i

X

u = 0

u

i u

e

t + m i

LettingV

y 0

= 0

the variance of the m-step-ahead forecast error of y

t + m

V

y m

is

V

y m

=

m 1

X

i = 0

i

X

u = 0

u

i u

e e

i

X

u = 0

u

i u

0

= V

y m 1

+

m 1

X

u = 0

u

m 1 u

e e

m 1

X

u = 0

u

m 1 u

0

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Chapter 18 Details

Relation of ARMA and State Space Forms

Every state space model has an ARMA representation and conversely every ARMA

model has a state space representation This section discusses this equivalence The

following material is adapted from Akaike (1974) where there is a more complete

discussion Pham-Dinh-Tuan (1978) also contains a discussion of this material

Suppose you are given the following ARMA model

B x

t

= B e

t

or in more detail

x

t

1

x

t 1

p

x

t p

= e

t

+

1

e

t 1

+ +

q

e

t q

(1)

wheree

t

is a sequence of independent multivariate normal random vectors with mean

0 and variance matrix

e e

B is the backshift operator (B x

t

= x

t 1

) B

and

B

are matrix polynomials in B andx

t

is the observed process

If the roots of the determinantial equationj B j = 0

are outside the unit circle in

the complex plane the model can also be written as

x

t

=

1

B B e

t

=

1

X

i = 0

i

e

t i

The

i

matrices are known as the impulse response matrices and can be computed

as

1

B B

You can assume p q

since if this is not initially true you can add more terms

i

that are identically 0 without changing the model

To write this set of equations in a state space form proceed as follows Letx

t + i j t

be

the conditional expectation of x

t + i

givenx

w

forw t

The following relations hold

x

t + i j t

=

1

X

j = i

j

e

t + i j

x

t + i j t + 1

= x

t + i j t

+

i 1

e

t + 1

However from equation (1) you can derive the following relationship

x

t + p j t

=

1

x

t + p 1 j t

+ +

p

x

t

(2)

Hence wheni = p

you can substitute forx

t + p j t

in the right-hand side of equation

(2) and close the system of equations

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Part 2 General Information

This substitution results in the following model in the state space form

z

t + 1

= F z

t

+ G e

t + 1

2

6

6

6

4

x

t + 1

x

t + 2 j t + 1

x

t + p j t + 1

3

7

7

7

5

=

2

6

6

4

0 I 0 0

0 0 I 0

p

p 1

1

3

7

7

5

2

6

6

6

4

x

t

x

t + 1 j t

x

t + p 1 j t

3

7

7

7

5

+

2

6

6

4

I

1

p 1

3

7

7

5

e

t + 1

Note that the state vectorz

t

is composed of conditional expectations of x

t

and the

first r components of z

t

are equal tox

t

The state space form can be cast into an ARMA form by solving the system of dif-

ference equations for the first r components

When converting from an ARMA form to a state space form you can generate a

state vector larger than needed that is the state space model may not be a minimal

representation When going from a state space form to an ARMA form you can have

nontrivial common factors in the autoregressive and moving average operators that

yield an ARMA model larger than necessary

If the state space form used is not a minimal representation some but not all compo-

nents of x

t + i j t

may be linearly dependent This situation corresponds to

p

p 1

being of less than full rank when B

and B

have no common nontrivial

left factors In this casez

t

consists of a subset of the possible components of

x

t + i j t

i = 1 2 p 1

However once a component of x

t + i j t

(for example the

jth one) is linearly dependent on the previous conditional expectations then all sub-

sequent jth components of x

t + k j t

fork i

must also be linearly dependent Note

that in this case equivalent but seemingly different structures can arise if the order of

the components withinx

t

is changed

OUT= Data Set

The forecasts are contained in the output data set specified by the OUT= option on the

PROC STATESPACE statement The OUT= data set contains the following variables

the BY variables

the ID variable

the VAR statement variables These variables contain the actual values from

the input data set

FORi numeric variables containing the forecasts The variable FORi contains

the forecasts for the ith variable in the VAR statement list Forecasts are one-

step-ahead predictions until the end of the data or until the observation specified

by the BACK= option

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Chapter 18 Details

RESi numeric variables containing the residual for the forecast of the ith vari-

able in the VAR statement list For forecast observations the actual values are

missing and the RESi variables contain missing values

STDi numeric variables containing the standard deviation for the forecast of

the ith variable in the VAR statement list The values of the STDi variables can

be used to construct univariate confidence limits for the corresponding fore-casts However such confidence limits do not take into account the covariance

of the forecasts

OUTAR= Data Set

The OUTAR= data set contains the estimates of the preliminary autoregressive mod-

els The OUTAR= data set contains the following variables

ORDER a numeric variable containing the order p of the autoregressive model

that the observation represents

AIC a numeric variable containing the value of the information criterionA I C

p

SIGFl numeric variables containing the estimate of the innovation covariance

matrices for the forward autoregressive models The variable SIGFl contains

the lth column of b

p

in the observations with ORDER= p

SIGBl numeric variables containing the estimate of the innovation covariance

matrices for the backward autoregressive models The variable SIGBl contains

the lth column of b

p

in the observations with ORDER= p

FORk ndashl numeric variables containing the estimates of the autoregressive pa-

rameter matrices for the forward models The variable FORk ndashl contains the

lth column of the lag k autoregressive parameter matrix b

p

k

in the observations

with ORDER= p

BACk ndashl numeric variables containing the estimates of the autoregressive pa-

rameter matrices for the backward models The variable BACk ndashl contains the

lth column of the lag k autoregressive parameter matrix b

p

k

in the observations

with ORDER= p

The estimates for the order p autoregressive model can be selected as those observa-

tions with ORDER= p Within these observations the k lth element of

p

i

is given by

the value of the FORindashl variable in the k th observation The k lth element of

p

i

is

given by the value of BACindashl variable in the k th observation The k lth element of

p

is given by SIGFl in the k th observation The k lth element of

p

is given by SIGBlin the k th observation

Table 181 shows an example of the OUTAR= data set with ARMAX=3 andx

t

of

dimension 2 In Table 181 i j

indicate the ijth element of the matrix

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Part 2 General Information

Table 181 Values in the OUTAR= Data Set

Obs ORDER AIC SIGF1 SIGF2 SIGB1 SIGB2 FOR1ndash 1 FOR1ndash 2 FOR2ndash 1 FOR2ndash 2 FOR3ndash 1

1 0 AIC0

0 1 1

0 1 2

0 1 1

0 1 2

2 0 AIC0

0 2 1

0 2 2

0 2 1

0 2 2

3 1 AIC1

1 1 1

1 1 2

1 1 1

1 1 2

1

1

1 1

1

1

1 2

4 1 AIC1

1 2 1

1 1 2

1 2 1

1 1 2

1

1

2 1

1

1

2 2

5 2 AIC2

2 1 1

2 1 2

2 1 1

2 1 2

2

1

1 1

2

1

1 2

2

2

1 1

2

2

1 2

6 2 AIC2

2 2 1

2 1 2

2 2 1

2 1 2

2

1

2 1

2

1

2 2

2

2

2 1

2

2

2 2

7 3 AIC3

3 1 1

3 1 2

3 1 1

3 1 2

3

1

1 1

3

1

1 2

3

2

1 1

3

2

1 2

3

3

1 1

8 3 AIC3

3 2 1

3 1 2

3 2 1

3 1 2

3

1

2 1

3

1

2 2

3

2

2 1

3

2

2 2

3

3

2 1

Obs FOR3ndash 2 BACK1ndash 1 BACK1ndash 2 BACK2ndash 1 BACK2ndash 2 BACK3ndash 1 BACK3ndash 2

1

2

3

1

1

1 1

1

1

1 2

4

1

1

2 1

1

1

2 2

5

2

1

1 1

2

1

1 2

2

2

1 1

2

2

1 2

6

2

1

2 1

2

1

2 2

2

2

2 1

2

2

2 2

7

3

3

1 2

3

1

1 1

3

1

1 2

3

2

1 1

3

2

1 2

3

3

1 1

3

3

1 2

8

3

3

2 2

3

1

2 1

3

1

2 2

3

2

2 1

3

2

2 2

3

3

2 1

3

3

2 2

The estimated autoregressive parameters can be used in the IML procedure to obtain

autoregressive estimates of the spectral density function or forecasts based on the

autoregressive models

OUTMODEL= Data Set

The OUTMODEL= data set contains the estimates of theF

andG

matrices and their

standard errors the names of the components of the state vector and the estimates of

the innovation covariance matrix The variables contained in the OUTMODEL= data

set are as follows

the BY variables

STATEVEC a character variable containing the name of the component of the

state vector corresponding to the observation The STATEVEC variable has

the value STD for standard deviations observations which contain the standarderrors for the estimates given in the preceding observation

Fndash j numeric variables containing the columns of theF

matrix The variable

Fndash j contains the jth column of F

The number of Fndash j variables is equal to

the value of the DIMMAX= option If the model is of smaller dimension the

extraneous variables are set to missing

Gndash j numeric variables containing the columns of theG

matrix The variable

Gndash j contains the jth column of G

The number of Gndash j variables is equal to r

the dimension of x

t

given by the number of variables in the VAR statement

SIGndash j numeric variables containing the columns of the innovation covariance

matrix The variable SIGndash j contains the jth column of

e e

There are r vari-ables SIGndash j

Table 182 shows an example of the OUTMODEL= data set withx

t

= x

t

y

t

0

z

t

= x

t

y

t

x

t + 1 j t

0 and DIMMAX=4 In Table 182F

i j

andG

i j

are the ijth

elements of F

andG

respectively Note that all elements for Fndash4 are missing because

F

is a3 3

matrix

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Chapter 18 Details

Table 182 Value in the OUTMODEL= Data Set

Obs STATEVEC Fndash 1 Fndash 2 Fndash 3 Fndash 4 Gndash 1 Gndash 2 SIGndash 1 SIGndash 2

1 X(TT) 0 0 1 1 0

1 1

1 2

2 STD

3 Y(TT) F2 1

F2 2

F2 3

0 1

2 1

2 2

4 STD std F2 1

std F2 2

std F2 3

5 X(T+1T) F3 1

F3 2

F3 3

G3 1

G3 2

6 STD std F3 1

std F3 2

std F3 3

std G3 1

std G3 2

Printed Output

The printed output produced by the STATESPACE procedure is described in the fol-

lowing

1 descriptive statistics which include the number of observations used the

names of the variables their means and standard deviations (Std) and the dif-

ferencing operations used

2 the Akaike information criteria for the sequence of preliminary autoregressive

models

3 if the PRINTOUT=LONG option is specified the sample autocovariance ma-

trices of the input series at various lags

4 if the PRINTOUT=LONG option is specified the sample autocorrelation ma-

trices of the input series

5 a schematic representation of the autocorrelation matrices showing the signif-

icant autocorrelations

6 if the PRINTOUT=LONG option is specified the partial autoregressive ma-

trices (These are

p

p

as described in Preliminary Autoregressive Modelsearlier in this chapter)

7 a schematic representation of the partial autocorrelation matrices showing the

significant partial autocorrelations

8 the Yule-Walker estimates of the autoregressive parameters for the autoregres-

sive model with the minimum AIC

9 if the PRINTOUT=LONG option is specified the autocovariance matrices of

the residuals of the minimum AIC model This is the sequence of estimated

innovation variance matrices for the solutions of the Yule-Walker equations

10 if the PRINTOUT=LONG option is specified the autocorrelation matrices of the residuals of the minimum AIC model

11 If the CANCORR option is specified the canonical correlations analysis for

each potential state vector considered in the state vector selection process

This includes the potential state vector the canonical correlations the informa-

tion criterion for the smallest canonical correlation Bartlettrsquos

2 statistic (ldquoChi

Squarerdquo) for the smallest canonical correlation and the degrees of freedom of

Bartlettrsquos

2

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Part 2 General Information

12 the components of the chosen state vector

13 the preliminary estimate of the transition matrixF

the input matrixG

and

the variance matrix for the innovations

e e

14 if the ITPRINT option is specified the iteration history of the likelihood max-

imization For each iteration this shows the iteration number the number of step halvings the determinant of the innovation variance matrix the damping

factor Lambda and the values of the parameters

15 the state vector printed again to aid interpretation of the following listing of F

andG

16 the final estimate of the transition matrixF

17 the final estimate of the input matrixG

18 the final estimate of the variance matrix for the innovations

e e

19 a table listing the estimates of the free parameters inF

andG

and their standarderrors and t statistics

20 if the COVB option is specified the covariance matrix of the parameter esti-

mates

21 if the COVB option is specified the correlation matrix of the parameter esti-

mates

22 if the PRINT option is specified the forecasts and their standard errors

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Chapter 18 Details

ODS Table Names

PROC STATESPACE assigns a name to each table it creates You can use these

names to reference the table when using the Output Delivery System (ODS) to select

tables and create output data sets These names are listed in the following table For

more information on ODS see Chapter 6 ldquoUsing the Output Delivery Systemrdquo

Table 183 ODS Tables Produced in PROC STATESPACE

ODS Table Name Description Option

NObs Number of observations default

Summary Simple summary statistics table default

InfoCriterion Information criterion table default

CovLags Covariance Matrices of Input Series PRINTOUT=LONG

CorrLags Correlation Matrices of Input Series PRINTOUT=LONG

PartialAR Partial Autoregressive Matrices PRINTOUT=LONG

YWEstimates Yule-Walker Estimates for Minimum AIC default

CovResiduals Covariance of Residuals PRINTOUT=LONGCorrResiduals Residual Correlations from AR Models PRINTOUT=LONG

StateVector State vector table default

CorrGraph Schematic Representation of Correlations default

TransitionMatrix Transition Matrix default

InputMatrix Input Matrix default

VarInnov Variance Matrix for the Innovation default

CovB Covariance of Parameter Estimates COVB

CorrB Correlation of Parameter Estimates COVB

CanCorr Canonical Correlation Analysis CANCORR

IterHistory Iterative Fitting table ITPRINT

ParameterEstimates Parameter Estimates Table default

Forecasts Forecasts Table PRINT

ConvergenceStatus Convergence Status Table default

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Part 2 General Information

Example

Example 181 Series J from Box and Jenkins

This example analyzes the gas furnace data (series J) from Box and Jenkins (The

data are not shown Refer to Box and Jenkins (1976) for the data)

First a model is selected and fit automatically using the following statements

title1 rsquoGas Furnace Datarsquo

title2 rsquoBox amp Jenkins Series Jrsquo

title3 rsquoAutomatically Selected Modelrsquo

proc statespace data=seriesj cancorr

var x y

run

The results for the automatically selected model are shown in Output 1811

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Chapter 18 Example

Output 1811 Results for Automatically Selected Model

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Number of Observations 296

Standard

Variable Mean Error

x -005683 1072766

y 5350912 3202121

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Information Criterion for Autoregressive Models

Lag=0 Lag=1 Lag=2 Lag=3 Lag=4 Lag=5 Lag=6 Lag=7 Lag=8

6513862 -103357 -163296 -164512 -165152 -164891 -164934 -164315 -163856

Information

Criterion for

Autoregressive

Models

Lag=9 Lag=10

-16348 -163359

Schematic Representation of Correlations

NameLag 0 1 2 3 4 5 6 7 8 9 10

x +- +- +- +- +- +- +- +- +- +- +-

y -+ -+ -+ -+ -+ -+ -+ -+ -+ -+ -+

+ is gt 2std error - is lt -2std error is between

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Part 2 General Information

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Schematic Representation of Partial Autocorrelations

NameLag 1 2 3 4 5 6 7 8 9 10

x + - + -

y -+ -- - + +

+ is gt 2std error - is lt -2std error is between

Yule-Walker Estimates for Minimum AIC

------Lag=1------ ------Lag=2------ ------Lag=3------ ------Lag=4------

x y x y x y x y

x 1925887 -000124 -120166 0004224 0116918 -000867 0104236 0003268

y 0050496 1299793 -002046 -03277 -071182 -025701 0195411 0133417

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Canonical Correlations Analysis

Information Chi

x(TT) y(TT) x(T+1T) Criterion Square DF

1 1 0804883 2929228 3047481 8

Information Chix(TT) y(TT) x(T+1T) y(T+1T) Criterion Square DF

1 1 0906681 0607529 1223358 1347237 7

Information Chi

x(TT) y(TT) x (T+1T) y (T+1T) x (T+2T) Criterion Square DF

1 1 0909434 0610278 0186274 -154701 1034705 6

Information Chi

x(TT) y(TT) x (T+1T) y (T+1T) y (T+2T) Criterion Square DF

1 1 091014 0618937 0206823 0940392 1280924 6

Information Chi

x(TT) y(TT) x(T+1T) y(T+1T) y(T+2T) y(T+3T) Criterion Square DF

1 1 0912963 0628785 0226598 0083258 -794103 2041584 5

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Chapter 18 Example

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Selected Statespace Form and Preliminary Estimates

State Vector

x(TT) y(TT) x(T+1T) y(T+1T) y(T+2T)

Estimate of Transition Matrix

0 0 1 0 0

0 0 0 1 0

-084718 0026794 1711715 -005019 0

0 0 0 0 1

-019785 0334274 -018174 -123557 1787475

Input Matrix for Innovation

1 0

0 11925887 -000124

0050496 1299793

0142421 1361696

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Selected Statespace Form and Preliminary Estimates

Variance Matrix for Innovation

0035274 -000734

-000734 0097569

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Part 2 General Information

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Selected Statespace Form and Fitted Model

State Vector

x(TT) y(TT) x(T+1T) y(T+1T) y(T+2T)

Estimate of Transition Matrix

0 0 1 0 0

0 0 0 1 0

-086192 0030609 1724235 -005483 0

0 0 0 0 1

-034839 0292124 -009435 -109823 1671418

Input Matrix for Innovation

1 0

0 1192442 -000416

0015621 1258495

008058 1353204

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Selected Statespace Form and Fitted Model

Variance Matrix for Innovation

0035579 -000728

-000728 0095577

Parameter Estimates

Standard

Parameter Estimate Error t Value

F(31) -086192 0072961 -1181

F(32) 0030609 0026167 117

F(33) 1724235 0061599 2799

F(34) -005483 0030169 -182

F(51) -034839 0135253 -258

F(52) 0292124 0046299 631

F(53) -009435 0096527 -098F(54) -109823 0109525 -1003

F(55) 1671418 0083737 1996

G(31) 1924420 0058162 3309

G(32) -000416 0035255 -012

G(41) 0015621 0095771 016

G(42) 1258495 0055742 2258

G(51) 0080580 0151622 053

G(52) 1353204 0091388 1481

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Chapter 18 Example

The two series are believed to have a transfer function relation with the gas rate

(variable X) as the input and the CO2

concentration (variable Y) as the output Since

the parameter estimates shown in Output 1811 support this kind of model the model

is reestimated with the feedback parameters restricted to 0 The following statements

fit the transfer function (no feedback) model

title3 rsquoTransfer Function Modelrsquo

proc statespace data=seriesj printout=none

var x y

restrict f(32)=0 f(34)=0

g(32)=0 g(41)=0 g(51)=0

run

The last 2 pages of the output are shown in Output 1812

Output 1812 STATESPACE Output for Transfer Function Model

Gas Furnace Data

Box amp Jenkins Series J

Transfer Function Model

The STATESPACE Procedure

Selected Statespace Form and Fitted Model

State Vector

x(TT) y(TT) x(T+1T) y(T+1T) y(T+2T)

Estimate of Transition Matrix

0 0 1 0 0

0 0 0 1 0

-068882 0 1598717 0 0

0 0 0 0 1

-035944 0284179 -00963 -107313 1650047

Input Matrix for Innovation

1 0

0 1

1923446 0

0 1260856

0 1346332

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Part 2 General Information

Gas Furnace Data

Box amp Jenkins Series J

Transfer Function Model

The STATESPACE Procedure

Selected Statespace Form and Fitted Model

Variance Matrix for Innovation

0036995 -00072

-00072 0095712

Parameter Estimates

Standard

Parameter Estimate Error t Value

F(31) -068882 0050549 -1363

F(33) 1598717 0050924 3139

F(51) -035944 0229044 -157

F(52) 0284179 0096944 293

F(53) -009630 0140876 -068

F(54) -107313 0250385 -429

F(55) 1650047 0188533 875G(31) 1923446 0056328 3415

G(42) 1260856 0056464 2233

G(52) 1346332 0091086 1478

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Chapter 18 References

References

Akaike H (1974) Markovian Representation of Stochastic Processes and Its Ap-

plication to the Analysis of Autoregressive Moving Average Processes Annals

of the Institute of Statistical Mathematics 26 363-387

Akaike H (1976) Canonical Correlations Analysis of Time Series and the Use of

an Information Criterion in Advances and Case Studies in System Identification

eds R Mehra and DG Lainiotis New York Academic Press

Anderson TW (1971) The Statistical Analysis of Time Series New York John

Wiley amp Sons

Ansley CF and Newbold P (1979) Multivariate Partial Autocorrelations Pro-

ceedings of the Business and Economic Statistics Section American Statistical

Association 349-353

Box GEP and Jenkins G (1976) Time Series Analysis Forecasting and Control

San Francisco Holden-Day

Brockwell PJ and Davis RA (1991) Time Series Theory and Methods 2nd Edi-

tion Springer-Verlag

Hannan EJ (1970) Multiple Time Series New York John Wiley amp Sons

Hannan EJ (1976) The Identification and Parameterization of ARMAX and State

Space Forms Econometrica 44 713-722

Harvey AC (1981a) The Econometric Analysis of Time Series New York John

Wiley amp Sons

Harvey AC (1981b) Time Series Models New York John Wiley amp Sons

Jones RH (1974) Identification and Autoregressive Spectrum Estimation IEEE Transactions on Automatic Control AC-19 894-897

Pham-Dinh-Tuan (1978) On the Fitting of Multivariate Processes of the

Autoregressive-Moving Average Type Biometrika 65 99-107

Priestley MB (1980) System Identification Kalman Filtering and Stochastic Con-

trol in Directions in Time Series eds DR Brillinger and GC Tiao Institute

of Mathematical Statistics

Whittle P (1963) On the Fitting of Multivariate Autoregressions and the Approxi-

mate Canonical Factorization of a Spectral Density Matrix Biometrika 50 129-

134

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The correct bibliographic citation for this manual is as follows SAS Institute Inc SAS

ET S U serrsquos Gui de Version 8 Cary NC SAS Institute Inc 1999 1546 pp

SASET S Userrsquos Guide Version 8

Copyright copy 1999 by S AS I ns t it u t e I nc Ca ry N C U S A

ISBN 1ndash58025ndash489ndash6

All r ight s res erved P r int ed in t he U n it ed St a t es of A merica N o pa rt of t his p u blica t ionma y b e reprodu ced s t ored in a ret r ieva l s ys t em or t ra ns mit t ed in a ny form or b y a nymeans electronic mechanical photocopying or otherwise without the prior writtenpermission of the publisher SAS Institute Inc

US Government Restricted Rights Notice Use du plicat ion or disclosure of the

software by the government is subject to restrictions as set forth in FAR 52227ndash19Commercial Computer Softw ar e-Restricted Rights (J une 1987)

SAS Institute Inc SAS Campus Drive Cary North Carolina 27513

1st print ing October 1999

S AS reg a nd a ll ot her SA S I ns t it u t e I nc p rodu ct or s ervice na m es a re regis t ered t ra dema rkst d k f SA S I t it t I i t h U S A d t h t i reg i di t U SA

Page 30: Statespace Model

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Part 2 General Information

Canonical Correlation Analysis

Given the order p letp

t

be the vector of current and past values relevant to prediction

of x

t + 1

p

t

= x

0

t

x

0

t 1

x

0

t p

0

Letf

t

be the vector of current and future values

f

t

= x

0

t

x

0

t + 1

x

0

t + p

0

In the canonical correlation analysis consider submatrices of the sample covariance

matrix of p

t

andf

t

This covariance matrixV

has a block Hankel form

V =

2

6

6

6

4

C

0

C

0

1

C

0

2

C

0

p

C

0

1

C

0

2

C

0

3

C

0

p + 1

C

0

p

C

0

p + 1

C

0

p + 2

C

0

2 p

3

7

7

7

5

State Vector Selection Process

The canonical correlation analysis forms a sequence of potential state vectorsz

j

t

Examine a sequencef

j

t

of subvectors of f

t

and form the submatrixV

j consisting

of the rows and columns of V

corresponding to the components of f

j

t

and compute

its canonical correlations

The smallest canonical correlation of V

j is then used in the selection of the com-

ponents of the state vector The selection process is described in the following For

more details about this process refer to Akaike (1976)

In the following discussion the notationx

t + k j t

denotes the wide sense conditional

expectation (best linear predictor) of x

t + k

given allx

s

with s less than or equal to t

In the notationx

i t + 1

the first subscript denotes the ith component of x

t + 1

The initial state vectorz

1

t

is set tox

t

The sequencef

j

t

is initialized by setting

f

1

t

= z

1

0

t

x

1 t + 1 j t

0

= x

0

t

x

1 t + 1 j t

0

That is start by considering whether to addx

1 t + 1 j t

to the initial state vectorz

1

t

The procedure forms the submatrixV

1 corresponding tof

1

t

and computes its canon-

ical correlations Denote the smallest canonical correlation of V

1 as

m i n

If

m i n

is

significantly greater than 0x

1 t + 1 j t

is added to the state vector

If the smallest canonical correlation of V

1 is not significantly greater than 0 then a

linear combination of f

1

t

is uncorrelated with the pastp

t

Assuming that the determi-

nant of C

0

is not 0 (that is no input series is a constant) you can take the coefficient

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Chapter 18 Details

of x

1 t + 1 j t

in this linear combination to be 1 Denote the coefficients of z

1

t

in this

linear combination as

This gives the relationship

x

1 t + 1 j t

=

0

x

t

Therefore the current state vector already contains all the past information useful forpredicting

x

1 t + 1

and any greater leads of x

1 t

The variablex

1 t + 1 j t

is not added to

the state vector nor are any termsx

1 t + k j t

considered as possible components of the

state vector The variablex

1

is no longer active for state vector selection

The process described forx

1 t + 1 j t

is repeated for the remaining elements of f

t

The

next candidate for inclusion in the state vector is the next component of f

t

corre-

sponding to an active variable Components of f

t

corresponding to inactive variables

that produced a zero

m i n

in a previous step are skipped

Denote the next candidate asx

l t + k j t

The vectorf

j

t

is formed from the current state

vector andx

l t + k j t

as follows

f

j

t

= z

j

0

t

x

l t + k j t

0

The matrixV

j is formed fromf

j

t

and its canonical correlations are computed The

smallest canonical correlation of V

j is judged to be either greater than or equal to 0

If it is judged to be greater than 0x

l t + k j t

is added to the state vector If it is judged

to be 0 then a linear combination of f

j

t

is uncorrelated with thep

t

and the variable

x

l

is now inactive

The state vector selection process continues until no active variables remain

Testing Significance of Canonical Correlations For each step in the canonical correlation sequence the significance of the small-

est canonical correlation

m i n

is judged by an information criterion from Akaike

(1976) This information criterion is

n l n 1

2

m i n

r p + 1 q + 1

where q is the dimension of f

j

t

at the current step r is the order of the state vector p

is the order of the vector autoregressive process and

is the value of the SIGCORR=

option The default is SIGCORR=2 If this information criterion is less than or equal

to 0

m i n

is taken to be 0 otherwise it is taken to be significantly greater than 0

(Do not confuse this information criterion with the AIC)

Variables inx

t + p j t

are not added in the model even with positive information crite-

rion because of the singularity of V

You can force the consideration of more candi-

date state variables by increasing the size of theV

matrix by specifying a PASTMIN=

option value larger than p

Printing the Canonical Correlations

To print the details of the canonical correlation analysis process specify the CAN-

CORR option in the PROC STATESPACE statement The CANCORR option prints

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Part 2 General Information

the candidate state vectors the canonical correlations and the information criteria for

testing the significance of the smallest canonical correlation

Bartlettrsquos

2 and its degrees of freedom are also printed when the CANCORR option

is specified The formula used for Bartlettrsquos

2 is

2

= n 5 r p + 1 q + 1 l n 1

2

m i n

withr p + 1 q + 1

degrees of freedom

Figure 1811 shows the output of the CANCORR option for the introductory example

shown in the Getting Started section of this chapter

The STATESPACE Procedure

Canonical Correlations Analysis

Information Chi

x(TT) y(TT) x(T+1T) Criterion Square DF

1 1 0237045 3566167 114505 4

Information Chi

x(TT) y(TT) x(T+1T) y(T+1T) Criterion Square DF

1 1 0238244 0056565 -535906 0636134 3

Information Chi

x(TT) y(TT) x(T+1T) x(T+2T) Criterion Square DF

1 1 0237602 0087493 -446312 1525353 3

Figure 1811 Canonical Correlations Analysis

New variables are added to the state vector if the information criteria are positive Inthis example

y

t + 1 j t

andx

t + 2 j t

are not added to the state space vector because the

information criteria for these models are negative

If the information criterion is nearly 0 then you may want to investigate models

that arise if the opposite decision is made regarding

m i n

This investigation can be

accomplished by using a FORM statement to specify part or all of the state vector

Preliminary Estimates of F

When a candidate variablex

l t + k j t

yields a zero

m i n

and is not added to the state

vector a linear combination of f

j

t

is uncorrelated with thep

t

Because of the method

used to construct thef

j

t

sequence the coefficient of x

l t + k j t

inl

can be taken as 1

Denote the coefficients of z

j

t

in this linear combination as l

This gives the relationship

x

l t + k j t

= l

0

z

j

t

The vectorl

is used as a preliminary estimate of the first r columns of the row of the

transition matrixF

corresponding tox

l t + k 1 j t

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Chapter 18 Details

Parameter Estimation

The model isz

t + 1

= F z

t

+ G e

t + 1

wheree

t

is a sequence of independent multivari-

ate normal innovations with mean vector 0 and variance

e e

The observed sequence

x

t

composes the first r components of z

t

and thusx

t

= H z

t

where H is ther s

matrix I

r

0

LetE

be ther n

matrix of innovations

E = e

1

e

n

If the number of observations n is reasonably large the log likelihood L can be

approximated up to an additive constant as follows

L =

n

2

l n j

e e

j

1

2

t r a c e

1

e e

E E

0

The elements of

e e

are taken as free parameters and are estimated as follows

S

0

=

1

n

E E

0

Replacing

e e

byS

0

in the likelihood equation the log likelihood up to an additive

constant is

L =

n

2

l n j S

0

j

Letting B be the backshift operator the formal relation betweenx

t

ande

t

is

x

t

= H I B F

1

G e

t

e

t

= H I B F

1

G

1

x

t

=

1

X

i = 0

i

x

t i

LettingC

i

be the ith lagged sample covariance of x

t

and neglecting end effects the

matrixS

0

is

S

0

=

1

X

i j = 0

i

C

i + j

0

j

For the computation of S

0

the infinite sum is truncated at the value of the KLAG=

option The value of the KLAG= option should be large enough that the sequence

i

is approximately 0 beyond that point

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Part 2 General Information

Let

be the vector of free parameters in theF

andG

matrices The derivative of the

log likelihood with respect to the parameter

is

L

=

n

2

t r a c e

S

1

0

S

0

The second derivative is

2

L

0

=

n

2

t r a c e

S

1

0

S

0

0

S

1

0

S

0

t r a c e

S

1

0

2

S

0

0

Near the maximum the first term is unimportant and the second term can be approx-

imated to give the following second derivative approximation

2

L

0

=

n t r a c e

S

1

0

E

E

0

0

The first derivative matrix and this second derivative matrix approximation are com-

puted from the sample covariance matrixC

0

and the truncated sequence

i

The

approximate likelihood function is maximized by a modified Newton-Raphson algo-

rithm employing these derivative matrices

The matrixS

0

is used as the estimate of the innovation covariance matrix

e e

The

negative of the inverse of the second derivative matrix at the maximum is used as an

approximate covariance matrix for the parameter estimates The standard errors of

the parameter estimates printed in the parameter estimates tables are taken from the

diagonal of this covariance matrix The parameter covariance matrix is printed when

the COVB option is specified

If the data are nearly nonstationary a better estimate of

e e

and the other parameters

can sometimes be obtained by specifying the RESIDEST option The RESIDEST

option estimates the parameters using conditional least squares instead of maximum

likelihood

The residuals are computed using the state space equation and the sample mean val-

ues of the variables in the model as start-up values The estimate of S

0

is then com-

puted using the residuals from the ith observation on where i is the maximum number

of times any variable occurs in the state vector A multivariate Gauss-Marquardt al-

gorithm is used to minimizej S

0

j

Refer to Harvey (1981a) for a further description

of this method

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Chapter 18 Details

Forecasting

Given estimates of F

G

and

e e

forecasts of x

t

are computed from the conditional

expectation of z

t

In forecasting the parameters F G and

e e

are replaced with the estimates or by val-

ues specified in the RESTRICT statement One-step-ahead forecasting is performedfor the observation

x

t

wheret n b

Heren

is the number of observations and

b is the value of the BACK= option For the observationx

t

wheret n b

m-

step-ahead forecasting is performed form = t n + b

The forecasts are generated

recursively with the initial conditionz

0

= 0

The m-step-ahead forecast of z

t + m

isz

t + m j t

wherez

t + m j t

denotes the conditional

expectation of z

t + m

given the information available at time t The m-step-ahead

forecast of x

t + m

isx

t + m j t

= H z

t + m j t

where the matrixH = I

r

0

Let

i

= F

i

G

Note that the lasts r

elements of z

t

consist of the elements of x

u j t

foru t

The state vector z

t + m

can be represented as

z

t + m

= F

m

z

t

+

m 1

X

i = 0

i

e

t + m i

Sincee

t + i j t

= 0

fori 0

the m-step-ahead forecastz

t + m j t

is

z

t + m j t

= F

m

z

t

= F z

t + m 1 j t

Therefore the m-step-ahead forecast of x

t + m

is

x

t + m j t

= H z

t + m j t

The m-step-ahead forecast error is

z

t + m

z

t + m j t

=

m 1

X

i = 0

i

e

t + m i

The variance of the m-step-ahead forecast error is

V

z m

=

m 1

X

i = 0

i

e e

0

i

LettingV

z 0

= 0

the variance of the m-step-ahead forecast error of z

t + m

V

z m

can

be computed recursively as follows

V

z m

= V

z m 1

+

m 1

e e

0

m 1

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Part 2 General Information

The variance of the m-step-ahead forecast error of x

t + m

is ther r

left upper sub-

matrix of V

z m

that is

V

x m

= H V

z m

H

0

Unless the NOCENTER option is specified the sample mean vector is added to the

forecast When differencing is specified the forecasts xt + m j t

plus the sample mean

vector are integrated back to produce forecasts for the original series

Lety

t

be the original series specified by the VAR statement with some 0 values

appended corresponding to the unobserved past observations Let B be the backshift

operator and let B

be thes s

matrix polynomial in the backshift operator

corresponding to the differencing specified by the VAR statement The off-diagonal

elements of

i

are 0 Note that

0

= I

s

whereI

s

is thes s

identity matrix Then

z

t

= B y

t

This gives the relationship

y

t

=

1

B z

t

=

1

X

i = 0

i

z

t i

where

1

B =

P

1

i = 0

i

B

i and

0

= I

s

The m-step-ahead forecast of y

t + m

is

y

t + m j t

=

m 1

X

i = 0

i

z

t + m i j t

+

1

X

i = m

i

z

t + m i

The m-step-ahead forecast error of y

t + m

is

m 1

X

i = 0

i

z

t + m i

z

t + m i j t

=

m 1

X

i = 0

i

X

u = 0

u

i u

e

t + m i

LettingV

y 0

= 0

the variance of the m-step-ahead forecast error of y

t + m

V

y m

is

V

y m

=

m 1

X

i = 0

i

X

u = 0

u

i u

e e

i

X

u = 0

u

i u

0

= V

y m 1

+

m 1

X

u = 0

u

m 1 u

e e

m 1

X

u = 0

u

m 1 u

0

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Chapter 18 Details

Relation of ARMA and State Space Forms

Every state space model has an ARMA representation and conversely every ARMA

model has a state space representation This section discusses this equivalence The

following material is adapted from Akaike (1974) where there is a more complete

discussion Pham-Dinh-Tuan (1978) also contains a discussion of this material

Suppose you are given the following ARMA model

B x

t

= B e

t

or in more detail

x

t

1

x

t 1

p

x

t p

= e

t

+

1

e

t 1

+ +

q

e

t q

(1)

wheree

t

is a sequence of independent multivariate normal random vectors with mean

0 and variance matrix

e e

B is the backshift operator (B x

t

= x

t 1

) B

and

B

are matrix polynomials in B andx

t

is the observed process

If the roots of the determinantial equationj B j = 0

are outside the unit circle in

the complex plane the model can also be written as

x

t

=

1

B B e

t

=

1

X

i = 0

i

e

t i

The

i

matrices are known as the impulse response matrices and can be computed

as

1

B B

You can assume p q

since if this is not initially true you can add more terms

i

that are identically 0 without changing the model

To write this set of equations in a state space form proceed as follows Letx

t + i j t

be

the conditional expectation of x

t + i

givenx

w

forw t

The following relations hold

x

t + i j t

=

1

X

j = i

j

e

t + i j

x

t + i j t + 1

= x

t + i j t

+

i 1

e

t + 1

However from equation (1) you can derive the following relationship

x

t + p j t

=

1

x

t + p 1 j t

+ +

p

x

t

(2)

Hence wheni = p

you can substitute forx

t + p j t

in the right-hand side of equation

(2) and close the system of equations

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Part 2 General Information

This substitution results in the following model in the state space form

z

t + 1

= F z

t

+ G e

t + 1

2

6

6

6

4

x

t + 1

x

t + 2 j t + 1

x

t + p j t + 1

3

7

7

7

5

=

2

6

6

4

0 I 0 0

0 0 I 0

p

p 1

1

3

7

7

5

2

6

6

6

4

x

t

x

t + 1 j t

x

t + p 1 j t

3

7

7

7

5

+

2

6

6

4

I

1

p 1

3

7

7

5

e

t + 1

Note that the state vectorz

t

is composed of conditional expectations of x

t

and the

first r components of z

t

are equal tox

t

The state space form can be cast into an ARMA form by solving the system of dif-

ference equations for the first r components

When converting from an ARMA form to a state space form you can generate a

state vector larger than needed that is the state space model may not be a minimal

representation When going from a state space form to an ARMA form you can have

nontrivial common factors in the autoregressive and moving average operators that

yield an ARMA model larger than necessary

If the state space form used is not a minimal representation some but not all compo-

nents of x

t + i j t

may be linearly dependent This situation corresponds to

p

p 1

being of less than full rank when B

and B

have no common nontrivial

left factors In this casez

t

consists of a subset of the possible components of

x

t + i j t

i = 1 2 p 1

However once a component of x

t + i j t

(for example the

jth one) is linearly dependent on the previous conditional expectations then all sub-

sequent jth components of x

t + k j t

fork i

must also be linearly dependent Note

that in this case equivalent but seemingly different structures can arise if the order of

the components withinx

t

is changed

OUT= Data Set

The forecasts are contained in the output data set specified by the OUT= option on the

PROC STATESPACE statement The OUT= data set contains the following variables

the BY variables

the ID variable

the VAR statement variables These variables contain the actual values from

the input data set

FORi numeric variables containing the forecasts The variable FORi contains

the forecasts for the ith variable in the VAR statement list Forecasts are one-

step-ahead predictions until the end of the data or until the observation specified

by the BACK= option

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Chapter 18 Details

RESi numeric variables containing the residual for the forecast of the ith vari-

able in the VAR statement list For forecast observations the actual values are

missing and the RESi variables contain missing values

STDi numeric variables containing the standard deviation for the forecast of

the ith variable in the VAR statement list The values of the STDi variables can

be used to construct univariate confidence limits for the corresponding fore-casts However such confidence limits do not take into account the covariance

of the forecasts

OUTAR= Data Set

The OUTAR= data set contains the estimates of the preliminary autoregressive mod-

els The OUTAR= data set contains the following variables

ORDER a numeric variable containing the order p of the autoregressive model

that the observation represents

AIC a numeric variable containing the value of the information criterionA I C

p

SIGFl numeric variables containing the estimate of the innovation covariance

matrices for the forward autoregressive models The variable SIGFl contains

the lth column of b

p

in the observations with ORDER= p

SIGBl numeric variables containing the estimate of the innovation covariance

matrices for the backward autoregressive models The variable SIGBl contains

the lth column of b

p

in the observations with ORDER= p

FORk ndashl numeric variables containing the estimates of the autoregressive pa-

rameter matrices for the forward models The variable FORk ndashl contains the

lth column of the lag k autoregressive parameter matrix b

p

k

in the observations

with ORDER= p

BACk ndashl numeric variables containing the estimates of the autoregressive pa-

rameter matrices for the backward models The variable BACk ndashl contains the

lth column of the lag k autoregressive parameter matrix b

p

k

in the observations

with ORDER= p

The estimates for the order p autoregressive model can be selected as those observa-

tions with ORDER= p Within these observations the k lth element of

p

i

is given by

the value of the FORindashl variable in the k th observation The k lth element of

p

i

is

given by the value of BACindashl variable in the k th observation The k lth element of

p

is given by SIGFl in the k th observation The k lth element of

p

is given by SIGBlin the k th observation

Table 181 shows an example of the OUTAR= data set with ARMAX=3 andx

t

of

dimension 2 In Table 181 i j

indicate the ijth element of the matrix

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Part 2 General Information

Table 181 Values in the OUTAR= Data Set

Obs ORDER AIC SIGF1 SIGF2 SIGB1 SIGB2 FOR1ndash 1 FOR1ndash 2 FOR2ndash 1 FOR2ndash 2 FOR3ndash 1

1 0 AIC0

0 1 1

0 1 2

0 1 1

0 1 2

2 0 AIC0

0 2 1

0 2 2

0 2 1

0 2 2

3 1 AIC1

1 1 1

1 1 2

1 1 1

1 1 2

1

1

1 1

1

1

1 2

4 1 AIC1

1 2 1

1 1 2

1 2 1

1 1 2

1

1

2 1

1

1

2 2

5 2 AIC2

2 1 1

2 1 2

2 1 1

2 1 2

2

1

1 1

2

1

1 2

2

2

1 1

2

2

1 2

6 2 AIC2

2 2 1

2 1 2

2 2 1

2 1 2

2

1

2 1

2

1

2 2

2

2

2 1

2

2

2 2

7 3 AIC3

3 1 1

3 1 2

3 1 1

3 1 2

3

1

1 1

3

1

1 2

3

2

1 1

3

2

1 2

3

3

1 1

8 3 AIC3

3 2 1

3 1 2

3 2 1

3 1 2

3

1

2 1

3

1

2 2

3

2

2 1

3

2

2 2

3

3

2 1

Obs FOR3ndash 2 BACK1ndash 1 BACK1ndash 2 BACK2ndash 1 BACK2ndash 2 BACK3ndash 1 BACK3ndash 2

1

2

3

1

1

1 1

1

1

1 2

4

1

1

2 1

1

1

2 2

5

2

1

1 1

2

1

1 2

2

2

1 1

2

2

1 2

6

2

1

2 1

2

1

2 2

2

2

2 1

2

2

2 2

7

3

3

1 2

3

1

1 1

3

1

1 2

3

2

1 1

3

2

1 2

3

3

1 1

3

3

1 2

8

3

3

2 2

3

1

2 1

3

1

2 2

3

2

2 1

3

2

2 2

3

3

2 1

3

3

2 2

The estimated autoregressive parameters can be used in the IML procedure to obtain

autoregressive estimates of the spectral density function or forecasts based on the

autoregressive models

OUTMODEL= Data Set

The OUTMODEL= data set contains the estimates of theF

andG

matrices and their

standard errors the names of the components of the state vector and the estimates of

the innovation covariance matrix The variables contained in the OUTMODEL= data

set are as follows

the BY variables

STATEVEC a character variable containing the name of the component of the

state vector corresponding to the observation The STATEVEC variable has

the value STD for standard deviations observations which contain the standarderrors for the estimates given in the preceding observation

Fndash j numeric variables containing the columns of theF

matrix The variable

Fndash j contains the jth column of F

The number of Fndash j variables is equal to

the value of the DIMMAX= option If the model is of smaller dimension the

extraneous variables are set to missing

Gndash j numeric variables containing the columns of theG

matrix The variable

Gndash j contains the jth column of G

The number of Gndash j variables is equal to r

the dimension of x

t

given by the number of variables in the VAR statement

SIGndash j numeric variables containing the columns of the innovation covariance

matrix The variable SIGndash j contains the jth column of

e e

There are r vari-ables SIGndash j

Table 182 shows an example of the OUTMODEL= data set withx

t

= x

t

y

t

0

z

t

= x

t

y

t

x

t + 1 j t

0 and DIMMAX=4 In Table 182F

i j

andG

i j

are the ijth

elements of F

andG

respectively Note that all elements for Fndash4 are missing because

F

is a3 3

matrix

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Chapter 18 Details

Table 182 Value in the OUTMODEL= Data Set

Obs STATEVEC Fndash 1 Fndash 2 Fndash 3 Fndash 4 Gndash 1 Gndash 2 SIGndash 1 SIGndash 2

1 X(TT) 0 0 1 1 0

1 1

1 2

2 STD

3 Y(TT) F2 1

F2 2

F2 3

0 1

2 1

2 2

4 STD std F2 1

std F2 2

std F2 3

5 X(T+1T) F3 1

F3 2

F3 3

G3 1

G3 2

6 STD std F3 1

std F3 2

std F3 3

std G3 1

std G3 2

Printed Output

The printed output produced by the STATESPACE procedure is described in the fol-

lowing

1 descriptive statistics which include the number of observations used the

names of the variables their means and standard deviations (Std) and the dif-

ferencing operations used

2 the Akaike information criteria for the sequence of preliminary autoregressive

models

3 if the PRINTOUT=LONG option is specified the sample autocovariance ma-

trices of the input series at various lags

4 if the PRINTOUT=LONG option is specified the sample autocorrelation ma-

trices of the input series

5 a schematic representation of the autocorrelation matrices showing the signif-

icant autocorrelations

6 if the PRINTOUT=LONG option is specified the partial autoregressive ma-

trices (These are

p

p

as described in Preliminary Autoregressive Modelsearlier in this chapter)

7 a schematic representation of the partial autocorrelation matrices showing the

significant partial autocorrelations

8 the Yule-Walker estimates of the autoregressive parameters for the autoregres-

sive model with the minimum AIC

9 if the PRINTOUT=LONG option is specified the autocovariance matrices of

the residuals of the minimum AIC model This is the sequence of estimated

innovation variance matrices for the solutions of the Yule-Walker equations

10 if the PRINTOUT=LONG option is specified the autocorrelation matrices of the residuals of the minimum AIC model

11 If the CANCORR option is specified the canonical correlations analysis for

each potential state vector considered in the state vector selection process

This includes the potential state vector the canonical correlations the informa-

tion criterion for the smallest canonical correlation Bartlettrsquos

2 statistic (ldquoChi

Squarerdquo) for the smallest canonical correlation and the degrees of freedom of

Bartlettrsquos

2

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Part 2 General Information

12 the components of the chosen state vector

13 the preliminary estimate of the transition matrixF

the input matrixG

and

the variance matrix for the innovations

e e

14 if the ITPRINT option is specified the iteration history of the likelihood max-

imization For each iteration this shows the iteration number the number of step halvings the determinant of the innovation variance matrix the damping

factor Lambda and the values of the parameters

15 the state vector printed again to aid interpretation of the following listing of F

andG

16 the final estimate of the transition matrixF

17 the final estimate of the input matrixG

18 the final estimate of the variance matrix for the innovations

e e

19 a table listing the estimates of the free parameters inF

andG

and their standarderrors and t statistics

20 if the COVB option is specified the covariance matrix of the parameter esti-

mates

21 if the COVB option is specified the correlation matrix of the parameter esti-

mates

22 if the PRINT option is specified the forecasts and their standard errors

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Chapter 18 Details

ODS Table Names

PROC STATESPACE assigns a name to each table it creates You can use these

names to reference the table when using the Output Delivery System (ODS) to select

tables and create output data sets These names are listed in the following table For

more information on ODS see Chapter 6 ldquoUsing the Output Delivery Systemrdquo

Table 183 ODS Tables Produced in PROC STATESPACE

ODS Table Name Description Option

NObs Number of observations default

Summary Simple summary statistics table default

InfoCriterion Information criterion table default

CovLags Covariance Matrices of Input Series PRINTOUT=LONG

CorrLags Correlation Matrices of Input Series PRINTOUT=LONG

PartialAR Partial Autoregressive Matrices PRINTOUT=LONG

YWEstimates Yule-Walker Estimates for Minimum AIC default

CovResiduals Covariance of Residuals PRINTOUT=LONGCorrResiduals Residual Correlations from AR Models PRINTOUT=LONG

StateVector State vector table default

CorrGraph Schematic Representation of Correlations default

TransitionMatrix Transition Matrix default

InputMatrix Input Matrix default

VarInnov Variance Matrix for the Innovation default

CovB Covariance of Parameter Estimates COVB

CorrB Correlation of Parameter Estimates COVB

CanCorr Canonical Correlation Analysis CANCORR

IterHistory Iterative Fitting table ITPRINT

ParameterEstimates Parameter Estimates Table default

Forecasts Forecasts Table PRINT

ConvergenceStatus Convergence Status Table default

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Part 2 General Information

Example

Example 181 Series J from Box and Jenkins

This example analyzes the gas furnace data (series J) from Box and Jenkins (The

data are not shown Refer to Box and Jenkins (1976) for the data)

First a model is selected and fit automatically using the following statements

title1 rsquoGas Furnace Datarsquo

title2 rsquoBox amp Jenkins Series Jrsquo

title3 rsquoAutomatically Selected Modelrsquo

proc statespace data=seriesj cancorr

var x y

run

The results for the automatically selected model are shown in Output 1811

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Chapter 18 Example

Output 1811 Results for Automatically Selected Model

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Number of Observations 296

Standard

Variable Mean Error

x -005683 1072766

y 5350912 3202121

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Information Criterion for Autoregressive Models

Lag=0 Lag=1 Lag=2 Lag=3 Lag=4 Lag=5 Lag=6 Lag=7 Lag=8

6513862 -103357 -163296 -164512 -165152 -164891 -164934 -164315 -163856

Information

Criterion for

Autoregressive

Models

Lag=9 Lag=10

-16348 -163359

Schematic Representation of Correlations

NameLag 0 1 2 3 4 5 6 7 8 9 10

x +- +- +- +- +- +- +- +- +- +- +-

y -+ -+ -+ -+ -+ -+ -+ -+ -+ -+ -+

+ is gt 2std error - is lt -2std error is between

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Part 2 General Information

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Schematic Representation of Partial Autocorrelations

NameLag 1 2 3 4 5 6 7 8 9 10

x + - + -

y -+ -- - + +

+ is gt 2std error - is lt -2std error is between

Yule-Walker Estimates for Minimum AIC

------Lag=1------ ------Lag=2------ ------Lag=3------ ------Lag=4------

x y x y x y x y

x 1925887 -000124 -120166 0004224 0116918 -000867 0104236 0003268

y 0050496 1299793 -002046 -03277 -071182 -025701 0195411 0133417

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Canonical Correlations Analysis

Information Chi

x(TT) y(TT) x(T+1T) Criterion Square DF

1 1 0804883 2929228 3047481 8

Information Chix(TT) y(TT) x(T+1T) y(T+1T) Criterion Square DF

1 1 0906681 0607529 1223358 1347237 7

Information Chi

x(TT) y(TT) x (T+1T) y (T+1T) x (T+2T) Criterion Square DF

1 1 0909434 0610278 0186274 -154701 1034705 6

Information Chi

x(TT) y(TT) x (T+1T) y (T+1T) y (T+2T) Criterion Square DF

1 1 091014 0618937 0206823 0940392 1280924 6

Information Chi

x(TT) y(TT) x(T+1T) y(T+1T) y(T+2T) y(T+3T) Criterion Square DF

1 1 0912963 0628785 0226598 0083258 -794103 2041584 5

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Chapter 18 Example

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Selected Statespace Form and Preliminary Estimates

State Vector

x(TT) y(TT) x(T+1T) y(T+1T) y(T+2T)

Estimate of Transition Matrix

0 0 1 0 0

0 0 0 1 0

-084718 0026794 1711715 -005019 0

0 0 0 0 1

-019785 0334274 -018174 -123557 1787475

Input Matrix for Innovation

1 0

0 11925887 -000124

0050496 1299793

0142421 1361696

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Selected Statespace Form and Preliminary Estimates

Variance Matrix for Innovation

0035274 -000734

-000734 0097569

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Part 2 General Information

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Selected Statespace Form and Fitted Model

State Vector

x(TT) y(TT) x(T+1T) y(T+1T) y(T+2T)

Estimate of Transition Matrix

0 0 1 0 0

0 0 0 1 0

-086192 0030609 1724235 -005483 0

0 0 0 0 1

-034839 0292124 -009435 -109823 1671418

Input Matrix for Innovation

1 0

0 1192442 -000416

0015621 1258495

008058 1353204

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Selected Statespace Form and Fitted Model

Variance Matrix for Innovation

0035579 -000728

-000728 0095577

Parameter Estimates

Standard

Parameter Estimate Error t Value

F(31) -086192 0072961 -1181

F(32) 0030609 0026167 117

F(33) 1724235 0061599 2799

F(34) -005483 0030169 -182

F(51) -034839 0135253 -258

F(52) 0292124 0046299 631

F(53) -009435 0096527 -098F(54) -109823 0109525 -1003

F(55) 1671418 0083737 1996

G(31) 1924420 0058162 3309

G(32) -000416 0035255 -012

G(41) 0015621 0095771 016

G(42) 1258495 0055742 2258

G(51) 0080580 0151622 053

G(52) 1353204 0091388 1481

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Chapter 18 Example

The two series are believed to have a transfer function relation with the gas rate

(variable X) as the input and the CO2

concentration (variable Y) as the output Since

the parameter estimates shown in Output 1811 support this kind of model the model

is reestimated with the feedback parameters restricted to 0 The following statements

fit the transfer function (no feedback) model

title3 rsquoTransfer Function Modelrsquo

proc statespace data=seriesj printout=none

var x y

restrict f(32)=0 f(34)=0

g(32)=0 g(41)=0 g(51)=0

run

The last 2 pages of the output are shown in Output 1812

Output 1812 STATESPACE Output for Transfer Function Model

Gas Furnace Data

Box amp Jenkins Series J

Transfer Function Model

The STATESPACE Procedure

Selected Statespace Form and Fitted Model

State Vector

x(TT) y(TT) x(T+1T) y(T+1T) y(T+2T)

Estimate of Transition Matrix

0 0 1 0 0

0 0 0 1 0

-068882 0 1598717 0 0

0 0 0 0 1

-035944 0284179 -00963 -107313 1650047

Input Matrix for Innovation

1 0

0 1

1923446 0

0 1260856

0 1346332

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Part 2 General Information

Gas Furnace Data

Box amp Jenkins Series J

Transfer Function Model

The STATESPACE Procedure

Selected Statespace Form and Fitted Model

Variance Matrix for Innovation

0036995 -00072

-00072 0095712

Parameter Estimates

Standard

Parameter Estimate Error t Value

F(31) -068882 0050549 -1363

F(33) 1598717 0050924 3139

F(51) -035944 0229044 -157

F(52) 0284179 0096944 293

F(53) -009630 0140876 -068

F(54) -107313 0250385 -429

F(55) 1650047 0188533 875G(31) 1923446 0056328 3415

G(42) 1260856 0056464 2233

G(52) 1346332 0091086 1478

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Chapter 18 References

References

Akaike H (1974) Markovian Representation of Stochastic Processes and Its Ap-

plication to the Analysis of Autoregressive Moving Average Processes Annals

of the Institute of Statistical Mathematics 26 363-387

Akaike H (1976) Canonical Correlations Analysis of Time Series and the Use of

an Information Criterion in Advances and Case Studies in System Identification

eds R Mehra and DG Lainiotis New York Academic Press

Anderson TW (1971) The Statistical Analysis of Time Series New York John

Wiley amp Sons

Ansley CF and Newbold P (1979) Multivariate Partial Autocorrelations Pro-

ceedings of the Business and Economic Statistics Section American Statistical

Association 349-353

Box GEP and Jenkins G (1976) Time Series Analysis Forecasting and Control

San Francisco Holden-Day

Brockwell PJ and Davis RA (1991) Time Series Theory and Methods 2nd Edi-

tion Springer-Verlag

Hannan EJ (1970) Multiple Time Series New York John Wiley amp Sons

Hannan EJ (1976) The Identification and Parameterization of ARMAX and State

Space Forms Econometrica 44 713-722

Harvey AC (1981a) The Econometric Analysis of Time Series New York John

Wiley amp Sons

Harvey AC (1981b) Time Series Models New York John Wiley amp Sons

Jones RH (1974) Identification and Autoregressive Spectrum Estimation IEEE Transactions on Automatic Control AC-19 894-897

Pham-Dinh-Tuan (1978) On the Fitting of Multivariate Processes of the

Autoregressive-Moving Average Type Biometrika 65 99-107

Priestley MB (1980) System Identification Kalman Filtering and Stochastic Con-

trol in Directions in Time Series eds DR Brillinger and GC Tiao Institute

of Mathematical Statistics

Whittle P (1963) On the Fitting of Multivariate Autoregressions and the Approxi-

mate Canonical Factorization of a Spectral Density Matrix Biometrika 50 129-

134

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The correct bibliographic citation for this manual is as follows SAS Institute Inc SAS

ET S U serrsquos Gui de Version 8 Cary NC SAS Institute Inc 1999 1546 pp

SASET S Userrsquos Guide Version 8

Copyright copy 1999 by S AS I ns t it u t e I nc Ca ry N C U S A

ISBN 1ndash58025ndash489ndash6

All r ight s res erved P r int ed in t he U n it ed St a t es of A merica N o pa rt of t his p u blica t ionma y b e reprodu ced s t ored in a ret r ieva l s ys t em or t ra ns mit t ed in a ny form or b y a nymeans electronic mechanical photocopying or otherwise without the prior writtenpermission of the publisher SAS Institute Inc

US Government Restricted Rights Notice Use du plicat ion or disclosure of the

software by the government is subject to restrictions as set forth in FAR 52227ndash19Commercial Computer Softw ar e-Restricted Rights (J une 1987)

SAS Institute Inc SAS Campus Drive Cary North Carolina 27513

1st print ing October 1999

S AS reg a nd a ll ot her SA S I ns t it u t e I nc p rodu ct or s ervice na m es a re regis t ered t ra dema rkst d k f SA S I t it t I i t h U S A d t h t i reg i di t U SA

Page 31: Statespace Model

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Chapter 18 Details

of x

1 t + 1 j t

in this linear combination to be 1 Denote the coefficients of z

1

t

in this

linear combination as

This gives the relationship

x

1 t + 1 j t

=

0

x

t

Therefore the current state vector already contains all the past information useful forpredicting

x

1 t + 1

and any greater leads of x

1 t

The variablex

1 t + 1 j t

is not added to

the state vector nor are any termsx

1 t + k j t

considered as possible components of the

state vector The variablex

1

is no longer active for state vector selection

The process described forx

1 t + 1 j t

is repeated for the remaining elements of f

t

The

next candidate for inclusion in the state vector is the next component of f

t

corre-

sponding to an active variable Components of f

t

corresponding to inactive variables

that produced a zero

m i n

in a previous step are skipped

Denote the next candidate asx

l t + k j t

The vectorf

j

t

is formed from the current state

vector andx

l t + k j t

as follows

f

j

t

= z

j

0

t

x

l t + k j t

0

The matrixV

j is formed fromf

j

t

and its canonical correlations are computed The

smallest canonical correlation of V

j is judged to be either greater than or equal to 0

If it is judged to be greater than 0x

l t + k j t

is added to the state vector If it is judged

to be 0 then a linear combination of f

j

t

is uncorrelated with thep

t

and the variable

x

l

is now inactive

The state vector selection process continues until no active variables remain

Testing Significance of Canonical Correlations For each step in the canonical correlation sequence the significance of the small-

est canonical correlation

m i n

is judged by an information criterion from Akaike

(1976) This information criterion is

n l n 1

2

m i n

r p + 1 q + 1

where q is the dimension of f

j

t

at the current step r is the order of the state vector p

is the order of the vector autoregressive process and

is the value of the SIGCORR=

option The default is SIGCORR=2 If this information criterion is less than or equal

to 0

m i n

is taken to be 0 otherwise it is taken to be significantly greater than 0

(Do not confuse this information criterion with the AIC)

Variables inx

t + p j t

are not added in the model even with positive information crite-

rion because of the singularity of V

You can force the consideration of more candi-

date state variables by increasing the size of theV

matrix by specifying a PASTMIN=

option value larger than p

Printing the Canonical Correlations

To print the details of the canonical correlation analysis process specify the CAN-

CORR option in the PROC STATESPACE statement The CANCORR option prints

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the candidate state vectors the canonical correlations and the information criteria for

testing the significance of the smallest canonical correlation

Bartlettrsquos

2 and its degrees of freedom are also printed when the CANCORR option

is specified The formula used for Bartlettrsquos

2 is

2

= n 5 r p + 1 q + 1 l n 1

2

m i n

withr p + 1 q + 1

degrees of freedom

Figure 1811 shows the output of the CANCORR option for the introductory example

shown in the Getting Started section of this chapter

The STATESPACE Procedure

Canonical Correlations Analysis

Information Chi

x(TT) y(TT) x(T+1T) Criterion Square DF

1 1 0237045 3566167 114505 4

Information Chi

x(TT) y(TT) x(T+1T) y(T+1T) Criterion Square DF

1 1 0238244 0056565 -535906 0636134 3

Information Chi

x(TT) y(TT) x(T+1T) x(T+2T) Criterion Square DF

1 1 0237602 0087493 -446312 1525353 3

Figure 1811 Canonical Correlations Analysis

New variables are added to the state vector if the information criteria are positive Inthis example

y

t + 1 j t

andx

t + 2 j t

are not added to the state space vector because the

information criteria for these models are negative

If the information criterion is nearly 0 then you may want to investigate models

that arise if the opposite decision is made regarding

m i n

This investigation can be

accomplished by using a FORM statement to specify part or all of the state vector

Preliminary Estimates of F

When a candidate variablex

l t + k j t

yields a zero

m i n

and is not added to the state

vector a linear combination of f

j

t

is uncorrelated with thep

t

Because of the method

used to construct thef

j

t

sequence the coefficient of x

l t + k j t

inl

can be taken as 1

Denote the coefficients of z

j

t

in this linear combination as l

This gives the relationship

x

l t + k j t

= l

0

z

j

t

The vectorl

is used as a preliminary estimate of the first r columns of the row of the

transition matrixF

corresponding tox

l t + k 1 j t

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Chapter 18 Details

Parameter Estimation

The model isz

t + 1

= F z

t

+ G e

t + 1

wheree

t

is a sequence of independent multivari-

ate normal innovations with mean vector 0 and variance

e e

The observed sequence

x

t

composes the first r components of z

t

and thusx

t

= H z

t

where H is ther s

matrix I

r

0

LetE

be ther n

matrix of innovations

E = e

1

e

n

If the number of observations n is reasonably large the log likelihood L can be

approximated up to an additive constant as follows

L =

n

2

l n j

e e

j

1

2

t r a c e

1

e e

E E

0

The elements of

e e

are taken as free parameters and are estimated as follows

S

0

=

1

n

E E

0

Replacing

e e

byS

0

in the likelihood equation the log likelihood up to an additive

constant is

L =

n

2

l n j S

0

j

Letting B be the backshift operator the formal relation betweenx

t

ande

t

is

x

t

= H I B F

1

G e

t

e

t

= H I B F

1

G

1

x

t

=

1

X

i = 0

i

x

t i

LettingC

i

be the ith lagged sample covariance of x

t

and neglecting end effects the

matrixS

0

is

S

0

=

1

X

i j = 0

i

C

i + j

0

j

For the computation of S

0

the infinite sum is truncated at the value of the KLAG=

option The value of the KLAG= option should be large enough that the sequence

i

is approximately 0 beyond that point

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Part 2 General Information

Let

be the vector of free parameters in theF

andG

matrices The derivative of the

log likelihood with respect to the parameter

is

L

=

n

2

t r a c e

S

1

0

S

0

The second derivative is

2

L

0

=

n

2

t r a c e

S

1

0

S

0

0

S

1

0

S

0

t r a c e

S

1

0

2

S

0

0

Near the maximum the first term is unimportant and the second term can be approx-

imated to give the following second derivative approximation

2

L

0

=

n t r a c e

S

1

0

E

E

0

0

The first derivative matrix and this second derivative matrix approximation are com-

puted from the sample covariance matrixC

0

and the truncated sequence

i

The

approximate likelihood function is maximized by a modified Newton-Raphson algo-

rithm employing these derivative matrices

The matrixS

0

is used as the estimate of the innovation covariance matrix

e e

The

negative of the inverse of the second derivative matrix at the maximum is used as an

approximate covariance matrix for the parameter estimates The standard errors of

the parameter estimates printed in the parameter estimates tables are taken from the

diagonal of this covariance matrix The parameter covariance matrix is printed when

the COVB option is specified

If the data are nearly nonstationary a better estimate of

e e

and the other parameters

can sometimes be obtained by specifying the RESIDEST option The RESIDEST

option estimates the parameters using conditional least squares instead of maximum

likelihood

The residuals are computed using the state space equation and the sample mean val-

ues of the variables in the model as start-up values The estimate of S

0

is then com-

puted using the residuals from the ith observation on where i is the maximum number

of times any variable occurs in the state vector A multivariate Gauss-Marquardt al-

gorithm is used to minimizej S

0

j

Refer to Harvey (1981a) for a further description

of this method

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Chapter 18 Details

Forecasting

Given estimates of F

G

and

e e

forecasts of x

t

are computed from the conditional

expectation of z

t

In forecasting the parameters F G and

e e

are replaced with the estimates or by val-

ues specified in the RESTRICT statement One-step-ahead forecasting is performedfor the observation

x

t

wheret n b

Heren

is the number of observations and

b is the value of the BACK= option For the observationx

t

wheret n b

m-

step-ahead forecasting is performed form = t n + b

The forecasts are generated

recursively with the initial conditionz

0

= 0

The m-step-ahead forecast of z

t + m

isz

t + m j t

wherez

t + m j t

denotes the conditional

expectation of z

t + m

given the information available at time t The m-step-ahead

forecast of x

t + m

isx

t + m j t

= H z

t + m j t

where the matrixH = I

r

0

Let

i

= F

i

G

Note that the lasts r

elements of z

t

consist of the elements of x

u j t

foru t

The state vector z

t + m

can be represented as

z

t + m

= F

m

z

t

+

m 1

X

i = 0

i

e

t + m i

Sincee

t + i j t

= 0

fori 0

the m-step-ahead forecastz

t + m j t

is

z

t + m j t

= F

m

z

t

= F z

t + m 1 j t

Therefore the m-step-ahead forecast of x

t + m

is

x

t + m j t

= H z

t + m j t

The m-step-ahead forecast error is

z

t + m

z

t + m j t

=

m 1

X

i = 0

i

e

t + m i

The variance of the m-step-ahead forecast error is

V

z m

=

m 1

X

i = 0

i

e e

0

i

LettingV

z 0

= 0

the variance of the m-step-ahead forecast error of z

t + m

V

z m

can

be computed recursively as follows

V

z m

= V

z m 1

+

m 1

e e

0

m 1

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Part 2 General Information

The variance of the m-step-ahead forecast error of x

t + m

is ther r

left upper sub-

matrix of V

z m

that is

V

x m

= H V

z m

H

0

Unless the NOCENTER option is specified the sample mean vector is added to the

forecast When differencing is specified the forecasts xt + m j t

plus the sample mean

vector are integrated back to produce forecasts for the original series

Lety

t

be the original series specified by the VAR statement with some 0 values

appended corresponding to the unobserved past observations Let B be the backshift

operator and let B

be thes s

matrix polynomial in the backshift operator

corresponding to the differencing specified by the VAR statement The off-diagonal

elements of

i

are 0 Note that

0

= I

s

whereI

s

is thes s

identity matrix Then

z

t

= B y

t

This gives the relationship

y

t

=

1

B z

t

=

1

X

i = 0

i

z

t i

where

1

B =

P

1

i = 0

i

B

i and

0

= I

s

The m-step-ahead forecast of y

t + m

is

y

t + m j t

=

m 1

X

i = 0

i

z

t + m i j t

+

1

X

i = m

i

z

t + m i

The m-step-ahead forecast error of y

t + m

is

m 1

X

i = 0

i

z

t + m i

z

t + m i j t

=

m 1

X

i = 0

i

X

u = 0

u

i u

e

t + m i

LettingV

y 0

= 0

the variance of the m-step-ahead forecast error of y

t + m

V

y m

is

V

y m

=

m 1

X

i = 0

i

X

u = 0

u

i u

e e

i

X

u = 0

u

i u

0

= V

y m 1

+

m 1

X

u = 0

u

m 1 u

e e

m 1

X

u = 0

u

m 1 u

0

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Chapter 18 Details

Relation of ARMA and State Space Forms

Every state space model has an ARMA representation and conversely every ARMA

model has a state space representation This section discusses this equivalence The

following material is adapted from Akaike (1974) where there is a more complete

discussion Pham-Dinh-Tuan (1978) also contains a discussion of this material

Suppose you are given the following ARMA model

B x

t

= B e

t

or in more detail

x

t

1

x

t 1

p

x

t p

= e

t

+

1

e

t 1

+ +

q

e

t q

(1)

wheree

t

is a sequence of independent multivariate normal random vectors with mean

0 and variance matrix

e e

B is the backshift operator (B x

t

= x

t 1

) B

and

B

are matrix polynomials in B andx

t

is the observed process

If the roots of the determinantial equationj B j = 0

are outside the unit circle in

the complex plane the model can also be written as

x

t

=

1

B B e

t

=

1

X

i = 0

i

e

t i

The

i

matrices are known as the impulse response matrices and can be computed

as

1

B B

You can assume p q

since if this is not initially true you can add more terms

i

that are identically 0 without changing the model

To write this set of equations in a state space form proceed as follows Letx

t + i j t

be

the conditional expectation of x

t + i

givenx

w

forw t

The following relations hold

x

t + i j t

=

1

X

j = i

j

e

t + i j

x

t + i j t + 1

= x

t + i j t

+

i 1

e

t + 1

However from equation (1) you can derive the following relationship

x

t + p j t

=

1

x

t + p 1 j t

+ +

p

x

t

(2)

Hence wheni = p

you can substitute forx

t + p j t

in the right-hand side of equation

(2) and close the system of equations

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Part 2 General Information

This substitution results in the following model in the state space form

z

t + 1

= F z

t

+ G e

t + 1

2

6

6

6

4

x

t + 1

x

t + 2 j t + 1

x

t + p j t + 1

3

7

7

7

5

=

2

6

6

4

0 I 0 0

0 0 I 0

p

p 1

1

3

7

7

5

2

6

6

6

4

x

t

x

t + 1 j t

x

t + p 1 j t

3

7

7

7

5

+

2

6

6

4

I

1

p 1

3

7

7

5

e

t + 1

Note that the state vectorz

t

is composed of conditional expectations of x

t

and the

first r components of z

t

are equal tox

t

The state space form can be cast into an ARMA form by solving the system of dif-

ference equations for the first r components

When converting from an ARMA form to a state space form you can generate a

state vector larger than needed that is the state space model may not be a minimal

representation When going from a state space form to an ARMA form you can have

nontrivial common factors in the autoregressive and moving average operators that

yield an ARMA model larger than necessary

If the state space form used is not a minimal representation some but not all compo-

nents of x

t + i j t

may be linearly dependent This situation corresponds to

p

p 1

being of less than full rank when B

and B

have no common nontrivial

left factors In this casez

t

consists of a subset of the possible components of

x

t + i j t

i = 1 2 p 1

However once a component of x

t + i j t

(for example the

jth one) is linearly dependent on the previous conditional expectations then all sub-

sequent jth components of x

t + k j t

fork i

must also be linearly dependent Note

that in this case equivalent but seemingly different structures can arise if the order of

the components withinx

t

is changed

OUT= Data Set

The forecasts are contained in the output data set specified by the OUT= option on the

PROC STATESPACE statement The OUT= data set contains the following variables

the BY variables

the ID variable

the VAR statement variables These variables contain the actual values from

the input data set

FORi numeric variables containing the forecasts The variable FORi contains

the forecasts for the ith variable in the VAR statement list Forecasts are one-

step-ahead predictions until the end of the data or until the observation specified

by the BACK= option

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Chapter 18 Details

RESi numeric variables containing the residual for the forecast of the ith vari-

able in the VAR statement list For forecast observations the actual values are

missing and the RESi variables contain missing values

STDi numeric variables containing the standard deviation for the forecast of

the ith variable in the VAR statement list The values of the STDi variables can

be used to construct univariate confidence limits for the corresponding fore-casts However such confidence limits do not take into account the covariance

of the forecasts

OUTAR= Data Set

The OUTAR= data set contains the estimates of the preliminary autoregressive mod-

els The OUTAR= data set contains the following variables

ORDER a numeric variable containing the order p of the autoregressive model

that the observation represents

AIC a numeric variable containing the value of the information criterionA I C

p

SIGFl numeric variables containing the estimate of the innovation covariance

matrices for the forward autoregressive models The variable SIGFl contains

the lth column of b

p

in the observations with ORDER= p

SIGBl numeric variables containing the estimate of the innovation covariance

matrices for the backward autoregressive models The variable SIGBl contains

the lth column of b

p

in the observations with ORDER= p

FORk ndashl numeric variables containing the estimates of the autoregressive pa-

rameter matrices for the forward models The variable FORk ndashl contains the

lth column of the lag k autoregressive parameter matrix b

p

k

in the observations

with ORDER= p

BACk ndashl numeric variables containing the estimates of the autoregressive pa-

rameter matrices for the backward models The variable BACk ndashl contains the

lth column of the lag k autoregressive parameter matrix b

p

k

in the observations

with ORDER= p

The estimates for the order p autoregressive model can be selected as those observa-

tions with ORDER= p Within these observations the k lth element of

p

i

is given by

the value of the FORindashl variable in the k th observation The k lth element of

p

i

is

given by the value of BACindashl variable in the k th observation The k lth element of

p

is given by SIGFl in the k th observation The k lth element of

p

is given by SIGBlin the k th observation

Table 181 shows an example of the OUTAR= data set with ARMAX=3 andx

t

of

dimension 2 In Table 181 i j

indicate the ijth element of the matrix

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Part 2 General Information

Table 181 Values in the OUTAR= Data Set

Obs ORDER AIC SIGF1 SIGF2 SIGB1 SIGB2 FOR1ndash 1 FOR1ndash 2 FOR2ndash 1 FOR2ndash 2 FOR3ndash 1

1 0 AIC0

0 1 1

0 1 2

0 1 1

0 1 2

2 0 AIC0

0 2 1

0 2 2

0 2 1

0 2 2

3 1 AIC1

1 1 1

1 1 2

1 1 1

1 1 2

1

1

1 1

1

1

1 2

4 1 AIC1

1 2 1

1 1 2

1 2 1

1 1 2

1

1

2 1

1

1

2 2

5 2 AIC2

2 1 1

2 1 2

2 1 1

2 1 2

2

1

1 1

2

1

1 2

2

2

1 1

2

2

1 2

6 2 AIC2

2 2 1

2 1 2

2 2 1

2 1 2

2

1

2 1

2

1

2 2

2

2

2 1

2

2

2 2

7 3 AIC3

3 1 1

3 1 2

3 1 1

3 1 2

3

1

1 1

3

1

1 2

3

2

1 1

3

2

1 2

3

3

1 1

8 3 AIC3

3 2 1

3 1 2

3 2 1

3 1 2

3

1

2 1

3

1

2 2

3

2

2 1

3

2

2 2

3

3

2 1

Obs FOR3ndash 2 BACK1ndash 1 BACK1ndash 2 BACK2ndash 1 BACK2ndash 2 BACK3ndash 1 BACK3ndash 2

1

2

3

1

1

1 1

1

1

1 2

4

1

1

2 1

1

1

2 2

5

2

1

1 1

2

1

1 2

2

2

1 1

2

2

1 2

6

2

1

2 1

2

1

2 2

2

2

2 1

2

2

2 2

7

3

3

1 2

3

1

1 1

3

1

1 2

3

2

1 1

3

2

1 2

3

3

1 1

3

3

1 2

8

3

3

2 2

3

1

2 1

3

1

2 2

3

2

2 1

3

2

2 2

3

3

2 1

3

3

2 2

The estimated autoregressive parameters can be used in the IML procedure to obtain

autoregressive estimates of the spectral density function or forecasts based on the

autoregressive models

OUTMODEL= Data Set

The OUTMODEL= data set contains the estimates of theF

andG

matrices and their

standard errors the names of the components of the state vector and the estimates of

the innovation covariance matrix The variables contained in the OUTMODEL= data

set are as follows

the BY variables

STATEVEC a character variable containing the name of the component of the

state vector corresponding to the observation The STATEVEC variable has

the value STD for standard deviations observations which contain the standarderrors for the estimates given in the preceding observation

Fndash j numeric variables containing the columns of theF

matrix The variable

Fndash j contains the jth column of F

The number of Fndash j variables is equal to

the value of the DIMMAX= option If the model is of smaller dimension the

extraneous variables are set to missing

Gndash j numeric variables containing the columns of theG

matrix The variable

Gndash j contains the jth column of G

The number of Gndash j variables is equal to r

the dimension of x

t

given by the number of variables in the VAR statement

SIGndash j numeric variables containing the columns of the innovation covariance

matrix The variable SIGndash j contains the jth column of

e e

There are r vari-ables SIGndash j

Table 182 shows an example of the OUTMODEL= data set withx

t

= x

t

y

t

0

z

t

= x

t

y

t

x

t + 1 j t

0 and DIMMAX=4 In Table 182F

i j

andG

i j

are the ijth

elements of F

andG

respectively Note that all elements for Fndash4 are missing because

F

is a3 3

matrix

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Chapter 18 Details

Table 182 Value in the OUTMODEL= Data Set

Obs STATEVEC Fndash 1 Fndash 2 Fndash 3 Fndash 4 Gndash 1 Gndash 2 SIGndash 1 SIGndash 2

1 X(TT) 0 0 1 1 0

1 1

1 2

2 STD

3 Y(TT) F2 1

F2 2

F2 3

0 1

2 1

2 2

4 STD std F2 1

std F2 2

std F2 3

5 X(T+1T) F3 1

F3 2

F3 3

G3 1

G3 2

6 STD std F3 1

std F3 2

std F3 3

std G3 1

std G3 2

Printed Output

The printed output produced by the STATESPACE procedure is described in the fol-

lowing

1 descriptive statistics which include the number of observations used the

names of the variables their means and standard deviations (Std) and the dif-

ferencing operations used

2 the Akaike information criteria for the sequence of preliminary autoregressive

models

3 if the PRINTOUT=LONG option is specified the sample autocovariance ma-

trices of the input series at various lags

4 if the PRINTOUT=LONG option is specified the sample autocorrelation ma-

trices of the input series

5 a schematic representation of the autocorrelation matrices showing the signif-

icant autocorrelations

6 if the PRINTOUT=LONG option is specified the partial autoregressive ma-

trices (These are

p

p

as described in Preliminary Autoregressive Modelsearlier in this chapter)

7 a schematic representation of the partial autocorrelation matrices showing the

significant partial autocorrelations

8 the Yule-Walker estimates of the autoregressive parameters for the autoregres-

sive model with the minimum AIC

9 if the PRINTOUT=LONG option is specified the autocovariance matrices of

the residuals of the minimum AIC model This is the sequence of estimated

innovation variance matrices for the solutions of the Yule-Walker equations

10 if the PRINTOUT=LONG option is specified the autocorrelation matrices of the residuals of the minimum AIC model

11 If the CANCORR option is specified the canonical correlations analysis for

each potential state vector considered in the state vector selection process

This includes the potential state vector the canonical correlations the informa-

tion criterion for the smallest canonical correlation Bartlettrsquos

2 statistic (ldquoChi

Squarerdquo) for the smallest canonical correlation and the degrees of freedom of

Bartlettrsquos

2

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Part 2 General Information

12 the components of the chosen state vector

13 the preliminary estimate of the transition matrixF

the input matrixG

and

the variance matrix for the innovations

e e

14 if the ITPRINT option is specified the iteration history of the likelihood max-

imization For each iteration this shows the iteration number the number of step halvings the determinant of the innovation variance matrix the damping

factor Lambda and the values of the parameters

15 the state vector printed again to aid interpretation of the following listing of F

andG

16 the final estimate of the transition matrixF

17 the final estimate of the input matrixG

18 the final estimate of the variance matrix for the innovations

e e

19 a table listing the estimates of the free parameters inF

andG

and their standarderrors and t statistics

20 if the COVB option is specified the covariance matrix of the parameter esti-

mates

21 if the COVB option is specified the correlation matrix of the parameter esti-

mates

22 if the PRINT option is specified the forecasts and their standard errors

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Chapter 18 Details

ODS Table Names

PROC STATESPACE assigns a name to each table it creates You can use these

names to reference the table when using the Output Delivery System (ODS) to select

tables and create output data sets These names are listed in the following table For

more information on ODS see Chapter 6 ldquoUsing the Output Delivery Systemrdquo

Table 183 ODS Tables Produced in PROC STATESPACE

ODS Table Name Description Option

NObs Number of observations default

Summary Simple summary statistics table default

InfoCriterion Information criterion table default

CovLags Covariance Matrices of Input Series PRINTOUT=LONG

CorrLags Correlation Matrices of Input Series PRINTOUT=LONG

PartialAR Partial Autoregressive Matrices PRINTOUT=LONG

YWEstimates Yule-Walker Estimates for Minimum AIC default

CovResiduals Covariance of Residuals PRINTOUT=LONGCorrResiduals Residual Correlations from AR Models PRINTOUT=LONG

StateVector State vector table default

CorrGraph Schematic Representation of Correlations default

TransitionMatrix Transition Matrix default

InputMatrix Input Matrix default

VarInnov Variance Matrix for the Innovation default

CovB Covariance of Parameter Estimates COVB

CorrB Correlation of Parameter Estimates COVB

CanCorr Canonical Correlation Analysis CANCORR

IterHistory Iterative Fitting table ITPRINT

ParameterEstimates Parameter Estimates Table default

Forecasts Forecasts Table PRINT

ConvergenceStatus Convergence Status Table default

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Part 2 General Information

Example

Example 181 Series J from Box and Jenkins

This example analyzes the gas furnace data (series J) from Box and Jenkins (The

data are not shown Refer to Box and Jenkins (1976) for the data)

First a model is selected and fit automatically using the following statements

title1 rsquoGas Furnace Datarsquo

title2 rsquoBox amp Jenkins Series Jrsquo

title3 rsquoAutomatically Selected Modelrsquo

proc statespace data=seriesj cancorr

var x y

run

The results for the automatically selected model are shown in Output 1811

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Chapter 18 Example

Output 1811 Results for Automatically Selected Model

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Number of Observations 296

Standard

Variable Mean Error

x -005683 1072766

y 5350912 3202121

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Information Criterion for Autoregressive Models

Lag=0 Lag=1 Lag=2 Lag=3 Lag=4 Lag=5 Lag=6 Lag=7 Lag=8

6513862 -103357 -163296 -164512 -165152 -164891 -164934 -164315 -163856

Information

Criterion for

Autoregressive

Models

Lag=9 Lag=10

-16348 -163359

Schematic Representation of Correlations

NameLag 0 1 2 3 4 5 6 7 8 9 10

x +- +- +- +- +- +- +- +- +- +- +-

y -+ -+ -+ -+ -+ -+ -+ -+ -+ -+ -+

+ is gt 2std error - is lt -2std error is between

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Part 2 General Information

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Schematic Representation of Partial Autocorrelations

NameLag 1 2 3 4 5 6 7 8 9 10

x + - + -

y -+ -- - + +

+ is gt 2std error - is lt -2std error is between

Yule-Walker Estimates for Minimum AIC

------Lag=1------ ------Lag=2------ ------Lag=3------ ------Lag=4------

x y x y x y x y

x 1925887 -000124 -120166 0004224 0116918 -000867 0104236 0003268

y 0050496 1299793 -002046 -03277 -071182 -025701 0195411 0133417

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Canonical Correlations Analysis

Information Chi

x(TT) y(TT) x(T+1T) Criterion Square DF

1 1 0804883 2929228 3047481 8

Information Chix(TT) y(TT) x(T+1T) y(T+1T) Criterion Square DF

1 1 0906681 0607529 1223358 1347237 7

Information Chi

x(TT) y(TT) x (T+1T) y (T+1T) x (T+2T) Criterion Square DF

1 1 0909434 0610278 0186274 -154701 1034705 6

Information Chi

x(TT) y(TT) x (T+1T) y (T+1T) y (T+2T) Criterion Square DF

1 1 091014 0618937 0206823 0940392 1280924 6

Information Chi

x(TT) y(TT) x(T+1T) y(T+1T) y(T+2T) y(T+3T) Criterion Square DF

1 1 0912963 0628785 0226598 0083258 -794103 2041584 5

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Chapter 18 Example

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Selected Statespace Form and Preliminary Estimates

State Vector

x(TT) y(TT) x(T+1T) y(T+1T) y(T+2T)

Estimate of Transition Matrix

0 0 1 0 0

0 0 0 1 0

-084718 0026794 1711715 -005019 0

0 0 0 0 1

-019785 0334274 -018174 -123557 1787475

Input Matrix for Innovation

1 0

0 11925887 -000124

0050496 1299793

0142421 1361696

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Selected Statespace Form and Preliminary Estimates

Variance Matrix for Innovation

0035274 -000734

-000734 0097569

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Part 2 General Information

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Selected Statespace Form and Fitted Model

State Vector

x(TT) y(TT) x(T+1T) y(T+1T) y(T+2T)

Estimate of Transition Matrix

0 0 1 0 0

0 0 0 1 0

-086192 0030609 1724235 -005483 0

0 0 0 0 1

-034839 0292124 -009435 -109823 1671418

Input Matrix for Innovation

1 0

0 1192442 -000416

0015621 1258495

008058 1353204

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Selected Statespace Form and Fitted Model

Variance Matrix for Innovation

0035579 -000728

-000728 0095577

Parameter Estimates

Standard

Parameter Estimate Error t Value

F(31) -086192 0072961 -1181

F(32) 0030609 0026167 117

F(33) 1724235 0061599 2799

F(34) -005483 0030169 -182

F(51) -034839 0135253 -258

F(52) 0292124 0046299 631

F(53) -009435 0096527 -098F(54) -109823 0109525 -1003

F(55) 1671418 0083737 1996

G(31) 1924420 0058162 3309

G(32) -000416 0035255 -012

G(41) 0015621 0095771 016

G(42) 1258495 0055742 2258

G(51) 0080580 0151622 053

G(52) 1353204 0091388 1481

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Chapter 18 Example

The two series are believed to have a transfer function relation with the gas rate

(variable X) as the input and the CO2

concentration (variable Y) as the output Since

the parameter estimates shown in Output 1811 support this kind of model the model

is reestimated with the feedback parameters restricted to 0 The following statements

fit the transfer function (no feedback) model

title3 rsquoTransfer Function Modelrsquo

proc statespace data=seriesj printout=none

var x y

restrict f(32)=0 f(34)=0

g(32)=0 g(41)=0 g(51)=0

run

The last 2 pages of the output are shown in Output 1812

Output 1812 STATESPACE Output for Transfer Function Model

Gas Furnace Data

Box amp Jenkins Series J

Transfer Function Model

The STATESPACE Procedure

Selected Statespace Form and Fitted Model

State Vector

x(TT) y(TT) x(T+1T) y(T+1T) y(T+2T)

Estimate of Transition Matrix

0 0 1 0 0

0 0 0 1 0

-068882 0 1598717 0 0

0 0 0 0 1

-035944 0284179 -00963 -107313 1650047

Input Matrix for Innovation

1 0

0 1

1923446 0

0 1260856

0 1346332

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Part 2 General Information

Gas Furnace Data

Box amp Jenkins Series J

Transfer Function Model

The STATESPACE Procedure

Selected Statespace Form and Fitted Model

Variance Matrix for Innovation

0036995 -00072

-00072 0095712

Parameter Estimates

Standard

Parameter Estimate Error t Value

F(31) -068882 0050549 -1363

F(33) 1598717 0050924 3139

F(51) -035944 0229044 -157

F(52) 0284179 0096944 293

F(53) -009630 0140876 -068

F(54) -107313 0250385 -429

F(55) 1650047 0188533 875G(31) 1923446 0056328 3415

G(42) 1260856 0056464 2233

G(52) 1346332 0091086 1478

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Chapter 18 References

References

Akaike H (1974) Markovian Representation of Stochastic Processes and Its Ap-

plication to the Analysis of Autoregressive Moving Average Processes Annals

of the Institute of Statistical Mathematics 26 363-387

Akaike H (1976) Canonical Correlations Analysis of Time Series and the Use of

an Information Criterion in Advances and Case Studies in System Identification

eds R Mehra and DG Lainiotis New York Academic Press

Anderson TW (1971) The Statistical Analysis of Time Series New York John

Wiley amp Sons

Ansley CF and Newbold P (1979) Multivariate Partial Autocorrelations Pro-

ceedings of the Business and Economic Statistics Section American Statistical

Association 349-353

Box GEP and Jenkins G (1976) Time Series Analysis Forecasting and Control

San Francisco Holden-Day

Brockwell PJ and Davis RA (1991) Time Series Theory and Methods 2nd Edi-

tion Springer-Verlag

Hannan EJ (1970) Multiple Time Series New York John Wiley amp Sons

Hannan EJ (1976) The Identification and Parameterization of ARMAX and State

Space Forms Econometrica 44 713-722

Harvey AC (1981a) The Econometric Analysis of Time Series New York John

Wiley amp Sons

Harvey AC (1981b) Time Series Models New York John Wiley amp Sons

Jones RH (1974) Identification and Autoregressive Spectrum Estimation IEEE Transactions on Automatic Control AC-19 894-897

Pham-Dinh-Tuan (1978) On the Fitting of Multivariate Processes of the

Autoregressive-Moving Average Type Biometrika 65 99-107

Priestley MB (1980) System Identification Kalman Filtering and Stochastic Con-

trol in Directions in Time Series eds DR Brillinger and GC Tiao Institute

of Mathematical Statistics

Whittle P (1963) On the Fitting of Multivariate Autoregressions and the Approxi-

mate Canonical Factorization of a Spectral Density Matrix Biometrika 50 129-

134

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The correct bibliographic citation for this manual is as follows SAS Institute Inc SAS

ET S U serrsquos Gui de Version 8 Cary NC SAS Institute Inc 1999 1546 pp

SASET S Userrsquos Guide Version 8

Copyright copy 1999 by S AS I ns t it u t e I nc Ca ry N C U S A

ISBN 1ndash58025ndash489ndash6

All r ight s res erved P r int ed in t he U n it ed St a t es of A merica N o pa rt of t his p u blica t ionma y b e reprodu ced s t ored in a ret r ieva l s ys t em or t ra ns mit t ed in a ny form or b y a nymeans electronic mechanical photocopying or otherwise without the prior writtenpermission of the publisher SAS Institute Inc

US Government Restricted Rights Notice Use du plicat ion or disclosure of the

software by the government is subject to restrictions as set forth in FAR 52227ndash19Commercial Computer Softw ar e-Restricted Rights (J une 1987)

SAS Institute Inc SAS Campus Drive Cary North Carolina 27513

1st print ing October 1999

S AS reg a nd a ll ot her SA S I ns t it u t e I nc p rodu ct or s ervice na m es a re regis t ered t ra dema rkst d k f SA S I t it t I i t h U S A d t h t i reg i di t U SA

Page 32: Statespace Model

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Part 2 General Information

the candidate state vectors the canonical correlations and the information criteria for

testing the significance of the smallest canonical correlation

Bartlettrsquos

2 and its degrees of freedom are also printed when the CANCORR option

is specified The formula used for Bartlettrsquos

2 is

2

= n 5 r p + 1 q + 1 l n 1

2

m i n

withr p + 1 q + 1

degrees of freedom

Figure 1811 shows the output of the CANCORR option for the introductory example

shown in the Getting Started section of this chapter

The STATESPACE Procedure

Canonical Correlations Analysis

Information Chi

x(TT) y(TT) x(T+1T) Criterion Square DF

1 1 0237045 3566167 114505 4

Information Chi

x(TT) y(TT) x(T+1T) y(T+1T) Criterion Square DF

1 1 0238244 0056565 -535906 0636134 3

Information Chi

x(TT) y(TT) x(T+1T) x(T+2T) Criterion Square DF

1 1 0237602 0087493 -446312 1525353 3

Figure 1811 Canonical Correlations Analysis

New variables are added to the state vector if the information criteria are positive Inthis example

y

t + 1 j t

andx

t + 2 j t

are not added to the state space vector because the

information criteria for these models are negative

If the information criterion is nearly 0 then you may want to investigate models

that arise if the opposite decision is made regarding

m i n

This investigation can be

accomplished by using a FORM statement to specify part or all of the state vector

Preliminary Estimates of F

When a candidate variablex

l t + k j t

yields a zero

m i n

and is not added to the state

vector a linear combination of f

j

t

is uncorrelated with thep

t

Because of the method

used to construct thef

j

t

sequence the coefficient of x

l t + k j t

inl

can be taken as 1

Denote the coefficients of z

j

t

in this linear combination as l

This gives the relationship

x

l t + k j t

= l

0

z

j

t

The vectorl

is used as a preliminary estimate of the first r columns of the row of the

transition matrixF

corresponding tox

l t + k 1 j t

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Chapter 18 Details

Parameter Estimation

The model isz

t + 1

= F z

t

+ G e

t + 1

wheree

t

is a sequence of independent multivari-

ate normal innovations with mean vector 0 and variance

e e

The observed sequence

x

t

composes the first r components of z

t

and thusx

t

= H z

t

where H is ther s

matrix I

r

0

LetE

be ther n

matrix of innovations

E = e

1

e

n

If the number of observations n is reasonably large the log likelihood L can be

approximated up to an additive constant as follows

L =

n

2

l n j

e e

j

1

2

t r a c e

1

e e

E E

0

The elements of

e e

are taken as free parameters and are estimated as follows

S

0

=

1

n

E E

0

Replacing

e e

byS

0

in the likelihood equation the log likelihood up to an additive

constant is

L =

n

2

l n j S

0

j

Letting B be the backshift operator the formal relation betweenx

t

ande

t

is

x

t

= H I B F

1

G e

t

e

t

= H I B F

1

G

1

x

t

=

1

X

i = 0

i

x

t i

LettingC

i

be the ith lagged sample covariance of x

t

and neglecting end effects the

matrixS

0

is

S

0

=

1

X

i j = 0

i

C

i + j

0

j

For the computation of S

0

the infinite sum is truncated at the value of the KLAG=

option The value of the KLAG= option should be large enough that the sequence

i

is approximately 0 beyond that point

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Part 2 General Information

Let

be the vector of free parameters in theF

andG

matrices The derivative of the

log likelihood with respect to the parameter

is

L

=

n

2

t r a c e

S

1

0

S

0

The second derivative is

2

L

0

=

n

2

t r a c e

S

1

0

S

0

0

S

1

0

S

0

t r a c e

S

1

0

2

S

0

0

Near the maximum the first term is unimportant and the second term can be approx-

imated to give the following second derivative approximation

2

L

0

=

n t r a c e

S

1

0

E

E

0

0

The first derivative matrix and this second derivative matrix approximation are com-

puted from the sample covariance matrixC

0

and the truncated sequence

i

The

approximate likelihood function is maximized by a modified Newton-Raphson algo-

rithm employing these derivative matrices

The matrixS

0

is used as the estimate of the innovation covariance matrix

e e

The

negative of the inverse of the second derivative matrix at the maximum is used as an

approximate covariance matrix for the parameter estimates The standard errors of

the parameter estimates printed in the parameter estimates tables are taken from the

diagonal of this covariance matrix The parameter covariance matrix is printed when

the COVB option is specified

If the data are nearly nonstationary a better estimate of

e e

and the other parameters

can sometimes be obtained by specifying the RESIDEST option The RESIDEST

option estimates the parameters using conditional least squares instead of maximum

likelihood

The residuals are computed using the state space equation and the sample mean val-

ues of the variables in the model as start-up values The estimate of S

0

is then com-

puted using the residuals from the ith observation on where i is the maximum number

of times any variable occurs in the state vector A multivariate Gauss-Marquardt al-

gorithm is used to minimizej S

0

j

Refer to Harvey (1981a) for a further description

of this method

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Chapter 18 Details

Forecasting

Given estimates of F

G

and

e e

forecasts of x

t

are computed from the conditional

expectation of z

t

In forecasting the parameters F G and

e e

are replaced with the estimates or by val-

ues specified in the RESTRICT statement One-step-ahead forecasting is performedfor the observation

x

t

wheret n b

Heren

is the number of observations and

b is the value of the BACK= option For the observationx

t

wheret n b

m-

step-ahead forecasting is performed form = t n + b

The forecasts are generated

recursively with the initial conditionz

0

= 0

The m-step-ahead forecast of z

t + m

isz

t + m j t

wherez

t + m j t

denotes the conditional

expectation of z

t + m

given the information available at time t The m-step-ahead

forecast of x

t + m

isx

t + m j t

= H z

t + m j t

where the matrixH = I

r

0

Let

i

= F

i

G

Note that the lasts r

elements of z

t

consist of the elements of x

u j t

foru t

The state vector z

t + m

can be represented as

z

t + m

= F

m

z

t

+

m 1

X

i = 0

i

e

t + m i

Sincee

t + i j t

= 0

fori 0

the m-step-ahead forecastz

t + m j t

is

z

t + m j t

= F

m

z

t

= F z

t + m 1 j t

Therefore the m-step-ahead forecast of x

t + m

is

x

t + m j t

= H z

t + m j t

The m-step-ahead forecast error is

z

t + m

z

t + m j t

=

m 1

X

i = 0

i

e

t + m i

The variance of the m-step-ahead forecast error is

V

z m

=

m 1

X

i = 0

i

e e

0

i

LettingV

z 0

= 0

the variance of the m-step-ahead forecast error of z

t + m

V

z m

can

be computed recursively as follows

V

z m

= V

z m 1

+

m 1

e e

0

m 1

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Part 2 General Information

The variance of the m-step-ahead forecast error of x

t + m

is ther r

left upper sub-

matrix of V

z m

that is

V

x m

= H V

z m

H

0

Unless the NOCENTER option is specified the sample mean vector is added to the

forecast When differencing is specified the forecasts xt + m j t

plus the sample mean

vector are integrated back to produce forecasts for the original series

Lety

t

be the original series specified by the VAR statement with some 0 values

appended corresponding to the unobserved past observations Let B be the backshift

operator and let B

be thes s

matrix polynomial in the backshift operator

corresponding to the differencing specified by the VAR statement The off-diagonal

elements of

i

are 0 Note that

0

= I

s

whereI

s

is thes s

identity matrix Then

z

t

= B y

t

This gives the relationship

y

t

=

1

B z

t

=

1

X

i = 0

i

z

t i

where

1

B =

P

1

i = 0

i

B

i and

0

= I

s

The m-step-ahead forecast of y

t + m

is

y

t + m j t

=

m 1

X

i = 0

i

z

t + m i j t

+

1

X

i = m

i

z

t + m i

The m-step-ahead forecast error of y

t + m

is

m 1

X

i = 0

i

z

t + m i

z

t + m i j t

=

m 1

X

i = 0

i

X

u = 0

u

i u

e

t + m i

LettingV

y 0

= 0

the variance of the m-step-ahead forecast error of y

t + m

V

y m

is

V

y m

=

m 1

X

i = 0

i

X

u = 0

u

i u

e e

i

X

u = 0

u

i u

0

= V

y m 1

+

m 1

X

u = 0

u

m 1 u

e e

m 1

X

u = 0

u

m 1 u

0

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Chapter 18 Details

Relation of ARMA and State Space Forms

Every state space model has an ARMA representation and conversely every ARMA

model has a state space representation This section discusses this equivalence The

following material is adapted from Akaike (1974) where there is a more complete

discussion Pham-Dinh-Tuan (1978) also contains a discussion of this material

Suppose you are given the following ARMA model

B x

t

= B e

t

or in more detail

x

t

1

x

t 1

p

x

t p

= e

t

+

1

e

t 1

+ +

q

e

t q

(1)

wheree

t

is a sequence of independent multivariate normal random vectors with mean

0 and variance matrix

e e

B is the backshift operator (B x

t

= x

t 1

) B

and

B

are matrix polynomials in B andx

t

is the observed process

If the roots of the determinantial equationj B j = 0

are outside the unit circle in

the complex plane the model can also be written as

x

t

=

1

B B e

t

=

1

X

i = 0

i

e

t i

The

i

matrices are known as the impulse response matrices and can be computed

as

1

B B

You can assume p q

since if this is not initially true you can add more terms

i

that are identically 0 without changing the model

To write this set of equations in a state space form proceed as follows Letx

t + i j t

be

the conditional expectation of x

t + i

givenx

w

forw t

The following relations hold

x

t + i j t

=

1

X

j = i

j

e

t + i j

x

t + i j t + 1

= x

t + i j t

+

i 1

e

t + 1

However from equation (1) you can derive the following relationship

x

t + p j t

=

1

x

t + p 1 j t

+ +

p

x

t

(2)

Hence wheni = p

you can substitute forx

t + p j t

in the right-hand side of equation

(2) and close the system of equations

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Part 2 General Information

This substitution results in the following model in the state space form

z

t + 1

= F z

t

+ G e

t + 1

2

6

6

6

4

x

t + 1

x

t + 2 j t + 1

x

t + p j t + 1

3

7

7

7

5

=

2

6

6

4

0 I 0 0

0 0 I 0

p

p 1

1

3

7

7

5

2

6

6

6

4

x

t

x

t + 1 j t

x

t + p 1 j t

3

7

7

7

5

+

2

6

6

4

I

1

p 1

3

7

7

5

e

t + 1

Note that the state vectorz

t

is composed of conditional expectations of x

t

and the

first r components of z

t

are equal tox

t

The state space form can be cast into an ARMA form by solving the system of dif-

ference equations for the first r components

When converting from an ARMA form to a state space form you can generate a

state vector larger than needed that is the state space model may not be a minimal

representation When going from a state space form to an ARMA form you can have

nontrivial common factors in the autoregressive and moving average operators that

yield an ARMA model larger than necessary

If the state space form used is not a minimal representation some but not all compo-

nents of x

t + i j t

may be linearly dependent This situation corresponds to

p

p 1

being of less than full rank when B

and B

have no common nontrivial

left factors In this casez

t

consists of a subset of the possible components of

x

t + i j t

i = 1 2 p 1

However once a component of x

t + i j t

(for example the

jth one) is linearly dependent on the previous conditional expectations then all sub-

sequent jth components of x

t + k j t

fork i

must also be linearly dependent Note

that in this case equivalent but seemingly different structures can arise if the order of

the components withinx

t

is changed

OUT= Data Set

The forecasts are contained in the output data set specified by the OUT= option on the

PROC STATESPACE statement The OUT= data set contains the following variables

the BY variables

the ID variable

the VAR statement variables These variables contain the actual values from

the input data set

FORi numeric variables containing the forecasts The variable FORi contains

the forecasts for the ith variable in the VAR statement list Forecasts are one-

step-ahead predictions until the end of the data or until the observation specified

by the BACK= option

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Chapter 18 Details

RESi numeric variables containing the residual for the forecast of the ith vari-

able in the VAR statement list For forecast observations the actual values are

missing and the RESi variables contain missing values

STDi numeric variables containing the standard deviation for the forecast of

the ith variable in the VAR statement list The values of the STDi variables can

be used to construct univariate confidence limits for the corresponding fore-casts However such confidence limits do not take into account the covariance

of the forecasts

OUTAR= Data Set

The OUTAR= data set contains the estimates of the preliminary autoregressive mod-

els The OUTAR= data set contains the following variables

ORDER a numeric variable containing the order p of the autoregressive model

that the observation represents

AIC a numeric variable containing the value of the information criterionA I C

p

SIGFl numeric variables containing the estimate of the innovation covariance

matrices for the forward autoregressive models The variable SIGFl contains

the lth column of b

p

in the observations with ORDER= p

SIGBl numeric variables containing the estimate of the innovation covariance

matrices for the backward autoregressive models The variable SIGBl contains

the lth column of b

p

in the observations with ORDER= p

FORk ndashl numeric variables containing the estimates of the autoregressive pa-

rameter matrices for the forward models The variable FORk ndashl contains the

lth column of the lag k autoregressive parameter matrix b

p

k

in the observations

with ORDER= p

BACk ndashl numeric variables containing the estimates of the autoregressive pa-

rameter matrices for the backward models The variable BACk ndashl contains the

lth column of the lag k autoregressive parameter matrix b

p

k

in the observations

with ORDER= p

The estimates for the order p autoregressive model can be selected as those observa-

tions with ORDER= p Within these observations the k lth element of

p

i

is given by

the value of the FORindashl variable in the k th observation The k lth element of

p

i

is

given by the value of BACindashl variable in the k th observation The k lth element of

p

is given by SIGFl in the k th observation The k lth element of

p

is given by SIGBlin the k th observation

Table 181 shows an example of the OUTAR= data set with ARMAX=3 andx

t

of

dimension 2 In Table 181 i j

indicate the ijth element of the matrix

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Part 2 General Information

Table 181 Values in the OUTAR= Data Set

Obs ORDER AIC SIGF1 SIGF2 SIGB1 SIGB2 FOR1ndash 1 FOR1ndash 2 FOR2ndash 1 FOR2ndash 2 FOR3ndash 1

1 0 AIC0

0 1 1

0 1 2

0 1 1

0 1 2

2 0 AIC0

0 2 1

0 2 2

0 2 1

0 2 2

3 1 AIC1

1 1 1

1 1 2

1 1 1

1 1 2

1

1

1 1

1

1

1 2

4 1 AIC1

1 2 1

1 1 2

1 2 1

1 1 2

1

1

2 1

1

1

2 2

5 2 AIC2

2 1 1

2 1 2

2 1 1

2 1 2

2

1

1 1

2

1

1 2

2

2

1 1

2

2

1 2

6 2 AIC2

2 2 1

2 1 2

2 2 1

2 1 2

2

1

2 1

2

1

2 2

2

2

2 1

2

2

2 2

7 3 AIC3

3 1 1

3 1 2

3 1 1

3 1 2

3

1

1 1

3

1

1 2

3

2

1 1

3

2

1 2

3

3

1 1

8 3 AIC3

3 2 1

3 1 2

3 2 1

3 1 2

3

1

2 1

3

1

2 2

3

2

2 1

3

2

2 2

3

3

2 1

Obs FOR3ndash 2 BACK1ndash 1 BACK1ndash 2 BACK2ndash 1 BACK2ndash 2 BACK3ndash 1 BACK3ndash 2

1

2

3

1

1

1 1

1

1

1 2

4

1

1

2 1

1

1

2 2

5

2

1

1 1

2

1

1 2

2

2

1 1

2

2

1 2

6

2

1

2 1

2

1

2 2

2

2

2 1

2

2

2 2

7

3

3

1 2

3

1

1 1

3

1

1 2

3

2

1 1

3

2

1 2

3

3

1 1

3

3

1 2

8

3

3

2 2

3

1

2 1

3

1

2 2

3

2

2 1

3

2

2 2

3

3

2 1

3

3

2 2

The estimated autoregressive parameters can be used in the IML procedure to obtain

autoregressive estimates of the spectral density function or forecasts based on the

autoregressive models

OUTMODEL= Data Set

The OUTMODEL= data set contains the estimates of theF

andG

matrices and their

standard errors the names of the components of the state vector and the estimates of

the innovation covariance matrix The variables contained in the OUTMODEL= data

set are as follows

the BY variables

STATEVEC a character variable containing the name of the component of the

state vector corresponding to the observation The STATEVEC variable has

the value STD for standard deviations observations which contain the standarderrors for the estimates given in the preceding observation

Fndash j numeric variables containing the columns of theF

matrix The variable

Fndash j contains the jth column of F

The number of Fndash j variables is equal to

the value of the DIMMAX= option If the model is of smaller dimension the

extraneous variables are set to missing

Gndash j numeric variables containing the columns of theG

matrix The variable

Gndash j contains the jth column of G

The number of Gndash j variables is equal to r

the dimension of x

t

given by the number of variables in the VAR statement

SIGndash j numeric variables containing the columns of the innovation covariance

matrix The variable SIGndash j contains the jth column of

e e

There are r vari-ables SIGndash j

Table 182 shows an example of the OUTMODEL= data set withx

t

= x

t

y

t

0

z

t

= x

t

y

t

x

t + 1 j t

0 and DIMMAX=4 In Table 182F

i j

andG

i j

are the ijth

elements of F

andG

respectively Note that all elements for Fndash4 are missing because

F

is a3 3

matrix

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Chapter 18 Details

Table 182 Value in the OUTMODEL= Data Set

Obs STATEVEC Fndash 1 Fndash 2 Fndash 3 Fndash 4 Gndash 1 Gndash 2 SIGndash 1 SIGndash 2

1 X(TT) 0 0 1 1 0

1 1

1 2

2 STD

3 Y(TT) F2 1

F2 2

F2 3

0 1

2 1

2 2

4 STD std F2 1

std F2 2

std F2 3

5 X(T+1T) F3 1

F3 2

F3 3

G3 1

G3 2

6 STD std F3 1

std F3 2

std F3 3

std G3 1

std G3 2

Printed Output

The printed output produced by the STATESPACE procedure is described in the fol-

lowing

1 descriptive statistics which include the number of observations used the

names of the variables their means and standard deviations (Std) and the dif-

ferencing operations used

2 the Akaike information criteria for the sequence of preliminary autoregressive

models

3 if the PRINTOUT=LONG option is specified the sample autocovariance ma-

trices of the input series at various lags

4 if the PRINTOUT=LONG option is specified the sample autocorrelation ma-

trices of the input series

5 a schematic representation of the autocorrelation matrices showing the signif-

icant autocorrelations

6 if the PRINTOUT=LONG option is specified the partial autoregressive ma-

trices (These are

p

p

as described in Preliminary Autoregressive Modelsearlier in this chapter)

7 a schematic representation of the partial autocorrelation matrices showing the

significant partial autocorrelations

8 the Yule-Walker estimates of the autoregressive parameters for the autoregres-

sive model with the minimum AIC

9 if the PRINTOUT=LONG option is specified the autocovariance matrices of

the residuals of the minimum AIC model This is the sequence of estimated

innovation variance matrices for the solutions of the Yule-Walker equations

10 if the PRINTOUT=LONG option is specified the autocorrelation matrices of the residuals of the minimum AIC model

11 If the CANCORR option is specified the canonical correlations analysis for

each potential state vector considered in the state vector selection process

This includes the potential state vector the canonical correlations the informa-

tion criterion for the smallest canonical correlation Bartlettrsquos

2 statistic (ldquoChi

Squarerdquo) for the smallest canonical correlation and the degrees of freedom of

Bartlettrsquos

2

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Part 2 General Information

12 the components of the chosen state vector

13 the preliminary estimate of the transition matrixF

the input matrixG

and

the variance matrix for the innovations

e e

14 if the ITPRINT option is specified the iteration history of the likelihood max-

imization For each iteration this shows the iteration number the number of step halvings the determinant of the innovation variance matrix the damping

factor Lambda and the values of the parameters

15 the state vector printed again to aid interpretation of the following listing of F

andG

16 the final estimate of the transition matrixF

17 the final estimate of the input matrixG

18 the final estimate of the variance matrix for the innovations

e e

19 a table listing the estimates of the free parameters inF

andG

and their standarderrors and t statistics

20 if the COVB option is specified the covariance matrix of the parameter esti-

mates

21 if the COVB option is specified the correlation matrix of the parameter esti-

mates

22 if the PRINT option is specified the forecasts and their standard errors

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Chapter 18 Details

ODS Table Names

PROC STATESPACE assigns a name to each table it creates You can use these

names to reference the table when using the Output Delivery System (ODS) to select

tables and create output data sets These names are listed in the following table For

more information on ODS see Chapter 6 ldquoUsing the Output Delivery Systemrdquo

Table 183 ODS Tables Produced in PROC STATESPACE

ODS Table Name Description Option

NObs Number of observations default

Summary Simple summary statistics table default

InfoCriterion Information criterion table default

CovLags Covariance Matrices of Input Series PRINTOUT=LONG

CorrLags Correlation Matrices of Input Series PRINTOUT=LONG

PartialAR Partial Autoregressive Matrices PRINTOUT=LONG

YWEstimates Yule-Walker Estimates for Minimum AIC default

CovResiduals Covariance of Residuals PRINTOUT=LONGCorrResiduals Residual Correlations from AR Models PRINTOUT=LONG

StateVector State vector table default

CorrGraph Schematic Representation of Correlations default

TransitionMatrix Transition Matrix default

InputMatrix Input Matrix default

VarInnov Variance Matrix for the Innovation default

CovB Covariance of Parameter Estimates COVB

CorrB Correlation of Parameter Estimates COVB

CanCorr Canonical Correlation Analysis CANCORR

IterHistory Iterative Fitting table ITPRINT

ParameterEstimates Parameter Estimates Table default

Forecasts Forecasts Table PRINT

ConvergenceStatus Convergence Status Table default

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Part 2 General Information

Example

Example 181 Series J from Box and Jenkins

This example analyzes the gas furnace data (series J) from Box and Jenkins (The

data are not shown Refer to Box and Jenkins (1976) for the data)

First a model is selected and fit automatically using the following statements

title1 rsquoGas Furnace Datarsquo

title2 rsquoBox amp Jenkins Series Jrsquo

title3 rsquoAutomatically Selected Modelrsquo

proc statespace data=seriesj cancorr

var x y

run

The results for the automatically selected model are shown in Output 1811

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Chapter 18 Example

Output 1811 Results for Automatically Selected Model

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Number of Observations 296

Standard

Variable Mean Error

x -005683 1072766

y 5350912 3202121

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Information Criterion for Autoregressive Models

Lag=0 Lag=1 Lag=2 Lag=3 Lag=4 Lag=5 Lag=6 Lag=7 Lag=8

6513862 -103357 -163296 -164512 -165152 -164891 -164934 -164315 -163856

Information

Criterion for

Autoregressive

Models

Lag=9 Lag=10

-16348 -163359

Schematic Representation of Correlations

NameLag 0 1 2 3 4 5 6 7 8 9 10

x +- +- +- +- +- +- +- +- +- +- +-

y -+ -+ -+ -+ -+ -+ -+ -+ -+ -+ -+

+ is gt 2std error - is lt -2std error is between

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Part 2 General Information

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Schematic Representation of Partial Autocorrelations

NameLag 1 2 3 4 5 6 7 8 9 10

x + - + -

y -+ -- - + +

+ is gt 2std error - is lt -2std error is between

Yule-Walker Estimates for Minimum AIC

------Lag=1------ ------Lag=2------ ------Lag=3------ ------Lag=4------

x y x y x y x y

x 1925887 -000124 -120166 0004224 0116918 -000867 0104236 0003268

y 0050496 1299793 -002046 -03277 -071182 -025701 0195411 0133417

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Canonical Correlations Analysis

Information Chi

x(TT) y(TT) x(T+1T) Criterion Square DF

1 1 0804883 2929228 3047481 8

Information Chix(TT) y(TT) x(T+1T) y(T+1T) Criterion Square DF

1 1 0906681 0607529 1223358 1347237 7

Information Chi

x(TT) y(TT) x (T+1T) y (T+1T) x (T+2T) Criterion Square DF

1 1 0909434 0610278 0186274 -154701 1034705 6

Information Chi

x(TT) y(TT) x (T+1T) y (T+1T) y (T+2T) Criterion Square DF

1 1 091014 0618937 0206823 0940392 1280924 6

Information Chi

x(TT) y(TT) x(T+1T) y(T+1T) y(T+2T) y(T+3T) Criterion Square DF

1 1 0912963 0628785 0226598 0083258 -794103 2041584 5

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Chapter 18 Example

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Selected Statespace Form and Preliminary Estimates

State Vector

x(TT) y(TT) x(T+1T) y(T+1T) y(T+2T)

Estimate of Transition Matrix

0 0 1 0 0

0 0 0 1 0

-084718 0026794 1711715 -005019 0

0 0 0 0 1

-019785 0334274 -018174 -123557 1787475

Input Matrix for Innovation

1 0

0 11925887 -000124

0050496 1299793

0142421 1361696

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Selected Statespace Form and Preliminary Estimates

Variance Matrix for Innovation

0035274 -000734

-000734 0097569

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Part 2 General Information

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Selected Statespace Form and Fitted Model

State Vector

x(TT) y(TT) x(T+1T) y(T+1T) y(T+2T)

Estimate of Transition Matrix

0 0 1 0 0

0 0 0 1 0

-086192 0030609 1724235 -005483 0

0 0 0 0 1

-034839 0292124 -009435 -109823 1671418

Input Matrix for Innovation

1 0

0 1192442 -000416

0015621 1258495

008058 1353204

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Selected Statespace Form and Fitted Model

Variance Matrix for Innovation

0035579 -000728

-000728 0095577

Parameter Estimates

Standard

Parameter Estimate Error t Value

F(31) -086192 0072961 -1181

F(32) 0030609 0026167 117

F(33) 1724235 0061599 2799

F(34) -005483 0030169 -182

F(51) -034839 0135253 -258

F(52) 0292124 0046299 631

F(53) -009435 0096527 -098F(54) -109823 0109525 -1003

F(55) 1671418 0083737 1996

G(31) 1924420 0058162 3309

G(32) -000416 0035255 -012

G(41) 0015621 0095771 016

G(42) 1258495 0055742 2258

G(51) 0080580 0151622 053

G(52) 1353204 0091388 1481

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Chapter 18 Example

The two series are believed to have a transfer function relation with the gas rate

(variable X) as the input and the CO2

concentration (variable Y) as the output Since

the parameter estimates shown in Output 1811 support this kind of model the model

is reestimated with the feedback parameters restricted to 0 The following statements

fit the transfer function (no feedback) model

title3 rsquoTransfer Function Modelrsquo

proc statespace data=seriesj printout=none

var x y

restrict f(32)=0 f(34)=0

g(32)=0 g(41)=0 g(51)=0

run

The last 2 pages of the output are shown in Output 1812

Output 1812 STATESPACE Output for Transfer Function Model

Gas Furnace Data

Box amp Jenkins Series J

Transfer Function Model

The STATESPACE Procedure

Selected Statespace Form and Fitted Model

State Vector

x(TT) y(TT) x(T+1T) y(T+1T) y(T+2T)

Estimate of Transition Matrix

0 0 1 0 0

0 0 0 1 0

-068882 0 1598717 0 0

0 0 0 0 1

-035944 0284179 -00963 -107313 1650047

Input Matrix for Innovation

1 0

0 1

1923446 0

0 1260856

0 1346332

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Part 2 General Information

Gas Furnace Data

Box amp Jenkins Series J

Transfer Function Model

The STATESPACE Procedure

Selected Statespace Form and Fitted Model

Variance Matrix for Innovation

0036995 -00072

-00072 0095712

Parameter Estimates

Standard

Parameter Estimate Error t Value

F(31) -068882 0050549 -1363

F(33) 1598717 0050924 3139

F(51) -035944 0229044 -157

F(52) 0284179 0096944 293

F(53) -009630 0140876 -068

F(54) -107313 0250385 -429

F(55) 1650047 0188533 875G(31) 1923446 0056328 3415

G(42) 1260856 0056464 2233

G(52) 1346332 0091086 1478

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Chapter 18 References

References

Akaike H (1974) Markovian Representation of Stochastic Processes and Its Ap-

plication to the Analysis of Autoregressive Moving Average Processes Annals

of the Institute of Statistical Mathematics 26 363-387

Akaike H (1976) Canonical Correlations Analysis of Time Series and the Use of

an Information Criterion in Advances and Case Studies in System Identification

eds R Mehra and DG Lainiotis New York Academic Press

Anderson TW (1971) The Statistical Analysis of Time Series New York John

Wiley amp Sons

Ansley CF and Newbold P (1979) Multivariate Partial Autocorrelations Pro-

ceedings of the Business and Economic Statistics Section American Statistical

Association 349-353

Box GEP and Jenkins G (1976) Time Series Analysis Forecasting and Control

San Francisco Holden-Day

Brockwell PJ and Davis RA (1991) Time Series Theory and Methods 2nd Edi-

tion Springer-Verlag

Hannan EJ (1970) Multiple Time Series New York John Wiley amp Sons

Hannan EJ (1976) The Identification and Parameterization of ARMAX and State

Space Forms Econometrica 44 713-722

Harvey AC (1981a) The Econometric Analysis of Time Series New York John

Wiley amp Sons

Harvey AC (1981b) Time Series Models New York John Wiley amp Sons

Jones RH (1974) Identification and Autoregressive Spectrum Estimation IEEE Transactions on Automatic Control AC-19 894-897

Pham-Dinh-Tuan (1978) On the Fitting of Multivariate Processes of the

Autoregressive-Moving Average Type Biometrika 65 99-107

Priestley MB (1980) System Identification Kalman Filtering and Stochastic Con-

trol in Directions in Time Series eds DR Brillinger and GC Tiao Institute

of Mathematical Statistics

Whittle P (1963) On the Fitting of Multivariate Autoregressions and the Approxi-

mate Canonical Factorization of a Spectral Density Matrix Biometrika 50 129-

134

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The correct bibliographic citation for this manual is as follows SAS Institute Inc SAS

ET S U serrsquos Gui de Version 8 Cary NC SAS Institute Inc 1999 1546 pp

SASET S Userrsquos Guide Version 8

Copyright copy 1999 by S AS I ns t it u t e I nc Ca ry N C U S A

ISBN 1ndash58025ndash489ndash6

All r ight s res erved P r int ed in t he U n it ed St a t es of A merica N o pa rt of t his p u blica t ionma y b e reprodu ced s t ored in a ret r ieva l s ys t em or t ra ns mit t ed in a ny form or b y a nymeans electronic mechanical photocopying or otherwise without the prior writtenpermission of the publisher SAS Institute Inc

US Government Restricted Rights Notice Use du plicat ion or disclosure of the

software by the government is subject to restrictions as set forth in FAR 52227ndash19Commercial Computer Softw ar e-Restricted Rights (J une 1987)

SAS Institute Inc SAS Campus Drive Cary North Carolina 27513

1st print ing October 1999

S AS reg a nd a ll ot her SA S I ns t it u t e I nc p rodu ct or s ervice na m es a re regis t ered t ra dema rkst d k f SA S I t it t I i t h U S A d t h t i reg i di t U SA

Page 33: Statespace Model

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Chapter 18 Details

Parameter Estimation

The model isz

t + 1

= F z

t

+ G e

t + 1

wheree

t

is a sequence of independent multivari-

ate normal innovations with mean vector 0 and variance

e e

The observed sequence

x

t

composes the first r components of z

t

and thusx

t

= H z

t

where H is ther s

matrix I

r

0

LetE

be ther n

matrix of innovations

E = e

1

e

n

If the number of observations n is reasonably large the log likelihood L can be

approximated up to an additive constant as follows

L =

n

2

l n j

e e

j

1

2

t r a c e

1

e e

E E

0

The elements of

e e

are taken as free parameters and are estimated as follows

S

0

=

1

n

E E

0

Replacing

e e

byS

0

in the likelihood equation the log likelihood up to an additive

constant is

L =

n

2

l n j S

0

j

Letting B be the backshift operator the formal relation betweenx

t

ande

t

is

x

t

= H I B F

1

G e

t

e

t

= H I B F

1

G

1

x

t

=

1

X

i = 0

i

x

t i

LettingC

i

be the ith lagged sample covariance of x

t

and neglecting end effects the

matrixS

0

is

S

0

=

1

X

i j = 0

i

C

i + j

0

j

For the computation of S

0

the infinite sum is truncated at the value of the KLAG=

option The value of the KLAG= option should be large enough that the sequence

i

is approximately 0 beyond that point

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Part 2 General Information

Let

be the vector of free parameters in theF

andG

matrices The derivative of the

log likelihood with respect to the parameter

is

L

=

n

2

t r a c e

S

1

0

S

0

The second derivative is

2

L

0

=

n

2

t r a c e

S

1

0

S

0

0

S

1

0

S

0

t r a c e

S

1

0

2

S

0

0

Near the maximum the first term is unimportant and the second term can be approx-

imated to give the following second derivative approximation

2

L

0

=

n t r a c e

S

1

0

E

E

0

0

The first derivative matrix and this second derivative matrix approximation are com-

puted from the sample covariance matrixC

0

and the truncated sequence

i

The

approximate likelihood function is maximized by a modified Newton-Raphson algo-

rithm employing these derivative matrices

The matrixS

0

is used as the estimate of the innovation covariance matrix

e e

The

negative of the inverse of the second derivative matrix at the maximum is used as an

approximate covariance matrix for the parameter estimates The standard errors of

the parameter estimates printed in the parameter estimates tables are taken from the

diagonal of this covariance matrix The parameter covariance matrix is printed when

the COVB option is specified

If the data are nearly nonstationary a better estimate of

e e

and the other parameters

can sometimes be obtained by specifying the RESIDEST option The RESIDEST

option estimates the parameters using conditional least squares instead of maximum

likelihood

The residuals are computed using the state space equation and the sample mean val-

ues of the variables in the model as start-up values The estimate of S

0

is then com-

puted using the residuals from the ith observation on where i is the maximum number

of times any variable occurs in the state vector A multivariate Gauss-Marquardt al-

gorithm is used to minimizej S

0

j

Refer to Harvey (1981a) for a further description

of this method

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Chapter 18 Details

Forecasting

Given estimates of F

G

and

e e

forecasts of x

t

are computed from the conditional

expectation of z

t

In forecasting the parameters F G and

e e

are replaced with the estimates or by val-

ues specified in the RESTRICT statement One-step-ahead forecasting is performedfor the observation

x

t

wheret n b

Heren

is the number of observations and

b is the value of the BACK= option For the observationx

t

wheret n b

m-

step-ahead forecasting is performed form = t n + b

The forecasts are generated

recursively with the initial conditionz

0

= 0

The m-step-ahead forecast of z

t + m

isz

t + m j t

wherez

t + m j t

denotes the conditional

expectation of z

t + m

given the information available at time t The m-step-ahead

forecast of x

t + m

isx

t + m j t

= H z

t + m j t

where the matrixH = I

r

0

Let

i

= F

i

G

Note that the lasts r

elements of z

t

consist of the elements of x

u j t

foru t

The state vector z

t + m

can be represented as

z

t + m

= F

m

z

t

+

m 1

X

i = 0

i

e

t + m i

Sincee

t + i j t

= 0

fori 0

the m-step-ahead forecastz

t + m j t

is

z

t + m j t

= F

m

z

t

= F z

t + m 1 j t

Therefore the m-step-ahead forecast of x

t + m

is

x

t + m j t

= H z

t + m j t

The m-step-ahead forecast error is

z

t + m

z

t + m j t

=

m 1

X

i = 0

i

e

t + m i

The variance of the m-step-ahead forecast error is

V

z m

=

m 1

X

i = 0

i

e e

0

i

LettingV

z 0

= 0

the variance of the m-step-ahead forecast error of z

t + m

V

z m

can

be computed recursively as follows

V

z m

= V

z m 1

+

m 1

e e

0

m 1

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Part 2 General Information

The variance of the m-step-ahead forecast error of x

t + m

is ther r

left upper sub-

matrix of V

z m

that is

V

x m

= H V

z m

H

0

Unless the NOCENTER option is specified the sample mean vector is added to the

forecast When differencing is specified the forecasts xt + m j t

plus the sample mean

vector are integrated back to produce forecasts for the original series

Lety

t

be the original series specified by the VAR statement with some 0 values

appended corresponding to the unobserved past observations Let B be the backshift

operator and let B

be thes s

matrix polynomial in the backshift operator

corresponding to the differencing specified by the VAR statement The off-diagonal

elements of

i

are 0 Note that

0

= I

s

whereI

s

is thes s

identity matrix Then

z

t

= B y

t

This gives the relationship

y

t

=

1

B z

t

=

1

X

i = 0

i

z

t i

where

1

B =

P

1

i = 0

i

B

i and

0

= I

s

The m-step-ahead forecast of y

t + m

is

y

t + m j t

=

m 1

X

i = 0

i

z

t + m i j t

+

1

X

i = m

i

z

t + m i

The m-step-ahead forecast error of y

t + m

is

m 1

X

i = 0

i

z

t + m i

z

t + m i j t

=

m 1

X

i = 0

i

X

u = 0

u

i u

e

t + m i

LettingV

y 0

= 0

the variance of the m-step-ahead forecast error of y

t + m

V

y m

is

V

y m

=

m 1

X

i = 0

i

X

u = 0

u

i u

e e

i

X

u = 0

u

i u

0

= V

y m 1

+

m 1

X

u = 0

u

m 1 u

e e

m 1

X

u = 0

u

m 1 u

0

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Chapter 18 Details

Relation of ARMA and State Space Forms

Every state space model has an ARMA representation and conversely every ARMA

model has a state space representation This section discusses this equivalence The

following material is adapted from Akaike (1974) where there is a more complete

discussion Pham-Dinh-Tuan (1978) also contains a discussion of this material

Suppose you are given the following ARMA model

B x

t

= B e

t

or in more detail

x

t

1

x

t 1

p

x

t p

= e

t

+

1

e

t 1

+ +

q

e

t q

(1)

wheree

t

is a sequence of independent multivariate normal random vectors with mean

0 and variance matrix

e e

B is the backshift operator (B x

t

= x

t 1

) B

and

B

are matrix polynomials in B andx

t

is the observed process

If the roots of the determinantial equationj B j = 0

are outside the unit circle in

the complex plane the model can also be written as

x

t

=

1

B B e

t

=

1

X

i = 0

i

e

t i

The

i

matrices are known as the impulse response matrices and can be computed

as

1

B B

You can assume p q

since if this is not initially true you can add more terms

i

that are identically 0 without changing the model

To write this set of equations in a state space form proceed as follows Letx

t + i j t

be

the conditional expectation of x

t + i

givenx

w

forw t

The following relations hold

x

t + i j t

=

1

X

j = i

j

e

t + i j

x

t + i j t + 1

= x

t + i j t

+

i 1

e

t + 1

However from equation (1) you can derive the following relationship

x

t + p j t

=

1

x

t + p 1 j t

+ +

p

x

t

(2)

Hence wheni = p

you can substitute forx

t + p j t

in the right-hand side of equation

(2) and close the system of equations

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Part 2 General Information

This substitution results in the following model in the state space form

z

t + 1

= F z

t

+ G e

t + 1

2

6

6

6

4

x

t + 1

x

t + 2 j t + 1

x

t + p j t + 1

3

7

7

7

5

=

2

6

6

4

0 I 0 0

0 0 I 0

p

p 1

1

3

7

7

5

2

6

6

6

4

x

t

x

t + 1 j t

x

t + p 1 j t

3

7

7

7

5

+

2

6

6

4

I

1

p 1

3

7

7

5

e

t + 1

Note that the state vectorz

t

is composed of conditional expectations of x

t

and the

first r components of z

t

are equal tox

t

The state space form can be cast into an ARMA form by solving the system of dif-

ference equations for the first r components

When converting from an ARMA form to a state space form you can generate a

state vector larger than needed that is the state space model may not be a minimal

representation When going from a state space form to an ARMA form you can have

nontrivial common factors in the autoregressive and moving average operators that

yield an ARMA model larger than necessary

If the state space form used is not a minimal representation some but not all compo-

nents of x

t + i j t

may be linearly dependent This situation corresponds to

p

p 1

being of less than full rank when B

and B

have no common nontrivial

left factors In this casez

t

consists of a subset of the possible components of

x

t + i j t

i = 1 2 p 1

However once a component of x

t + i j t

(for example the

jth one) is linearly dependent on the previous conditional expectations then all sub-

sequent jth components of x

t + k j t

fork i

must also be linearly dependent Note

that in this case equivalent but seemingly different structures can arise if the order of

the components withinx

t

is changed

OUT= Data Set

The forecasts are contained in the output data set specified by the OUT= option on the

PROC STATESPACE statement The OUT= data set contains the following variables

the BY variables

the ID variable

the VAR statement variables These variables contain the actual values from

the input data set

FORi numeric variables containing the forecasts The variable FORi contains

the forecasts for the ith variable in the VAR statement list Forecasts are one-

step-ahead predictions until the end of the data or until the observation specified

by the BACK= option

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Chapter 18 Details

RESi numeric variables containing the residual for the forecast of the ith vari-

able in the VAR statement list For forecast observations the actual values are

missing and the RESi variables contain missing values

STDi numeric variables containing the standard deviation for the forecast of

the ith variable in the VAR statement list The values of the STDi variables can

be used to construct univariate confidence limits for the corresponding fore-casts However such confidence limits do not take into account the covariance

of the forecasts

OUTAR= Data Set

The OUTAR= data set contains the estimates of the preliminary autoregressive mod-

els The OUTAR= data set contains the following variables

ORDER a numeric variable containing the order p of the autoregressive model

that the observation represents

AIC a numeric variable containing the value of the information criterionA I C

p

SIGFl numeric variables containing the estimate of the innovation covariance

matrices for the forward autoregressive models The variable SIGFl contains

the lth column of b

p

in the observations with ORDER= p

SIGBl numeric variables containing the estimate of the innovation covariance

matrices for the backward autoregressive models The variable SIGBl contains

the lth column of b

p

in the observations with ORDER= p

FORk ndashl numeric variables containing the estimates of the autoregressive pa-

rameter matrices for the forward models The variable FORk ndashl contains the

lth column of the lag k autoregressive parameter matrix b

p

k

in the observations

with ORDER= p

BACk ndashl numeric variables containing the estimates of the autoregressive pa-

rameter matrices for the backward models The variable BACk ndashl contains the

lth column of the lag k autoregressive parameter matrix b

p

k

in the observations

with ORDER= p

The estimates for the order p autoregressive model can be selected as those observa-

tions with ORDER= p Within these observations the k lth element of

p

i

is given by

the value of the FORindashl variable in the k th observation The k lth element of

p

i

is

given by the value of BACindashl variable in the k th observation The k lth element of

p

is given by SIGFl in the k th observation The k lth element of

p

is given by SIGBlin the k th observation

Table 181 shows an example of the OUTAR= data set with ARMAX=3 andx

t

of

dimension 2 In Table 181 i j

indicate the ijth element of the matrix

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Part 2 General Information

Table 181 Values in the OUTAR= Data Set

Obs ORDER AIC SIGF1 SIGF2 SIGB1 SIGB2 FOR1ndash 1 FOR1ndash 2 FOR2ndash 1 FOR2ndash 2 FOR3ndash 1

1 0 AIC0

0 1 1

0 1 2

0 1 1

0 1 2

2 0 AIC0

0 2 1

0 2 2

0 2 1

0 2 2

3 1 AIC1

1 1 1

1 1 2

1 1 1

1 1 2

1

1

1 1

1

1

1 2

4 1 AIC1

1 2 1

1 1 2

1 2 1

1 1 2

1

1

2 1

1

1

2 2

5 2 AIC2

2 1 1

2 1 2

2 1 1

2 1 2

2

1

1 1

2

1

1 2

2

2

1 1

2

2

1 2

6 2 AIC2

2 2 1

2 1 2

2 2 1

2 1 2

2

1

2 1

2

1

2 2

2

2

2 1

2

2

2 2

7 3 AIC3

3 1 1

3 1 2

3 1 1

3 1 2

3

1

1 1

3

1

1 2

3

2

1 1

3

2

1 2

3

3

1 1

8 3 AIC3

3 2 1

3 1 2

3 2 1

3 1 2

3

1

2 1

3

1

2 2

3

2

2 1

3

2

2 2

3

3

2 1

Obs FOR3ndash 2 BACK1ndash 1 BACK1ndash 2 BACK2ndash 1 BACK2ndash 2 BACK3ndash 1 BACK3ndash 2

1

2

3

1

1

1 1

1

1

1 2

4

1

1

2 1

1

1

2 2

5

2

1

1 1

2

1

1 2

2

2

1 1

2

2

1 2

6

2

1

2 1

2

1

2 2

2

2

2 1

2

2

2 2

7

3

3

1 2

3

1

1 1

3

1

1 2

3

2

1 1

3

2

1 2

3

3

1 1

3

3

1 2

8

3

3

2 2

3

1

2 1

3

1

2 2

3

2

2 1

3

2

2 2

3

3

2 1

3

3

2 2

The estimated autoregressive parameters can be used in the IML procedure to obtain

autoregressive estimates of the spectral density function or forecasts based on the

autoregressive models

OUTMODEL= Data Set

The OUTMODEL= data set contains the estimates of theF

andG

matrices and their

standard errors the names of the components of the state vector and the estimates of

the innovation covariance matrix The variables contained in the OUTMODEL= data

set are as follows

the BY variables

STATEVEC a character variable containing the name of the component of the

state vector corresponding to the observation The STATEVEC variable has

the value STD for standard deviations observations which contain the standarderrors for the estimates given in the preceding observation

Fndash j numeric variables containing the columns of theF

matrix The variable

Fndash j contains the jth column of F

The number of Fndash j variables is equal to

the value of the DIMMAX= option If the model is of smaller dimension the

extraneous variables are set to missing

Gndash j numeric variables containing the columns of theG

matrix The variable

Gndash j contains the jth column of G

The number of Gndash j variables is equal to r

the dimension of x

t

given by the number of variables in the VAR statement

SIGndash j numeric variables containing the columns of the innovation covariance

matrix The variable SIGndash j contains the jth column of

e e

There are r vari-ables SIGndash j

Table 182 shows an example of the OUTMODEL= data set withx

t

= x

t

y

t

0

z

t

= x

t

y

t

x

t + 1 j t

0 and DIMMAX=4 In Table 182F

i j

andG

i j

are the ijth

elements of F

andG

respectively Note that all elements for Fndash4 are missing because

F

is a3 3

matrix

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Chapter 18 Details

Table 182 Value in the OUTMODEL= Data Set

Obs STATEVEC Fndash 1 Fndash 2 Fndash 3 Fndash 4 Gndash 1 Gndash 2 SIGndash 1 SIGndash 2

1 X(TT) 0 0 1 1 0

1 1

1 2

2 STD

3 Y(TT) F2 1

F2 2

F2 3

0 1

2 1

2 2

4 STD std F2 1

std F2 2

std F2 3

5 X(T+1T) F3 1

F3 2

F3 3

G3 1

G3 2

6 STD std F3 1

std F3 2

std F3 3

std G3 1

std G3 2

Printed Output

The printed output produced by the STATESPACE procedure is described in the fol-

lowing

1 descriptive statistics which include the number of observations used the

names of the variables their means and standard deviations (Std) and the dif-

ferencing operations used

2 the Akaike information criteria for the sequence of preliminary autoregressive

models

3 if the PRINTOUT=LONG option is specified the sample autocovariance ma-

trices of the input series at various lags

4 if the PRINTOUT=LONG option is specified the sample autocorrelation ma-

trices of the input series

5 a schematic representation of the autocorrelation matrices showing the signif-

icant autocorrelations

6 if the PRINTOUT=LONG option is specified the partial autoregressive ma-

trices (These are

p

p

as described in Preliminary Autoregressive Modelsearlier in this chapter)

7 a schematic representation of the partial autocorrelation matrices showing the

significant partial autocorrelations

8 the Yule-Walker estimates of the autoregressive parameters for the autoregres-

sive model with the minimum AIC

9 if the PRINTOUT=LONG option is specified the autocovariance matrices of

the residuals of the minimum AIC model This is the sequence of estimated

innovation variance matrices for the solutions of the Yule-Walker equations

10 if the PRINTOUT=LONG option is specified the autocorrelation matrices of the residuals of the minimum AIC model

11 If the CANCORR option is specified the canonical correlations analysis for

each potential state vector considered in the state vector selection process

This includes the potential state vector the canonical correlations the informa-

tion criterion for the smallest canonical correlation Bartlettrsquos

2 statistic (ldquoChi

Squarerdquo) for the smallest canonical correlation and the degrees of freedom of

Bartlettrsquos

2

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Part 2 General Information

12 the components of the chosen state vector

13 the preliminary estimate of the transition matrixF

the input matrixG

and

the variance matrix for the innovations

e e

14 if the ITPRINT option is specified the iteration history of the likelihood max-

imization For each iteration this shows the iteration number the number of step halvings the determinant of the innovation variance matrix the damping

factor Lambda and the values of the parameters

15 the state vector printed again to aid interpretation of the following listing of F

andG

16 the final estimate of the transition matrixF

17 the final estimate of the input matrixG

18 the final estimate of the variance matrix for the innovations

e e

19 a table listing the estimates of the free parameters inF

andG

and their standarderrors and t statistics

20 if the COVB option is specified the covariance matrix of the parameter esti-

mates

21 if the COVB option is specified the correlation matrix of the parameter esti-

mates

22 if the PRINT option is specified the forecasts and their standard errors

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Chapter 18 Details

ODS Table Names

PROC STATESPACE assigns a name to each table it creates You can use these

names to reference the table when using the Output Delivery System (ODS) to select

tables and create output data sets These names are listed in the following table For

more information on ODS see Chapter 6 ldquoUsing the Output Delivery Systemrdquo

Table 183 ODS Tables Produced in PROC STATESPACE

ODS Table Name Description Option

NObs Number of observations default

Summary Simple summary statistics table default

InfoCriterion Information criterion table default

CovLags Covariance Matrices of Input Series PRINTOUT=LONG

CorrLags Correlation Matrices of Input Series PRINTOUT=LONG

PartialAR Partial Autoregressive Matrices PRINTOUT=LONG

YWEstimates Yule-Walker Estimates for Minimum AIC default

CovResiduals Covariance of Residuals PRINTOUT=LONGCorrResiduals Residual Correlations from AR Models PRINTOUT=LONG

StateVector State vector table default

CorrGraph Schematic Representation of Correlations default

TransitionMatrix Transition Matrix default

InputMatrix Input Matrix default

VarInnov Variance Matrix for the Innovation default

CovB Covariance of Parameter Estimates COVB

CorrB Correlation of Parameter Estimates COVB

CanCorr Canonical Correlation Analysis CANCORR

IterHistory Iterative Fitting table ITPRINT

ParameterEstimates Parameter Estimates Table default

Forecasts Forecasts Table PRINT

ConvergenceStatus Convergence Status Table default

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Part 2 General Information

Example

Example 181 Series J from Box and Jenkins

This example analyzes the gas furnace data (series J) from Box and Jenkins (The

data are not shown Refer to Box and Jenkins (1976) for the data)

First a model is selected and fit automatically using the following statements

title1 rsquoGas Furnace Datarsquo

title2 rsquoBox amp Jenkins Series Jrsquo

title3 rsquoAutomatically Selected Modelrsquo

proc statespace data=seriesj cancorr

var x y

run

The results for the automatically selected model are shown in Output 1811

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Chapter 18 Example

Output 1811 Results for Automatically Selected Model

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Number of Observations 296

Standard

Variable Mean Error

x -005683 1072766

y 5350912 3202121

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Information Criterion for Autoregressive Models

Lag=0 Lag=1 Lag=2 Lag=3 Lag=4 Lag=5 Lag=6 Lag=7 Lag=8

6513862 -103357 -163296 -164512 -165152 -164891 -164934 -164315 -163856

Information

Criterion for

Autoregressive

Models

Lag=9 Lag=10

-16348 -163359

Schematic Representation of Correlations

NameLag 0 1 2 3 4 5 6 7 8 9 10

x +- +- +- +- +- +- +- +- +- +- +-

y -+ -+ -+ -+ -+ -+ -+ -+ -+ -+ -+

+ is gt 2std error - is lt -2std error is between

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Part 2 General Information

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Schematic Representation of Partial Autocorrelations

NameLag 1 2 3 4 5 6 7 8 9 10

x + - + -

y -+ -- - + +

+ is gt 2std error - is lt -2std error is between

Yule-Walker Estimates for Minimum AIC

------Lag=1------ ------Lag=2------ ------Lag=3------ ------Lag=4------

x y x y x y x y

x 1925887 -000124 -120166 0004224 0116918 -000867 0104236 0003268

y 0050496 1299793 -002046 -03277 -071182 -025701 0195411 0133417

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Canonical Correlations Analysis

Information Chi

x(TT) y(TT) x(T+1T) Criterion Square DF

1 1 0804883 2929228 3047481 8

Information Chix(TT) y(TT) x(T+1T) y(T+1T) Criterion Square DF

1 1 0906681 0607529 1223358 1347237 7

Information Chi

x(TT) y(TT) x (T+1T) y (T+1T) x (T+2T) Criterion Square DF

1 1 0909434 0610278 0186274 -154701 1034705 6

Information Chi

x(TT) y(TT) x (T+1T) y (T+1T) y (T+2T) Criterion Square DF

1 1 091014 0618937 0206823 0940392 1280924 6

Information Chi

x(TT) y(TT) x(T+1T) y(T+1T) y(T+2T) y(T+3T) Criterion Square DF

1 1 0912963 0628785 0226598 0083258 -794103 2041584 5

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Chapter 18 Example

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Selected Statespace Form and Preliminary Estimates

State Vector

x(TT) y(TT) x(T+1T) y(T+1T) y(T+2T)

Estimate of Transition Matrix

0 0 1 0 0

0 0 0 1 0

-084718 0026794 1711715 -005019 0

0 0 0 0 1

-019785 0334274 -018174 -123557 1787475

Input Matrix for Innovation

1 0

0 11925887 -000124

0050496 1299793

0142421 1361696

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Selected Statespace Form and Preliminary Estimates

Variance Matrix for Innovation

0035274 -000734

-000734 0097569

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Part 2 General Information

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Selected Statespace Form and Fitted Model

State Vector

x(TT) y(TT) x(T+1T) y(T+1T) y(T+2T)

Estimate of Transition Matrix

0 0 1 0 0

0 0 0 1 0

-086192 0030609 1724235 -005483 0

0 0 0 0 1

-034839 0292124 -009435 -109823 1671418

Input Matrix for Innovation

1 0

0 1192442 -000416

0015621 1258495

008058 1353204

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Selected Statespace Form and Fitted Model

Variance Matrix for Innovation

0035579 -000728

-000728 0095577

Parameter Estimates

Standard

Parameter Estimate Error t Value

F(31) -086192 0072961 -1181

F(32) 0030609 0026167 117

F(33) 1724235 0061599 2799

F(34) -005483 0030169 -182

F(51) -034839 0135253 -258

F(52) 0292124 0046299 631

F(53) -009435 0096527 -098F(54) -109823 0109525 -1003

F(55) 1671418 0083737 1996

G(31) 1924420 0058162 3309

G(32) -000416 0035255 -012

G(41) 0015621 0095771 016

G(42) 1258495 0055742 2258

G(51) 0080580 0151622 053

G(52) 1353204 0091388 1481

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Chapter 18 Example

The two series are believed to have a transfer function relation with the gas rate

(variable X) as the input and the CO2

concentration (variable Y) as the output Since

the parameter estimates shown in Output 1811 support this kind of model the model

is reestimated with the feedback parameters restricted to 0 The following statements

fit the transfer function (no feedback) model

title3 rsquoTransfer Function Modelrsquo

proc statespace data=seriesj printout=none

var x y

restrict f(32)=0 f(34)=0

g(32)=0 g(41)=0 g(51)=0

run

The last 2 pages of the output are shown in Output 1812

Output 1812 STATESPACE Output for Transfer Function Model

Gas Furnace Data

Box amp Jenkins Series J

Transfer Function Model

The STATESPACE Procedure

Selected Statespace Form and Fitted Model

State Vector

x(TT) y(TT) x(T+1T) y(T+1T) y(T+2T)

Estimate of Transition Matrix

0 0 1 0 0

0 0 0 1 0

-068882 0 1598717 0 0

0 0 0 0 1

-035944 0284179 -00963 -107313 1650047

Input Matrix for Innovation

1 0

0 1

1923446 0

0 1260856

0 1346332

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Part 2 General Information

Gas Furnace Data

Box amp Jenkins Series J

Transfer Function Model

The STATESPACE Procedure

Selected Statespace Form and Fitted Model

Variance Matrix for Innovation

0036995 -00072

-00072 0095712

Parameter Estimates

Standard

Parameter Estimate Error t Value

F(31) -068882 0050549 -1363

F(33) 1598717 0050924 3139

F(51) -035944 0229044 -157

F(52) 0284179 0096944 293

F(53) -009630 0140876 -068

F(54) -107313 0250385 -429

F(55) 1650047 0188533 875G(31) 1923446 0056328 3415

G(42) 1260856 0056464 2233

G(52) 1346332 0091086 1478

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Chapter 18 References

References

Akaike H (1974) Markovian Representation of Stochastic Processes and Its Ap-

plication to the Analysis of Autoregressive Moving Average Processes Annals

of the Institute of Statistical Mathematics 26 363-387

Akaike H (1976) Canonical Correlations Analysis of Time Series and the Use of

an Information Criterion in Advances and Case Studies in System Identification

eds R Mehra and DG Lainiotis New York Academic Press

Anderson TW (1971) The Statistical Analysis of Time Series New York John

Wiley amp Sons

Ansley CF and Newbold P (1979) Multivariate Partial Autocorrelations Pro-

ceedings of the Business and Economic Statistics Section American Statistical

Association 349-353

Box GEP and Jenkins G (1976) Time Series Analysis Forecasting and Control

San Francisco Holden-Day

Brockwell PJ and Davis RA (1991) Time Series Theory and Methods 2nd Edi-

tion Springer-Verlag

Hannan EJ (1970) Multiple Time Series New York John Wiley amp Sons

Hannan EJ (1976) The Identification and Parameterization of ARMAX and State

Space Forms Econometrica 44 713-722

Harvey AC (1981a) The Econometric Analysis of Time Series New York John

Wiley amp Sons

Harvey AC (1981b) Time Series Models New York John Wiley amp Sons

Jones RH (1974) Identification and Autoregressive Spectrum Estimation IEEE Transactions on Automatic Control AC-19 894-897

Pham-Dinh-Tuan (1978) On the Fitting of Multivariate Processes of the

Autoregressive-Moving Average Type Biometrika 65 99-107

Priestley MB (1980) System Identification Kalman Filtering and Stochastic Con-

trol in Directions in Time Series eds DR Brillinger and GC Tiao Institute

of Mathematical Statistics

Whittle P (1963) On the Fitting of Multivariate Autoregressions and the Approxi-

mate Canonical Factorization of a Spectral Density Matrix Biometrika 50 129-

134

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The correct bibliographic citation for this manual is as follows SAS Institute Inc SAS

ET S U serrsquos Gui de Version 8 Cary NC SAS Institute Inc 1999 1546 pp

SASET S Userrsquos Guide Version 8

Copyright copy 1999 by S AS I ns t it u t e I nc Ca ry N C U S A

ISBN 1ndash58025ndash489ndash6

All r ight s res erved P r int ed in t he U n it ed St a t es of A merica N o pa rt of t his p u blica t ionma y b e reprodu ced s t ored in a ret r ieva l s ys t em or t ra ns mit t ed in a ny form or b y a nymeans electronic mechanical photocopying or otherwise without the prior writtenpermission of the publisher SAS Institute Inc

US Government Restricted Rights Notice Use du plicat ion or disclosure of the

software by the government is subject to restrictions as set forth in FAR 52227ndash19Commercial Computer Softw ar e-Restricted Rights (J une 1987)

SAS Institute Inc SAS Campus Drive Cary North Carolina 27513

1st print ing October 1999

S AS reg a nd a ll ot her SA S I ns t it u t e I nc p rodu ct or s ervice na m es a re regis t ered t ra dema rkst d k f SA S I t it t I i t h U S A d t h t i reg i di t U SA

Page 34: Statespace Model

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Part 2 General Information

Let

be the vector of free parameters in theF

andG

matrices The derivative of the

log likelihood with respect to the parameter

is

L

=

n

2

t r a c e

S

1

0

S

0

The second derivative is

2

L

0

=

n

2

t r a c e

S

1

0

S

0

0

S

1

0

S

0

t r a c e

S

1

0

2

S

0

0

Near the maximum the first term is unimportant and the second term can be approx-

imated to give the following second derivative approximation

2

L

0

=

n t r a c e

S

1

0

E

E

0

0

The first derivative matrix and this second derivative matrix approximation are com-

puted from the sample covariance matrixC

0

and the truncated sequence

i

The

approximate likelihood function is maximized by a modified Newton-Raphson algo-

rithm employing these derivative matrices

The matrixS

0

is used as the estimate of the innovation covariance matrix

e e

The

negative of the inverse of the second derivative matrix at the maximum is used as an

approximate covariance matrix for the parameter estimates The standard errors of

the parameter estimates printed in the parameter estimates tables are taken from the

diagonal of this covariance matrix The parameter covariance matrix is printed when

the COVB option is specified

If the data are nearly nonstationary a better estimate of

e e

and the other parameters

can sometimes be obtained by specifying the RESIDEST option The RESIDEST

option estimates the parameters using conditional least squares instead of maximum

likelihood

The residuals are computed using the state space equation and the sample mean val-

ues of the variables in the model as start-up values The estimate of S

0

is then com-

puted using the residuals from the ith observation on where i is the maximum number

of times any variable occurs in the state vector A multivariate Gauss-Marquardt al-

gorithm is used to minimizej S

0

j

Refer to Harvey (1981a) for a further description

of this method

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Chapter 18 Details

Forecasting

Given estimates of F

G

and

e e

forecasts of x

t

are computed from the conditional

expectation of z

t

In forecasting the parameters F G and

e e

are replaced with the estimates or by val-

ues specified in the RESTRICT statement One-step-ahead forecasting is performedfor the observation

x

t

wheret n b

Heren

is the number of observations and

b is the value of the BACK= option For the observationx

t

wheret n b

m-

step-ahead forecasting is performed form = t n + b

The forecasts are generated

recursively with the initial conditionz

0

= 0

The m-step-ahead forecast of z

t + m

isz

t + m j t

wherez

t + m j t

denotes the conditional

expectation of z

t + m

given the information available at time t The m-step-ahead

forecast of x

t + m

isx

t + m j t

= H z

t + m j t

where the matrixH = I

r

0

Let

i

= F

i

G

Note that the lasts r

elements of z

t

consist of the elements of x

u j t

foru t

The state vector z

t + m

can be represented as

z

t + m

= F

m

z

t

+

m 1

X

i = 0

i

e

t + m i

Sincee

t + i j t

= 0

fori 0

the m-step-ahead forecastz

t + m j t

is

z

t + m j t

= F

m

z

t

= F z

t + m 1 j t

Therefore the m-step-ahead forecast of x

t + m

is

x

t + m j t

= H z

t + m j t

The m-step-ahead forecast error is

z

t + m

z

t + m j t

=

m 1

X

i = 0

i

e

t + m i

The variance of the m-step-ahead forecast error is

V

z m

=

m 1

X

i = 0

i

e e

0

i

LettingV

z 0

= 0

the variance of the m-step-ahead forecast error of z

t + m

V

z m

can

be computed recursively as follows

V

z m

= V

z m 1

+

m 1

e e

0

m 1

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Part 2 General Information

The variance of the m-step-ahead forecast error of x

t + m

is ther r

left upper sub-

matrix of V

z m

that is

V

x m

= H V

z m

H

0

Unless the NOCENTER option is specified the sample mean vector is added to the

forecast When differencing is specified the forecasts xt + m j t

plus the sample mean

vector are integrated back to produce forecasts for the original series

Lety

t

be the original series specified by the VAR statement with some 0 values

appended corresponding to the unobserved past observations Let B be the backshift

operator and let B

be thes s

matrix polynomial in the backshift operator

corresponding to the differencing specified by the VAR statement The off-diagonal

elements of

i

are 0 Note that

0

= I

s

whereI

s

is thes s

identity matrix Then

z

t

= B y

t

This gives the relationship

y

t

=

1

B z

t

=

1

X

i = 0

i

z

t i

where

1

B =

P

1

i = 0

i

B

i and

0

= I

s

The m-step-ahead forecast of y

t + m

is

y

t + m j t

=

m 1

X

i = 0

i

z

t + m i j t

+

1

X

i = m

i

z

t + m i

The m-step-ahead forecast error of y

t + m

is

m 1

X

i = 0

i

z

t + m i

z

t + m i j t

=

m 1

X

i = 0

i

X

u = 0

u

i u

e

t + m i

LettingV

y 0

= 0

the variance of the m-step-ahead forecast error of y

t + m

V

y m

is

V

y m

=

m 1

X

i = 0

i

X

u = 0

u

i u

e e

i

X

u = 0

u

i u

0

= V

y m 1

+

m 1

X

u = 0

u

m 1 u

e e

m 1

X

u = 0

u

m 1 u

0

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Chapter 18 Details

Relation of ARMA and State Space Forms

Every state space model has an ARMA representation and conversely every ARMA

model has a state space representation This section discusses this equivalence The

following material is adapted from Akaike (1974) where there is a more complete

discussion Pham-Dinh-Tuan (1978) also contains a discussion of this material

Suppose you are given the following ARMA model

B x

t

= B e

t

or in more detail

x

t

1

x

t 1

p

x

t p

= e

t

+

1

e

t 1

+ +

q

e

t q

(1)

wheree

t

is a sequence of independent multivariate normal random vectors with mean

0 and variance matrix

e e

B is the backshift operator (B x

t

= x

t 1

) B

and

B

are matrix polynomials in B andx

t

is the observed process

If the roots of the determinantial equationj B j = 0

are outside the unit circle in

the complex plane the model can also be written as

x

t

=

1

B B e

t

=

1

X

i = 0

i

e

t i

The

i

matrices are known as the impulse response matrices and can be computed

as

1

B B

You can assume p q

since if this is not initially true you can add more terms

i

that are identically 0 without changing the model

To write this set of equations in a state space form proceed as follows Letx

t + i j t

be

the conditional expectation of x

t + i

givenx

w

forw t

The following relations hold

x

t + i j t

=

1

X

j = i

j

e

t + i j

x

t + i j t + 1

= x

t + i j t

+

i 1

e

t + 1

However from equation (1) you can derive the following relationship

x

t + p j t

=

1

x

t + p 1 j t

+ +

p

x

t

(2)

Hence wheni = p

you can substitute forx

t + p j t

in the right-hand side of equation

(2) and close the system of equations

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Part 2 General Information

This substitution results in the following model in the state space form

z

t + 1

= F z

t

+ G e

t + 1

2

6

6

6

4

x

t + 1

x

t + 2 j t + 1

x

t + p j t + 1

3

7

7

7

5

=

2

6

6

4

0 I 0 0

0 0 I 0

p

p 1

1

3

7

7

5

2

6

6

6

4

x

t

x

t + 1 j t

x

t + p 1 j t

3

7

7

7

5

+

2

6

6

4

I

1

p 1

3

7

7

5

e

t + 1

Note that the state vectorz

t

is composed of conditional expectations of x

t

and the

first r components of z

t

are equal tox

t

The state space form can be cast into an ARMA form by solving the system of dif-

ference equations for the first r components

When converting from an ARMA form to a state space form you can generate a

state vector larger than needed that is the state space model may not be a minimal

representation When going from a state space form to an ARMA form you can have

nontrivial common factors in the autoregressive and moving average operators that

yield an ARMA model larger than necessary

If the state space form used is not a minimal representation some but not all compo-

nents of x

t + i j t

may be linearly dependent This situation corresponds to

p

p 1

being of less than full rank when B

and B

have no common nontrivial

left factors In this casez

t

consists of a subset of the possible components of

x

t + i j t

i = 1 2 p 1

However once a component of x

t + i j t

(for example the

jth one) is linearly dependent on the previous conditional expectations then all sub-

sequent jth components of x

t + k j t

fork i

must also be linearly dependent Note

that in this case equivalent but seemingly different structures can arise if the order of

the components withinx

t

is changed

OUT= Data Set

The forecasts are contained in the output data set specified by the OUT= option on the

PROC STATESPACE statement The OUT= data set contains the following variables

the BY variables

the ID variable

the VAR statement variables These variables contain the actual values from

the input data set

FORi numeric variables containing the forecasts The variable FORi contains

the forecasts for the ith variable in the VAR statement list Forecasts are one-

step-ahead predictions until the end of the data or until the observation specified

by the BACK= option

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Chapter 18 Details

RESi numeric variables containing the residual for the forecast of the ith vari-

able in the VAR statement list For forecast observations the actual values are

missing and the RESi variables contain missing values

STDi numeric variables containing the standard deviation for the forecast of

the ith variable in the VAR statement list The values of the STDi variables can

be used to construct univariate confidence limits for the corresponding fore-casts However such confidence limits do not take into account the covariance

of the forecasts

OUTAR= Data Set

The OUTAR= data set contains the estimates of the preliminary autoregressive mod-

els The OUTAR= data set contains the following variables

ORDER a numeric variable containing the order p of the autoregressive model

that the observation represents

AIC a numeric variable containing the value of the information criterionA I C

p

SIGFl numeric variables containing the estimate of the innovation covariance

matrices for the forward autoregressive models The variable SIGFl contains

the lth column of b

p

in the observations with ORDER= p

SIGBl numeric variables containing the estimate of the innovation covariance

matrices for the backward autoregressive models The variable SIGBl contains

the lth column of b

p

in the observations with ORDER= p

FORk ndashl numeric variables containing the estimates of the autoregressive pa-

rameter matrices for the forward models The variable FORk ndashl contains the

lth column of the lag k autoregressive parameter matrix b

p

k

in the observations

with ORDER= p

BACk ndashl numeric variables containing the estimates of the autoregressive pa-

rameter matrices for the backward models The variable BACk ndashl contains the

lth column of the lag k autoregressive parameter matrix b

p

k

in the observations

with ORDER= p

The estimates for the order p autoregressive model can be selected as those observa-

tions with ORDER= p Within these observations the k lth element of

p

i

is given by

the value of the FORindashl variable in the k th observation The k lth element of

p

i

is

given by the value of BACindashl variable in the k th observation The k lth element of

p

is given by SIGFl in the k th observation The k lth element of

p

is given by SIGBlin the k th observation

Table 181 shows an example of the OUTAR= data set with ARMAX=3 andx

t

of

dimension 2 In Table 181 i j

indicate the ijth element of the matrix

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Part 2 General Information

Table 181 Values in the OUTAR= Data Set

Obs ORDER AIC SIGF1 SIGF2 SIGB1 SIGB2 FOR1ndash 1 FOR1ndash 2 FOR2ndash 1 FOR2ndash 2 FOR3ndash 1

1 0 AIC0

0 1 1

0 1 2

0 1 1

0 1 2

2 0 AIC0

0 2 1

0 2 2

0 2 1

0 2 2

3 1 AIC1

1 1 1

1 1 2

1 1 1

1 1 2

1

1

1 1

1

1

1 2

4 1 AIC1

1 2 1

1 1 2

1 2 1

1 1 2

1

1

2 1

1

1

2 2

5 2 AIC2

2 1 1

2 1 2

2 1 1

2 1 2

2

1

1 1

2

1

1 2

2

2

1 1

2

2

1 2

6 2 AIC2

2 2 1

2 1 2

2 2 1

2 1 2

2

1

2 1

2

1

2 2

2

2

2 1

2

2

2 2

7 3 AIC3

3 1 1

3 1 2

3 1 1

3 1 2

3

1

1 1

3

1

1 2

3

2

1 1

3

2

1 2

3

3

1 1

8 3 AIC3

3 2 1

3 1 2

3 2 1

3 1 2

3

1

2 1

3

1

2 2

3

2

2 1

3

2

2 2

3

3

2 1

Obs FOR3ndash 2 BACK1ndash 1 BACK1ndash 2 BACK2ndash 1 BACK2ndash 2 BACK3ndash 1 BACK3ndash 2

1

2

3

1

1

1 1

1

1

1 2

4

1

1

2 1

1

1

2 2

5

2

1

1 1

2

1

1 2

2

2

1 1

2

2

1 2

6

2

1

2 1

2

1

2 2

2

2

2 1

2

2

2 2

7

3

3

1 2

3

1

1 1

3

1

1 2

3

2

1 1

3

2

1 2

3

3

1 1

3

3

1 2

8

3

3

2 2

3

1

2 1

3

1

2 2

3

2

2 1

3

2

2 2

3

3

2 1

3

3

2 2

The estimated autoregressive parameters can be used in the IML procedure to obtain

autoregressive estimates of the spectral density function or forecasts based on the

autoregressive models

OUTMODEL= Data Set

The OUTMODEL= data set contains the estimates of theF

andG

matrices and their

standard errors the names of the components of the state vector and the estimates of

the innovation covariance matrix The variables contained in the OUTMODEL= data

set are as follows

the BY variables

STATEVEC a character variable containing the name of the component of the

state vector corresponding to the observation The STATEVEC variable has

the value STD for standard deviations observations which contain the standarderrors for the estimates given in the preceding observation

Fndash j numeric variables containing the columns of theF

matrix The variable

Fndash j contains the jth column of F

The number of Fndash j variables is equal to

the value of the DIMMAX= option If the model is of smaller dimension the

extraneous variables are set to missing

Gndash j numeric variables containing the columns of theG

matrix The variable

Gndash j contains the jth column of G

The number of Gndash j variables is equal to r

the dimension of x

t

given by the number of variables in the VAR statement

SIGndash j numeric variables containing the columns of the innovation covariance

matrix The variable SIGndash j contains the jth column of

e e

There are r vari-ables SIGndash j

Table 182 shows an example of the OUTMODEL= data set withx

t

= x

t

y

t

0

z

t

= x

t

y

t

x

t + 1 j t

0 and DIMMAX=4 In Table 182F

i j

andG

i j

are the ijth

elements of F

andG

respectively Note that all elements for Fndash4 are missing because

F

is a3 3

matrix

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Chapter 18 Details

Table 182 Value in the OUTMODEL= Data Set

Obs STATEVEC Fndash 1 Fndash 2 Fndash 3 Fndash 4 Gndash 1 Gndash 2 SIGndash 1 SIGndash 2

1 X(TT) 0 0 1 1 0

1 1

1 2

2 STD

3 Y(TT) F2 1

F2 2

F2 3

0 1

2 1

2 2

4 STD std F2 1

std F2 2

std F2 3

5 X(T+1T) F3 1

F3 2

F3 3

G3 1

G3 2

6 STD std F3 1

std F3 2

std F3 3

std G3 1

std G3 2

Printed Output

The printed output produced by the STATESPACE procedure is described in the fol-

lowing

1 descriptive statistics which include the number of observations used the

names of the variables their means and standard deviations (Std) and the dif-

ferencing operations used

2 the Akaike information criteria for the sequence of preliminary autoregressive

models

3 if the PRINTOUT=LONG option is specified the sample autocovariance ma-

trices of the input series at various lags

4 if the PRINTOUT=LONG option is specified the sample autocorrelation ma-

trices of the input series

5 a schematic representation of the autocorrelation matrices showing the signif-

icant autocorrelations

6 if the PRINTOUT=LONG option is specified the partial autoregressive ma-

trices (These are

p

p

as described in Preliminary Autoregressive Modelsearlier in this chapter)

7 a schematic representation of the partial autocorrelation matrices showing the

significant partial autocorrelations

8 the Yule-Walker estimates of the autoregressive parameters for the autoregres-

sive model with the minimum AIC

9 if the PRINTOUT=LONG option is specified the autocovariance matrices of

the residuals of the minimum AIC model This is the sequence of estimated

innovation variance matrices for the solutions of the Yule-Walker equations

10 if the PRINTOUT=LONG option is specified the autocorrelation matrices of the residuals of the minimum AIC model

11 If the CANCORR option is specified the canonical correlations analysis for

each potential state vector considered in the state vector selection process

This includes the potential state vector the canonical correlations the informa-

tion criterion for the smallest canonical correlation Bartlettrsquos

2 statistic (ldquoChi

Squarerdquo) for the smallest canonical correlation and the degrees of freedom of

Bartlettrsquos

2

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Part 2 General Information

12 the components of the chosen state vector

13 the preliminary estimate of the transition matrixF

the input matrixG

and

the variance matrix for the innovations

e e

14 if the ITPRINT option is specified the iteration history of the likelihood max-

imization For each iteration this shows the iteration number the number of step halvings the determinant of the innovation variance matrix the damping

factor Lambda and the values of the parameters

15 the state vector printed again to aid interpretation of the following listing of F

andG

16 the final estimate of the transition matrixF

17 the final estimate of the input matrixG

18 the final estimate of the variance matrix for the innovations

e e

19 a table listing the estimates of the free parameters inF

andG

and their standarderrors and t statistics

20 if the COVB option is specified the covariance matrix of the parameter esti-

mates

21 if the COVB option is specified the correlation matrix of the parameter esti-

mates

22 if the PRINT option is specified the forecasts and their standard errors

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Chapter 18 Details

ODS Table Names

PROC STATESPACE assigns a name to each table it creates You can use these

names to reference the table when using the Output Delivery System (ODS) to select

tables and create output data sets These names are listed in the following table For

more information on ODS see Chapter 6 ldquoUsing the Output Delivery Systemrdquo

Table 183 ODS Tables Produced in PROC STATESPACE

ODS Table Name Description Option

NObs Number of observations default

Summary Simple summary statistics table default

InfoCriterion Information criterion table default

CovLags Covariance Matrices of Input Series PRINTOUT=LONG

CorrLags Correlation Matrices of Input Series PRINTOUT=LONG

PartialAR Partial Autoregressive Matrices PRINTOUT=LONG

YWEstimates Yule-Walker Estimates for Minimum AIC default

CovResiduals Covariance of Residuals PRINTOUT=LONGCorrResiduals Residual Correlations from AR Models PRINTOUT=LONG

StateVector State vector table default

CorrGraph Schematic Representation of Correlations default

TransitionMatrix Transition Matrix default

InputMatrix Input Matrix default

VarInnov Variance Matrix for the Innovation default

CovB Covariance of Parameter Estimates COVB

CorrB Correlation of Parameter Estimates COVB

CanCorr Canonical Correlation Analysis CANCORR

IterHistory Iterative Fitting table ITPRINT

ParameterEstimates Parameter Estimates Table default

Forecasts Forecasts Table PRINT

ConvergenceStatus Convergence Status Table default

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Part 2 General Information

Example

Example 181 Series J from Box and Jenkins

This example analyzes the gas furnace data (series J) from Box and Jenkins (The

data are not shown Refer to Box and Jenkins (1976) for the data)

First a model is selected and fit automatically using the following statements

title1 rsquoGas Furnace Datarsquo

title2 rsquoBox amp Jenkins Series Jrsquo

title3 rsquoAutomatically Selected Modelrsquo

proc statespace data=seriesj cancorr

var x y

run

The results for the automatically selected model are shown in Output 1811

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Chapter 18 Example

Output 1811 Results for Automatically Selected Model

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Number of Observations 296

Standard

Variable Mean Error

x -005683 1072766

y 5350912 3202121

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Information Criterion for Autoregressive Models

Lag=0 Lag=1 Lag=2 Lag=3 Lag=4 Lag=5 Lag=6 Lag=7 Lag=8

6513862 -103357 -163296 -164512 -165152 -164891 -164934 -164315 -163856

Information

Criterion for

Autoregressive

Models

Lag=9 Lag=10

-16348 -163359

Schematic Representation of Correlations

NameLag 0 1 2 3 4 5 6 7 8 9 10

x +- +- +- +- +- +- +- +- +- +- +-

y -+ -+ -+ -+ -+ -+ -+ -+ -+ -+ -+

+ is gt 2std error - is lt -2std error is between

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Part 2 General Information

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Schematic Representation of Partial Autocorrelations

NameLag 1 2 3 4 5 6 7 8 9 10

x + - + -

y -+ -- - + +

+ is gt 2std error - is lt -2std error is between

Yule-Walker Estimates for Minimum AIC

------Lag=1------ ------Lag=2------ ------Lag=3------ ------Lag=4------

x y x y x y x y

x 1925887 -000124 -120166 0004224 0116918 -000867 0104236 0003268

y 0050496 1299793 -002046 -03277 -071182 -025701 0195411 0133417

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Canonical Correlations Analysis

Information Chi

x(TT) y(TT) x(T+1T) Criterion Square DF

1 1 0804883 2929228 3047481 8

Information Chix(TT) y(TT) x(T+1T) y(T+1T) Criterion Square DF

1 1 0906681 0607529 1223358 1347237 7

Information Chi

x(TT) y(TT) x (T+1T) y (T+1T) x (T+2T) Criterion Square DF

1 1 0909434 0610278 0186274 -154701 1034705 6

Information Chi

x(TT) y(TT) x (T+1T) y (T+1T) y (T+2T) Criterion Square DF

1 1 091014 0618937 0206823 0940392 1280924 6

Information Chi

x(TT) y(TT) x(T+1T) y(T+1T) y(T+2T) y(T+3T) Criterion Square DF

1 1 0912963 0628785 0226598 0083258 -794103 2041584 5

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Chapter 18 Example

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Selected Statespace Form and Preliminary Estimates

State Vector

x(TT) y(TT) x(T+1T) y(T+1T) y(T+2T)

Estimate of Transition Matrix

0 0 1 0 0

0 0 0 1 0

-084718 0026794 1711715 -005019 0

0 0 0 0 1

-019785 0334274 -018174 -123557 1787475

Input Matrix for Innovation

1 0

0 11925887 -000124

0050496 1299793

0142421 1361696

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Selected Statespace Form and Preliminary Estimates

Variance Matrix for Innovation

0035274 -000734

-000734 0097569

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Part 2 General Information

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Selected Statespace Form and Fitted Model

State Vector

x(TT) y(TT) x(T+1T) y(T+1T) y(T+2T)

Estimate of Transition Matrix

0 0 1 0 0

0 0 0 1 0

-086192 0030609 1724235 -005483 0

0 0 0 0 1

-034839 0292124 -009435 -109823 1671418

Input Matrix for Innovation

1 0

0 1192442 -000416

0015621 1258495

008058 1353204

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Selected Statespace Form and Fitted Model

Variance Matrix for Innovation

0035579 -000728

-000728 0095577

Parameter Estimates

Standard

Parameter Estimate Error t Value

F(31) -086192 0072961 -1181

F(32) 0030609 0026167 117

F(33) 1724235 0061599 2799

F(34) -005483 0030169 -182

F(51) -034839 0135253 -258

F(52) 0292124 0046299 631

F(53) -009435 0096527 -098F(54) -109823 0109525 -1003

F(55) 1671418 0083737 1996

G(31) 1924420 0058162 3309

G(32) -000416 0035255 -012

G(41) 0015621 0095771 016

G(42) 1258495 0055742 2258

G(51) 0080580 0151622 053

G(52) 1353204 0091388 1481

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Chapter 18 Example

The two series are believed to have a transfer function relation with the gas rate

(variable X) as the input and the CO2

concentration (variable Y) as the output Since

the parameter estimates shown in Output 1811 support this kind of model the model

is reestimated with the feedback parameters restricted to 0 The following statements

fit the transfer function (no feedback) model

title3 rsquoTransfer Function Modelrsquo

proc statespace data=seriesj printout=none

var x y

restrict f(32)=0 f(34)=0

g(32)=0 g(41)=0 g(51)=0

run

The last 2 pages of the output are shown in Output 1812

Output 1812 STATESPACE Output for Transfer Function Model

Gas Furnace Data

Box amp Jenkins Series J

Transfer Function Model

The STATESPACE Procedure

Selected Statespace Form and Fitted Model

State Vector

x(TT) y(TT) x(T+1T) y(T+1T) y(T+2T)

Estimate of Transition Matrix

0 0 1 0 0

0 0 0 1 0

-068882 0 1598717 0 0

0 0 0 0 1

-035944 0284179 -00963 -107313 1650047

Input Matrix for Innovation

1 0

0 1

1923446 0

0 1260856

0 1346332

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Part 2 General Information

Gas Furnace Data

Box amp Jenkins Series J

Transfer Function Model

The STATESPACE Procedure

Selected Statespace Form and Fitted Model

Variance Matrix for Innovation

0036995 -00072

-00072 0095712

Parameter Estimates

Standard

Parameter Estimate Error t Value

F(31) -068882 0050549 -1363

F(33) 1598717 0050924 3139

F(51) -035944 0229044 -157

F(52) 0284179 0096944 293

F(53) -009630 0140876 -068

F(54) -107313 0250385 -429

F(55) 1650047 0188533 875G(31) 1923446 0056328 3415

G(42) 1260856 0056464 2233

G(52) 1346332 0091086 1478

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Chapter 18 References

References

Akaike H (1974) Markovian Representation of Stochastic Processes and Its Ap-

plication to the Analysis of Autoregressive Moving Average Processes Annals

of the Institute of Statistical Mathematics 26 363-387

Akaike H (1976) Canonical Correlations Analysis of Time Series and the Use of

an Information Criterion in Advances and Case Studies in System Identification

eds R Mehra and DG Lainiotis New York Academic Press

Anderson TW (1971) The Statistical Analysis of Time Series New York John

Wiley amp Sons

Ansley CF and Newbold P (1979) Multivariate Partial Autocorrelations Pro-

ceedings of the Business and Economic Statistics Section American Statistical

Association 349-353

Box GEP and Jenkins G (1976) Time Series Analysis Forecasting and Control

San Francisco Holden-Day

Brockwell PJ and Davis RA (1991) Time Series Theory and Methods 2nd Edi-

tion Springer-Verlag

Hannan EJ (1970) Multiple Time Series New York John Wiley amp Sons

Hannan EJ (1976) The Identification and Parameterization of ARMAX and State

Space Forms Econometrica 44 713-722

Harvey AC (1981a) The Econometric Analysis of Time Series New York John

Wiley amp Sons

Harvey AC (1981b) Time Series Models New York John Wiley amp Sons

Jones RH (1974) Identification and Autoregressive Spectrum Estimation IEEE Transactions on Automatic Control AC-19 894-897

Pham-Dinh-Tuan (1978) On the Fitting of Multivariate Processes of the

Autoregressive-Moving Average Type Biometrika 65 99-107

Priestley MB (1980) System Identification Kalman Filtering and Stochastic Con-

trol in Directions in Time Series eds DR Brillinger and GC Tiao Institute

of Mathematical Statistics

Whittle P (1963) On the Fitting of Multivariate Autoregressions and the Approxi-

mate Canonical Factorization of a Spectral Density Matrix Biometrika 50 129-

134

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The correct bibliographic citation for this manual is as follows SAS Institute Inc SAS

ET S U serrsquos Gui de Version 8 Cary NC SAS Institute Inc 1999 1546 pp

SASET S Userrsquos Guide Version 8

Copyright copy 1999 by S AS I ns t it u t e I nc Ca ry N C U S A

ISBN 1ndash58025ndash489ndash6

All r ight s res erved P r int ed in t he U n it ed St a t es of A merica N o pa rt of t his p u blica t ionma y b e reprodu ced s t ored in a ret r ieva l s ys t em or t ra ns mit t ed in a ny form or b y a nymeans electronic mechanical photocopying or otherwise without the prior writtenpermission of the publisher SAS Institute Inc

US Government Restricted Rights Notice Use du plicat ion or disclosure of the

software by the government is subject to restrictions as set forth in FAR 52227ndash19Commercial Computer Softw ar e-Restricted Rights (J une 1987)

SAS Institute Inc SAS Campus Drive Cary North Carolina 27513

1st print ing October 1999

S AS reg a nd a ll ot her SA S I ns t it u t e I nc p rodu ct or s ervice na m es a re regis t ered t ra dema rkst d k f SA S I t it t I i t h U S A d t h t i reg i di t U SA

Page 35: Statespace Model

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Chapter 18 Details

Forecasting

Given estimates of F

G

and

e e

forecasts of x

t

are computed from the conditional

expectation of z

t

In forecasting the parameters F G and

e e

are replaced with the estimates or by val-

ues specified in the RESTRICT statement One-step-ahead forecasting is performedfor the observation

x

t

wheret n b

Heren

is the number of observations and

b is the value of the BACK= option For the observationx

t

wheret n b

m-

step-ahead forecasting is performed form = t n + b

The forecasts are generated

recursively with the initial conditionz

0

= 0

The m-step-ahead forecast of z

t + m

isz

t + m j t

wherez

t + m j t

denotes the conditional

expectation of z

t + m

given the information available at time t The m-step-ahead

forecast of x

t + m

isx

t + m j t

= H z

t + m j t

where the matrixH = I

r

0

Let

i

= F

i

G

Note that the lasts r

elements of z

t

consist of the elements of x

u j t

foru t

The state vector z

t + m

can be represented as

z

t + m

= F

m

z

t

+

m 1

X

i = 0

i

e

t + m i

Sincee

t + i j t

= 0

fori 0

the m-step-ahead forecastz

t + m j t

is

z

t + m j t

= F

m

z

t

= F z

t + m 1 j t

Therefore the m-step-ahead forecast of x

t + m

is

x

t + m j t

= H z

t + m j t

The m-step-ahead forecast error is

z

t + m

z

t + m j t

=

m 1

X

i = 0

i

e

t + m i

The variance of the m-step-ahead forecast error is

V

z m

=

m 1

X

i = 0

i

e e

0

i

LettingV

z 0

= 0

the variance of the m-step-ahead forecast error of z

t + m

V

z m

can

be computed recursively as follows

V

z m

= V

z m 1

+

m 1

e e

0

m 1

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Part 2 General Information

The variance of the m-step-ahead forecast error of x

t + m

is ther r

left upper sub-

matrix of V

z m

that is

V

x m

= H V

z m

H

0

Unless the NOCENTER option is specified the sample mean vector is added to the

forecast When differencing is specified the forecasts xt + m j t

plus the sample mean

vector are integrated back to produce forecasts for the original series

Lety

t

be the original series specified by the VAR statement with some 0 values

appended corresponding to the unobserved past observations Let B be the backshift

operator and let B

be thes s

matrix polynomial in the backshift operator

corresponding to the differencing specified by the VAR statement The off-diagonal

elements of

i

are 0 Note that

0

= I

s

whereI

s

is thes s

identity matrix Then

z

t

= B y

t

This gives the relationship

y

t

=

1

B z

t

=

1

X

i = 0

i

z

t i

where

1

B =

P

1

i = 0

i

B

i and

0

= I

s

The m-step-ahead forecast of y

t + m

is

y

t + m j t

=

m 1

X

i = 0

i

z

t + m i j t

+

1

X

i = m

i

z

t + m i

The m-step-ahead forecast error of y

t + m

is

m 1

X

i = 0

i

z

t + m i

z

t + m i j t

=

m 1

X

i = 0

i

X

u = 0

u

i u

e

t + m i

LettingV

y 0

= 0

the variance of the m-step-ahead forecast error of y

t + m

V

y m

is

V

y m

=

m 1

X

i = 0

i

X

u = 0

u

i u

e e

i

X

u = 0

u

i u

0

= V

y m 1

+

m 1

X

u = 0

u

m 1 u

e e

m 1

X

u = 0

u

m 1 u

0

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Chapter 18 Details

Relation of ARMA and State Space Forms

Every state space model has an ARMA representation and conversely every ARMA

model has a state space representation This section discusses this equivalence The

following material is adapted from Akaike (1974) where there is a more complete

discussion Pham-Dinh-Tuan (1978) also contains a discussion of this material

Suppose you are given the following ARMA model

B x

t

= B e

t

or in more detail

x

t

1

x

t 1

p

x

t p

= e

t

+

1

e

t 1

+ +

q

e

t q

(1)

wheree

t

is a sequence of independent multivariate normal random vectors with mean

0 and variance matrix

e e

B is the backshift operator (B x

t

= x

t 1

) B

and

B

are matrix polynomials in B andx

t

is the observed process

If the roots of the determinantial equationj B j = 0

are outside the unit circle in

the complex plane the model can also be written as

x

t

=

1

B B e

t

=

1

X

i = 0

i

e

t i

The

i

matrices are known as the impulse response matrices and can be computed

as

1

B B

You can assume p q

since if this is not initially true you can add more terms

i

that are identically 0 without changing the model

To write this set of equations in a state space form proceed as follows Letx

t + i j t

be

the conditional expectation of x

t + i

givenx

w

forw t

The following relations hold

x

t + i j t

=

1

X

j = i

j

e

t + i j

x

t + i j t + 1

= x

t + i j t

+

i 1

e

t + 1

However from equation (1) you can derive the following relationship

x

t + p j t

=

1

x

t + p 1 j t

+ +

p

x

t

(2)

Hence wheni = p

you can substitute forx

t + p j t

in the right-hand side of equation

(2) and close the system of equations

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Part 2 General Information

This substitution results in the following model in the state space form

z

t + 1

= F z

t

+ G e

t + 1

2

6

6

6

4

x

t + 1

x

t + 2 j t + 1

x

t + p j t + 1

3

7

7

7

5

=

2

6

6

4

0 I 0 0

0 0 I 0

p

p 1

1

3

7

7

5

2

6

6

6

4

x

t

x

t + 1 j t

x

t + p 1 j t

3

7

7

7

5

+

2

6

6

4

I

1

p 1

3

7

7

5

e

t + 1

Note that the state vectorz

t

is composed of conditional expectations of x

t

and the

first r components of z

t

are equal tox

t

The state space form can be cast into an ARMA form by solving the system of dif-

ference equations for the first r components

When converting from an ARMA form to a state space form you can generate a

state vector larger than needed that is the state space model may not be a minimal

representation When going from a state space form to an ARMA form you can have

nontrivial common factors in the autoregressive and moving average operators that

yield an ARMA model larger than necessary

If the state space form used is not a minimal representation some but not all compo-

nents of x

t + i j t

may be linearly dependent This situation corresponds to

p

p 1

being of less than full rank when B

and B

have no common nontrivial

left factors In this casez

t

consists of a subset of the possible components of

x

t + i j t

i = 1 2 p 1

However once a component of x

t + i j t

(for example the

jth one) is linearly dependent on the previous conditional expectations then all sub-

sequent jth components of x

t + k j t

fork i

must also be linearly dependent Note

that in this case equivalent but seemingly different structures can arise if the order of

the components withinx

t

is changed

OUT= Data Set

The forecasts are contained in the output data set specified by the OUT= option on the

PROC STATESPACE statement The OUT= data set contains the following variables

the BY variables

the ID variable

the VAR statement variables These variables contain the actual values from

the input data set

FORi numeric variables containing the forecasts The variable FORi contains

the forecasts for the ith variable in the VAR statement list Forecasts are one-

step-ahead predictions until the end of the data or until the observation specified

by the BACK= option

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Chapter 18 Details

RESi numeric variables containing the residual for the forecast of the ith vari-

able in the VAR statement list For forecast observations the actual values are

missing and the RESi variables contain missing values

STDi numeric variables containing the standard deviation for the forecast of

the ith variable in the VAR statement list The values of the STDi variables can

be used to construct univariate confidence limits for the corresponding fore-casts However such confidence limits do not take into account the covariance

of the forecasts

OUTAR= Data Set

The OUTAR= data set contains the estimates of the preliminary autoregressive mod-

els The OUTAR= data set contains the following variables

ORDER a numeric variable containing the order p of the autoregressive model

that the observation represents

AIC a numeric variable containing the value of the information criterionA I C

p

SIGFl numeric variables containing the estimate of the innovation covariance

matrices for the forward autoregressive models The variable SIGFl contains

the lth column of b

p

in the observations with ORDER= p

SIGBl numeric variables containing the estimate of the innovation covariance

matrices for the backward autoregressive models The variable SIGBl contains

the lth column of b

p

in the observations with ORDER= p

FORk ndashl numeric variables containing the estimates of the autoregressive pa-

rameter matrices for the forward models The variable FORk ndashl contains the

lth column of the lag k autoregressive parameter matrix b

p

k

in the observations

with ORDER= p

BACk ndashl numeric variables containing the estimates of the autoregressive pa-

rameter matrices for the backward models The variable BACk ndashl contains the

lth column of the lag k autoregressive parameter matrix b

p

k

in the observations

with ORDER= p

The estimates for the order p autoregressive model can be selected as those observa-

tions with ORDER= p Within these observations the k lth element of

p

i

is given by

the value of the FORindashl variable in the k th observation The k lth element of

p

i

is

given by the value of BACindashl variable in the k th observation The k lth element of

p

is given by SIGFl in the k th observation The k lth element of

p

is given by SIGBlin the k th observation

Table 181 shows an example of the OUTAR= data set with ARMAX=3 andx

t

of

dimension 2 In Table 181 i j

indicate the ijth element of the matrix

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Part 2 General Information

Table 181 Values in the OUTAR= Data Set

Obs ORDER AIC SIGF1 SIGF2 SIGB1 SIGB2 FOR1ndash 1 FOR1ndash 2 FOR2ndash 1 FOR2ndash 2 FOR3ndash 1

1 0 AIC0

0 1 1

0 1 2

0 1 1

0 1 2

2 0 AIC0

0 2 1

0 2 2

0 2 1

0 2 2

3 1 AIC1

1 1 1

1 1 2

1 1 1

1 1 2

1

1

1 1

1

1

1 2

4 1 AIC1

1 2 1

1 1 2

1 2 1

1 1 2

1

1

2 1

1

1

2 2

5 2 AIC2

2 1 1

2 1 2

2 1 1

2 1 2

2

1

1 1

2

1

1 2

2

2

1 1

2

2

1 2

6 2 AIC2

2 2 1

2 1 2

2 2 1

2 1 2

2

1

2 1

2

1

2 2

2

2

2 1

2

2

2 2

7 3 AIC3

3 1 1

3 1 2

3 1 1

3 1 2

3

1

1 1

3

1

1 2

3

2

1 1

3

2

1 2

3

3

1 1

8 3 AIC3

3 2 1

3 1 2

3 2 1

3 1 2

3

1

2 1

3

1

2 2

3

2

2 1

3

2

2 2

3

3

2 1

Obs FOR3ndash 2 BACK1ndash 1 BACK1ndash 2 BACK2ndash 1 BACK2ndash 2 BACK3ndash 1 BACK3ndash 2

1

2

3

1

1

1 1

1

1

1 2

4

1

1

2 1

1

1

2 2

5

2

1

1 1

2

1

1 2

2

2

1 1

2

2

1 2

6

2

1

2 1

2

1

2 2

2

2

2 1

2

2

2 2

7

3

3

1 2

3

1

1 1

3

1

1 2

3

2

1 1

3

2

1 2

3

3

1 1

3

3

1 2

8

3

3

2 2

3

1

2 1

3

1

2 2

3

2

2 1

3

2

2 2

3

3

2 1

3

3

2 2

The estimated autoregressive parameters can be used in the IML procedure to obtain

autoregressive estimates of the spectral density function or forecasts based on the

autoregressive models

OUTMODEL= Data Set

The OUTMODEL= data set contains the estimates of theF

andG

matrices and their

standard errors the names of the components of the state vector and the estimates of

the innovation covariance matrix The variables contained in the OUTMODEL= data

set are as follows

the BY variables

STATEVEC a character variable containing the name of the component of the

state vector corresponding to the observation The STATEVEC variable has

the value STD for standard deviations observations which contain the standarderrors for the estimates given in the preceding observation

Fndash j numeric variables containing the columns of theF

matrix The variable

Fndash j contains the jth column of F

The number of Fndash j variables is equal to

the value of the DIMMAX= option If the model is of smaller dimension the

extraneous variables are set to missing

Gndash j numeric variables containing the columns of theG

matrix The variable

Gndash j contains the jth column of G

The number of Gndash j variables is equal to r

the dimension of x

t

given by the number of variables in the VAR statement

SIGndash j numeric variables containing the columns of the innovation covariance

matrix The variable SIGndash j contains the jth column of

e e

There are r vari-ables SIGndash j

Table 182 shows an example of the OUTMODEL= data set withx

t

= x

t

y

t

0

z

t

= x

t

y

t

x

t + 1 j t

0 and DIMMAX=4 In Table 182F

i j

andG

i j

are the ijth

elements of F

andG

respectively Note that all elements for Fndash4 are missing because

F

is a3 3

matrix

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Chapter 18 Details

Table 182 Value in the OUTMODEL= Data Set

Obs STATEVEC Fndash 1 Fndash 2 Fndash 3 Fndash 4 Gndash 1 Gndash 2 SIGndash 1 SIGndash 2

1 X(TT) 0 0 1 1 0

1 1

1 2

2 STD

3 Y(TT) F2 1

F2 2

F2 3

0 1

2 1

2 2

4 STD std F2 1

std F2 2

std F2 3

5 X(T+1T) F3 1

F3 2

F3 3

G3 1

G3 2

6 STD std F3 1

std F3 2

std F3 3

std G3 1

std G3 2

Printed Output

The printed output produced by the STATESPACE procedure is described in the fol-

lowing

1 descriptive statistics which include the number of observations used the

names of the variables their means and standard deviations (Std) and the dif-

ferencing operations used

2 the Akaike information criteria for the sequence of preliminary autoregressive

models

3 if the PRINTOUT=LONG option is specified the sample autocovariance ma-

trices of the input series at various lags

4 if the PRINTOUT=LONG option is specified the sample autocorrelation ma-

trices of the input series

5 a schematic representation of the autocorrelation matrices showing the signif-

icant autocorrelations

6 if the PRINTOUT=LONG option is specified the partial autoregressive ma-

trices (These are

p

p

as described in Preliminary Autoregressive Modelsearlier in this chapter)

7 a schematic representation of the partial autocorrelation matrices showing the

significant partial autocorrelations

8 the Yule-Walker estimates of the autoregressive parameters for the autoregres-

sive model with the minimum AIC

9 if the PRINTOUT=LONG option is specified the autocovariance matrices of

the residuals of the minimum AIC model This is the sequence of estimated

innovation variance matrices for the solutions of the Yule-Walker equations

10 if the PRINTOUT=LONG option is specified the autocorrelation matrices of the residuals of the minimum AIC model

11 If the CANCORR option is specified the canonical correlations analysis for

each potential state vector considered in the state vector selection process

This includes the potential state vector the canonical correlations the informa-

tion criterion for the smallest canonical correlation Bartlettrsquos

2 statistic (ldquoChi

Squarerdquo) for the smallest canonical correlation and the degrees of freedom of

Bartlettrsquos

2

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Part 2 General Information

12 the components of the chosen state vector

13 the preliminary estimate of the transition matrixF

the input matrixG

and

the variance matrix for the innovations

e e

14 if the ITPRINT option is specified the iteration history of the likelihood max-

imization For each iteration this shows the iteration number the number of step halvings the determinant of the innovation variance matrix the damping

factor Lambda and the values of the parameters

15 the state vector printed again to aid interpretation of the following listing of F

andG

16 the final estimate of the transition matrixF

17 the final estimate of the input matrixG

18 the final estimate of the variance matrix for the innovations

e e

19 a table listing the estimates of the free parameters inF

andG

and their standarderrors and t statistics

20 if the COVB option is specified the covariance matrix of the parameter esti-

mates

21 if the COVB option is specified the correlation matrix of the parameter esti-

mates

22 if the PRINT option is specified the forecasts and their standard errors

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Chapter 18 Details

ODS Table Names

PROC STATESPACE assigns a name to each table it creates You can use these

names to reference the table when using the Output Delivery System (ODS) to select

tables and create output data sets These names are listed in the following table For

more information on ODS see Chapter 6 ldquoUsing the Output Delivery Systemrdquo

Table 183 ODS Tables Produced in PROC STATESPACE

ODS Table Name Description Option

NObs Number of observations default

Summary Simple summary statistics table default

InfoCriterion Information criterion table default

CovLags Covariance Matrices of Input Series PRINTOUT=LONG

CorrLags Correlation Matrices of Input Series PRINTOUT=LONG

PartialAR Partial Autoregressive Matrices PRINTOUT=LONG

YWEstimates Yule-Walker Estimates for Minimum AIC default

CovResiduals Covariance of Residuals PRINTOUT=LONGCorrResiduals Residual Correlations from AR Models PRINTOUT=LONG

StateVector State vector table default

CorrGraph Schematic Representation of Correlations default

TransitionMatrix Transition Matrix default

InputMatrix Input Matrix default

VarInnov Variance Matrix for the Innovation default

CovB Covariance of Parameter Estimates COVB

CorrB Correlation of Parameter Estimates COVB

CanCorr Canonical Correlation Analysis CANCORR

IterHistory Iterative Fitting table ITPRINT

ParameterEstimates Parameter Estimates Table default

Forecasts Forecasts Table PRINT

ConvergenceStatus Convergence Status Table default

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Part 2 General Information

Example

Example 181 Series J from Box and Jenkins

This example analyzes the gas furnace data (series J) from Box and Jenkins (The

data are not shown Refer to Box and Jenkins (1976) for the data)

First a model is selected and fit automatically using the following statements

title1 rsquoGas Furnace Datarsquo

title2 rsquoBox amp Jenkins Series Jrsquo

title3 rsquoAutomatically Selected Modelrsquo

proc statespace data=seriesj cancorr

var x y

run

The results for the automatically selected model are shown in Output 1811

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Chapter 18 Example

Output 1811 Results for Automatically Selected Model

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Number of Observations 296

Standard

Variable Mean Error

x -005683 1072766

y 5350912 3202121

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Information Criterion for Autoregressive Models

Lag=0 Lag=1 Lag=2 Lag=3 Lag=4 Lag=5 Lag=6 Lag=7 Lag=8

6513862 -103357 -163296 -164512 -165152 -164891 -164934 -164315 -163856

Information

Criterion for

Autoregressive

Models

Lag=9 Lag=10

-16348 -163359

Schematic Representation of Correlations

NameLag 0 1 2 3 4 5 6 7 8 9 10

x +- +- +- +- +- +- +- +- +- +- +-

y -+ -+ -+ -+ -+ -+ -+ -+ -+ -+ -+

+ is gt 2std error - is lt -2std error is between

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Part 2 General Information

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Schematic Representation of Partial Autocorrelations

NameLag 1 2 3 4 5 6 7 8 9 10

x + - + -

y -+ -- - + +

+ is gt 2std error - is lt -2std error is between

Yule-Walker Estimates for Minimum AIC

------Lag=1------ ------Lag=2------ ------Lag=3------ ------Lag=4------

x y x y x y x y

x 1925887 -000124 -120166 0004224 0116918 -000867 0104236 0003268

y 0050496 1299793 -002046 -03277 -071182 -025701 0195411 0133417

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Canonical Correlations Analysis

Information Chi

x(TT) y(TT) x(T+1T) Criterion Square DF

1 1 0804883 2929228 3047481 8

Information Chix(TT) y(TT) x(T+1T) y(T+1T) Criterion Square DF

1 1 0906681 0607529 1223358 1347237 7

Information Chi

x(TT) y(TT) x (T+1T) y (T+1T) x (T+2T) Criterion Square DF

1 1 0909434 0610278 0186274 -154701 1034705 6

Information Chi

x(TT) y(TT) x (T+1T) y (T+1T) y (T+2T) Criterion Square DF

1 1 091014 0618937 0206823 0940392 1280924 6

Information Chi

x(TT) y(TT) x(T+1T) y(T+1T) y(T+2T) y(T+3T) Criterion Square DF

1 1 0912963 0628785 0226598 0083258 -794103 2041584 5

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Chapter 18 Example

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Selected Statespace Form and Preliminary Estimates

State Vector

x(TT) y(TT) x(T+1T) y(T+1T) y(T+2T)

Estimate of Transition Matrix

0 0 1 0 0

0 0 0 1 0

-084718 0026794 1711715 -005019 0

0 0 0 0 1

-019785 0334274 -018174 -123557 1787475

Input Matrix for Innovation

1 0

0 11925887 -000124

0050496 1299793

0142421 1361696

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Selected Statespace Form and Preliminary Estimates

Variance Matrix for Innovation

0035274 -000734

-000734 0097569

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Part 2 General Information

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Selected Statespace Form and Fitted Model

State Vector

x(TT) y(TT) x(T+1T) y(T+1T) y(T+2T)

Estimate of Transition Matrix

0 0 1 0 0

0 0 0 1 0

-086192 0030609 1724235 -005483 0

0 0 0 0 1

-034839 0292124 -009435 -109823 1671418

Input Matrix for Innovation

1 0

0 1192442 -000416

0015621 1258495

008058 1353204

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Selected Statespace Form and Fitted Model

Variance Matrix for Innovation

0035579 -000728

-000728 0095577

Parameter Estimates

Standard

Parameter Estimate Error t Value

F(31) -086192 0072961 -1181

F(32) 0030609 0026167 117

F(33) 1724235 0061599 2799

F(34) -005483 0030169 -182

F(51) -034839 0135253 -258

F(52) 0292124 0046299 631

F(53) -009435 0096527 -098F(54) -109823 0109525 -1003

F(55) 1671418 0083737 1996

G(31) 1924420 0058162 3309

G(32) -000416 0035255 -012

G(41) 0015621 0095771 016

G(42) 1258495 0055742 2258

G(51) 0080580 0151622 053

G(52) 1353204 0091388 1481

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Chapter 18 Example

The two series are believed to have a transfer function relation with the gas rate

(variable X) as the input and the CO2

concentration (variable Y) as the output Since

the parameter estimates shown in Output 1811 support this kind of model the model

is reestimated with the feedback parameters restricted to 0 The following statements

fit the transfer function (no feedback) model

title3 rsquoTransfer Function Modelrsquo

proc statespace data=seriesj printout=none

var x y

restrict f(32)=0 f(34)=0

g(32)=0 g(41)=0 g(51)=0

run

The last 2 pages of the output are shown in Output 1812

Output 1812 STATESPACE Output for Transfer Function Model

Gas Furnace Data

Box amp Jenkins Series J

Transfer Function Model

The STATESPACE Procedure

Selected Statespace Form and Fitted Model

State Vector

x(TT) y(TT) x(T+1T) y(T+1T) y(T+2T)

Estimate of Transition Matrix

0 0 1 0 0

0 0 0 1 0

-068882 0 1598717 0 0

0 0 0 0 1

-035944 0284179 -00963 -107313 1650047

Input Matrix for Innovation

1 0

0 1

1923446 0

0 1260856

0 1346332

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Part 2 General Information

Gas Furnace Data

Box amp Jenkins Series J

Transfer Function Model

The STATESPACE Procedure

Selected Statespace Form and Fitted Model

Variance Matrix for Innovation

0036995 -00072

-00072 0095712

Parameter Estimates

Standard

Parameter Estimate Error t Value

F(31) -068882 0050549 -1363

F(33) 1598717 0050924 3139

F(51) -035944 0229044 -157

F(52) 0284179 0096944 293

F(53) -009630 0140876 -068

F(54) -107313 0250385 -429

F(55) 1650047 0188533 875G(31) 1923446 0056328 3415

G(42) 1260856 0056464 2233

G(52) 1346332 0091086 1478

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Chapter 18 References

References

Akaike H (1974) Markovian Representation of Stochastic Processes and Its Ap-

plication to the Analysis of Autoregressive Moving Average Processes Annals

of the Institute of Statistical Mathematics 26 363-387

Akaike H (1976) Canonical Correlations Analysis of Time Series and the Use of

an Information Criterion in Advances and Case Studies in System Identification

eds R Mehra and DG Lainiotis New York Academic Press

Anderson TW (1971) The Statistical Analysis of Time Series New York John

Wiley amp Sons

Ansley CF and Newbold P (1979) Multivariate Partial Autocorrelations Pro-

ceedings of the Business and Economic Statistics Section American Statistical

Association 349-353

Box GEP and Jenkins G (1976) Time Series Analysis Forecasting and Control

San Francisco Holden-Day

Brockwell PJ and Davis RA (1991) Time Series Theory and Methods 2nd Edi-

tion Springer-Verlag

Hannan EJ (1970) Multiple Time Series New York John Wiley amp Sons

Hannan EJ (1976) The Identification and Parameterization of ARMAX and State

Space Forms Econometrica 44 713-722

Harvey AC (1981a) The Econometric Analysis of Time Series New York John

Wiley amp Sons

Harvey AC (1981b) Time Series Models New York John Wiley amp Sons

Jones RH (1974) Identification and Autoregressive Spectrum Estimation IEEE Transactions on Automatic Control AC-19 894-897

Pham-Dinh-Tuan (1978) On the Fitting of Multivariate Processes of the

Autoregressive-Moving Average Type Biometrika 65 99-107

Priestley MB (1980) System Identification Kalman Filtering and Stochastic Con-

trol in Directions in Time Series eds DR Brillinger and GC Tiao Institute

of Mathematical Statistics

Whittle P (1963) On the Fitting of Multivariate Autoregressions and the Approxi-

mate Canonical Factorization of a Spectral Density Matrix Biometrika 50 129-

134

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The correct bibliographic citation for this manual is as follows SAS Institute Inc SAS

ET S U serrsquos Gui de Version 8 Cary NC SAS Institute Inc 1999 1546 pp

SASET S Userrsquos Guide Version 8

Copyright copy 1999 by S AS I ns t it u t e I nc Ca ry N C U S A

ISBN 1ndash58025ndash489ndash6

All r ight s res erved P r int ed in t he U n it ed St a t es of A merica N o pa rt of t his p u blica t ionma y b e reprodu ced s t ored in a ret r ieva l s ys t em or t ra ns mit t ed in a ny form or b y a nymeans electronic mechanical photocopying or otherwise without the prior writtenpermission of the publisher SAS Institute Inc

US Government Restricted Rights Notice Use du plicat ion or disclosure of the

software by the government is subject to restrictions as set forth in FAR 52227ndash19Commercial Computer Softw ar e-Restricted Rights (J une 1987)

SAS Institute Inc SAS Campus Drive Cary North Carolina 27513

1st print ing October 1999

S AS reg a nd a ll ot her SA S I ns t it u t e I nc p rodu ct or s ervice na m es a re regis t ered t ra dema rkst d k f SA S I t it t I i t h U S A d t h t i reg i di t U SA

Page 36: Statespace Model

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Part 2 General Information

The variance of the m-step-ahead forecast error of x

t + m

is ther r

left upper sub-

matrix of V

z m

that is

V

x m

= H V

z m

H

0

Unless the NOCENTER option is specified the sample mean vector is added to the

forecast When differencing is specified the forecasts xt + m j t

plus the sample mean

vector are integrated back to produce forecasts for the original series

Lety

t

be the original series specified by the VAR statement with some 0 values

appended corresponding to the unobserved past observations Let B be the backshift

operator and let B

be thes s

matrix polynomial in the backshift operator

corresponding to the differencing specified by the VAR statement The off-diagonal

elements of

i

are 0 Note that

0

= I

s

whereI

s

is thes s

identity matrix Then

z

t

= B y

t

This gives the relationship

y

t

=

1

B z

t

=

1

X

i = 0

i

z

t i

where

1

B =

P

1

i = 0

i

B

i and

0

= I

s

The m-step-ahead forecast of y

t + m

is

y

t + m j t

=

m 1

X

i = 0

i

z

t + m i j t

+

1

X

i = m

i

z

t + m i

The m-step-ahead forecast error of y

t + m

is

m 1

X

i = 0

i

z

t + m i

z

t + m i j t

=

m 1

X

i = 0

i

X

u = 0

u

i u

e

t + m i

LettingV

y 0

= 0

the variance of the m-step-ahead forecast error of y

t + m

V

y m

is

V

y m

=

m 1

X

i = 0

i

X

u = 0

u

i u

e e

i

X

u = 0

u

i u

0

= V

y m 1

+

m 1

X

u = 0

u

m 1 u

e e

m 1

X

u = 0

u

m 1 u

0

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Chapter 18 Details

Relation of ARMA and State Space Forms

Every state space model has an ARMA representation and conversely every ARMA

model has a state space representation This section discusses this equivalence The

following material is adapted from Akaike (1974) where there is a more complete

discussion Pham-Dinh-Tuan (1978) also contains a discussion of this material

Suppose you are given the following ARMA model

B x

t

= B e

t

or in more detail

x

t

1

x

t 1

p

x

t p

= e

t

+

1

e

t 1

+ +

q

e

t q

(1)

wheree

t

is a sequence of independent multivariate normal random vectors with mean

0 and variance matrix

e e

B is the backshift operator (B x

t

= x

t 1

) B

and

B

are matrix polynomials in B andx

t

is the observed process

If the roots of the determinantial equationj B j = 0

are outside the unit circle in

the complex plane the model can also be written as

x

t

=

1

B B e

t

=

1

X

i = 0

i

e

t i

The

i

matrices are known as the impulse response matrices and can be computed

as

1

B B

You can assume p q

since if this is not initially true you can add more terms

i

that are identically 0 without changing the model

To write this set of equations in a state space form proceed as follows Letx

t + i j t

be

the conditional expectation of x

t + i

givenx

w

forw t

The following relations hold

x

t + i j t

=

1

X

j = i

j

e

t + i j

x

t + i j t + 1

= x

t + i j t

+

i 1

e

t + 1

However from equation (1) you can derive the following relationship

x

t + p j t

=

1

x

t + p 1 j t

+ +

p

x

t

(2)

Hence wheni = p

you can substitute forx

t + p j t

in the right-hand side of equation

(2) and close the system of equations

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Part 2 General Information

This substitution results in the following model in the state space form

z

t + 1

= F z

t

+ G e

t + 1

2

6

6

6

4

x

t + 1

x

t + 2 j t + 1

x

t + p j t + 1

3

7

7

7

5

=

2

6

6

4

0 I 0 0

0 0 I 0

p

p 1

1

3

7

7

5

2

6

6

6

4

x

t

x

t + 1 j t

x

t + p 1 j t

3

7

7

7

5

+

2

6

6

4

I

1

p 1

3

7

7

5

e

t + 1

Note that the state vectorz

t

is composed of conditional expectations of x

t

and the

first r components of z

t

are equal tox

t

The state space form can be cast into an ARMA form by solving the system of dif-

ference equations for the first r components

When converting from an ARMA form to a state space form you can generate a

state vector larger than needed that is the state space model may not be a minimal

representation When going from a state space form to an ARMA form you can have

nontrivial common factors in the autoregressive and moving average operators that

yield an ARMA model larger than necessary

If the state space form used is not a minimal representation some but not all compo-

nents of x

t + i j t

may be linearly dependent This situation corresponds to

p

p 1

being of less than full rank when B

and B

have no common nontrivial

left factors In this casez

t

consists of a subset of the possible components of

x

t + i j t

i = 1 2 p 1

However once a component of x

t + i j t

(for example the

jth one) is linearly dependent on the previous conditional expectations then all sub-

sequent jth components of x

t + k j t

fork i

must also be linearly dependent Note

that in this case equivalent but seemingly different structures can arise if the order of

the components withinx

t

is changed

OUT= Data Set

The forecasts are contained in the output data set specified by the OUT= option on the

PROC STATESPACE statement The OUT= data set contains the following variables

the BY variables

the ID variable

the VAR statement variables These variables contain the actual values from

the input data set

FORi numeric variables containing the forecasts The variable FORi contains

the forecasts for the ith variable in the VAR statement list Forecasts are one-

step-ahead predictions until the end of the data or until the observation specified

by the BACK= option

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Chapter 18 Details

RESi numeric variables containing the residual for the forecast of the ith vari-

able in the VAR statement list For forecast observations the actual values are

missing and the RESi variables contain missing values

STDi numeric variables containing the standard deviation for the forecast of

the ith variable in the VAR statement list The values of the STDi variables can

be used to construct univariate confidence limits for the corresponding fore-casts However such confidence limits do not take into account the covariance

of the forecasts

OUTAR= Data Set

The OUTAR= data set contains the estimates of the preliminary autoregressive mod-

els The OUTAR= data set contains the following variables

ORDER a numeric variable containing the order p of the autoregressive model

that the observation represents

AIC a numeric variable containing the value of the information criterionA I C

p

SIGFl numeric variables containing the estimate of the innovation covariance

matrices for the forward autoregressive models The variable SIGFl contains

the lth column of b

p

in the observations with ORDER= p

SIGBl numeric variables containing the estimate of the innovation covariance

matrices for the backward autoregressive models The variable SIGBl contains

the lth column of b

p

in the observations with ORDER= p

FORk ndashl numeric variables containing the estimates of the autoregressive pa-

rameter matrices for the forward models The variable FORk ndashl contains the

lth column of the lag k autoregressive parameter matrix b

p

k

in the observations

with ORDER= p

BACk ndashl numeric variables containing the estimates of the autoregressive pa-

rameter matrices for the backward models The variable BACk ndashl contains the

lth column of the lag k autoregressive parameter matrix b

p

k

in the observations

with ORDER= p

The estimates for the order p autoregressive model can be selected as those observa-

tions with ORDER= p Within these observations the k lth element of

p

i

is given by

the value of the FORindashl variable in the k th observation The k lth element of

p

i

is

given by the value of BACindashl variable in the k th observation The k lth element of

p

is given by SIGFl in the k th observation The k lth element of

p

is given by SIGBlin the k th observation

Table 181 shows an example of the OUTAR= data set with ARMAX=3 andx

t

of

dimension 2 In Table 181 i j

indicate the ijth element of the matrix

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Part 2 General Information

Table 181 Values in the OUTAR= Data Set

Obs ORDER AIC SIGF1 SIGF2 SIGB1 SIGB2 FOR1ndash 1 FOR1ndash 2 FOR2ndash 1 FOR2ndash 2 FOR3ndash 1

1 0 AIC0

0 1 1

0 1 2

0 1 1

0 1 2

2 0 AIC0

0 2 1

0 2 2

0 2 1

0 2 2

3 1 AIC1

1 1 1

1 1 2

1 1 1

1 1 2

1

1

1 1

1

1

1 2

4 1 AIC1

1 2 1

1 1 2

1 2 1

1 1 2

1

1

2 1

1

1

2 2

5 2 AIC2

2 1 1

2 1 2

2 1 1

2 1 2

2

1

1 1

2

1

1 2

2

2

1 1

2

2

1 2

6 2 AIC2

2 2 1

2 1 2

2 2 1

2 1 2

2

1

2 1

2

1

2 2

2

2

2 1

2

2

2 2

7 3 AIC3

3 1 1

3 1 2

3 1 1

3 1 2

3

1

1 1

3

1

1 2

3

2

1 1

3

2

1 2

3

3

1 1

8 3 AIC3

3 2 1

3 1 2

3 2 1

3 1 2

3

1

2 1

3

1

2 2

3

2

2 1

3

2

2 2

3

3

2 1

Obs FOR3ndash 2 BACK1ndash 1 BACK1ndash 2 BACK2ndash 1 BACK2ndash 2 BACK3ndash 1 BACK3ndash 2

1

2

3

1

1

1 1

1

1

1 2

4

1

1

2 1

1

1

2 2

5

2

1

1 1

2

1

1 2

2

2

1 1

2

2

1 2

6

2

1

2 1

2

1

2 2

2

2

2 1

2

2

2 2

7

3

3

1 2

3

1

1 1

3

1

1 2

3

2

1 1

3

2

1 2

3

3

1 1

3

3

1 2

8

3

3

2 2

3

1

2 1

3

1

2 2

3

2

2 1

3

2

2 2

3

3

2 1

3

3

2 2

The estimated autoregressive parameters can be used in the IML procedure to obtain

autoregressive estimates of the spectral density function or forecasts based on the

autoregressive models

OUTMODEL= Data Set

The OUTMODEL= data set contains the estimates of theF

andG

matrices and their

standard errors the names of the components of the state vector and the estimates of

the innovation covariance matrix The variables contained in the OUTMODEL= data

set are as follows

the BY variables

STATEVEC a character variable containing the name of the component of the

state vector corresponding to the observation The STATEVEC variable has

the value STD for standard deviations observations which contain the standarderrors for the estimates given in the preceding observation

Fndash j numeric variables containing the columns of theF

matrix The variable

Fndash j contains the jth column of F

The number of Fndash j variables is equal to

the value of the DIMMAX= option If the model is of smaller dimension the

extraneous variables are set to missing

Gndash j numeric variables containing the columns of theG

matrix The variable

Gndash j contains the jth column of G

The number of Gndash j variables is equal to r

the dimension of x

t

given by the number of variables in the VAR statement

SIGndash j numeric variables containing the columns of the innovation covariance

matrix The variable SIGndash j contains the jth column of

e e

There are r vari-ables SIGndash j

Table 182 shows an example of the OUTMODEL= data set withx

t

= x

t

y

t

0

z

t

= x

t

y

t

x

t + 1 j t

0 and DIMMAX=4 In Table 182F

i j

andG

i j

are the ijth

elements of F

andG

respectively Note that all elements for Fndash4 are missing because

F

is a3 3

matrix

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Chapter 18 Details

Table 182 Value in the OUTMODEL= Data Set

Obs STATEVEC Fndash 1 Fndash 2 Fndash 3 Fndash 4 Gndash 1 Gndash 2 SIGndash 1 SIGndash 2

1 X(TT) 0 0 1 1 0

1 1

1 2

2 STD

3 Y(TT) F2 1

F2 2

F2 3

0 1

2 1

2 2

4 STD std F2 1

std F2 2

std F2 3

5 X(T+1T) F3 1

F3 2

F3 3

G3 1

G3 2

6 STD std F3 1

std F3 2

std F3 3

std G3 1

std G3 2

Printed Output

The printed output produced by the STATESPACE procedure is described in the fol-

lowing

1 descriptive statistics which include the number of observations used the

names of the variables their means and standard deviations (Std) and the dif-

ferencing operations used

2 the Akaike information criteria for the sequence of preliminary autoregressive

models

3 if the PRINTOUT=LONG option is specified the sample autocovariance ma-

trices of the input series at various lags

4 if the PRINTOUT=LONG option is specified the sample autocorrelation ma-

trices of the input series

5 a schematic representation of the autocorrelation matrices showing the signif-

icant autocorrelations

6 if the PRINTOUT=LONG option is specified the partial autoregressive ma-

trices (These are

p

p

as described in Preliminary Autoregressive Modelsearlier in this chapter)

7 a schematic representation of the partial autocorrelation matrices showing the

significant partial autocorrelations

8 the Yule-Walker estimates of the autoregressive parameters for the autoregres-

sive model with the minimum AIC

9 if the PRINTOUT=LONG option is specified the autocovariance matrices of

the residuals of the minimum AIC model This is the sequence of estimated

innovation variance matrices for the solutions of the Yule-Walker equations

10 if the PRINTOUT=LONG option is specified the autocorrelation matrices of the residuals of the minimum AIC model

11 If the CANCORR option is specified the canonical correlations analysis for

each potential state vector considered in the state vector selection process

This includes the potential state vector the canonical correlations the informa-

tion criterion for the smallest canonical correlation Bartlettrsquos

2 statistic (ldquoChi

Squarerdquo) for the smallest canonical correlation and the degrees of freedom of

Bartlettrsquos

2

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Part 2 General Information

12 the components of the chosen state vector

13 the preliminary estimate of the transition matrixF

the input matrixG

and

the variance matrix for the innovations

e e

14 if the ITPRINT option is specified the iteration history of the likelihood max-

imization For each iteration this shows the iteration number the number of step halvings the determinant of the innovation variance matrix the damping

factor Lambda and the values of the parameters

15 the state vector printed again to aid interpretation of the following listing of F

andG

16 the final estimate of the transition matrixF

17 the final estimate of the input matrixG

18 the final estimate of the variance matrix for the innovations

e e

19 a table listing the estimates of the free parameters inF

andG

and their standarderrors and t statistics

20 if the COVB option is specified the covariance matrix of the parameter esti-

mates

21 if the COVB option is specified the correlation matrix of the parameter esti-

mates

22 if the PRINT option is specified the forecasts and their standard errors

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Chapter 18 Details

ODS Table Names

PROC STATESPACE assigns a name to each table it creates You can use these

names to reference the table when using the Output Delivery System (ODS) to select

tables and create output data sets These names are listed in the following table For

more information on ODS see Chapter 6 ldquoUsing the Output Delivery Systemrdquo

Table 183 ODS Tables Produced in PROC STATESPACE

ODS Table Name Description Option

NObs Number of observations default

Summary Simple summary statistics table default

InfoCriterion Information criterion table default

CovLags Covariance Matrices of Input Series PRINTOUT=LONG

CorrLags Correlation Matrices of Input Series PRINTOUT=LONG

PartialAR Partial Autoregressive Matrices PRINTOUT=LONG

YWEstimates Yule-Walker Estimates for Minimum AIC default

CovResiduals Covariance of Residuals PRINTOUT=LONGCorrResiduals Residual Correlations from AR Models PRINTOUT=LONG

StateVector State vector table default

CorrGraph Schematic Representation of Correlations default

TransitionMatrix Transition Matrix default

InputMatrix Input Matrix default

VarInnov Variance Matrix for the Innovation default

CovB Covariance of Parameter Estimates COVB

CorrB Correlation of Parameter Estimates COVB

CanCorr Canonical Correlation Analysis CANCORR

IterHistory Iterative Fitting table ITPRINT

ParameterEstimates Parameter Estimates Table default

Forecasts Forecasts Table PRINT

ConvergenceStatus Convergence Status Table default

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Part 2 General Information

Example

Example 181 Series J from Box and Jenkins

This example analyzes the gas furnace data (series J) from Box and Jenkins (The

data are not shown Refer to Box and Jenkins (1976) for the data)

First a model is selected and fit automatically using the following statements

title1 rsquoGas Furnace Datarsquo

title2 rsquoBox amp Jenkins Series Jrsquo

title3 rsquoAutomatically Selected Modelrsquo

proc statespace data=seriesj cancorr

var x y

run

The results for the automatically selected model are shown in Output 1811

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Chapter 18 Example

Output 1811 Results for Automatically Selected Model

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Number of Observations 296

Standard

Variable Mean Error

x -005683 1072766

y 5350912 3202121

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Information Criterion for Autoregressive Models

Lag=0 Lag=1 Lag=2 Lag=3 Lag=4 Lag=5 Lag=6 Lag=7 Lag=8

6513862 -103357 -163296 -164512 -165152 -164891 -164934 -164315 -163856

Information

Criterion for

Autoregressive

Models

Lag=9 Lag=10

-16348 -163359

Schematic Representation of Correlations

NameLag 0 1 2 3 4 5 6 7 8 9 10

x +- +- +- +- +- +- +- +- +- +- +-

y -+ -+ -+ -+ -+ -+ -+ -+ -+ -+ -+

+ is gt 2std error - is lt -2std error is between

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Part 2 General Information

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Schematic Representation of Partial Autocorrelations

NameLag 1 2 3 4 5 6 7 8 9 10

x + - + -

y -+ -- - + +

+ is gt 2std error - is lt -2std error is between

Yule-Walker Estimates for Minimum AIC

------Lag=1------ ------Lag=2------ ------Lag=3------ ------Lag=4------

x y x y x y x y

x 1925887 -000124 -120166 0004224 0116918 -000867 0104236 0003268

y 0050496 1299793 -002046 -03277 -071182 -025701 0195411 0133417

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Canonical Correlations Analysis

Information Chi

x(TT) y(TT) x(T+1T) Criterion Square DF

1 1 0804883 2929228 3047481 8

Information Chix(TT) y(TT) x(T+1T) y(T+1T) Criterion Square DF

1 1 0906681 0607529 1223358 1347237 7

Information Chi

x(TT) y(TT) x (T+1T) y (T+1T) x (T+2T) Criterion Square DF

1 1 0909434 0610278 0186274 -154701 1034705 6

Information Chi

x(TT) y(TT) x (T+1T) y (T+1T) y (T+2T) Criterion Square DF

1 1 091014 0618937 0206823 0940392 1280924 6

Information Chi

x(TT) y(TT) x(T+1T) y(T+1T) y(T+2T) y(T+3T) Criterion Square DF

1 1 0912963 0628785 0226598 0083258 -794103 2041584 5

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Chapter 18 Example

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Selected Statespace Form and Preliminary Estimates

State Vector

x(TT) y(TT) x(T+1T) y(T+1T) y(T+2T)

Estimate of Transition Matrix

0 0 1 0 0

0 0 0 1 0

-084718 0026794 1711715 -005019 0

0 0 0 0 1

-019785 0334274 -018174 -123557 1787475

Input Matrix for Innovation

1 0

0 11925887 -000124

0050496 1299793

0142421 1361696

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Selected Statespace Form and Preliminary Estimates

Variance Matrix for Innovation

0035274 -000734

-000734 0097569

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Part 2 General Information

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Selected Statespace Form and Fitted Model

State Vector

x(TT) y(TT) x(T+1T) y(T+1T) y(T+2T)

Estimate of Transition Matrix

0 0 1 0 0

0 0 0 1 0

-086192 0030609 1724235 -005483 0

0 0 0 0 1

-034839 0292124 -009435 -109823 1671418

Input Matrix for Innovation

1 0

0 1192442 -000416

0015621 1258495

008058 1353204

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Selected Statespace Form and Fitted Model

Variance Matrix for Innovation

0035579 -000728

-000728 0095577

Parameter Estimates

Standard

Parameter Estimate Error t Value

F(31) -086192 0072961 -1181

F(32) 0030609 0026167 117

F(33) 1724235 0061599 2799

F(34) -005483 0030169 -182

F(51) -034839 0135253 -258

F(52) 0292124 0046299 631

F(53) -009435 0096527 -098F(54) -109823 0109525 -1003

F(55) 1671418 0083737 1996

G(31) 1924420 0058162 3309

G(32) -000416 0035255 -012

G(41) 0015621 0095771 016

G(42) 1258495 0055742 2258

G(51) 0080580 0151622 053

G(52) 1353204 0091388 1481

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Chapter 18 Example

The two series are believed to have a transfer function relation with the gas rate

(variable X) as the input and the CO2

concentration (variable Y) as the output Since

the parameter estimates shown in Output 1811 support this kind of model the model

is reestimated with the feedback parameters restricted to 0 The following statements

fit the transfer function (no feedback) model

title3 rsquoTransfer Function Modelrsquo

proc statespace data=seriesj printout=none

var x y

restrict f(32)=0 f(34)=0

g(32)=0 g(41)=0 g(51)=0

run

The last 2 pages of the output are shown in Output 1812

Output 1812 STATESPACE Output for Transfer Function Model

Gas Furnace Data

Box amp Jenkins Series J

Transfer Function Model

The STATESPACE Procedure

Selected Statespace Form and Fitted Model

State Vector

x(TT) y(TT) x(T+1T) y(T+1T) y(T+2T)

Estimate of Transition Matrix

0 0 1 0 0

0 0 0 1 0

-068882 0 1598717 0 0

0 0 0 0 1

-035944 0284179 -00963 -107313 1650047

Input Matrix for Innovation

1 0

0 1

1923446 0

0 1260856

0 1346332

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Part 2 General Information

Gas Furnace Data

Box amp Jenkins Series J

Transfer Function Model

The STATESPACE Procedure

Selected Statespace Form and Fitted Model

Variance Matrix for Innovation

0036995 -00072

-00072 0095712

Parameter Estimates

Standard

Parameter Estimate Error t Value

F(31) -068882 0050549 -1363

F(33) 1598717 0050924 3139

F(51) -035944 0229044 -157

F(52) 0284179 0096944 293

F(53) -009630 0140876 -068

F(54) -107313 0250385 -429

F(55) 1650047 0188533 875G(31) 1923446 0056328 3415

G(42) 1260856 0056464 2233

G(52) 1346332 0091086 1478

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Chapter 18 References

References

Akaike H (1974) Markovian Representation of Stochastic Processes and Its Ap-

plication to the Analysis of Autoregressive Moving Average Processes Annals

of the Institute of Statistical Mathematics 26 363-387

Akaike H (1976) Canonical Correlations Analysis of Time Series and the Use of

an Information Criterion in Advances and Case Studies in System Identification

eds R Mehra and DG Lainiotis New York Academic Press

Anderson TW (1971) The Statistical Analysis of Time Series New York John

Wiley amp Sons

Ansley CF and Newbold P (1979) Multivariate Partial Autocorrelations Pro-

ceedings of the Business and Economic Statistics Section American Statistical

Association 349-353

Box GEP and Jenkins G (1976) Time Series Analysis Forecasting and Control

San Francisco Holden-Day

Brockwell PJ and Davis RA (1991) Time Series Theory and Methods 2nd Edi-

tion Springer-Verlag

Hannan EJ (1970) Multiple Time Series New York John Wiley amp Sons

Hannan EJ (1976) The Identification and Parameterization of ARMAX and State

Space Forms Econometrica 44 713-722

Harvey AC (1981a) The Econometric Analysis of Time Series New York John

Wiley amp Sons

Harvey AC (1981b) Time Series Models New York John Wiley amp Sons

Jones RH (1974) Identification and Autoregressive Spectrum Estimation IEEE Transactions on Automatic Control AC-19 894-897

Pham-Dinh-Tuan (1978) On the Fitting of Multivariate Processes of the

Autoregressive-Moving Average Type Biometrika 65 99-107

Priestley MB (1980) System Identification Kalman Filtering and Stochastic Con-

trol in Directions in Time Series eds DR Brillinger and GC Tiao Institute

of Mathematical Statistics

Whittle P (1963) On the Fitting of Multivariate Autoregressions and the Approxi-

mate Canonical Factorization of a Spectral Density Matrix Biometrika 50 129-

134

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The correct bibliographic citation for this manual is as follows SAS Institute Inc SAS

ET S U serrsquos Gui de Version 8 Cary NC SAS Institute Inc 1999 1546 pp

SASET S Userrsquos Guide Version 8

Copyright copy 1999 by S AS I ns t it u t e I nc Ca ry N C U S A

ISBN 1ndash58025ndash489ndash6

All r ight s res erved P r int ed in t he U n it ed St a t es of A merica N o pa rt of t his p u blica t ionma y b e reprodu ced s t ored in a ret r ieva l s ys t em or t ra ns mit t ed in a ny form or b y a nymeans electronic mechanical photocopying or otherwise without the prior writtenpermission of the publisher SAS Institute Inc

US Government Restricted Rights Notice Use du plicat ion or disclosure of the

software by the government is subject to restrictions as set forth in FAR 52227ndash19Commercial Computer Softw ar e-Restricted Rights (J une 1987)

SAS Institute Inc SAS Campus Drive Cary North Carolina 27513

1st print ing October 1999

S AS reg a nd a ll ot her SA S I ns t it u t e I nc p rodu ct or s ervice na m es a re regis t ered t ra dema rkst d k f SA S I t it t I i t h U S A d t h t i reg i di t U SA

Page 37: Statespace Model

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Chapter 18 Details

Relation of ARMA and State Space Forms

Every state space model has an ARMA representation and conversely every ARMA

model has a state space representation This section discusses this equivalence The

following material is adapted from Akaike (1974) where there is a more complete

discussion Pham-Dinh-Tuan (1978) also contains a discussion of this material

Suppose you are given the following ARMA model

B x

t

= B e

t

or in more detail

x

t

1

x

t 1

p

x

t p

= e

t

+

1

e

t 1

+ +

q

e

t q

(1)

wheree

t

is a sequence of independent multivariate normal random vectors with mean

0 and variance matrix

e e

B is the backshift operator (B x

t

= x

t 1

) B

and

B

are matrix polynomials in B andx

t

is the observed process

If the roots of the determinantial equationj B j = 0

are outside the unit circle in

the complex plane the model can also be written as

x

t

=

1

B B e

t

=

1

X

i = 0

i

e

t i

The

i

matrices are known as the impulse response matrices and can be computed

as

1

B B

You can assume p q

since if this is not initially true you can add more terms

i

that are identically 0 without changing the model

To write this set of equations in a state space form proceed as follows Letx

t + i j t

be

the conditional expectation of x

t + i

givenx

w

forw t

The following relations hold

x

t + i j t

=

1

X

j = i

j

e

t + i j

x

t + i j t + 1

= x

t + i j t

+

i 1

e

t + 1

However from equation (1) you can derive the following relationship

x

t + p j t

=

1

x

t + p 1 j t

+ +

p

x

t

(2)

Hence wheni = p

you can substitute forx

t + p j t

in the right-hand side of equation

(2) and close the system of equations

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Part 2 General Information

This substitution results in the following model in the state space form

z

t + 1

= F z

t

+ G e

t + 1

2

6

6

6

4

x

t + 1

x

t + 2 j t + 1

x

t + p j t + 1

3

7

7

7

5

=

2

6

6

4

0 I 0 0

0 0 I 0

p

p 1

1

3

7

7

5

2

6

6

6

4

x

t

x

t + 1 j t

x

t + p 1 j t

3

7

7

7

5

+

2

6

6

4

I

1

p 1

3

7

7

5

e

t + 1

Note that the state vectorz

t

is composed of conditional expectations of x

t

and the

first r components of z

t

are equal tox

t

The state space form can be cast into an ARMA form by solving the system of dif-

ference equations for the first r components

When converting from an ARMA form to a state space form you can generate a

state vector larger than needed that is the state space model may not be a minimal

representation When going from a state space form to an ARMA form you can have

nontrivial common factors in the autoregressive and moving average operators that

yield an ARMA model larger than necessary

If the state space form used is not a minimal representation some but not all compo-

nents of x

t + i j t

may be linearly dependent This situation corresponds to

p

p 1

being of less than full rank when B

and B

have no common nontrivial

left factors In this casez

t

consists of a subset of the possible components of

x

t + i j t

i = 1 2 p 1

However once a component of x

t + i j t

(for example the

jth one) is linearly dependent on the previous conditional expectations then all sub-

sequent jth components of x

t + k j t

fork i

must also be linearly dependent Note

that in this case equivalent but seemingly different structures can arise if the order of

the components withinx

t

is changed

OUT= Data Set

The forecasts are contained in the output data set specified by the OUT= option on the

PROC STATESPACE statement The OUT= data set contains the following variables

the BY variables

the ID variable

the VAR statement variables These variables contain the actual values from

the input data set

FORi numeric variables containing the forecasts The variable FORi contains

the forecasts for the ith variable in the VAR statement list Forecasts are one-

step-ahead predictions until the end of the data or until the observation specified

by the BACK= option

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Chapter 18 Details

RESi numeric variables containing the residual for the forecast of the ith vari-

able in the VAR statement list For forecast observations the actual values are

missing and the RESi variables contain missing values

STDi numeric variables containing the standard deviation for the forecast of

the ith variable in the VAR statement list The values of the STDi variables can

be used to construct univariate confidence limits for the corresponding fore-casts However such confidence limits do not take into account the covariance

of the forecasts

OUTAR= Data Set

The OUTAR= data set contains the estimates of the preliminary autoregressive mod-

els The OUTAR= data set contains the following variables

ORDER a numeric variable containing the order p of the autoregressive model

that the observation represents

AIC a numeric variable containing the value of the information criterionA I C

p

SIGFl numeric variables containing the estimate of the innovation covariance

matrices for the forward autoregressive models The variable SIGFl contains

the lth column of b

p

in the observations with ORDER= p

SIGBl numeric variables containing the estimate of the innovation covariance

matrices for the backward autoregressive models The variable SIGBl contains

the lth column of b

p

in the observations with ORDER= p

FORk ndashl numeric variables containing the estimates of the autoregressive pa-

rameter matrices for the forward models The variable FORk ndashl contains the

lth column of the lag k autoregressive parameter matrix b

p

k

in the observations

with ORDER= p

BACk ndashl numeric variables containing the estimates of the autoregressive pa-

rameter matrices for the backward models The variable BACk ndashl contains the

lth column of the lag k autoregressive parameter matrix b

p

k

in the observations

with ORDER= p

The estimates for the order p autoregressive model can be selected as those observa-

tions with ORDER= p Within these observations the k lth element of

p

i

is given by

the value of the FORindashl variable in the k th observation The k lth element of

p

i

is

given by the value of BACindashl variable in the k th observation The k lth element of

p

is given by SIGFl in the k th observation The k lth element of

p

is given by SIGBlin the k th observation

Table 181 shows an example of the OUTAR= data set with ARMAX=3 andx

t

of

dimension 2 In Table 181 i j

indicate the ijth element of the matrix

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Part 2 General Information

Table 181 Values in the OUTAR= Data Set

Obs ORDER AIC SIGF1 SIGF2 SIGB1 SIGB2 FOR1ndash 1 FOR1ndash 2 FOR2ndash 1 FOR2ndash 2 FOR3ndash 1

1 0 AIC0

0 1 1

0 1 2

0 1 1

0 1 2

2 0 AIC0

0 2 1

0 2 2

0 2 1

0 2 2

3 1 AIC1

1 1 1

1 1 2

1 1 1

1 1 2

1

1

1 1

1

1

1 2

4 1 AIC1

1 2 1

1 1 2

1 2 1

1 1 2

1

1

2 1

1

1

2 2

5 2 AIC2

2 1 1

2 1 2

2 1 1

2 1 2

2

1

1 1

2

1

1 2

2

2

1 1

2

2

1 2

6 2 AIC2

2 2 1

2 1 2

2 2 1

2 1 2

2

1

2 1

2

1

2 2

2

2

2 1

2

2

2 2

7 3 AIC3

3 1 1

3 1 2

3 1 1

3 1 2

3

1

1 1

3

1

1 2

3

2

1 1

3

2

1 2

3

3

1 1

8 3 AIC3

3 2 1

3 1 2

3 2 1

3 1 2

3

1

2 1

3

1

2 2

3

2

2 1

3

2

2 2

3

3

2 1

Obs FOR3ndash 2 BACK1ndash 1 BACK1ndash 2 BACK2ndash 1 BACK2ndash 2 BACK3ndash 1 BACK3ndash 2

1

2

3

1

1

1 1

1

1

1 2

4

1

1

2 1

1

1

2 2

5

2

1

1 1

2

1

1 2

2

2

1 1

2

2

1 2

6

2

1

2 1

2

1

2 2

2

2

2 1

2

2

2 2

7

3

3

1 2

3

1

1 1

3

1

1 2

3

2

1 1

3

2

1 2

3

3

1 1

3

3

1 2

8

3

3

2 2

3

1

2 1

3

1

2 2

3

2

2 1

3

2

2 2

3

3

2 1

3

3

2 2

The estimated autoregressive parameters can be used in the IML procedure to obtain

autoregressive estimates of the spectral density function or forecasts based on the

autoregressive models

OUTMODEL= Data Set

The OUTMODEL= data set contains the estimates of theF

andG

matrices and their

standard errors the names of the components of the state vector and the estimates of

the innovation covariance matrix The variables contained in the OUTMODEL= data

set are as follows

the BY variables

STATEVEC a character variable containing the name of the component of the

state vector corresponding to the observation The STATEVEC variable has

the value STD for standard deviations observations which contain the standarderrors for the estimates given in the preceding observation

Fndash j numeric variables containing the columns of theF

matrix The variable

Fndash j contains the jth column of F

The number of Fndash j variables is equal to

the value of the DIMMAX= option If the model is of smaller dimension the

extraneous variables are set to missing

Gndash j numeric variables containing the columns of theG

matrix The variable

Gndash j contains the jth column of G

The number of Gndash j variables is equal to r

the dimension of x

t

given by the number of variables in the VAR statement

SIGndash j numeric variables containing the columns of the innovation covariance

matrix The variable SIGndash j contains the jth column of

e e

There are r vari-ables SIGndash j

Table 182 shows an example of the OUTMODEL= data set withx

t

= x

t

y

t

0

z

t

= x

t

y

t

x

t + 1 j t

0 and DIMMAX=4 In Table 182F

i j

andG

i j

are the ijth

elements of F

andG

respectively Note that all elements for Fndash4 are missing because

F

is a3 3

matrix

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Chapter 18 Details

Table 182 Value in the OUTMODEL= Data Set

Obs STATEVEC Fndash 1 Fndash 2 Fndash 3 Fndash 4 Gndash 1 Gndash 2 SIGndash 1 SIGndash 2

1 X(TT) 0 0 1 1 0

1 1

1 2

2 STD

3 Y(TT) F2 1

F2 2

F2 3

0 1

2 1

2 2

4 STD std F2 1

std F2 2

std F2 3

5 X(T+1T) F3 1

F3 2

F3 3

G3 1

G3 2

6 STD std F3 1

std F3 2

std F3 3

std G3 1

std G3 2

Printed Output

The printed output produced by the STATESPACE procedure is described in the fol-

lowing

1 descriptive statistics which include the number of observations used the

names of the variables their means and standard deviations (Std) and the dif-

ferencing operations used

2 the Akaike information criteria for the sequence of preliminary autoregressive

models

3 if the PRINTOUT=LONG option is specified the sample autocovariance ma-

trices of the input series at various lags

4 if the PRINTOUT=LONG option is specified the sample autocorrelation ma-

trices of the input series

5 a schematic representation of the autocorrelation matrices showing the signif-

icant autocorrelations

6 if the PRINTOUT=LONG option is specified the partial autoregressive ma-

trices (These are

p

p

as described in Preliminary Autoregressive Modelsearlier in this chapter)

7 a schematic representation of the partial autocorrelation matrices showing the

significant partial autocorrelations

8 the Yule-Walker estimates of the autoregressive parameters for the autoregres-

sive model with the minimum AIC

9 if the PRINTOUT=LONG option is specified the autocovariance matrices of

the residuals of the minimum AIC model This is the sequence of estimated

innovation variance matrices for the solutions of the Yule-Walker equations

10 if the PRINTOUT=LONG option is specified the autocorrelation matrices of the residuals of the minimum AIC model

11 If the CANCORR option is specified the canonical correlations analysis for

each potential state vector considered in the state vector selection process

This includes the potential state vector the canonical correlations the informa-

tion criterion for the smallest canonical correlation Bartlettrsquos

2 statistic (ldquoChi

Squarerdquo) for the smallest canonical correlation and the degrees of freedom of

Bartlettrsquos

2

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Part 2 General Information

12 the components of the chosen state vector

13 the preliminary estimate of the transition matrixF

the input matrixG

and

the variance matrix for the innovations

e e

14 if the ITPRINT option is specified the iteration history of the likelihood max-

imization For each iteration this shows the iteration number the number of step halvings the determinant of the innovation variance matrix the damping

factor Lambda and the values of the parameters

15 the state vector printed again to aid interpretation of the following listing of F

andG

16 the final estimate of the transition matrixF

17 the final estimate of the input matrixG

18 the final estimate of the variance matrix for the innovations

e e

19 a table listing the estimates of the free parameters inF

andG

and their standarderrors and t statistics

20 if the COVB option is specified the covariance matrix of the parameter esti-

mates

21 if the COVB option is specified the correlation matrix of the parameter esti-

mates

22 if the PRINT option is specified the forecasts and their standard errors

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Chapter 18 Details

ODS Table Names

PROC STATESPACE assigns a name to each table it creates You can use these

names to reference the table when using the Output Delivery System (ODS) to select

tables and create output data sets These names are listed in the following table For

more information on ODS see Chapter 6 ldquoUsing the Output Delivery Systemrdquo

Table 183 ODS Tables Produced in PROC STATESPACE

ODS Table Name Description Option

NObs Number of observations default

Summary Simple summary statistics table default

InfoCriterion Information criterion table default

CovLags Covariance Matrices of Input Series PRINTOUT=LONG

CorrLags Correlation Matrices of Input Series PRINTOUT=LONG

PartialAR Partial Autoregressive Matrices PRINTOUT=LONG

YWEstimates Yule-Walker Estimates for Minimum AIC default

CovResiduals Covariance of Residuals PRINTOUT=LONGCorrResiduals Residual Correlations from AR Models PRINTOUT=LONG

StateVector State vector table default

CorrGraph Schematic Representation of Correlations default

TransitionMatrix Transition Matrix default

InputMatrix Input Matrix default

VarInnov Variance Matrix for the Innovation default

CovB Covariance of Parameter Estimates COVB

CorrB Correlation of Parameter Estimates COVB

CanCorr Canonical Correlation Analysis CANCORR

IterHistory Iterative Fitting table ITPRINT

ParameterEstimates Parameter Estimates Table default

Forecasts Forecasts Table PRINT

ConvergenceStatus Convergence Status Table default

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Part 2 General Information

Example

Example 181 Series J from Box and Jenkins

This example analyzes the gas furnace data (series J) from Box and Jenkins (The

data are not shown Refer to Box and Jenkins (1976) for the data)

First a model is selected and fit automatically using the following statements

title1 rsquoGas Furnace Datarsquo

title2 rsquoBox amp Jenkins Series Jrsquo

title3 rsquoAutomatically Selected Modelrsquo

proc statespace data=seriesj cancorr

var x y

run

The results for the automatically selected model are shown in Output 1811

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Chapter 18 Example

Output 1811 Results for Automatically Selected Model

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Number of Observations 296

Standard

Variable Mean Error

x -005683 1072766

y 5350912 3202121

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Information Criterion for Autoregressive Models

Lag=0 Lag=1 Lag=2 Lag=3 Lag=4 Lag=5 Lag=6 Lag=7 Lag=8

6513862 -103357 -163296 -164512 -165152 -164891 -164934 -164315 -163856

Information

Criterion for

Autoregressive

Models

Lag=9 Lag=10

-16348 -163359

Schematic Representation of Correlations

NameLag 0 1 2 3 4 5 6 7 8 9 10

x +- +- +- +- +- +- +- +- +- +- +-

y -+ -+ -+ -+ -+ -+ -+ -+ -+ -+ -+

+ is gt 2std error - is lt -2std error is between

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Part 2 General Information

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Schematic Representation of Partial Autocorrelations

NameLag 1 2 3 4 5 6 7 8 9 10

x + - + -

y -+ -- - + +

+ is gt 2std error - is lt -2std error is between

Yule-Walker Estimates for Minimum AIC

------Lag=1------ ------Lag=2------ ------Lag=3------ ------Lag=4------

x y x y x y x y

x 1925887 -000124 -120166 0004224 0116918 -000867 0104236 0003268

y 0050496 1299793 -002046 -03277 -071182 -025701 0195411 0133417

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Canonical Correlations Analysis

Information Chi

x(TT) y(TT) x(T+1T) Criterion Square DF

1 1 0804883 2929228 3047481 8

Information Chix(TT) y(TT) x(T+1T) y(T+1T) Criterion Square DF

1 1 0906681 0607529 1223358 1347237 7

Information Chi

x(TT) y(TT) x (T+1T) y (T+1T) x (T+2T) Criterion Square DF

1 1 0909434 0610278 0186274 -154701 1034705 6

Information Chi

x(TT) y(TT) x (T+1T) y (T+1T) y (T+2T) Criterion Square DF

1 1 091014 0618937 0206823 0940392 1280924 6

Information Chi

x(TT) y(TT) x(T+1T) y(T+1T) y(T+2T) y(T+3T) Criterion Square DF

1 1 0912963 0628785 0226598 0083258 -794103 2041584 5

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Chapter 18 Example

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Selected Statespace Form and Preliminary Estimates

State Vector

x(TT) y(TT) x(T+1T) y(T+1T) y(T+2T)

Estimate of Transition Matrix

0 0 1 0 0

0 0 0 1 0

-084718 0026794 1711715 -005019 0

0 0 0 0 1

-019785 0334274 -018174 -123557 1787475

Input Matrix for Innovation

1 0

0 11925887 -000124

0050496 1299793

0142421 1361696

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Selected Statespace Form and Preliminary Estimates

Variance Matrix for Innovation

0035274 -000734

-000734 0097569

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Part 2 General Information

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Selected Statespace Form and Fitted Model

State Vector

x(TT) y(TT) x(T+1T) y(T+1T) y(T+2T)

Estimate of Transition Matrix

0 0 1 0 0

0 0 0 1 0

-086192 0030609 1724235 -005483 0

0 0 0 0 1

-034839 0292124 -009435 -109823 1671418

Input Matrix for Innovation

1 0

0 1192442 -000416

0015621 1258495

008058 1353204

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Selected Statespace Form and Fitted Model

Variance Matrix for Innovation

0035579 -000728

-000728 0095577

Parameter Estimates

Standard

Parameter Estimate Error t Value

F(31) -086192 0072961 -1181

F(32) 0030609 0026167 117

F(33) 1724235 0061599 2799

F(34) -005483 0030169 -182

F(51) -034839 0135253 -258

F(52) 0292124 0046299 631

F(53) -009435 0096527 -098F(54) -109823 0109525 -1003

F(55) 1671418 0083737 1996

G(31) 1924420 0058162 3309

G(32) -000416 0035255 -012

G(41) 0015621 0095771 016

G(42) 1258495 0055742 2258

G(51) 0080580 0151622 053

G(52) 1353204 0091388 1481

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Chapter 18 Example

The two series are believed to have a transfer function relation with the gas rate

(variable X) as the input and the CO2

concentration (variable Y) as the output Since

the parameter estimates shown in Output 1811 support this kind of model the model

is reestimated with the feedback parameters restricted to 0 The following statements

fit the transfer function (no feedback) model

title3 rsquoTransfer Function Modelrsquo

proc statespace data=seriesj printout=none

var x y

restrict f(32)=0 f(34)=0

g(32)=0 g(41)=0 g(51)=0

run

The last 2 pages of the output are shown in Output 1812

Output 1812 STATESPACE Output for Transfer Function Model

Gas Furnace Data

Box amp Jenkins Series J

Transfer Function Model

The STATESPACE Procedure

Selected Statespace Form and Fitted Model

State Vector

x(TT) y(TT) x(T+1T) y(T+1T) y(T+2T)

Estimate of Transition Matrix

0 0 1 0 0

0 0 0 1 0

-068882 0 1598717 0 0

0 0 0 0 1

-035944 0284179 -00963 -107313 1650047

Input Matrix for Innovation

1 0

0 1

1923446 0

0 1260856

0 1346332

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Part 2 General Information

Gas Furnace Data

Box amp Jenkins Series J

Transfer Function Model

The STATESPACE Procedure

Selected Statespace Form and Fitted Model

Variance Matrix for Innovation

0036995 -00072

-00072 0095712

Parameter Estimates

Standard

Parameter Estimate Error t Value

F(31) -068882 0050549 -1363

F(33) 1598717 0050924 3139

F(51) -035944 0229044 -157

F(52) 0284179 0096944 293

F(53) -009630 0140876 -068

F(54) -107313 0250385 -429

F(55) 1650047 0188533 875G(31) 1923446 0056328 3415

G(42) 1260856 0056464 2233

G(52) 1346332 0091086 1478

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Chapter 18 References

References

Akaike H (1974) Markovian Representation of Stochastic Processes and Its Ap-

plication to the Analysis of Autoregressive Moving Average Processes Annals

of the Institute of Statistical Mathematics 26 363-387

Akaike H (1976) Canonical Correlations Analysis of Time Series and the Use of

an Information Criterion in Advances and Case Studies in System Identification

eds R Mehra and DG Lainiotis New York Academic Press

Anderson TW (1971) The Statistical Analysis of Time Series New York John

Wiley amp Sons

Ansley CF and Newbold P (1979) Multivariate Partial Autocorrelations Pro-

ceedings of the Business and Economic Statistics Section American Statistical

Association 349-353

Box GEP and Jenkins G (1976) Time Series Analysis Forecasting and Control

San Francisco Holden-Day

Brockwell PJ and Davis RA (1991) Time Series Theory and Methods 2nd Edi-

tion Springer-Verlag

Hannan EJ (1970) Multiple Time Series New York John Wiley amp Sons

Hannan EJ (1976) The Identification and Parameterization of ARMAX and State

Space Forms Econometrica 44 713-722

Harvey AC (1981a) The Econometric Analysis of Time Series New York John

Wiley amp Sons

Harvey AC (1981b) Time Series Models New York John Wiley amp Sons

Jones RH (1974) Identification and Autoregressive Spectrum Estimation IEEE Transactions on Automatic Control AC-19 894-897

Pham-Dinh-Tuan (1978) On the Fitting of Multivariate Processes of the

Autoregressive-Moving Average Type Biometrika 65 99-107

Priestley MB (1980) System Identification Kalman Filtering and Stochastic Con-

trol in Directions in Time Series eds DR Brillinger and GC Tiao Institute

of Mathematical Statistics

Whittle P (1963) On the Fitting of Multivariate Autoregressions and the Approxi-

mate Canonical Factorization of a Spectral Density Matrix Biometrika 50 129-

134

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The correct bibliographic citation for this manual is as follows SAS Institute Inc SAS

ET S U serrsquos Gui de Version 8 Cary NC SAS Institute Inc 1999 1546 pp

SASET S Userrsquos Guide Version 8

Copyright copy 1999 by S AS I ns t it u t e I nc Ca ry N C U S A

ISBN 1ndash58025ndash489ndash6

All r ight s res erved P r int ed in t he U n it ed St a t es of A merica N o pa rt of t his p u blica t ionma y b e reprodu ced s t ored in a ret r ieva l s ys t em or t ra ns mit t ed in a ny form or b y a nymeans electronic mechanical photocopying or otherwise without the prior writtenpermission of the publisher SAS Institute Inc

US Government Restricted Rights Notice Use du plicat ion or disclosure of the

software by the government is subject to restrictions as set forth in FAR 52227ndash19Commercial Computer Softw ar e-Restricted Rights (J une 1987)

SAS Institute Inc SAS Campus Drive Cary North Carolina 27513

1st print ing October 1999

S AS reg a nd a ll ot her SA S I ns t it u t e I nc p rodu ct or s ervice na m es a re regis t ered t ra dema rkst d k f SA S I t it t I i t h U S A d t h t i reg i di t U SA

Page 38: Statespace Model

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Part 2 General Information

This substitution results in the following model in the state space form

z

t + 1

= F z

t

+ G e

t + 1

2

6

6

6

4

x

t + 1

x

t + 2 j t + 1

x

t + p j t + 1

3

7

7

7

5

=

2

6

6

4

0 I 0 0

0 0 I 0

p

p 1

1

3

7

7

5

2

6

6

6

4

x

t

x

t + 1 j t

x

t + p 1 j t

3

7

7

7

5

+

2

6

6

4

I

1

p 1

3

7

7

5

e

t + 1

Note that the state vectorz

t

is composed of conditional expectations of x

t

and the

first r components of z

t

are equal tox

t

The state space form can be cast into an ARMA form by solving the system of dif-

ference equations for the first r components

When converting from an ARMA form to a state space form you can generate a

state vector larger than needed that is the state space model may not be a minimal

representation When going from a state space form to an ARMA form you can have

nontrivial common factors in the autoregressive and moving average operators that

yield an ARMA model larger than necessary

If the state space form used is not a minimal representation some but not all compo-

nents of x

t + i j t

may be linearly dependent This situation corresponds to

p

p 1

being of less than full rank when B

and B

have no common nontrivial

left factors In this casez

t

consists of a subset of the possible components of

x

t + i j t

i = 1 2 p 1

However once a component of x

t + i j t

(for example the

jth one) is linearly dependent on the previous conditional expectations then all sub-

sequent jth components of x

t + k j t

fork i

must also be linearly dependent Note

that in this case equivalent but seemingly different structures can arise if the order of

the components withinx

t

is changed

OUT= Data Set

The forecasts are contained in the output data set specified by the OUT= option on the

PROC STATESPACE statement The OUT= data set contains the following variables

the BY variables

the ID variable

the VAR statement variables These variables contain the actual values from

the input data set

FORi numeric variables containing the forecasts The variable FORi contains

the forecasts for the ith variable in the VAR statement list Forecasts are one-

step-ahead predictions until the end of the data or until the observation specified

by the BACK= option

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Chapter 18 Details

RESi numeric variables containing the residual for the forecast of the ith vari-

able in the VAR statement list For forecast observations the actual values are

missing and the RESi variables contain missing values

STDi numeric variables containing the standard deviation for the forecast of

the ith variable in the VAR statement list The values of the STDi variables can

be used to construct univariate confidence limits for the corresponding fore-casts However such confidence limits do not take into account the covariance

of the forecasts

OUTAR= Data Set

The OUTAR= data set contains the estimates of the preliminary autoregressive mod-

els The OUTAR= data set contains the following variables

ORDER a numeric variable containing the order p of the autoregressive model

that the observation represents

AIC a numeric variable containing the value of the information criterionA I C

p

SIGFl numeric variables containing the estimate of the innovation covariance

matrices for the forward autoregressive models The variable SIGFl contains

the lth column of b

p

in the observations with ORDER= p

SIGBl numeric variables containing the estimate of the innovation covariance

matrices for the backward autoregressive models The variable SIGBl contains

the lth column of b

p

in the observations with ORDER= p

FORk ndashl numeric variables containing the estimates of the autoregressive pa-

rameter matrices for the forward models The variable FORk ndashl contains the

lth column of the lag k autoregressive parameter matrix b

p

k

in the observations

with ORDER= p

BACk ndashl numeric variables containing the estimates of the autoregressive pa-

rameter matrices for the backward models The variable BACk ndashl contains the

lth column of the lag k autoregressive parameter matrix b

p

k

in the observations

with ORDER= p

The estimates for the order p autoregressive model can be selected as those observa-

tions with ORDER= p Within these observations the k lth element of

p

i

is given by

the value of the FORindashl variable in the k th observation The k lth element of

p

i

is

given by the value of BACindashl variable in the k th observation The k lth element of

p

is given by SIGFl in the k th observation The k lth element of

p

is given by SIGBlin the k th observation

Table 181 shows an example of the OUTAR= data set with ARMAX=3 andx

t

of

dimension 2 In Table 181 i j

indicate the ijth element of the matrix

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Part 2 General Information

Table 181 Values in the OUTAR= Data Set

Obs ORDER AIC SIGF1 SIGF2 SIGB1 SIGB2 FOR1ndash 1 FOR1ndash 2 FOR2ndash 1 FOR2ndash 2 FOR3ndash 1

1 0 AIC0

0 1 1

0 1 2

0 1 1

0 1 2

2 0 AIC0

0 2 1

0 2 2

0 2 1

0 2 2

3 1 AIC1

1 1 1

1 1 2

1 1 1

1 1 2

1

1

1 1

1

1

1 2

4 1 AIC1

1 2 1

1 1 2

1 2 1

1 1 2

1

1

2 1

1

1

2 2

5 2 AIC2

2 1 1

2 1 2

2 1 1

2 1 2

2

1

1 1

2

1

1 2

2

2

1 1

2

2

1 2

6 2 AIC2

2 2 1

2 1 2

2 2 1

2 1 2

2

1

2 1

2

1

2 2

2

2

2 1

2

2

2 2

7 3 AIC3

3 1 1

3 1 2

3 1 1

3 1 2

3

1

1 1

3

1

1 2

3

2

1 1

3

2

1 2

3

3

1 1

8 3 AIC3

3 2 1

3 1 2

3 2 1

3 1 2

3

1

2 1

3

1

2 2

3

2

2 1

3

2

2 2

3

3

2 1

Obs FOR3ndash 2 BACK1ndash 1 BACK1ndash 2 BACK2ndash 1 BACK2ndash 2 BACK3ndash 1 BACK3ndash 2

1

2

3

1

1

1 1

1

1

1 2

4

1

1

2 1

1

1

2 2

5

2

1

1 1

2

1

1 2

2

2

1 1

2

2

1 2

6

2

1

2 1

2

1

2 2

2

2

2 1

2

2

2 2

7

3

3

1 2

3

1

1 1

3

1

1 2

3

2

1 1

3

2

1 2

3

3

1 1

3

3

1 2

8

3

3

2 2

3

1

2 1

3

1

2 2

3

2

2 1

3

2

2 2

3

3

2 1

3

3

2 2

The estimated autoregressive parameters can be used in the IML procedure to obtain

autoregressive estimates of the spectral density function or forecasts based on the

autoregressive models

OUTMODEL= Data Set

The OUTMODEL= data set contains the estimates of theF

andG

matrices and their

standard errors the names of the components of the state vector and the estimates of

the innovation covariance matrix The variables contained in the OUTMODEL= data

set are as follows

the BY variables

STATEVEC a character variable containing the name of the component of the

state vector corresponding to the observation The STATEVEC variable has

the value STD for standard deviations observations which contain the standarderrors for the estimates given in the preceding observation

Fndash j numeric variables containing the columns of theF

matrix The variable

Fndash j contains the jth column of F

The number of Fndash j variables is equal to

the value of the DIMMAX= option If the model is of smaller dimension the

extraneous variables are set to missing

Gndash j numeric variables containing the columns of theG

matrix The variable

Gndash j contains the jth column of G

The number of Gndash j variables is equal to r

the dimension of x

t

given by the number of variables in the VAR statement

SIGndash j numeric variables containing the columns of the innovation covariance

matrix The variable SIGndash j contains the jth column of

e e

There are r vari-ables SIGndash j

Table 182 shows an example of the OUTMODEL= data set withx

t

= x

t

y

t

0

z

t

= x

t

y

t

x

t + 1 j t

0 and DIMMAX=4 In Table 182F

i j

andG

i j

are the ijth

elements of F

andG

respectively Note that all elements for Fndash4 are missing because

F

is a3 3

matrix

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Chapter 18 Details

Table 182 Value in the OUTMODEL= Data Set

Obs STATEVEC Fndash 1 Fndash 2 Fndash 3 Fndash 4 Gndash 1 Gndash 2 SIGndash 1 SIGndash 2

1 X(TT) 0 0 1 1 0

1 1

1 2

2 STD

3 Y(TT) F2 1

F2 2

F2 3

0 1

2 1

2 2

4 STD std F2 1

std F2 2

std F2 3

5 X(T+1T) F3 1

F3 2

F3 3

G3 1

G3 2

6 STD std F3 1

std F3 2

std F3 3

std G3 1

std G3 2

Printed Output

The printed output produced by the STATESPACE procedure is described in the fol-

lowing

1 descriptive statistics which include the number of observations used the

names of the variables their means and standard deviations (Std) and the dif-

ferencing operations used

2 the Akaike information criteria for the sequence of preliminary autoregressive

models

3 if the PRINTOUT=LONG option is specified the sample autocovariance ma-

trices of the input series at various lags

4 if the PRINTOUT=LONG option is specified the sample autocorrelation ma-

trices of the input series

5 a schematic representation of the autocorrelation matrices showing the signif-

icant autocorrelations

6 if the PRINTOUT=LONG option is specified the partial autoregressive ma-

trices (These are

p

p

as described in Preliminary Autoregressive Modelsearlier in this chapter)

7 a schematic representation of the partial autocorrelation matrices showing the

significant partial autocorrelations

8 the Yule-Walker estimates of the autoregressive parameters for the autoregres-

sive model with the minimum AIC

9 if the PRINTOUT=LONG option is specified the autocovariance matrices of

the residuals of the minimum AIC model This is the sequence of estimated

innovation variance matrices for the solutions of the Yule-Walker equations

10 if the PRINTOUT=LONG option is specified the autocorrelation matrices of the residuals of the minimum AIC model

11 If the CANCORR option is specified the canonical correlations analysis for

each potential state vector considered in the state vector selection process

This includes the potential state vector the canonical correlations the informa-

tion criterion for the smallest canonical correlation Bartlettrsquos

2 statistic (ldquoChi

Squarerdquo) for the smallest canonical correlation and the degrees of freedom of

Bartlettrsquos

2

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Part 2 General Information

12 the components of the chosen state vector

13 the preliminary estimate of the transition matrixF

the input matrixG

and

the variance matrix for the innovations

e e

14 if the ITPRINT option is specified the iteration history of the likelihood max-

imization For each iteration this shows the iteration number the number of step halvings the determinant of the innovation variance matrix the damping

factor Lambda and the values of the parameters

15 the state vector printed again to aid interpretation of the following listing of F

andG

16 the final estimate of the transition matrixF

17 the final estimate of the input matrixG

18 the final estimate of the variance matrix for the innovations

e e

19 a table listing the estimates of the free parameters inF

andG

and their standarderrors and t statistics

20 if the COVB option is specified the covariance matrix of the parameter esti-

mates

21 if the COVB option is specified the correlation matrix of the parameter esti-

mates

22 if the PRINT option is specified the forecasts and their standard errors

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Chapter 18 Details

ODS Table Names

PROC STATESPACE assigns a name to each table it creates You can use these

names to reference the table when using the Output Delivery System (ODS) to select

tables and create output data sets These names are listed in the following table For

more information on ODS see Chapter 6 ldquoUsing the Output Delivery Systemrdquo

Table 183 ODS Tables Produced in PROC STATESPACE

ODS Table Name Description Option

NObs Number of observations default

Summary Simple summary statistics table default

InfoCriterion Information criterion table default

CovLags Covariance Matrices of Input Series PRINTOUT=LONG

CorrLags Correlation Matrices of Input Series PRINTOUT=LONG

PartialAR Partial Autoregressive Matrices PRINTOUT=LONG

YWEstimates Yule-Walker Estimates for Minimum AIC default

CovResiduals Covariance of Residuals PRINTOUT=LONGCorrResiduals Residual Correlations from AR Models PRINTOUT=LONG

StateVector State vector table default

CorrGraph Schematic Representation of Correlations default

TransitionMatrix Transition Matrix default

InputMatrix Input Matrix default

VarInnov Variance Matrix for the Innovation default

CovB Covariance of Parameter Estimates COVB

CorrB Correlation of Parameter Estimates COVB

CanCorr Canonical Correlation Analysis CANCORR

IterHistory Iterative Fitting table ITPRINT

ParameterEstimates Parameter Estimates Table default

Forecasts Forecasts Table PRINT

ConvergenceStatus Convergence Status Table default

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Part 2 General Information

Example

Example 181 Series J from Box and Jenkins

This example analyzes the gas furnace data (series J) from Box and Jenkins (The

data are not shown Refer to Box and Jenkins (1976) for the data)

First a model is selected and fit automatically using the following statements

title1 rsquoGas Furnace Datarsquo

title2 rsquoBox amp Jenkins Series Jrsquo

title3 rsquoAutomatically Selected Modelrsquo

proc statespace data=seriesj cancorr

var x y

run

The results for the automatically selected model are shown in Output 1811

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Chapter 18 Example

Output 1811 Results for Automatically Selected Model

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Number of Observations 296

Standard

Variable Mean Error

x -005683 1072766

y 5350912 3202121

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Information Criterion for Autoregressive Models

Lag=0 Lag=1 Lag=2 Lag=3 Lag=4 Lag=5 Lag=6 Lag=7 Lag=8

6513862 -103357 -163296 -164512 -165152 -164891 -164934 -164315 -163856

Information

Criterion for

Autoregressive

Models

Lag=9 Lag=10

-16348 -163359

Schematic Representation of Correlations

NameLag 0 1 2 3 4 5 6 7 8 9 10

x +- +- +- +- +- +- +- +- +- +- +-

y -+ -+ -+ -+ -+ -+ -+ -+ -+ -+ -+

+ is gt 2std error - is lt -2std error is between

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Part 2 General Information

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Schematic Representation of Partial Autocorrelations

NameLag 1 2 3 4 5 6 7 8 9 10

x + - + -

y -+ -- - + +

+ is gt 2std error - is lt -2std error is between

Yule-Walker Estimates for Minimum AIC

------Lag=1------ ------Lag=2------ ------Lag=3------ ------Lag=4------

x y x y x y x y

x 1925887 -000124 -120166 0004224 0116918 -000867 0104236 0003268

y 0050496 1299793 -002046 -03277 -071182 -025701 0195411 0133417

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Canonical Correlations Analysis

Information Chi

x(TT) y(TT) x(T+1T) Criterion Square DF

1 1 0804883 2929228 3047481 8

Information Chix(TT) y(TT) x(T+1T) y(T+1T) Criterion Square DF

1 1 0906681 0607529 1223358 1347237 7

Information Chi

x(TT) y(TT) x (T+1T) y (T+1T) x (T+2T) Criterion Square DF

1 1 0909434 0610278 0186274 -154701 1034705 6

Information Chi

x(TT) y(TT) x (T+1T) y (T+1T) y (T+2T) Criterion Square DF

1 1 091014 0618937 0206823 0940392 1280924 6

Information Chi

x(TT) y(TT) x(T+1T) y(T+1T) y(T+2T) y(T+3T) Criterion Square DF

1 1 0912963 0628785 0226598 0083258 -794103 2041584 5

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Chapter 18 Example

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Selected Statespace Form and Preliminary Estimates

State Vector

x(TT) y(TT) x(T+1T) y(T+1T) y(T+2T)

Estimate of Transition Matrix

0 0 1 0 0

0 0 0 1 0

-084718 0026794 1711715 -005019 0

0 0 0 0 1

-019785 0334274 -018174 -123557 1787475

Input Matrix for Innovation

1 0

0 11925887 -000124

0050496 1299793

0142421 1361696

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Selected Statespace Form and Preliminary Estimates

Variance Matrix for Innovation

0035274 -000734

-000734 0097569

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Part 2 General Information

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Selected Statespace Form and Fitted Model

State Vector

x(TT) y(TT) x(T+1T) y(T+1T) y(T+2T)

Estimate of Transition Matrix

0 0 1 0 0

0 0 0 1 0

-086192 0030609 1724235 -005483 0

0 0 0 0 1

-034839 0292124 -009435 -109823 1671418

Input Matrix for Innovation

1 0

0 1192442 -000416

0015621 1258495

008058 1353204

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Selected Statespace Form and Fitted Model

Variance Matrix for Innovation

0035579 -000728

-000728 0095577

Parameter Estimates

Standard

Parameter Estimate Error t Value

F(31) -086192 0072961 -1181

F(32) 0030609 0026167 117

F(33) 1724235 0061599 2799

F(34) -005483 0030169 -182

F(51) -034839 0135253 -258

F(52) 0292124 0046299 631

F(53) -009435 0096527 -098F(54) -109823 0109525 -1003

F(55) 1671418 0083737 1996

G(31) 1924420 0058162 3309

G(32) -000416 0035255 -012

G(41) 0015621 0095771 016

G(42) 1258495 0055742 2258

G(51) 0080580 0151622 053

G(52) 1353204 0091388 1481

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Chapter 18 Example

The two series are believed to have a transfer function relation with the gas rate

(variable X) as the input and the CO2

concentration (variable Y) as the output Since

the parameter estimates shown in Output 1811 support this kind of model the model

is reestimated with the feedback parameters restricted to 0 The following statements

fit the transfer function (no feedback) model

title3 rsquoTransfer Function Modelrsquo

proc statespace data=seriesj printout=none

var x y

restrict f(32)=0 f(34)=0

g(32)=0 g(41)=0 g(51)=0

run

The last 2 pages of the output are shown in Output 1812

Output 1812 STATESPACE Output for Transfer Function Model

Gas Furnace Data

Box amp Jenkins Series J

Transfer Function Model

The STATESPACE Procedure

Selected Statespace Form and Fitted Model

State Vector

x(TT) y(TT) x(T+1T) y(T+1T) y(T+2T)

Estimate of Transition Matrix

0 0 1 0 0

0 0 0 1 0

-068882 0 1598717 0 0

0 0 0 0 1

-035944 0284179 -00963 -107313 1650047

Input Matrix for Innovation

1 0

0 1

1923446 0

0 1260856

0 1346332

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Part 2 General Information

Gas Furnace Data

Box amp Jenkins Series J

Transfer Function Model

The STATESPACE Procedure

Selected Statespace Form and Fitted Model

Variance Matrix for Innovation

0036995 -00072

-00072 0095712

Parameter Estimates

Standard

Parameter Estimate Error t Value

F(31) -068882 0050549 -1363

F(33) 1598717 0050924 3139

F(51) -035944 0229044 -157

F(52) 0284179 0096944 293

F(53) -009630 0140876 -068

F(54) -107313 0250385 -429

F(55) 1650047 0188533 875G(31) 1923446 0056328 3415

G(42) 1260856 0056464 2233

G(52) 1346332 0091086 1478

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Chapter 18 References

References

Akaike H (1974) Markovian Representation of Stochastic Processes and Its Ap-

plication to the Analysis of Autoregressive Moving Average Processes Annals

of the Institute of Statistical Mathematics 26 363-387

Akaike H (1976) Canonical Correlations Analysis of Time Series and the Use of

an Information Criterion in Advances and Case Studies in System Identification

eds R Mehra and DG Lainiotis New York Academic Press

Anderson TW (1971) The Statistical Analysis of Time Series New York John

Wiley amp Sons

Ansley CF and Newbold P (1979) Multivariate Partial Autocorrelations Pro-

ceedings of the Business and Economic Statistics Section American Statistical

Association 349-353

Box GEP and Jenkins G (1976) Time Series Analysis Forecasting and Control

San Francisco Holden-Day

Brockwell PJ and Davis RA (1991) Time Series Theory and Methods 2nd Edi-

tion Springer-Verlag

Hannan EJ (1970) Multiple Time Series New York John Wiley amp Sons

Hannan EJ (1976) The Identification and Parameterization of ARMAX and State

Space Forms Econometrica 44 713-722

Harvey AC (1981a) The Econometric Analysis of Time Series New York John

Wiley amp Sons

Harvey AC (1981b) Time Series Models New York John Wiley amp Sons

Jones RH (1974) Identification and Autoregressive Spectrum Estimation IEEE Transactions on Automatic Control AC-19 894-897

Pham-Dinh-Tuan (1978) On the Fitting of Multivariate Processes of the

Autoregressive-Moving Average Type Biometrika 65 99-107

Priestley MB (1980) System Identification Kalman Filtering and Stochastic Con-

trol in Directions in Time Series eds DR Brillinger and GC Tiao Institute

of Mathematical Statistics

Whittle P (1963) On the Fitting of Multivariate Autoregressions and the Approxi-

mate Canonical Factorization of a Spectral Density Matrix Biometrika 50 129-

134

1047SAS OnlineDoc 991522 Version 8

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The correct bibliographic citation for this manual is as follows SAS Institute Inc SAS

ET S U serrsquos Gui de Version 8 Cary NC SAS Institute Inc 1999 1546 pp

SASET S Userrsquos Guide Version 8

Copyright copy 1999 by S AS I ns t it u t e I nc Ca ry N C U S A

ISBN 1ndash58025ndash489ndash6

All r ight s res erved P r int ed in t he U n it ed St a t es of A merica N o pa rt of t his p u blica t ionma y b e reprodu ced s t ored in a ret r ieva l s ys t em or t ra ns mit t ed in a ny form or b y a nymeans electronic mechanical photocopying or otherwise without the prior writtenpermission of the publisher SAS Institute Inc

US Government Restricted Rights Notice Use du plicat ion or disclosure of the

software by the government is subject to restrictions as set forth in FAR 52227ndash19Commercial Computer Softw ar e-Restricted Rights (J une 1987)

SAS Institute Inc SAS Campus Drive Cary North Carolina 27513

1st print ing October 1999

S AS reg a nd a ll ot her SA S I ns t it u t e I nc p rodu ct or s ervice na m es a re regis t ered t ra dema rkst d k f SA S I t it t I i t h U S A d t h t i reg i di t U SA

Page 39: Statespace Model

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Chapter 18 Details

RESi numeric variables containing the residual for the forecast of the ith vari-

able in the VAR statement list For forecast observations the actual values are

missing and the RESi variables contain missing values

STDi numeric variables containing the standard deviation for the forecast of

the ith variable in the VAR statement list The values of the STDi variables can

be used to construct univariate confidence limits for the corresponding fore-casts However such confidence limits do not take into account the covariance

of the forecasts

OUTAR= Data Set

The OUTAR= data set contains the estimates of the preliminary autoregressive mod-

els The OUTAR= data set contains the following variables

ORDER a numeric variable containing the order p of the autoregressive model

that the observation represents

AIC a numeric variable containing the value of the information criterionA I C

p

SIGFl numeric variables containing the estimate of the innovation covariance

matrices for the forward autoregressive models The variable SIGFl contains

the lth column of b

p

in the observations with ORDER= p

SIGBl numeric variables containing the estimate of the innovation covariance

matrices for the backward autoregressive models The variable SIGBl contains

the lth column of b

p

in the observations with ORDER= p

FORk ndashl numeric variables containing the estimates of the autoregressive pa-

rameter matrices for the forward models The variable FORk ndashl contains the

lth column of the lag k autoregressive parameter matrix b

p

k

in the observations

with ORDER= p

BACk ndashl numeric variables containing the estimates of the autoregressive pa-

rameter matrices for the backward models The variable BACk ndashl contains the

lth column of the lag k autoregressive parameter matrix b

p

k

in the observations

with ORDER= p

The estimates for the order p autoregressive model can be selected as those observa-

tions with ORDER= p Within these observations the k lth element of

p

i

is given by

the value of the FORindashl variable in the k th observation The k lth element of

p

i

is

given by the value of BACindashl variable in the k th observation The k lth element of

p

is given by SIGFl in the k th observation The k lth element of

p

is given by SIGBlin the k th observation

Table 181 shows an example of the OUTAR= data set with ARMAX=3 andx

t

of

dimension 2 In Table 181 i j

indicate the ijth element of the matrix

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Part 2 General Information

Table 181 Values in the OUTAR= Data Set

Obs ORDER AIC SIGF1 SIGF2 SIGB1 SIGB2 FOR1ndash 1 FOR1ndash 2 FOR2ndash 1 FOR2ndash 2 FOR3ndash 1

1 0 AIC0

0 1 1

0 1 2

0 1 1

0 1 2

2 0 AIC0

0 2 1

0 2 2

0 2 1

0 2 2

3 1 AIC1

1 1 1

1 1 2

1 1 1

1 1 2

1

1

1 1

1

1

1 2

4 1 AIC1

1 2 1

1 1 2

1 2 1

1 1 2

1

1

2 1

1

1

2 2

5 2 AIC2

2 1 1

2 1 2

2 1 1

2 1 2

2

1

1 1

2

1

1 2

2

2

1 1

2

2

1 2

6 2 AIC2

2 2 1

2 1 2

2 2 1

2 1 2

2

1

2 1

2

1

2 2

2

2

2 1

2

2

2 2

7 3 AIC3

3 1 1

3 1 2

3 1 1

3 1 2

3

1

1 1

3

1

1 2

3

2

1 1

3

2

1 2

3

3

1 1

8 3 AIC3

3 2 1

3 1 2

3 2 1

3 1 2

3

1

2 1

3

1

2 2

3

2

2 1

3

2

2 2

3

3

2 1

Obs FOR3ndash 2 BACK1ndash 1 BACK1ndash 2 BACK2ndash 1 BACK2ndash 2 BACK3ndash 1 BACK3ndash 2

1

2

3

1

1

1 1

1

1

1 2

4

1

1

2 1

1

1

2 2

5

2

1

1 1

2

1

1 2

2

2

1 1

2

2

1 2

6

2

1

2 1

2

1

2 2

2

2

2 1

2

2

2 2

7

3

3

1 2

3

1

1 1

3

1

1 2

3

2

1 1

3

2

1 2

3

3

1 1

3

3

1 2

8

3

3

2 2

3

1

2 1

3

1

2 2

3

2

2 1

3

2

2 2

3

3

2 1

3

3

2 2

The estimated autoregressive parameters can be used in the IML procedure to obtain

autoregressive estimates of the spectral density function or forecasts based on the

autoregressive models

OUTMODEL= Data Set

The OUTMODEL= data set contains the estimates of theF

andG

matrices and their

standard errors the names of the components of the state vector and the estimates of

the innovation covariance matrix The variables contained in the OUTMODEL= data

set are as follows

the BY variables

STATEVEC a character variable containing the name of the component of the

state vector corresponding to the observation The STATEVEC variable has

the value STD for standard deviations observations which contain the standarderrors for the estimates given in the preceding observation

Fndash j numeric variables containing the columns of theF

matrix The variable

Fndash j contains the jth column of F

The number of Fndash j variables is equal to

the value of the DIMMAX= option If the model is of smaller dimension the

extraneous variables are set to missing

Gndash j numeric variables containing the columns of theG

matrix The variable

Gndash j contains the jth column of G

The number of Gndash j variables is equal to r

the dimension of x

t

given by the number of variables in the VAR statement

SIGndash j numeric variables containing the columns of the innovation covariance

matrix The variable SIGndash j contains the jth column of

e e

There are r vari-ables SIGndash j

Table 182 shows an example of the OUTMODEL= data set withx

t

= x

t

y

t

0

z

t

= x

t

y

t

x

t + 1 j t

0 and DIMMAX=4 In Table 182F

i j

andG

i j

are the ijth

elements of F

andG

respectively Note that all elements for Fndash4 are missing because

F

is a3 3

matrix

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Chapter 18 Details

Table 182 Value in the OUTMODEL= Data Set

Obs STATEVEC Fndash 1 Fndash 2 Fndash 3 Fndash 4 Gndash 1 Gndash 2 SIGndash 1 SIGndash 2

1 X(TT) 0 0 1 1 0

1 1

1 2

2 STD

3 Y(TT) F2 1

F2 2

F2 3

0 1

2 1

2 2

4 STD std F2 1

std F2 2

std F2 3

5 X(T+1T) F3 1

F3 2

F3 3

G3 1

G3 2

6 STD std F3 1

std F3 2

std F3 3

std G3 1

std G3 2

Printed Output

The printed output produced by the STATESPACE procedure is described in the fol-

lowing

1 descriptive statistics which include the number of observations used the

names of the variables their means and standard deviations (Std) and the dif-

ferencing operations used

2 the Akaike information criteria for the sequence of preliminary autoregressive

models

3 if the PRINTOUT=LONG option is specified the sample autocovariance ma-

trices of the input series at various lags

4 if the PRINTOUT=LONG option is specified the sample autocorrelation ma-

trices of the input series

5 a schematic representation of the autocorrelation matrices showing the signif-

icant autocorrelations

6 if the PRINTOUT=LONG option is specified the partial autoregressive ma-

trices (These are

p

p

as described in Preliminary Autoregressive Modelsearlier in this chapter)

7 a schematic representation of the partial autocorrelation matrices showing the

significant partial autocorrelations

8 the Yule-Walker estimates of the autoregressive parameters for the autoregres-

sive model with the minimum AIC

9 if the PRINTOUT=LONG option is specified the autocovariance matrices of

the residuals of the minimum AIC model This is the sequence of estimated

innovation variance matrices for the solutions of the Yule-Walker equations

10 if the PRINTOUT=LONG option is specified the autocorrelation matrices of the residuals of the minimum AIC model

11 If the CANCORR option is specified the canonical correlations analysis for

each potential state vector considered in the state vector selection process

This includes the potential state vector the canonical correlations the informa-

tion criterion for the smallest canonical correlation Bartlettrsquos

2 statistic (ldquoChi

Squarerdquo) for the smallest canonical correlation and the degrees of freedom of

Bartlettrsquos

2

1037SAS OnlineDoc 991522 Version 8

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Part 2 General Information

12 the components of the chosen state vector

13 the preliminary estimate of the transition matrixF

the input matrixG

and

the variance matrix for the innovations

e e

14 if the ITPRINT option is specified the iteration history of the likelihood max-

imization For each iteration this shows the iteration number the number of step halvings the determinant of the innovation variance matrix the damping

factor Lambda and the values of the parameters

15 the state vector printed again to aid interpretation of the following listing of F

andG

16 the final estimate of the transition matrixF

17 the final estimate of the input matrixG

18 the final estimate of the variance matrix for the innovations

e e

19 a table listing the estimates of the free parameters inF

andG

and their standarderrors and t statistics

20 if the COVB option is specified the covariance matrix of the parameter esti-

mates

21 if the COVB option is specified the correlation matrix of the parameter esti-

mates

22 if the PRINT option is specified the forecasts and their standard errors

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Chapter 18 Details

ODS Table Names

PROC STATESPACE assigns a name to each table it creates You can use these

names to reference the table when using the Output Delivery System (ODS) to select

tables and create output data sets These names are listed in the following table For

more information on ODS see Chapter 6 ldquoUsing the Output Delivery Systemrdquo

Table 183 ODS Tables Produced in PROC STATESPACE

ODS Table Name Description Option

NObs Number of observations default

Summary Simple summary statistics table default

InfoCriterion Information criterion table default

CovLags Covariance Matrices of Input Series PRINTOUT=LONG

CorrLags Correlation Matrices of Input Series PRINTOUT=LONG

PartialAR Partial Autoregressive Matrices PRINTOUT=LONG

YWEstimates Yule-Walker Estimates for Minimum AIC default

CovResiduals Covariance of Residuals PRINTOUT=LONGCorrResiduals Residual Correlations from AR Models PRINTOUT=LONG

StateVector State vector table default

CorrGraph Schematic Representation of Correlations default

TransitionMatrix Transition Matrix default

InputMatrix Input Matrix default

VarInnov Variance Matrix for the Innovation default

CovB Covariance of Parameter Estimates COVB

CorrB Correlation of Parameter Estimates COVB

CanCorr Canonical Correlation Analysis CANCORR

IterHistory Iterative Fitting table ITPRINT

ParameterEstimates Parameter Estimates Table default

Forecasts Forecasts Table PRINT

ConvergenceStatus Convergence Status Table default

1039SAS OnlineDoc 991522 Version 8

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Part 2 General Information

Example

Example 181 Series J from Box and Jenkins

This example analyzes the gas furnace data (series J) from Box and Jenkins (The

data are not shown Refer to Box and Jenkins (1976) for the data)

First a model is selected and fit automatically using the following statements

title1 rsquoGas Furnace Datarsquo

title2 rsquoBox amp Jenkins Series Jrsquo

title3 rsquoAutomatically Selected Modelrsquo

proc statespace data=seriesj cancorr

var x y

run

The results for the automatically selected model are shown in Output 1811

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Chapter 18 Example

Output 1811 Results for Automatically Selected Model

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Number of Observations 296

Standard

Variable Mean Error

x -005683 1072766

y 5350912 3202121

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Information Criterion for Autoregressive Models

Lag=0 Lag=1 Lag=2 Lag=3 Lag=4 Lag=5 Lag=6 Lag=7 Lag=8

6513862 -103357 -163296 -164512 -165152 -164891 -164934 -164315 -163856

Information

Criterion for

Autoregressive

Models

Lag=9 Lag=10

-16348 -163359

Schematic Representation of Correlations

NameLag 0 1 2 3 4 5 6 7 8 9 10

x +- +- +- +- +- +- +- +- +- +- +-

y -+ -+ -+ -+ -+ -+ -+ -+ -+ -+ -+

+ is gt 2std error - is lt -2std error is between

1041SAS OnlineDoc 991522 Version 8

8132019 Statespace Model

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Part 2 General Information

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Schematic Representation of Partial Autocorrelations

NameLag 1 2 3 4 5 6 7 8 9 10

x + - + -

y -+ -- - + +

+ is gt 2std error - is lt -2std error is between

Yule-Walker Estimates for Minimum AIC

------Lag=1------ ------Lag=2------ ------Lag=3------ ------Lag=4------

x y x y x y x y

x 1925887 -000124 -120166 0004224 0116918 -000867 0104236 0003268

y 0050496 1299793 -002046 -03277 -071182 -025701 0195411 0133417

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Canonical Correlations Analysis

Information Chi

x(TT) y(TT) x(T+1T) Criterion Square DF

1 1 0804883 2929228 3047481 8

Information Chix(TT) y(TT) x(T+1T) y(T+1T) Criterion Square DF

1 1 0906681 0607529 1223358 1347237 7

Information Chi

x(TT) y(TT) x (T+1T) y (T+1T) x (T+2T) Criterion Square DF

1 1 0909434 0610278 0186274 -154701 1034705 6

Information Chi

x(TT) y(TT) x (T+1T) y (T+1T) y (T+2T) Criterion Square DF

1 1 091014 0618937 0206823 0940392 1280924 6

Information Chi

x(TT) y(TT) x(T+1T) y(T+1T) y(T+2T) y(T+3T) Criterion Square DF

1 1 0912963 0628785 0226598 0083258 -794103 2041584 5

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Chapter 18 Example

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Selected Statespace Form and Preliminary Estimates

State Vector

x(TT) y(TT) x(T+1T) y(T+1T) y(T+2T)

Estimate of Transition Matrix

0 0 1 0 0

0 0 0 1 0

-084718 0026794 1711715 -005019 0

0 0 0 0 1

-019785 0334274 -018174 -123557 1787475

Input Matrix for Innovation

1 0

0 11925887 -000124

0050496 1299793

0142421 1361696

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Selected Statespace Form and Preliminary Estimates

Variance Matrix for Innovation

0035274 -000734

-000734 0097569

1043SAS OnlineDoc 991522 Version 8

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Part 2 General Information

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Selected Statespace Form and Fitted Model

State Vector

x(TT) y(TT) x(T+1T) y(T+1T) y(T+2T)

Estimate of Transition Matrix

0 0 1 0 0

0 0 0 1 0

-086192 0030609 1724235 -005483 0

0 0 0 0 1

-034839 0292124 -009435 -109823 1671418

Input Matrix for Innovation

1 0

0 1192442 -000416

0015621 1258495

008058 1353204

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Selected Statespace Form and Fitted Model

Variance Matrix for Innovation

0035579 -000728

-000728 0095577

Parameter Estimates

Standard

Parameter Estimate Error t Value

F(31) -086192 0072961 -1181

F(32) 0030609 0026167 117

F(33) 1724235 0061599 2799

F(34) -005483 0030169 -182

F(51) -034839 0135253 -258

F(52) 0292124 0046299 631

F(53) -009435 0096527 -098F(54) -109823 0109525 -1003

F(55) 1671418 0083737 1996

G(31) 1924420 0058162 3309

G(32) -000416 0035255 -012

G(41) 0015621 0095771 016

G(42) 1258495 0055742 2258

G(51) 0080580 0151622 053

G(52) 1353204 0091388 1481

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Chapter 18 Example

The two series are believed to have a transfer function relation with the gas rate

(variable X) as the input and the CO2

concentration (variable Y) as the output Since

the parameter estimates shown in Output 1811 support this kind of model the model

is reestimated with the feedback parameters restricted to 0 The following statements

fit the transfer function (no feedback) model

title3 rsquoTransfer Function Modelrsquo

proc statespace data=seriesj printout=none

var x y

restrict f(32)=0 f(34)=0

g(32)=0 g(41)=0 g(51)=0

run

The last 2 pages of the output are shown in Output 1812

Output 1812 STATESPACE Output for Transfer Function Model

Gas Furnace Data

Box amp Jenkins Series J

Transfer Function Model

The STATESPACE Procedure

Selected Statespace Form and Fitted Model

State Vector

x(TT) y(TT) x(T+1T) y(T+1T) y(T+2T)

Estimate of Transition Matrix

0 0 1 0 0

0 0 0 1 0

-068882 0 1598717 0 0

0 0 0 0 1

-035944 0284179 -00963 -107313 1650047

Input Matrix for Innovation

1 0

0 1

1923446 0

0 1260856

0 1346332

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Part 2 General Information

Gas Furnace Data

Box amp Jenkins Series J

Transfer Function Model

The STATESPACE Procedure

Selected Statespace Form and Fitted Model

Variance Matrix for Innovation

0036995 -00072

-00072 0095712

Parameter Estimates

Standard

Parameter Estimate Error t Value

F(31) -068882 0050549 -1363

F(33) 1598717 0050924 3139

F(51) -035944 0229044 -157

F(52) 0284179 0096944 293

F(53) -009630 0140876 -068

F(54) -107313 0250385 -429

F(55) 1650047 0188533 875G(31) 1923446 0056328 3415

G(42) 1260856 0056464 2233

G(52) 1346332 0091086 1478

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Chapter 18 References

References

Akaike H (1974) Markovian Representation of Stochastic Processes and Its Ap-

plication to the Analysis of Autoregressive Moving Average Processes Annals

of the Institute of Statistical Mathematics 26 363-387

Akaike H (1976) Canonical Correlations Analysis of Time Series and the Use of

an Information Criterion in Advances and Case Studies in System Identification

eds R Mehra and DG Lainiotis New York Academic Press

Anderson TW (1971) The Statistical Analysis of Time Series New York John

Wiley amp Sons

Ansley CF and Newbold P (1979) Multivariate Partial Autocorrelations Pro-

ceedings of the Business and Economic Statistics Section American Statistical

Association 349-353

Box GEP and Jenkins G (1976) Time Series Analysis Forecasting and Control

San Francisco Holden-Day

Brockwell PJ and Davis RA (1991) Time Series Theory and Methods 2nd Edi-

tion Springer-Verlag

Hannan EJ (1970) Multiple Time Series New York John Wiley amp Sons

Hannan EJ (1976) The Identification and Parameterization of ARMAX and State

Space Forms Econometrica 44 713-722

Harvey AC (1981a) The Econometric Analysis of Time Series New York John

Wiley amp Sons

Harvey AC (1981b) Time Series Models New York John Wiley amp Sons

Jones RH (1974) Identification and Autoregressive Spectrum Estimation IEEE Transactions on Automatic Control AC-19 894-897

Pham-Dinh-Tuan (1978) On the Fitting of Multivariate Processes of the

Autoregressive-Moving Average Type Biometrika 65 99-107

Priestley MB (1980) System Identification Kalman Filtering and Stochastic Con-

trol in Directions in Time Series eds DR Brillinger and GC Tiao Institute

of Mathematical Statistics

Whittle P (1963) On the Fitting of Multivariate Autoregressions and the Approxi-

mate Canonical Factorization of a Spectral Density Matrix Biometrika 50 129-

134

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The correct bibliographic citation for this manual is as follows SAS Institute Inc SAS

ET S U serrsquos Gui de Version 8 Cary NC SAS Institute Inc 1999 1546 pp

SASET S Userrsquos Guide Version 8

Copyright copy 1999 by S AS I ns t it u t e I nc Ca ry N C U S A

ISBN 1ndash58025ndash489ndash6

All r ight s res erved P r int ed in t he U n it ed St a t es of A merica N o pa rt of t his p u blica t ionma y b e reprodu ced s t ored in a ret r ieva l s ys t em or t ra ns mit t ed in a ny form or b y a nymeans electronic mechanical photocopying or otherwise without the prior writtenpermission of the publisher SAS Institute Inc

US Government Restricted Rights Notice Use du plicat ion or disclosure of the

software by the government is subject to restrictions as set forth in FAR 52227ndash19Commercial Computer Softw ar e-Restricted Rights (J une 1987)

SAS Institute Inc SAS Campus Drive Cary North Carolina 27513

1st print ing October 1999

S AS reg a nd a ll ot her SA S I ns t it u t e I nc p rodu ct or s ervice na m es a re regis t ered t ra dema rkst d k f SA S I t it t I i t h U S A d t h t i reg i di t U SA

Page 40: Statespace Model

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Part 2 General Information

Table 181 Values in the OUTAR= Data Set

Obs ORDER AIC SIGF1 SIGF2 SIGB1 SIGB2 FOR1ndash 1 FOR1ndash 2 FOR2ndash 1 FOR2ndash 2 FOR3ndash 1

1 0 AIC0

0 1 1

0 1 2

0 1 1

0 1 2

2 0 AIC0

0 2 1

0 2 2

0 2 1

0 2 2

3 1 AIC1

1 1 1

1 1 2

1 1 1

1 1 2

1

1

1 1

1

1

1 2

4 1 AIC1

1 2 1

1 1 2

1 2 1

1 1 2

1

1

2 1

1

1

2 2

5 2 AIC2

2 1 1

2 1 2

2 1 1

2 1 2

2

1

1 1

2

1

1 2

2

2

1 1

2

2

1 2

6 2 AIC2

2 2 1

2 1 2

2 2 1

2 1 2

2

1

2 1

2

1

2 2

2

2

2 1

2

2

2 2

7 3 AIC3

3 1 1

3 1 2

3 1 1

3 1 2

3

1

1 1

3

1

1 2

3

2

1 1

3

2

1 2

3

3

1 1

8 3 AIC3

3 2 1

3 1 2

3 2 1

3 1 2

3

1

2 1

3

1

2 2

3

2

2 1

3

2

2 2

3

3

2 1

Obs FOR3ndash 2 BACK1ndash 1 BACK1ndash 2 BACK2ndash 1 BACK2ndash 2 BACK3ndash 1 BACK3ndash 2

1

2

3

1

1

1 1

1

1

1 2

4

1

1

2 1

1

1

2 2

5

2

1

1 1

2

1

1 2

2

2

1 1

2

2

1 2

6

2

1

2 1

2

1

2 2

2

2

2 1

2

2

2 2

7

3

3

1 2

3

1

1 1

3

1

1 2

3

2

1 1

3

2

1 2

3

3

1 1

3

3

1 2

8

3

3

2 2

3

1

2 1

3

1

2 2

3

2

2 1

3

2

2 2

3

3

2 1

3

3

2 2

The estimated autoregressive parameters can be used in the IML procedure to obtain

autoregressive estimates of the spectral density function or forecasts based on the

autoregressive models

OUTMODEL= Data Set

The OUTMODEL= data set contains the estimates of theF

andG

matrices and their

standard errors the names of the components of the state vector and the estimates of

the innovation covariance matrix The variables contained in the OUTMODEL= data

set are as follows

the BY variables

STATEVEC a character variable containing the name of the component of the

state vector corresponding to the observation The STATEVEC variable has

the value STD for standard deviations observations which contain the standarderrors for the estimates given in the preceding observation

Fndash j numeric variables containing the columns of theF

matrix The variable

Fndash j contains the jth column of F

The number of Fndash j variables is equal to

the value of the DIMMAX= option If the model is of smaller dimension the

extraneous variables are set to missing

Gndash j numeric variables containing the columns of theG

matrix The variable

Gndash j contains the jth column of G

The number of Gndash j variables is equal to r

the dimension of x

t

given by the number of variables in the VAR statement

SIGndash j numeric variables containing the columns of the innovation covariance

matrix The variable SIGndash j contains the jth column of

e e

There are r vari-ables SIGndash j

Table 182 shows an example of the OUTMODEL= data set withx

t

= x

t

y

t

0

z

t

= x

t

y

t

x

t + 1 j t

0 and DIMMAX=4 In Table 182F

i j

andG

i j

are the ijth

elements of F

andG

respectively Note that all elements for Fndash4 are missing because

F

is a3 3

matrix

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Chapter 18 Details

Table 182 Value in the OUTMODEL= Data Set

Obs STATEVEC Fndash 1 Fndash 2 Fndash 3 Fndash 4 Gndash 1 Gndash 2 SIGndash 1 SIGndash 2

1 X(TT) 0 0 1 1 0

1 1

1 2

2 STD

3 Y(TT) F2 1

F2 2

F2 3

0 1

2 1

2 2

4 STD std F2 1

std F2 2

std F2 3

5 X(T+1T) F3 1

F3 2

F3 3

G3 1

G3 2

6 STD std F3 1

std F3 2

std F3 3

std G3 1

std G3 2

Printed Output

The printed output produced by the STATESPACE procedure is described in the fol-

lowing

1 descriptive statistics which include the number of observations used the

names of the variables their means and standard deviations (Std) and the dif-

ferencing operations used

2 the Akaike information criteria for the sequence of preliminary autoregressive

models

3 if the PRINTOUT=LONG option is specified the sample autocovariance ma-

trices of the input series at various lags

4 if the PRINTOUT=LONG option is specified the sample autocorrelation ma-

trices of the input series

5 a schematic representation of the autocorrelation matrices showing the signif-

icant autocorrelations

6 if the PRINTOUT=LONG option is specified the partial autoregressive ma-

trices (These are

p

p

as described in Preliminary Autoregressive Modelsearlier in this chapter)

7 a schematic representation of the partial autocorrelation matrices showing the

significant partial autocorrelations

8 the Yule-Walker estimates of the autoregressive parameters for the autoregres-

sive model with the minimum AIC

9 if the PRINTOUT=LONG option is specified the autocovariance matrices of

the residuals of the minimum AIC model This is the sequence of estimated

innovation variance matrices for the solutions of the Yule-Walker equations

10 if the PRINTOUT=LONG option is specified the autocorrelation matrices of the residuals of the minimum AIC model

11 If the CANCORR option is specified the canonical correlations analysis for

each potential state vector considered in the state vector selection process

This includes the potential state vector the canonical correlations the informa-

tion criterion for the smallest canonical correlation Bartlettrsquos

2 statistic (ldquoChi

Squarerdquo) for the smallest canonical correlation and the degrees of freedom of

Bartlettrsquos

2

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Part 2 General Information

12 the components of the chosen state vector

13 the preliminary estimate of the transition matrixF

the input matrixG

and

the variance matrix for the innovations

e e

14 if the ITPRINT option is specified the iteration history of the likelihood max-

imization For each iteration this shows the iteration number the number of step halvings the determinant of the innovation variance matrix the damping

factor Lambda and the values of the parameters

15 the state vector printed again to aid interpretation of the following listing of F

andG

16 the final estimate of the transition matrixF

17 the final estimate of the input matrixG

18 the final estimate of the variance matrix for the innovations

e e

19 a table listing the estimates of the free parameters inF

andG

and their standarderrors and t statistics

20 if the COVB option is specified the covariance matrix of the parameter esti-

mates

21 if the COVB option is specified the correlation matrix of the parameter esti-

mates

22 if the PRINT option is specified the forecasts and their standard errors

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Chapter 18 Details

ODS Table Names

PROC STATESPACE assigns a name to each table it creates You can use these

names to reference the table when using the Output Delivery System (ODS) to select

tables and create output data sets These names are listed in the following table For

more information on ODS see Chapter 6 ldquoUsing the Output Delivery Systemrdquo

Table 183 ODS Tables Produced in PROC STATESPACE

ODS Table Name Description Option

NObs Number of observations default

Summary Simple summary statistics table default

InfoCriterion Information criterion table default

CovLags Covariance Matrices of Input Series PRINTOUT=LONG

CorrLags Correlation Matrices of Input Series PRINTOUT=LONG

PartialAR Partial Autoregressive Matrices PRINTOUT=LONG

YWEstimates Yule-Walker Estimates for Minimum AIC default

CovResiduals Covariance of Residuals PRINTOUT=LONGCorrResiduals Residual Correlations from AR Models PRINTOUT=LONG

StateVector State vector table default

CorrGraph Schematic Representation of Correlations default

TransitionMatrix Transition Matrix default

InputMatrix Input Matrix default

VarInnov Variance Matrix for the Innovation default

CovB Covariance of Parameter Estimates COVB

CorrB Correlation of Parameter Estimates COVB

CanCorr Canonical Correlation Analysis CANCORR

IterHistory Iterative Fitting table ITPRINT

ParameterEstimates Parameter Estimates Table default

Forecasts Forecasts Table PRINT

ConvergenceStatus Convergence Status Table default

1039SAS OnlineDoc 991522 Version 8

8132019 Statespace Model

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Part 2 General Information

Example

Example 181 Series J from Box and Jenkins

This example analyzes the gas furnace data (series J) from Box and Jenkins (The

data are not shown Refer to Box and Jenkins (1976) for the data)

First a model is selected and fit automatically using the following statements

title1 rsquoGas Furnace Datarsquo

title2 rsquoBox amp Jenkins Series Jrsquo

title3 rsquoAutomatically Selected Modelrsquo

proc statespace data=seriesj cancorr

var x y

run

The results for the automatically selected model are shown in Output 1811

SAS OnlineDoc 991522 Version 8 1040

8132019 Statespace Model

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Chapter 18 Example

Output 1811 Results for Automatically Selected Model

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Number of Observations 296

Standard

Variable Mean Error

x -005683 1072766

y 5350912 3202121

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Information Criterion for Autoregressive Models

Lag=0 Lag=1 Lag=2 Lag=3 Lag=4 Lag=5 Lag=6 Lag=7 Lag=8

6513862 -103357 -163296 -164512 -165152 -164891 -164934 -164315 -163856

Information

Criterion for

Autoregressive

Models

Lag=9 Lag=10

-16348 -163359

Schematic Representation of Correlations

NameLag 0 1 2 3 4 5 6 7 8 9 10

x +- +- +- +- +- +- +- +- +- +- +-

y -+ -+ -+ -+ -+ -+ -+ -+ -+ -+ -+

+ is gt 2std error - is lt -2std error is between

1041SAS OnlineDoc 991522 Version 8

8132019 Statespace Model

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Part 2 General Information

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Schematic Representation of Partial Autocorrelations

NameLag 1 2 3 4 5 6 7 8 9 10

x + - + -

y -+ -- - + +

+ is gt 2std error - is lt -2std error is between

Yule-Walker Estimates for Minimum AIC

------Lag=1------ ------Lag=2------ ------Lag=3------ ------Lag=4------

x y x y x y x y

x 1925887 -000124 -120166 0004224 0116918 -000867 0104236 0003268

y 0050496 1299793 -002046 -03277 -071182 -025701 0195411 0133417

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Canonical Correlations Analysis

Information Chi

x(TT) y(TT) x(T+1T) Criterion Square DF

1 1 0804883 2929228 3047481 8

Information Chix(TT) y(TT) x(T+1T) y(T+1T) Criterion Square DF

1 1 0906681 0607529 1223358 1347237 7

Information Chi

x(TT) y(TT) x (T+1T) y (T+1T) x (T+2T) Criterion Square DF

1 1 0909434 0610278 0186274 -154701 1034705 6

Information Chi

x(TT) y(TT) x (T+1T) y (T+1T) y (T+2T) Criterion Square DF

1 1 091014 0618937 0206823 0940392 1280924 6

Information Chi

x(TT) y(TT) x(T+1T) y(T+1T) y(T+2T) y(T+3T) Criterion Square DF

1 1 0912963 0628785 0226598 0083258 -794103 2041584 5

SAS OnlineDoc 991522 Version 8 1042

8132019 Statespace Model

httpslidepdfcomreaderfullstatespace-model 4752

Chapter 18 Example

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Selected Statespace Form and Preliminary Estimates

State Vector

x(TT) y(TT) x(T+1T) y(T+1T) y(T+2T)

Estimate of Transition Matrix

0 0 1 0 0

0 0 0 1 0

-084718 0026794 1711715 -005019 0

0 0 0 0 1

-019785 0334274 -018174 -123557 1787475

Input Matrix for Innovation

1 0

0 11925887 -000124

0050496 1299793

0142421 1361696

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Selected Statespace Form and Preliminary Estimates

Variance Matrix for Innovation

0035274 -000734

-000734 0097569

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8132019 Statespace Model

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Part 2 General Information

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Selected Statespace Form and Fitted Model

State Vector

x(TT) y(TT) x(T+1T) y(T+1T) y(T+2T)

Estimate of Transition Matrix

0 0 1 0 0

0 0 0 1 0

-086192 0030609 1724235 -005483 0

0 0 0 0 1

-034839 0292124 -009435 -109823 1671418

Input Matrix for Innovation

1 0

0 1192442 -000416

0015621 1258495

008058 1353204

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Selected Statespace Form and Fitted Model

Variance Matrix for Innovation

0035579 -000728

-000728 0095577

Parameter Estimates

Standard

Parameter Estimate Error t Value

F(31) -086192 0072961 -1181

F(32) 0030609 0026167 117

F(33) 1724235 0061599 2799

F(34) -005483 0030169 -182

F(51) -034839 0135253 -258

F(52) 0292124 0046299 631

F(53) -009435 0096527 -098F(54) -109823 0109525 -1003

F(55) 1671418 0083737 1996

G(31) 1924420 0058162 3309

G(32) -000416 0035255 -012

G(41) 0015621 0095771 016

G(42) 1258495 0055742 2258

G(51) 0080580 0151622 053

G(52) 1353204 0091388 1481

SAS OnlineDoc 991522 Version 8 1044

8132019 Statespace Model

httpslidepdfcomreaderfullstatespace-model 4952

Chapter 18 Example

The two series are believed to have a transfer function relation with the gas rate

(variable X) as the input and the CO2

concentration (variable Y) as the output Since

the parameter estimates shown in Output 1811 support this kind of model the model

is reestimated with the feedback parameters restricted to 0 The following statements

fit the transfer function (no feedback) model

title3 rsquoTransfer Function Modelrsquo

proc statespace data=seriesj printout=none

var x y

restrict f(32)=0 f(34)=0

g(32)=0 g(41)=0 g(51)=0

run

The last 2 pages of the output are shown in Output 1812

Output 1812 STATESPACE Output for Transfer Function Model

Gas Furnace Data

Box amp Jenkins Series J

Transfer Function Model

The STATESPACE Procedure

Selected Statespace Form and Fitted Model

State Vector

x(TT) y(TT) x(T+1T) y(T+1T) y(T+2T)

Estimate of Transition Matrix

0 0 1 0 0

0 0 0 1 0

-068882 0 1598717 0 0

0 0 0 0 1

-035944 0284179 -00963 -107313 1650047

Input Matrix for Innovation

1 0

0 1

1923446 0

0 1260856

0 1346332

1045SAS OnlineDoc 991522 Version 8

8132019 Statespace Model

httpslidepdfcomreaderfullstatespace-model 5052

Part 2 General Information

Gas Furnace Data

Box amp Jenkins Series J

Transfer Function Model

The STATESPACE Procedure

Selected Statespace Form and Fitted Model

Variance Matrix for Innovation

0036995 -00072

-00072 0095712

Parameter Estimates

Standard

Parameter Estimate Error t Value

F(31) -068882 0050549 -1363

F(33) 1598717 0050924 3139

F(51) -035944 0229044 -157

F(52) 0284179 0096944 293

F(53) -009630 0140876 -068

F(54) -107313 0250385 -429

F(55) 1650047 0188533 875G(31) 1923446 0056328 3415

G(42) 1260856 0056464 2233

G(52) 1346332 0091086 1478

SAS OnlineDoc 991522 Version 8 1046

8132019 Statespace Model

httpslidepdfcomreaderfullstatespace-model 5152

Chapter 18 References

References

Akaike H (1974) Markovian Representation of Stochastic Processes and Its Ap-

plication to the Analysis of Autoregressive Moving Average Processes Annals

of the Institute of Statistical Mathematics 26 363-387

Akaike H (1976) Canonical Correlations Analysis of Time Series and the Use of

an Information Criterion in Advances and Case Studies in System Identification

eds R Mehra and DG Lainiotis New York Academic Press

Anderson TW (1971) The Statistical Analysis of Time Series New York John

Wiley amp Sons

Ansley CF and Newbold P (1979) Multivariate Partial Autocorrelations Pro-

ceedings of the Business and Economic Statistics Section American Statistical

Association 349-353

Box GEP and Jenkins G (1976) Time Series Analysis Forecasting and Control

San Francisco Holden-Day

Brockwell PJ and Davis RA (1991) Time Series Theory and Methods 2nd Edi-

tion Springer-Verlag

Hannan EJ (1970) Multiple Time Series New York John Wiley amp Sons

Hannan EJ (1976) The Identification and Parameterization of ARMAX and State

Space Forms Econometrica 44 713-722

Harvey AC (1981a) The Econometric Analysis of Time Series New York John

Wiley amp Sons

Harvey AC (1981b) Time Series Models New York John Wiley amp Sons

Jones RH (1974) Identification and Autoregressive Spectrum Estimation IEEE Transactions on Automatic Control AC-19 894-897

Pham-Dinh-Tuan (1978) On the Fitting of Multivariate Processes of the

Autoregressive-Moving Average Type Biometrika 65 99-107

Priestley MB (1980) System Identification Kalman Filtering and Stochastic Con-

trol in Directions in Time Series eds DR Brillinger and GC Tiao Institute

of Mathematical Statistics

Whittle P (1963) On the Fitting of Multivariate Autoregressions and the Approxi-

mate Canonical Factorization of a Spectral Density Matrix Biometrika 50 129-

134

1047SAS OnlineDoc 991522 Version 8

8132019 Statespace Model

httpslidepdfcomreaderfullstatespace-model 5252

The correct bibliographic citation for this manual is as follows SAS Institute Inc SAS

ET S U serrsquos Gui de Version 8 Cary NC SAS Institute Inc 1999 1546 pp

SASET S Userrsquos Guide Version 8

Copyright copy 1999 by S AS I ns t it u t e I nc Ca ry N C U S A

ISBN 1ndash58025ndash489ndash6

All r ight s res erved P r int ed in t he U n it ed St a t es of A merica N o pa rt of t his p u blica t ionma y b e reprodu ced s t ored in a ret r ieva l s ys t em or t ra ns mit t ed in a ny form or b y a nymeans electronic mechanical photocopying or otherwise without the prior writtenpermission of the publisher SAS Institute Inc

US Government Restricted Rights Notice Use du plicat ion or disclosure of the

software by the government is subject to restrictions as set forth in FAR 52227ndash19Commercial Computer Softw ar e-Restricted Rights (J une 1987)

SAS Institute Inc SAS Campus Drive Cary North Carolina 27513

1st print ing October 1999

S AS reg a nd a ll ot her SA S I ns t it u t e I nc p rodu ct or s ervice na m es a re regis t ered t ra dema rkst d k f SA S I t it t I i t h U S A d t h t i reg i di t U SA

Page 41: Statespace Model

8132019 Statespace Model

httpslidepdfcomreaderfullstatespace-model 4152

Chapter 18 Details

Table 182 Value in the OUTMODEL= Data Set

Obs STATEVEC Fndash 1 Fndash 2 Fndash 3 Fndash 4 Gndash 1 Gndash 2 SIGndash 1 SIGndash 2

1 X(TT) 0 0 1 1 0

1 1

1 2

2 STD

3 Y(TT) F2 1

F2 2

F2 3

0 1

2 1

2 2

4 STD std F2 1

std F2 2

std F2 3

5 X(T+1T) F3 1

F3 2

F3 3

G3 1

G3 2

6 STD std F3 1

std F3 2

std F3 3

std G3 1

std G3 2

Printed Output

The printed output produced by the STATESPACE procedure is described in the fol-

lowing

1 descriptive statistics which include the number of observations used the

names of the variables their means and standard deviations (Std) and the dif-

ferencing operations used

2 the Akaike information criteria for the sequence of preliminary autoregressive

models

3 if the PRINTOUT=LONG option is specified the sample autocovariance ma-

trices of the input series at various lags

4 if the PRINTOUT=LONG option is specified the sample autocorrelation ma-

trices of the input series

5 a schematic representation of the autocorrelation matrices showing the signif-

icant autocorrelations

6 if the PRINTOUT=LONG option is specified the partial autoregressive ma-

trices (These are

p

p

as described in Preliminary Autoregressive Modelsearlier in this chapter)

7 a schematic representation of the partial autocorrelation matrices showing the

significant partial autocorrelations

8 the Yule-Walker estimates of the autoregressive parameters for the autoregres-

sive model with the minimum AIC

9 if the PRINTOUT=LONG option is specified the autocovariance matrices of

the residuals of the minimum AIC model This is the sequence of estimated

innovation variance matrices for the solutions of the Yule-Walker equations

10 if the PRINTOUT=LONG option is specified the autocorrelation matrices of the residuals of the minimum AIC model

11 If the CANCORR option is specified the canonical correlations analysis for

each potential state vector considered in the state vector selection process

This includes the potential state vector the canonical correlations the informa-

tion criterion for the smallest canonical correlation Bartlettrsquos

2 statistic (ldquoChi

Squarerdquo) for the smallest canonical correlation and the degrees of freedom of

Bartlettrsquos

2

1037SAS OnlineDoc 991522 Version 8

8132019 Statespace Model

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Part 2 General Information

12 the components of the chosen state vector

13 the preliminary estimate of the transition matrixF

the input matrixG

and

the variance matrix for the innovations

e e

14 if the ITPRINT option is specified the iteration history of the likelihood max-

imization For each iteration this shows the iteration number the number of step halvings the determinant of the innovation variance matrix the damping

factor Lambda and the values of the parameters

15 the state vector printed again to aid interpretation of the following listing of F

andG

16 the final estimate of the transition matrixF

17 the final estimate of the input matrixG

18 the final estimate of the variance matrix for the innovations

e e

19 a table listing the estimates of the free parameters inF

andG

and their standarderrors and t statistics

20 if the COVB option is specified the covariance matrix of the parameter esti-

mates

21 if the COVB option is specified the correlation matrix of the parameter esti-

mates

22 if the PRINT option is specified the forecasts and their standard errors

SAS OnlineDoc 991522 Version 8 1038

8132019 Statespace Model

httpslidepdfcomreaderfullstatespace-model 4352

Chapter 18 Details

ODS Table Names

PROC STATESPACE assigns a name to each table it creates You can use these

names to reference the table when using the Output Delivery System (ODS) to select

tables and create output data sets These names are listed in the following table For

more information on ODS see Chapter 6 ldquoUsing the Output Delivery Systemrdquo

Table 183 ODS Tables Produced in PROC STATESPACE

ODS Table Name Description Option

NObs Number of observations default

Summary Simple summary statistics table default

InfoCriterion Information criterion table default

CovLags Covariance Matrices of Input Series PRINTOUT=LONG

CorrLags Correlation Matrices of Input Series PRINTOUT=LONG

PartialAR Partial Autoregressive Matrices PRINTOUT=LONG

YWEstimates Yule-Walker Estimates for Minimum AIC default

CovResiduals Covariance of Residuals PRINTOUT=LONGCorrResiduals Residual Correlations from AR Models PRINTOUT=LONG

StateVector State vector table default

CorrGraph Schematic Representation of Correlations default

TransitionMatrix Transition Matrix default

InputMatrix Input Matrix default

VarInnov Variance Matrix for the Innovation default

CovB Covariance of Parameter Estimates COVB

CorrB Correlation of Parameter Estimates COVB

CanCorr Canonical Correlation Analysis CANCORR

IterHistory Iterative Fitting table ITPRINT

ParameterEstimates Parameter Estimates Table default

Forecasts Forecasts Table PRINT

ConvergenceStatus Convergence Status Table default

1039SAS OnlineDoc 991522 Version 8

8132019 Statespace Model

httpslidepdfcomreaderfullstatespace-model 4452

Part 2 General Information

Example

Example 181 Series J from Box and Jenkins

This example analyzes the gas furnace data (series J) from Box and Jenkins (The

data are not shown Refer to Box and Jenkins (1976) for the data)

First a model is selected and fit automatically using the following statements

title1 rsquoGas Furnace Datarsquo

title2 rsquoBox amp Jenkins Series Jrsquo

title3 rsquoAutomatically Selected Modelrsquo

proc statespace data=seriesj cancorr

var x y

run

The results for the automatically selected model are shown in Output 1811

SAS OnlineDoc 991522 Version 8 1040

8132019 Statespace Model

httpslidepdfcomreaderfullstatespace-model 4552

Chapter 18 Example

Output 1811 Results for Automatically Selected Model

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Number of Observations 296

Standard

Variable Mean Error

x -005683 1072766

y 5350912 3202121

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Information Criterion for Autoregressive Models

Lag=0 Lag=1 Lag=2 Lag=3 Lag=4 Lag=5 Lag=6 Lag=7 Lag=8

6513862 -103357 -163296 -164512 -165152 -164891 -164934 -164315 -163856

Information

Criterion for

Autoregressive

Models

Lag=9 Lag=10

-16348 -163359

Schematic Representation of Correlations

NameLag 0 1 2 3 4 5 6 7 8 9 10

x +- +- +- +- +- +- +- +- +- +- +-

y -+ -+ -+ -+ -+ -+ -+ -+ -+ -+ -+

+ is gt 2std error - is lt -2std error is between

1041SAS OnlineDoc 991522 Version 8

8132019 Statespace Model

httpslidepdfcomreaderfullstatespace-model 4652

Part 2 General Information

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Schematic Representation of Partial Autocorrelations

NameLag 1 2 3 4 5 6 7 8 9 10

x + - + -

y -+ -- - + +

+ is gt 2std error - is lt -2std error is between

Yule-Walker Estimates for Minimum AIC

------Lag=1------ ------Lag=2------ ------Lag=3------ ------Lag=4------

x y x y x y x y

x 1925887 -000124 -120166 0004224 0116918 -000867 0104236 0003268

y 0050496 1299793 -002046 -03277 -071182 -025701 0195411 0133417

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Canonical Correlations Analysis

Information Chi

x(TT) y(TT) x(T+1T) Criterion Square DF

1 1 0804883 2929228 3047481 8

Information Chix(TT) y(TT) x(T+1T) y(T+1T) Criterion Square DF

1 1 0906681 0607529 1223358 1347237 7

Information Chi

x(TT) y(TT) x (T+1T) y (T+1T) x (T+2T) Criterion Square DF

1 1 0909434 0610278 0186274 -154701 1034705 6

Information Chi

x(TT) y(TT) x (T+1T) y (T+1T) y (T+2T) Criterion Square DF

1 1 091014 0618937 0206823 0940392 1280924 6

Information Chi

x(TT) y(TT) x(T+1T) y(T+1T) y(T+2T) y(T+3T) Criterion Square DF

1 1 0912963 0628785 0226598 0083258 -794103 2041584 5

SAS OnlineDoc 991522 Version 8 1042

8132019 Statespace Model

httpslidepdfcomreaderfullstatespace-model 4752

Chapter 18 Example

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Selected Statespace Form and Preliminary Estimates

State Vector

x(TT) y(TT) x(T+1T) y(T+1T) y(T+2T)

Estimate of Transition Matrix

0 0 1 0 0

0 0 0 1 0

-084718 0026794 1711715 -005019 0

0 0 0 0 1

-019785 0334274 -018174 -123557 1787475

Input Matrix for Innovation

1 0

0 11925887 -000124

0050496 1299793

0142421 1361696

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Selected Statespace Form and Preliminary Estimates

Variance Matrix for Innovation

0035274 -000734

-000734 0097569

1043SAS OnlineDoc 991522 Version 8

8132019 Statespace Model

httpslidepdfcomreaderfullstatespace-model 4852

Part 2 General Information

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Selected Statespace Form and Fitted Model

State Vector

x(TT) y(TT) x(T+1T) y(T+1T) y(T+2T)

Estimate of Transition Matrix

0 0 1 0 0

0 0 0 1 0

-086192 0030609 1724235 -005483 0

0 0 0 0 1

-034839 0292124 -009435 -109823 1671418

Input Matrix for Innovation

1 0

0 1192442 -000416

0015621 1258495

008058 1353204

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Selected Statespace Form and Fitted Model

Variance Matrix for Innovation

0035579 -000728

-000728 0095577

Parameter Estimates

Standard

Parameter Estimate Error t Value

F(31) -086192 0072961 -1181

F(32) 0030609 0026167 117

F(33) 1724235 0061599 2799

F(34) -005483 0030169 -182

F(51) -034839 0135253 -258

F(52) 0292124 0046299 631

F(53) -009435 0096527 -098F(54) -109823 0109525 -1003

F(55) 1671418 0083737 1996

G(31) 1924420 0058162 3309

G(32) -000416 0035255 -012

G(41) 0015621 0095771 016

G(42) 1258495 0055742 2258

G(51) 0080580 0151622 053

G(52) 1353204 0091388 1481

SAS OnlineDoc 991522 Version 8 1044

8132019 Statespace Model

httpslidepdfcomreaderfullstatespace-model 4952

Chapter 18 Example

The two series are believed to have a transfer function relation with the gas rate

(variable X) as the input and the CO2

concentration (variable Y) as the output Since

the parameter estimates shown in Output 1811 support this kind of model the model

is reestimated with the feedback parameters restricted to 0 The following statements

fit the transfer function (no feedback) model

title3 rsquoTransfer Function Modelrsquo

proc statespace data=seriesj printout=none

var x y

restrict f(32)=0 f(34)=0

g(32)=0 g(41)=0 g(51)=0

run

The last 2 pages of the output are shown in Output 1812

Output 1812 STATESPACE Output for Transfer Function Model

Gas Furnace Data

Box amp Jenkins Series J

Transfer Function Model

The STATESPACE Procedure

Selected Statespace Form and Fitted Model

State Vector

x(TT) y(TT) x(T+1T) y(T+1T) y(T+2T)

Estimate of Transition Matrix

0 0 1 0 0

0 0 0 1 0

-068882 0 1598717 0 0

0 0 0 0 1

-035944 0284179 -00963 -107313 1650047

Input Matrix for Innovation

1 0

0 1

1923446 0

0 1260856

0 1346332

1045SAS OnlineDoc 991522 Version 8

8132019 Statespace Model

httpslidepdfcomreaderfullstatespace-model 5052

Part 2 General Information

Gas Furnace Data

Box amp Jenkins Series J

Transfer Function Model

The STATESPACE Procedure

Selected Statespace Form and Fitted Model

Variance Matrix for Innovation

0036995 -00072

-00072 0095712

Parameter Estimates

Standard

Parameter Estimate Error t Value

F(31) -068882 0050549 -1363

F(33) 1598717 0050924 3139

F(51) -035944 0229044 -157

F(52) 0284179 0096944 293

F(53) -009630 0140876 -068

F(54) -107313 0250385 -429

F(55) 1650047 0188533 875G(31) 1923446 0056328 3415

G(42) 1260856 0056464 2233

G(52) 1346332 0091086 1478

SAS OnlineDoc 991522 Version 8 1046

8132019 Statespace Model

httpslidepdfcomreaderfullstatespace-model 5152

Chapter 18 References

References

Akaike H (1974) Markovian Representation of Stochastic Processes and Its Ap-

plication to the Analysis of Autoregressive Moving Average Processes Annals

of the Institute of Statistical Mathematics 26 363-387

Akaike H (1976) Canonical Correlations Analysis of Time Series and the Use of

an Information Criterion in Advances and Case Studies in System Identification

eds R Mehra and DG Lainiotis New York Academic Press

Anderson TW (1971) The Statistical Analysis of Time Series New York John

Wiley amp Sons

Ansley CF and Newbold P (1979) Multivariate Partial Autocorrelations Pro-

ceedings of the Business and Economic Statistics Section American Statistical

Association 349-353

Box GEP and Jenkins G (1976) Time Series Analysis Forecasting and Control

San Francisco Holden-Day

Brockwell PJ and Davis RA (1991) Time Series Theory and Methods 2nd Edi-

tion Springer-Verlag

Hannan EJ (1970) Multiple Time Series New York John Wiley amp Sons

Hannan EJ (1976) The Identification and Parameterization of ARMAX and State

Space Forms Econometrica 44 713-722

Harvey AC (1981a) The Econometric Analysis of Time Series New York John

Wiley amp Sons

Harvey AC (1981b) Time Series Models New York John Wiley amp Sons

Jones RH (1974) Identification and Autoregressive Spectrum Estimation IEEE Transactions on Automatic Control AC-19 894-897

Pham-Dinh-Tuan (1978) On the Fitting of Multivariate Processes of the

Autoregressive-Moving Average Type Biometrika 65 99-107

Priestley MB (1980) System Identification Kalman Filtering and Stochastic Con-

trol in Directions in Time Series eds DR Brillinger and GC Tiao Institute

of Mathematical Statistics

Whittle P (1963) On the Fitting of Multivariate Autoregressions and the Approxi-

mate Canonical Factorization of a Spectral Density Matrix Biometrika 50 129-

134

1047SAS OnlineDoc 991522 Version 8

8132019 Statespace Model

httpslidepdfcomreaderfullstatespace-model 5252

The correct bibliographic citation for this manual is as follows SAS Institute Inc SAS

ET S U serrsquos Gui de Version 8 Cary NC SAS Institute Inc 1999 1546 pp

SASET S Userrsquos Guide Version 8

Copyright copy 1999 by S AS I ns t it u t e I nc Ca ry N C U S A

ISBN 1ndash58025ndash489ndash6

All r ight s res erved P r int ed in t he U n it ed St a t es of A merica N o pa rt of t his p u blica t ionma y b e reprodu ced s t ored in a ret r ieva l s ys t em or t ra ns mit t ed in a ny form or b y a nymeans electronic mechanical photocopying or otherwise without the prior writtenpermission of the publisher SAS Institute Inc

US Government Restricted Rights Notice Use du plicat ion or disclosure of the

software by the government is subject to restrictions as set forth in FAR 52227ndash19Commercial Computer Softw ar e-Restricted Rights (J une 1987)

SAS Institute Inc SAS Campus Drive Cary North Carolina 27513

1st print ing October 1999

S AS reg a nd a ll ot her SA S I ns t it u t e I nc p rodu ct or s ervice na m es a re regis t ered t ra dema rkst d k f SA S I t it t I i t h U S A d t h t i reg i di t U SA

Page 42: Statespace Model

8132019 Statespace Model

httpslidepdfcomreaderfullstatespace-model 4252

Part 2 General Information

12 the components of the chosen state vector

13 the preliminary estimate of the transition matrixF

the input matrixG

and

the variance matrix for the innovations

e e

14 if the ITPRINT option is specified the iteration history of the likelihood max-

imization For each iteration this shows the iteration number the number of step halvings the determinant of the innovation variance matrix the damping

factor Lambda and the values of the parameters

15 the state vector printed again to aid interpretation of the following listing of F

andG

16 the final estimate of the transition matrixF

17 the final estimate of the input matrixG

18 the final estimate of the variance matrix for the innovations

e e

19 a table listing the estimates of the free parameters inF

andG

and their standarderrors and t statistics

20 if the COVB option is specified the covariance matrix of the parameter esti-

mates

21 if the COVB option is specified the correlation matrix of the parameter esti-

mates

22 if the PRINT option is specified the forecasts and their standard errors

SAS OnlineDoc 991522 Version 8 1038

8132019 Statespace Model

httpslidepdfcomreaderfullstatespace-model 4352

Chapter 18 Details

ODS Table Names

PROC STATESPACE assigns a name to each table it creates You can use these

names to reference the table when using the Output Delivery System (ODS) to select

tables and create output data sets These names are listed in the following table For

more information on ODS see Chapter 6 ldquoUsing the Output Delivery Systemrdquo

Table 183 ODS Tables Produced in PROC STATESPACE

ODS Table Name Description Option

NObs Number of observations default

Summary Simple summary statistics table default

InfoCriterion Information criterion table default

CovLags Covariance Matrices of Input Series PRINTOUT=LONG

CorrLags Correlation Matrices of Input Series PRINTOUT=LONG

PartialAR Partial Autoregressive Matrices PRINTOUT=LONG

YWEstimates Yule-Walker Estimates for Minimum AIC default

CovResiduals Covariance of Residuals PRINTOUT=LONGCorrResiduals Residual Correlations from AR Models PRINTOUT=LONG

StateVector State vector table default

CorrGraph Schematic Representation of Correlations default

TransitionMatrix Transition Matrix default

InputMatrix Input Matrix default

VarInnov Variance Matrix for the Innovation default

CovB Covariance of Parameter Estimates COVB

CorrB Correlation of Parameter Estimates COVB

CanCorr Canonical Correlation Analysis CANCORR

IterHistory Iterative Fitting table ITPRINT

ParameterEstimates Parameter Estimates Table default

Forecasts Forecasts Table PRINT

ConvergenceStatus Convergence Status Table default

1039SAS OnlineDoc 991522 Version 8

8132019 Statespace Model

httpslidepdfcomreaderfullstatespace-model 4452

Part 2 General Information

Example

Example 181 Series J from Box and Jenkins

This example analyzes the gas furnace data (series J) from Box and Jenkins (The

data are not shown Refer to Box and Jenkins (1976) for the data)

First a model is selected and fit automatically using the following statements

title1 rsquoGas Furnace Datarsquo

title2 rsquoBox amp Jenkins Series Jrsquo

title3 rsquoAutomatically Selected Modelrsquo

proc statespace data=seriesj cancorr

var x y

run

The results for the automatically selected model are shown in Output 1811

SAS OnlineDoc 991522 Version 8 1040

8132019 Statespace Model

httpslidepdfcomreaderfullstatespace-model 4552

Chapter 18 Example

Output 1811 Results for Automatically Selected Model

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Number of Observations 296

Standard

Variable Mean Error

x -005683 1072766

y 5350912 3202121

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Information Criterion for Autoregressive Models

Lag=0 Lag=1 Lag=2 Lag=3 Lag=4 Lag=5 Lag=6 Lag=7 Lag=8

6513862 -103357 -163296 -164512 -165152 -164891 -164934 -164315 -163856

Information

Criterion for

Autoregressive

Models

Lag=9 Lag=10

-16348 -163359

Schematic Representation of Correlations

NameLag 0 1 2 3 4 5 6 7 8 9 10

x +- +- +- +- +- +- +- +- +- +- +-

y -+ -+ -+ -+ -+ -+ -+ -+ -+ -+ -+

+ is gt 2std error - is lt -2std error is between

1041SAS OnlineDoc 991522 Version 8

8132019 Statespace Model

httpslidepdfcomreaderfullstatespace-model 4652

Part 2 General Information

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Schematic Representation of Partial Autocorrelations

NameLag 1 2 3 4 5 6 7 8 9 10

x + - + -

y -+ -- - + +

+ is gt 2std error - is lt -2std error is between

Yule-Walker Estimates for Minimum AIC

------Lag=1------ ------Lag=2------ ------Lag=3------ ------Lag=4------

x y x y x y x y

x 1925887 -000124 -120166 0004224 0116918 -000867 0104236 0003268

y 0050496 1299793 -002046 -03277 -071182 -025701 0195411 0133417

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Canonical Correlations Analysis

Information Chi

x(TT) y(TT) x(T+1T) Criterion Square DF

1 1 0804883 2929228 3047481 8

Information Chix(TT) y(TT) x(T+1T) y(T+1T) Criterion Square DF

1 1 0906681 0607529 1223358 1347237 7

Information Chi

x(TT) y(TT) x (T+1T) y (T+1T) x (T+2T) Criterion Square DF

1 1 0909434 0610278 0186274 -154701 1034705 6

Information Chi

x(TT) y(TT) x (T+1T) y (T+1T) y (T+2T) Criterion Square DF

1 1 091014 0618937 0206823 0940392 1280924 6

Information Chi

x(TT) y(TT) x(T+1T) y(T+1T) y(T+2T) y(T+3T) Criterion Square DF

1 1 0912963 0628785 0226598 0083258 -794103 2041584 5

SAS OnlineDoc 991522 Version 8 1042

8132019 Statespace Model

httpslidepdfcomreaderfullstatespace-model 4752

Chapter 18 Example

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Selected Statespace Form and Preliminary Estimates

State Vector

x(TT) y(TT) x(T+1T) y(T+1T) y(T+2T)

Estimate of Transition Matrix

0 0 1 0 0

0 0 0 1 0

-084718 0026794 1711715 -005019 0

0 0 0 0 1

-019785 0334274 -018174 -123557 1787475

Input Matrix for Innovation

1 0

0 11925887 -000124

0050496 1299793

0142421 1361696

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Selected Statespace Form and Preliminary Estimates

Variance Matrix for Innovation

0035274 -000734

-000734 0097569

1043SAS OnlineDoc 991522 Version 8

8132019 Statespace Model

httpslidepdfcomreaderfullstatespace-model 4852

Part 2 General Information

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Selected Statespace Form and Fitted Model

State Vector

x(TT) y(TT) x(T+1T) y(T+1T) y(T+2T)

Estimate of Transition Matrix

0 0 1 0 0

0 0 0 1 0

-086192 0030609 1724235 -005483 0

0 0 0 0 1

-034839 0292124 -009435 -109823 1671418

Input Matrix for Innovation

1 0

0 1192442 -000416

0015621 1258495

008058 1353204

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Selected Statespace Form and Fitted Model

Variance Matrix for Innovation

0035579 -000728

-000728 0095577

Parameter Estimates

Standard

Parameter Estimate Error t Value

F(31) -086192 0072961 -1181

F(32) 0030609 0026167 117

F(33) 1724235 0061599 2799

F(34) -005483 0030169 -182

F(51) -034839 0135253 -258

F(52) 0292124 0046299 631

F(53) -009435 0096527 -098F(54) -109823 0109525 -1003

F(55) 1671418 0083737 1996

G(31) 1924420 0058162 3309

G(32) -000416 0035255 -012

G(41) 0015621 0095771 016

G(42) 1258495 0055742 2258

G(51) 0080580 0151622 053

G(52) 1353204 0091388 1481

SAS OnlineDoc 991522 Version 8 1044

8132019 Statespace Model

httpslidepdfcomreaderfullstatespace-model 4952

Chapter 18 Example

The two series are believed to have a transfer function relation with the gas rate

(variable X) as the input and the CO2

concentration (variable Y) as the output Since

the parameter estimates shown in Output 1811 support this kind of model the model

is reestimated with the feedback parameters restricted to 0 The following statements

fit the transfer function (no feedback) model

title3 rsquoTransfer Function Modelrsquo

proc statespace data=seriesj printout=none

var x y

restrict f(32)=0 f(34)=0

g(32)=0 g(41)=0 g(51)=0

run

The last 2 pages of the output are shown in Output 1812

Output 1812 STATESPACE Output for Transfer Function Model

Gas Furnace Data

Box amp Jenkins Series J

Transfer Function Model

The STATESPACE Procedure

Selected Statespace Form and Fitted Model

State Vector

x(TT) y(TT) x(T+1T) y(T+1T) y(T+2T)

Estimate of Transition Matrix

0 0 1 0 0

0 0 0 1 0

-068882 0 1598717 0 0

0 0 0 0 1

-035944 0284179 -00963 -107313 1650047

Input Matrix for Innovation

1 0

0 1

1923446 0

0 1260856

0 1346332

1045SAS OnlineDoc 991522 Version 8

8132019 Statespace Model

httpslidepdfcomreaderfullstatespace-model 5052

Part 2 General Information

Gas Furnace Data

Box amp Jenkins Series J

Transfer Function Model

The STATESPACE Procedure

Selected Statespace Form and Fitted Model

Variance Matrix for Innovation

0036995 -00072

-00072 0095712

Parameter Estimates

Standard

Parameter Estimate Error t Value

F(31) -068882 0050549 -1363

F(33) 1598717 0050924 3139

F(51) -035944 0229044 -157

F(52) 0284179 0096944 293

F(53) -009630 0140876 -068

F(54) -107313 0250385 -429

F(55) 1650047 0188533 875G(31) 1923446 0056328 3415

G(42) 1260856 0056464 2233

G(52) 1346332 0091086 1478

SAS OnlineDoc 991522 Version 8 1046

8132019 Statespace Model

httpslidepdfcomreaderfullstatespace-model 5152

Chapter 18 References

References

Akaike H (1974) Markovian Representation of Stochastic Processes and Its Ap-

plication to the Analysis of Autoregressive Moving Average Processes Annals

of the Institute of Statistical Mathematics 26 363-387

Akaike H (1976) Canonical Correlations Analysis of Time Series and the Use of

an Information Criterion in Advances and Case Studies in System Identification

eds R Mehra and DG Lainiotis New York Academic Press

Anderson TW (1971) The Statistical Analysis of Time Series New York John

Wiley amp Sons

Ansley CF and Newbold P (1979) Multivariate Partial Autocorrelations Pro-

ceedings of the Business and Economic Statistics Section American Statistical

Association 349-353

Box GEP and Jenkins G (1976) Time Series Analysis Forecasting and Control

San Francisco Holden-Day

Brockwell PJ and Davis RA (1991) Time Series Theory and Methods 2nd Edi-

tion Springer-Verlag

Hannan EJ (1970) Multiple Time Series New York John Wiley amp Sons

Hannan EJ (1976) The Identification and Parameterization of ARMAX and State

Space Forms Econometrica 44 713-722

Harvey AC (1981a) The Econometric Analysis of Time Series New York John

Wiley amp Sons

Harvey AC (1981b) Time Series Models New York John Wiley amp Sons

Jones RH (1974) Identification and Autoregressive Spectrum Estimation IEEE Transactions on Automatic Control AC-19 894-897

Pham-Dinh-Tuan (1978) On the Fitting of Multivariate Processes of the

Autoregressive-Moving Average Type Biometrika 65 99-107

Priestley MB (1980) System Identification Kalman Filtering and Stochastic Con-

trol in Directions in Time Series eds DR Brillinger and GC Tiao Institute

of Mathematical Statistics

Whittle P (1963) On the Fitting of Multivariate Autoregressions and the Approxi-

mate Canonical Factorization of a Spectral Density Matrix Biometrika 50 129-

134

1047SAS OnlineDoc 991522 Version 8

8132019 Statespace Model

httpslidepdfcomreaderfullstatespace-model 5252

The correct bibliographic citation for this manual is as follows SAS Institute Inc SAS

ET S U serrsquos Gui de Version 8 Cary NC SAS Institute Inc 1999 1546 pp

SASET S Userrsquos Guide Version 8

Copyright copy 1999 by S AS I ns t it u t e I nc Ca ry N C U S A

ISBN 1ndash58025ndash489ndash6

All r ight s res erved P r int ed in t he U n it ed St a t es of A merica N o pa rt of t his p u blica t ionma y b e reprodu ced s t ored in a ret r ieva l s ys t em or t ra ns mit t ed in a ny form or b y a nymeans electronic mechanical photocopying or otherwise without the prior writtenpermission of the publisher SAS Institute Inc

US Government Restricted Rights Notice Use du plicat ion or disclosure of the

software by the government is subject to restrictions as set forth in FAR 52227ndash19Commercial Computer Softw ar e-Restricted Rights (J une 1987)

SAS Institute Inc SAS Campus Drive Cary North Carolina 27513

1st print ing October 1999

S AS reg a nd a ll ot her SA S I ns t it u t e I nc p rodu ct or s ervice na m es a re regis t ered t ra dema rkst d k f SA S I t it t I i t h U S A d t h t i reg i di t U SA

Page 43: Statespace Model

8132019 Statespace Model

httpslidepdfcomreaderfullstatespace-model 4352

Chapter 18 Details

ODS Table Names

PROC STATESPACE assigns a name to each table it creates You can use these

names to reference the table when using the Output Delivery System (ODS) to select

tables and create output data sets These names are listed in the following table For

more information on ODS see Chapter 6 ldquoUsing the Output Delivery Systemrdquo

Table 183 ODS Tables Produced in PROC STATESPACE

ODS Table Name Description Option

NObs Number of observations default

Summary Simple summary statistics table default

InfoCriterion Information criterion table default

CovLags Covariance Matrices of Input Series PRINTOUT=LONG

CorrLags Correlation Matrices of Input Series PRINTOUT=LONG

PartialAR Partial Autoregressive Matrices PRINTOUT=LONG

YWEstimates Yule-Walker Estimates for Minimum AIC default

CovResiduals Covariance of Residuals PRINTOUT=LONGCorrResiduals Residual Correlations from AR Models PRINTOUT=LONG

StateVector State vector table default

CorrGraph Schematic Representation of Correlations default

TransitionMatrix Transition Matrix default

InputMatrix Input Matrix default

VarInnov Variance Matrix for the Innovation default

CovB Covariance of Parameter Estimates COVB

CorrB Correlation of Parameter Estimates COVB

CanCorr Canonical Correlation Analysis CANCORR

IterHistory Iterative Fitting table ITPRINT

ParameterEstimates Parameter Estimates Table default

Forecasts Forecasts Table PRINT

ConvergenceStatus Convergence Status Table default

1039SAS OnlineDoc 991522 Version 8

8132019 Statespace Model

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Part 2 General Information

Example

Example 181 Series J from Box and Jenkins

This example analyzes the gas furnace data (series J) from Box and Jenkins (The

data are not shown Refer to Box and Jenkins (1976) for the data)

First a model is selected and fit automatically using the following statements

title1 rsquoGas Furnace Datarsquo

title2 rsquoBox amp Jenkins Series Jrsquo

title3 rsquoAutomatically Selected Modelrsquo

proc statespace data=seriesj cancorr

var x y

run

The results for the automatically selected model are shown in Output 1811

SAS OnlineDoc 991522 Version 8 1040

8132019 Statespace Model

httpslidepdfcomreaderfullstatespace-model 4552

Chapter 18 Example

Output 1811 Results for Automatically Selected Model

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Number of Observations 296

Standard

Variable Mean Error

x -005683 1072766

y 5350912 3202121

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Information Criterion for Autoregressive Models

Lag=0 Lag=1 Lag=2 Lag=3 Lag=4 Lag=5 Lag=6 Lag=7 Lag=8

6513862 -103357 -163296 -164512 -165152 -164891 -164934 -164315 -163856

Information

Criterion for

Autoregressive

Models

Lag=9 Lag=10

-16348 -163359

Schematic Representation of Correlations

NameLag 0 1 2 3 4 5 6 7 8 9 10

x +- +- +- +- +- +- +- +- +- +- +-

y -+ -+ -+ -+ -+ -+ -+ -+ -+ -+ -+

+ is gt 2std error - is lt -2std error is between

1041SAS OnlineDoc 991522 Version 8

8132019 Statespace Model

httpslidepdfcomreaderfullstatespace-model 4652

Part 2 General Information

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Schematic Representation of Partial Autocorrelations

NameLag 1 2 3 4 5 6 7 8 9 10

x + - + -

y -+ -- - + +

+ is gt 2std error - is lt -2std error is between

Yule-Walker Estimates for Minimum AIC

------Lag=1------ ------Lag=2------ ------Lag=3------ ------Lag=4------

x y x y x y x y

x 1925887 -000124 -120166 0004224 0116918 -000867 0104236 0003268

y 0050496 1299793 -002046 -03277 -071182 -025701 0195411 0133417

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Canonical Correlations Analysis

Information Chi

x(TT) y(TT) x(T+1T) Criterion Square DF

1 1 0804883 2929228 3047481 8

Information Chix(TT) y(TT) x(T+1T) y(T+1T) Criterion Square DF

1 1 0906681 0607529 1223358 1347237 7

Information Chi

x(TT) y(TT) x (T+1T) y (T+1T) x (T+2T) Criterion Square DF

1 1 0909434 0610278 0186274 -154701 1034705 6

Information Chi

x(TT) y(TT) x (T+1T) y (T+1T) y (T+2T) Criterion Square DF

1 1 091014 0618937 0206823 0940392 1280924 6

Information Chi

x(TT) y(TT) x(T+1T) y(T+1T) y(T+2T) y(T+3T) Criterion Square DF

1 1 0912963 0628785 0226598 0083258 -794103 2041584 5

SAS OnlineDoc 991522 Version 8 1042

8132019 Statespace Model

httpslidepdfcomreaderfullstatespace-model 4752

Chapter 18 Example

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Selected Statespace Form and Preliminary Estimates

State Vector

x(TT) y(TT) x(T+1T) y(T+1T) y(T+2T)

Estimate of Transition Matrix

0 0 1 0 0

0 0 0 1 0

-084718 0026794 1711715 -005019 0

0 0 0 0 1

-019785 0334274 -018174 -123557 1787475

Input Matrix for Innovation

1 0

0 11925887 -000124

0050496 1299793

0142421 1361696

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Selected Statespace Form and Preliminary Estimates

Variance Matrix for Innovation

0035274 -000734

-000734 0097569

1043SAS OnlineDoc 991522 Version 8

8132019 Statespace Model

httpslidepdfcomreaderfullstatespace-model 4852

Part 2 General Information

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Selected Statespace Form and Fitted Model

State Vector

x(TT) y(TT) x(T+1T) y(T+1T) y(T+2T)

Estimate of Transition Matrix

0 0 1 0 0

0 0 0 1 0

-086192 0030609 1724235 -005483 0

0 0 0 0 1

-034839 0292124 -009435 -109823 1671418

Input Matrix for Innovation

1 0

0 1192442 -000416

0015621 1258495

008058 1353204

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Selected Statespace Form and Fitted Model

Variance Matrix for Innovation

0035579 -000728

-000728 0095577

Parameter Estimates

Standard

Parameter Estimate Error t Value

F(31) -086192 0072961 -1181

F(32) 0030609 0026167 117

F(33) 1724235 0061599 2799

F(34) -005483 0030169 -182

F(51) -034839 0135253 -258

F(52) 0292124 0046299 631

F(53) -009435 0096527 -098F(54) -109823 0109525 -1003

F(55) 1671418 0083737 1996

G(31) 1924420 0058162 3309

G(32) -000416 0035255 -012

G(41) 0015621 0095771 016

G(42) 1258495 0055742 2258

G(51) 0080580 0151622 053

G(52) 1353204 0091388 1481

SAS OnlineDoc 991522 Version 8 1044

8132019 Statespace Model

httpslidepdfcomreaderfullstatespace-model 4952

Chapter 18 Example

The two series are believed to have a transfer function relation with the gas rate

(variable X) as the input and the CO2

concentration (variable Y) as the output Since

the parameter estimates shown in Output 1811 support this kind of model the model

is reestimated with the feedback parameters restricted to 0 The following statements

fit the transfer function (no feedback) model

title3 rsquoTransfer Function Modelrsquo

proc statespace data=seriesj printout=none

var x y

restrict f(32)=0 f(34)=0

g(32)=0 g(41)=0 g(51)=0

run

The last 2 pages of the output are shown in Output 1812

Output 1812 STATESPACE Output for Transfer Function Model

Gas Furnace Data

Box amp Jenkins Series J

Transfer Function Model

The STATESPACE Procedure

Selected Statespace Form and Fitted Model

State Vector

x(TT) y(TT) x(T+1T) y(T+1T) y(T+2T)

Estimate of Transition Matrix

0 0 1 0 0

0 0 0 1 0

-068882 0 1598717 0 0

0 0 0 0 1

-035944 0284179 -00963 -107313 1650047

Input Matrix for Innovation

1 0

0 1

1923446 0

0 1260856

0 1346332

1045SAS OnlineDoc 991522 Version 8

8132019 Statespace Model

httpslidepdfcomreaderfullstatespace-model 5052

Part 2 General Information

Gas Furnace Data

Box amp Jenkins Series J

Transfer Function Model

The STATESPACE Procedure

Selected Statespace Form and Fitted Model

Variance Matrix for Innovation

0036995 -00072

-00072 0095712

Parameter Estimates

Standard

Parameter Estimate Error t Value

F(31) -068882 0050549 -1363

F(33) 1598717 0050924 3139

F(51) -035944 0229044 -157

F(52) 0284179 0096944 293

F(53) -009630 0140876 -068

F(54) -107313 0250385 -429

F(55) 1650047 0188533 875G(31) 1923446 0056328 3415

G(42) 1260856 0056464 2233

G(52) 1346332 0091086 1478

SAS OnlineDoc 991522 Version 8 1046

8132019 Statespace Model

httpslidepdfcomreaderfullstatespace-model 5152

Chapter 18 References

References

Akaike H (1974) Markovian Representation of Stochastic Processes and Its Ap-

plication to the Analysis of Autoregressive Moving Average Processes Annals

of the Institute of Statistical Mathematics 26 363-387

Akaike H (1976) Canonical Correlations Analysis of Time Series and the Use of

an Information Criterion in Advances and Case Studies in System Identification

eds R Mehra and DG Lainiotis New York Academic Press

Anderson TW (1971) The Statistical Analysis of Time Series New York John

Wiley amp Sons

Ansley CF and Newbold P (1979) Multivariate Partial Autocorrelations Pro-

ceedings of the Business and Economic Statistics Section American Statistical

Association 349-353

Box GEP and Jenkins G (1976) Time Series Analysis Forecasting and Control

San Francisco Holden-Day

Brockwell PJ and Davis RA (1991) Time Series Theory and Methods 2nd Edi-

tion Springer-Verlag

Hannan EJ (1970) Multiple Time Series New York John Wiley amp Sons

Hannan EJ (1976) The Identification and Parameterization of ARMAX and State

Space Forms Econometrica 44 713-722

Harvey AC (1981a) The Econometric Analysis of Time Series New York John

Wiley amp Sons

Harvey AC (1981b) Time Series Models New York John Wiley amp Sons

Jones RH (1974) Identification and Autoregressive Spectrum Estimation IEEE Transactions on Automatic Control AC-19 894-897

Pham-Dinh-Tuan (1978) On the Fitting of Multivariate Processes of the

Autoregressive-Moving Average Type Biometrika 65 99-107

Priestley MB (1980) System Identification Kalman Filtering and Stochastic Con-

trol in Directions in Time Series eds DR Brillinger and GC Tiao Institute

of Mathematical Statistics

Whittle P (1963) On the Fitting of Multivariate Autoregressions and the Approxi-

mate Canonical Factorization of a Spectral Density Matrix Biometrika 50 129-

134

1047SAS OnlineDoc 991522 Version 8

8132019 Statespace Model

httpslidepdfcomreaderfullstatespace-model 5252

The correct bibliographic citation for this manual is as follows SAS Institute Inc SAS

ET S U serrsquos Gui de Version 8 Cary NC SAS Institute Inc 1999 1546 pp

SASET S Userrsquos Guide Version 8

Copyright copy 1999 by S AS I ns t it u t e I nc Ca ry N C U S A

ISBN 1ndash58025ndash489ndash6

All r ight s res erved P r int ed in t he U n it ed St a t es of A merica N o pa rt of t his p u blica t ionma y b e reprodu ced s t ored in a ret r ieva l s ys t em or t ra ns mit t ed in a ny form or b y a nymeans electronic mechanical photocopying or otherwise without the prior writtenpermission of the publisher SAS Institute Inc

US Government Restricted Rights Notice Use du plicat ion or disclosure of the

software by the government is subject to restrictions as set forth in FAR 52227ndash19Commercial Computer Softw ar e-Restricted Rights (J une 1987)

SAS Institute Inc SAS Campus Drive Cary North Carolina 27513

1st print ing October 1999

S AS reg a nd a ll ot her SA S I ns t it u t e I nc p rodu ct or s ervice na m es a re regis t ered t ra dema rkst d k f SA S I t it t I i t h U S A d t h t i reg i di t U SA

Page 44: Statespace Model

8132019 Statespace Model

httpslidepdfcomreaderfullstatespace-model 4452

Part 2 General Information

Example

Example 181 Series J from Box and Jenkins

This example analyzes the gas furnace data (series J) from Box and Jenkins (The

data are not shown Refer to Box and Jenkins (1976) for the data)

First a model is selected and fit automatically using the following statements

title1 rsquoGas Furnace Datarsquo

title2 rsquoBox amp Jenkins Series Jrsquo

title3 rsquoAutomatically Selected Modelrsquo

proc statespace data=seriesj cancorr

var x y

run

The results for the automatically selected model are shown in Output 1811

SAS OnlineDoc 991522 Version 8 1040

8132019 Statespace Model

httpslidepdfcomreaderfullstatespace-model 4552

Chapter 18 Example

Output 1811 Results for Automatically Selected Model

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Number of Observations 296

Standard

Variable Mean Error

x -005683 1072766

y 5350912 3202121

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Information Criterion for Autoregressive Models

Lag=0 Lag=1 Lag=2 Lag=3 Lag=4 Lag=5 Lag=6 Lag=7 Lag=8

6513862 -103357 -163296 -164512 -165152 -164891 -164934 -164315 -163856

Information

Criterion for

Autoregressive

Models

Lag=9 Lag=10

-16348 -163359

Schematic Representation of Correlations

NameLag 0 1 2 3 4 5 6 7 8 9 10

x +- +- +- +- +- +- +- +- +- +- +-

y -+ -+ -+ -+ -+ -+ -+ -+ -+ -+ -+

+ is gt 2std error - is lt -2std error is between

1041SAS OnlineDoc 991522 Version 8

8132019 Statespace Model

httpslidepdfcomreaderfullstatespace-model 4652

Part 2 General Information

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Schematic Representation of Partial Autocorrelations

NameLag 1 2 3 4 5 6 7 8 9 10

x + - + -

y -+ -- - + +

+ is gt 2std error - is lt -2std error is between

Yule-Walker Estimates for Minimum AIC

------Lag=1------ ------Lag=2------ ------Lag=3------ ------Lag=4------

x y x y x y x y

x 1925887 -000124 -120166 0004224 0116918 -000867 0104236 0003268

y 0050496 1299793 -002046 -03277 -071182 -025701 0195411 0133417

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Canonical Correlations Analysis

Information Chi

x(TT) y(TT) x(T+1T) Criterion Square DF

1 1 0804883 2929228 3047481 8

Information Chix(TT) y(TT) x(T+1T) y(T+1T) Criterion Square DF

1 1 0906681 0607529 1223358 1347237 7

Information Chi

x(TT) y(TT) x (T+1T) y (T+1T) x (T+2T) Criterion Square DF

1 1 0909434 0610278 0186274 -154701 1034705 6

Information Chi

x(TT) y(TT) x (T+1T) y (T+1T) y (T+2T) Criterion Square DF

1 1 091014 0618937 0206823 0940392 1280924 6

Information Chi

x(TT) y(TT) x(T+1T) y(T+1T) y(T+2T) y(T+3T) Criterion Square DF

1 1 0912963 0628785 0226598 0083258 -794103 2041584 5

SAS OnlineDoc 991522 Version 8 1042

8132019 Statespace Model

httpslidepdfcomreaderfullstatespace-model 4752

Chapter 18 Example

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Selected Statespace Form and Preliminary Estimates

State Vector

x(TT) y(TT) x(T+1T) y(T+1T) y(T+2T)

Estimate of Transition Matrix

0 0 1 0 0

0 0 0 1 0

-084718 0026794 1711715 -005019 0

0 0 0 0 1

-019785 0334274 -018174 -123557 1787475

Input Matrix for Innovation

1 0

0 11925887 -000124

0050496 1299793

0142421 1361696

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Selected Statespace Form and Preliminary Estimates

Variance Matrix for Innovation

0035274 -000734

-000734 0097569

1043SAS OnlineDoc 991522 Version 8

8132019 Statespace Model

httpslidepdfcomreaderfullstatespace-model 4852

Part 2 General Information

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Selected Statespace Form and Fitted Model

State Vector

x(TT) y(TT) x(T+1T) y(T+1T) y(T+2T)

Estimate of Transition Matrix

0 0 1 0 0

0 0 0 1 0

-086192 0030609 1724235 -005483 0

0 0 0 0 1

-034839 0292124 -009435 -109823 1671418

Input Matrix for Innovation

1 0

0 1192442 -000416

0015621 1258495

008058 1353204

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Selected Statespace Form and Fitted Model

Variance Matrix for Innovation

0035579 -000728

-000728 0095577

Parameter Estimates

Standard

Parameter Estimate Error t Value

F(31) -086192 0072961 -1181

F(32) 0030609 0026167 117

F(33) 1724235 0061599 2799

F(34) -005483 0030169 -182

F(51) -034839 0135253 -258

F(52) 0292124 0046299 631

F(53) -009435 0096527 -098F(54) -109823 0109525 -1003

F(55) 1671418 0083737 1996

G(31) 1924420 0058162 3309

G(32) -000416 0035255 -012

G(41) 0015621 0095771 016

G(42) 1258495 0055742 2258

G(51) 0080580 0151622 053

G(52) 1353204 0091388 1481

SAS OnlineDoc 991522 Version 8 1044

8132019 Statespace Model

httpslidepdfcomreaderfullstatespace-model 4952

Chapter 18 Example

The two series are believed to have a transfer function relation with the gas rate

(variable X) as the input and the CO2

concentration (variable Y) as the output Since

the parameter estimates shown in Output 1811 support this kind of model the model

is reestimated with the feedback parameters restricted to 0 The following statements

fit the transfer function (no feedback) model

title3 rsquoTransfer Function Modelrsquo

proc statespace data=seriesj printout=none

var x y

restrict f(32)=0 f(34)=0

g(32)=0 g(41)=0 g(51)=0

run

The last 2 pages of the output are shown in Output 1812

Output 1812 STATESPACE Output for Transfer Function Model

Gas Furnace Data

Box amp Jenkins Series J

Transfer Function Model

The STATESPACE Procedure

Selected Statespace Form and Fitted Model

State Vector

x(TT) y(TT) x(T+1T) y(T+1T) y(T+2T)

Estimate of Transition Matrix

0 0 1 0 0

0 0 0 1 0

-068882 0 1598717 0 0

0 0 0 0 1

-035944 0284179 -00963 -107313 1650047

Input Matrix for Innovation

1 0

0 1

1923446 0

0 1260856

0 1346332

1045SAS OnlineDoc 991522 Version 8

8132019 Statespace Model

httpslidepdfcomreaderfullstatespace-model 5052

Part 2 General Information

Gas Furnace Data

Box amp Jenkins Series J

Transfer Function Model

The STATESPACE Procedure

Selected Statespace Form and Fitted Model

Variance Matrix for Innovation

0036995 -00072

-00072 0095712

Parameter Estimates

Standard

Parameter Estimate Error t Value

F(31) -068882 0050549 -1363

F(33) 1598717 0050924 3139

F(51) -035944 0229044 -157

F(52) 0284179 0096944 293

F(53) -009630 0140876 -068

F(54) -107313 0250385 -429

F(55) 1650047 0188533 875G(31) 1923446 0056328 3415

G(42) 1260856 0056464 2233

G(52) 1346332 0091086 1478

SAS OnlineDoc 991522 Version 8 1046

8132019 Statespace Model

httpslidepdfcomreaderfullstatespace-model 5152

Chapter 18 References

References

Akaike H (1974) Markovian Representation of Stochastic Processes and Its Ap-

plication to the Analysis of Autoregressive Moving Average Processes Annals

of the Institute of Statistical Mathematics 26 363-387

Akaike H (1976) Canonical Correlations Analysis of Time Series and the Use of

an Information Criterion in Advances and Case Studies in System Identification

eds R Mehra and DG Lainiotis New York Academic Press

Anderson TW (1971) The Statistical Analysis of Time Series New York John

Wiley amp Sons

Ansley CF and Newbold P (1979) Multivariate Partial Autocorrelations Pro-

ceedings of the Business and Economic Statistics Section American Statistical

Association 349-353

Box GEP and Jenkins G (1976) Time Series Analysis Forecasting and Control

San Francisco Holden-Day

Brockwell PJ and Davis RA (1991) Time Series Theory and Methods 2nd Edi-

tion Springer-Verlag

Hannan EJ (1970) Multiple Time Series New York John Wiley amp Sons

Hannan EJ (1976) The Identification and Parameterization of ARMAX and State

Space Forms Econometrica 44 713-722

Harvey AC (1981a) The Econometric Analysis of Time Series New York John

Wiley amp Sons

Harvey AC (1981b) Time Series Models New York John Wiley amp Sons

Jones RH (1974) Identification and Autoregressive Spectrum Estimation IEEE Transactions on Automatic Control AC-19 894-897

Pham-Dinh-Tuan (1978) On the Fitting of Multivariate Processes of the

Autoregressive-Moving Average Type Biometrika 65 99-107

Priestley MB (1980) System Identification Kalman Filtering and Stochastic Con-

trol in Directions in Time Series eds DR Brillinger and GC Tiao Institute

of Mathematical Statistics

Whittle P (1963) On the Fitting of Multivariate Autoregressions and the Approxi-

mate Canonical Factorization of a Spectral Density Matrix Biometrika 50 129-

134

1047SAS OnlineDoc 991522 Version 8

8132019 Statespace Model

httpslidepdfcomreaderfullstatespace-model 5252

The correct bibliographic citation for this manual is as follows SAS Institute Inc SAS

ET S U serrsquos Gui de Version 8 Cary NC SAS Institute Inc 1999 1546 pp

SASET S Userrsquos Guide Version 8

Copyright copy 1999 by S AS I ns t it u t e I nc Ca ry N C U S A

ISBN 1ndash58025ndash489ndash6

All r ight s res erved P r int ed in t he U n it ed St a t es of A merica N o pa rt of t his p u blica t ionma y b e reprodu ced s t ored in a ret r ieva l s ys t em or t ra ns mit t ed in a ny form or b y a nymeans electronic mechanical photocopying or otherwise without the prior writtenpermission of the publisher SAS Institute Inc

US Government Restricted Rights Notice Use du plicat ion or disclosure of the

software by the government is subject to restrictions as set forth in FAR 52227ndash19Commercial Computer Softw ar e-Restricted Rights (J une 1987)

SAS Institute Inc SAS Campus Drive Cary North Carolina 27513

1st print ing October 1999

S AS reg a nd a ll ot her SA S I ns t it u t e I nc p rodu ct or s ervice na m es a re regis t ered t ra dema rkst d k f SA S I t it t I i t h U S A d t h t i reg i di t U SA

Page 45: Statespace Model

8132019 Statespace Model

httpslidepdfcomreaderfullstatespace-model 4552

Chapter 18 Example

Output 1811 Results for Automatically Selected Model

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Number of Observations 296

Standard

Variable Mean Error

x -005683 1072766

y 5350912 3202121

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Information Criterion for Autoregressive Models

Lag=0 Lag=1 Lag=2 Lag=3 Lag=4 Lag=5 Lag=6 Lag=7 Lag=8

6513862 -103357 -163296 -164512 -165152 -164891 -164934 -164315 -163856

Information

Criterion for

Autoregressive

Models

Lag=9 Lag=10

-16348 -163359

Schematic Representation of Correlations

NameLag 0 1 2 3 4 5 6 7 8 9 10

x +- +- +- +- +- +- +- +- +- +- +-

y -+ -+ -+ -+ -+ -+ -+ -+ -+ -+ -+

+ is gt 2std error - is lt -2std error is between

1041SAS OnlineDoc 991522 Version 8

8132019 Statespace Model

httpslidepdfcomreaderfullstatespace-model 4652

Part 2 General Information

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Schematic Representation of Partial Autocorrelations

NameLag 1 2 3 4 5 6 7 8 9 10

x + - + -

y -+ -- - + +

+ is gt 2std error - is lt -2std error is between

Yule-Walker Estimates for Minimum AIC

------Lag=1------ ------Lag=2------ ------Lag=3------ ------Lag=4------

x y x y x y x y

x 1925887 -000124 -120166 0004224 0116918 -000867 0104236 0003268

y 0050496 1299793 -002046 -03277 -071182 -025701 0195411 0133417

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Canonical Correlations Analysis

Information Chi

x(TT) y(TT) x(T+1T) Criterion Square DF

1 1 0804883 2929228 3047481 8

Information Chix(TT) y(TT) x(T+1T) y(T+1T) Criterion Square DF

1 1 0906681 0607529 1223358 1347237 7

Information Chi

x(TT) y(TT) x (T+1T) y (T+1T) x (T+2T) Criterion Square DF

1 1 0909434 0610278 0186274 -154701 1034705 6

Information Chi

x(TT) y(TT) x (T+1T) y (T+1T) y (T+2T) Criterion Square DF

1 1 091014 0618937 0206823 0940392 1280924 6

Information Chi

x(TT) y(TT) x(T+1T) y(T+1T) y(T+2T) y(T+3T) Criterion Square DF

1 1 0912963 0628785 0226598 0083258 -794103 2041584 5

SAS OnlineDoc 991522 Version 8 1042

8132019 Statespace Model

httpslidepdfcomreaderfullstatespace-model 4752

Chapter 18 Example

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Selected Statespace Form and Preliminary Estimates

State Vector

x(TT) y(TT) x(T+1T) y(T+1T) y(T+2T)

Estimate of Transition Matrix

0 0 1 0 0

0 0 0 1 0

-084718 0026794 1711715 -005019 0

0 0 0 0 1

-019785 0334274 -018174 -123557 1787475

Input Matrix for Innovation

1 0

0 11925887 -000124

0050496 1299793

0142421 1361696

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Selected Statespace Form and Preliminary Estimates

Variance Matrix for Innovation

0035274 -000734

-000734 0097569

1043SAS OnlineDoc 991522 Version 8

8132019 Statespace Model

httpslidepdfcomreaderfullstatespace-model 4852

Part 2 General Information

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Selected Statespace Form and Fitted Model

State Vector

x(TT) y(TT) x(T+1T) y(T+1T) y(T+2T)

Estimate of Transition Matrix

0 0 1 0 0

0 0 0 1 0

-086192 0030609 1724235 -005483 0

0 0 0 0 1

-034839 0292124 -009435 -109823 1671418

Input Matrix for Innovation

1 0

0 1192442 -000416

0015621 1258495

008058 1353204

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Selected Statespace Form and Fitted Model

Variance Matrix for Innovation

0035579 -000728

-000728 0095577

Parameter Estimates

Standard

Parameter Estimate Error t Value

F(31) -086192 0072961 -1181

F(32) 0030609 0026167 117

F(33) 1724235 0061599 2799

F(34) -005483 0030169 -182

F(51) -034839 0135253 -258

F(52) 0292124 0046299 631

F(53) -009435 0096527 -098F(54) -109823 0109525 -1003

F(55) 1671418 0083737 1996

G(31) 1924420 0058162 3309

G(32) -000416 0035255 -012

G(41) 0015621 0095771 016

G(42) 1258495 0055742 2258

G(51) 0080580 0151622 053

G(52) 1353204 0091388 1481

SAS OnlineDoc 991522 Version 8 1044

8132019 Statespace Model

httpslidepdfcomreaderfullstatespace-model 4952

Chapter 18 Example

The two series are believed to have a transfer function relation with the gas rate

(variable X) as the input and the CO2

concentration (variable Y) as the output Since

the parameter estimates shown in Output 1811 support this kind of model the model

is reestimated with the feedback parameters restricted to 0 The following statements

fit the transfer function (no feedback) model

title3 rsquoTransfer Function Modelrsquo

proc statespace data=seriesj printout=none

var x y

restrict f(32)=0 f(34)=0

g(32)=0 g(41)=0 g(51)=0

run

The last 2 pages of the output are shown in Output 1812

Output 1812 STATESPACE Output for Transfer Function Model

Gas Furnace Data

Box amp Jenkins Series J

Transfer Function Model

The STATESPACE Procedure

Selected Statespace Form and Fitted Model

State Vector

x(TT) y(TT) x(T+1T) y(T+1T) y(T+2T)

Estimate of Transition Matrix

0 0 1 0 0

0 0 0 1 0

-068882 0 1598717 0 0

0 0 0 0 1

-035944 0284179 -00963 -107313 1650047

Input Matrix for Innovation

1 0

0 1

1923446 0

0 1260856

0 1346332

1045SAS OnlineDoc 991522 Version 8

8132019 Statespace Model

httpslidepdfcomreaderfullstatespace-model 5052

Part 2 General Information

Gas Furnace Data

Box amp Jenkins Series J

Transfer Function Model

The STATESPACE Procedure

Selected Statespace Form and Fitted Model

Variance Matrix for Innovation

0036995 -00072

-00072 0095712

Parameter Estimates

Standard

Parameter Estimate Error t Value

F(31) -068882 0050549 -1363

F(33) 1598717 0050924 3139

F(51) -035944 0229044 -157

F(52) 0284179 0096944 293

F(53) -009630 0140876 -068

F(54) -107313 0250385 -429

F(55) 1650047 0188533 875G(31) 1923446 0056328 3415

G(42) 1260856 0056464 2233

G(52) 1346332 0091086 1478

SAS OnlineDoc 991522 Version 8 1046

8132019 Statespace Model

httpslidepdfcomreaderfullstatespace-model 5152

Chapter 18 References

References

Akaike H (1974) Markovian Representation of Stochastic Processes and Its Ap-

plication to the Analysis of Autoregressive Moving Average Processes Annals

of the Institute of Statistical Mathematics 26 363-387

Akaike H (1976) Canonical Correlations Analysis of Time Series and the Use of

an Information Criterion in Advances and Case Studies in System Identification

eds R Mehra and DG Lainiotis New York Academic Press

Anderson TW (1971) The Statistical Analysis of Time Series New York John

Wiley amp Sons

Ansley CF and Newbold P (1979) Multivariate Partial Autocorrelations Pro-

ceedings of the Business and Economic Statistics Section American Statistical

Association 349-353

Box GEP and Jenkins G (1976) Time Series Analysis Forecasting and Control

San Francisco Holden-Day

Brockwell PJ and Davis RA (1991) Time Series Theory and Methods 2nd Edi-

tion Springer-Verlag

Hannan EJ (1970) Multiple Time Series New York John Wiley amp Sons

Hannan EJ (1976) The Identification and Parameterization of ARMAX and State

Space Forms Econometrica 44 713-722

Harvey AC (1981a) The Econometric Analysis of Time Series New York John

Wiley amp Sons

Harvey AC (1981b) Time Series Models New York John Wiley amp Sons

Jones RH (1974) Identification and Autoregressive Spectrum Estimation IEEE Transactions on Automatic Control AC-19 894-897

Pham-Dinh-Tuan (1978) On the Fitting of Multivariate Processes of the

Autoregressive-Moving Average Type Biometrika 65 99-107

Priestley MB (1980) System Identification Kalman Filtering and Stochastic Con-

trol in Directions in Time Series eds DR Brillinger and GC Tiao Institute

of Mathematical Statistics

Whittle P (1963) On the Fitting of Multivariate Autoregressions and the Approxi-

mate Canonical Factorization of a Spectral Density Matrix Biometrika 50 129-

134

1047SAS OnlineDoc 991522 Version 8

8132019 Statespace Model

httpslidepdfcomreaderfullstatespace-model 5252

The correct bibliographic citation for this manual is as follows SAS Institute Inc SAS

ET S U serrsquos Gui de Version 8 Cary NC SAS Institute Inc 1999 1546 pp

SASET S Userrsquos Guide Version 8

Copyright copy 1999 by S AS I ns t it u t e I nc Ca ry N C U S A

ISBN 1ndash58025ndash489ndash6

All r ight s res erved P r int ed in t he U n it ed St a t es of A merica N o pa rt of t his p u blica t ionma y b e reprodu ced s t ored in a ret r ieva l s ys t em or t ra ns mit t ed in a ny form or b y a nymeans electronic mechanical photocopying or otherwise without the prior writtenpermission of the publisher SAS Institute Inc

US Government Restricted Rights Notice Use du plicat ion or disclosure of the

software by the government is subject to restrictions as set forth in FAR 52227ndash19Commercial Computer Softw ar e-Restricted Rights (J une 1987)

SAS Institute Inc SAS Campus Drive Cary North Carolina 27513

1st print ing October 1999

S AS reg a nd a ll ot her SA S I ns t it u t e I nc p rodu ct or s ervice na m es a re regis t ered t ra dema rkst d k f SA S I t it t I i t h U S A d t h t i reg i di t U SA

Page 46: Statespace Model

8132019 Statespace Model

httpslidepdfcomreaderfullstatespace-model 4652

Part 2 General Information

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Schematic Representation of Partial Autocorrelations

NameLag 1 2 3 4 5 6 7 8 9 10

x + - + -

y -+ -- - + +

+ is gt 2std error - is lt -2std error is between

Yule-Walker Estimates for Minimum AIC

------Lag=1------ ------Lag=2------ ------Lag=3------ ------Lag=4------

x y x y x y x y

x 1925887 -000124 -120166 0004224 0116918 -000867 0104236 0003268

y 0050496 1299793 -002046 -03277 -071182 -025701 0195411 0133417

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Canonical Correlations Analysis

Information Chi

x(TT) y(TT) x(T+1T) Criterion Square DF

1 1 0804883 2929228 3047481 8

Information Chix(TT) y(TT) x(T+1T) y(T+1T) Criterion Square DF

1 1 0906681 0607529 1223358 1347237 7

Information Chi

x(TT) y(TT) x (T+1T) y (T+1T) x (T+2T) Criterion Square DF

1 1 0909434 0610278 0186274 -154701 1034705 6

Information Chi

x(TT) y(TT) x (T+1T) y (T+1T) y (T+2T) Criterion Square DF

1 1 091014 0618937 0206823 0940392 1280924 6

Information Chi

x(TT) y(TT) x(T+1T) y(T+1T) y(T+2T) y(T+3T) Criterion Square DF

1 1 0912963 0628785 0226598 0083258 -794103 2041584 5

SAS OnlineDoc 991522 Version 8 1042

8132019 Statespace Model

httpslidepdfcomreaderfullstatespace-model 4752

Chapter 18 Example

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Selected Statespace Form and Preliminary Estimates

State Vector

x(TT) y(TT) x(T+1T) y(T+1T) y(T+2T)

Estimate of Transition Matrix

0 0 1 0 0

0 0 0 1 0

-084718 0026794 1711715 -005019 0

0 0 0 0 1

-019785 0334274 -018174 -123557 1787475

Input Matrix for Innovation

1 0

0 11925887 -000124

0050496 1299793

0142421 1361696

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Selected Statespace Form and Preliminary Estimates

Variance Matrix for Innovation

0035274 -000734

-000734 0097569

1043SAS OnlineDoc 991522 Version 8

8132019 Statespace Model

httpslidepdfcomreaderfullstatespace-model 4852

Part 2 General Information

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Selected Statespace Form and Fitted Model

State Vector

x(TT) y(TT) x(T+1T) y(T+1T) y(T+2T)

Estimate of Transition Matrix

0 0 1 0 0

0 0 0 1 0

-086192 0030609 1724235 -005483 0

0 0 0 0 1

-034839 0292124 -009435 -109823 1671418

Input Matrix for Innovation

1 0

0 1192442 -000416

0015621 1258495

008058 1353204

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Selected Statespace Form and Fitted Model

Variance Matrix for Innovation

0035579 -000728

-000728 0095577

Parameter Estimates

Standard

Parameter Estimate Error t Value

F(31) -086192 0072961 -1181

F(32) 0030609 0026167 117

F(33) 1724235 0061599 2799

F(34) -005483 0030169 -182

F(51) -034839 0135253 -258

F(52) 0292124 0046299 631

F(53) -009435 0096527 -098F(54) -109823 0109525 -1003

F(55) 1671418 0083737 1996

G(31) 1924420 0058162 3309

G(32) -000416 0035255 -012

G(41) 0015621 0095771 016

G(42) 1258495 0055742 2258

G(51) 0080580 0151622 053

G(52) 1353204 0091388 1481

SAS OnlineDoc 991522 Version 8 1044

8132019 Statespace Model

httpslidepdfcomreaderfullstatespace-model 4952

Chapter 18 Example

The two series are believed to have a transfer function relation with the gas rate

(variable X) as the input and the CO2

concentration (variable Y) as the output Since

the parameter estimates shown in Output 1811 support this kind of model the model

is reestimated with the feedback parameters restricted to 0 The following statements

fit the transfer function (no feedback) model

title3 rsquoTransfer Function Modelrsquo

proc statespace data=seriesj printout=none

var x y

restrict f(32)=0 f(34)=0

g(32)=0 g(41)=0 g(51)=0

run

The last 2 pages of the output are shown in Output 1812

Output 1812 STATESPACE Output for Transfer Function Model

Gas Furnace Data

Box amp Jenkins Series J

Transfer Function Model

The STATESPACE Procedure

Selected Statespace Form and Fitted Model

State Vector

x(TT) y(TT) x(T+1T) y(T+1T) y(T+2T)

Estimate of Transition Matrix

0 0 1 0 0

0 0 0 1 0

-068882 0 1598717 0 0

0 0 0 0 1

-035944 0284179 -00963 -107313 1650047

Input Matrix for Innovation

1 0

0 1

1923446 0

0 1260856

0 1346332

1045SAS OnlineDoc 991522 Version 8

8132019 Statespace Model

httpslidepdfcomreaderfullstatespace-model 5052

Part 2 General Information

Gas Furnace Data

Box amp Jenkins Series J

Transfer Function Model

The STATESPACE Procedure

Selected Statespace Form and Fitted Model

Variance Matrix for Innovation

0036995 -00072

-00072 0095712

Parameter Estimates

Standard

Parameter Estimate Error t Value

F(31) -068882 0050549 -1363

F(33) 1598717 0050924 3139

F(51) -035944 0229044 -157

F(52) 0284179 0096944 293

F(53) -009630 0140876 -068

F(54) -107313 0250385 -429

F(55) 1650047 0188533 875G(31) 1923446 0056328 3415

G(42) 1260856 0056464 2233

G(52) 1346332 0091086 1478

SAS OnlineDoc 991522 Version 8 1046

8132019 Statespace Model

httpslidepdfcomreaderfullstatespace-model 5152

Chapter 18 References

References

Akaike H (1974) Markovian Representation of Stochastic Processes and Its Ap-

plication to the Analysis of Autoregressive Moving Average Processes Annals

of the Institute of Statistical Mathematics 26 363-387

Akaike H (1976) Canonical Correlations Analysis of Time Series and the Use of

an Information Criterion in Advances and Case Studies in System Identification

eds R Mehra and DG Lainiotis New York Academic Press

Anderson TW (1971) The Statistical Analysis of Time Series New York John

Wiley amp Sons

Ansley CF and Newbold P (1979) Multivariate Partial Autocorrelations Pro-

ceedings of the Business and Economic Statistics Section American Statistical

Association 349-353

Box GEP and Jenkins G (1976) Time Series Analysis Forecasting and Control

San Francisco Holden-Day

Brockwell PJ and Davis RA (1991) Time Series Theory and Methods 2nd Edi-

tion Springer-Verlag

Hannan EJ (1970) Multiple Time Series New York John Wiley amp Sons

Hannan EJ (1976) The Identification and Parameterization of ARMAX and State

Space Forms Econometrica 44 713-722

Harvey AC (1981a) The Econometric Analysis of Time Series New York John

Wiley amp Sons

Harvey AC (1981b) Time Series Models New York John Wiley amp Sons

Jones RH (1974) Identification and Autoregressive Spectrum Estimation IEEE Transactions on Automatic Control AC-19 894-897

Pham-Dinh-Tuan (1978) On the Fitting of Multivariate Processes of the

Autoregressive-Moving Average Type Biometrika 65 99-107

Priestley MB (1980) System Identification Kalman Filtering and Stochastic Con-

trol in Directions in Time Series eds DR Brillinger and GC Tiao Institute

of Mathematical Statistics

Whittle P (1963) On the Fitting of Multivariate Autoregressions and the Approxi-

mate Canonical Factorization of a Spectral Density Matrix Biometrika 50 129-

134

1047SAS OnlineDoc 991522 Version 8

8132019 Statespace Model

httpslidepdfcomreaderfullstatespace-model 5252

The correct bibliographic citation for this manual is as follows SAS Institute Inc SAS

ET S U serrsquos Gui de Version 8 Cary NC SAS Institute Inc 1999 1546 pp

SASET S Userrsquos Guide Version 8

Copyright copy 1999 by S AS I ns t it u t e I nc Ca ry N C U S A

ISBN 1ndash58025ndash489ndash6

All r ight s res erved P r int ed in t he U n it ed St a t es of A merica N o pa rt of t his p u blica t ionma y b e reprodu ced s t ored in a ret r ieva l s ys t em or t ra ns mit t ed in a ny form or b y a nymeans electronic mechanical photocopying or otherwise without the prior writtenpermission of the publisher SAS Institute Inc

US Government Restricted Rights Notice Use du plicat ion or disclosure of the

software by the government is subject to restrictions as set forth in FAR 52227ndash19Commercial Computer Softw ar e-Restricted Rights (J une 1987)

SAS Institute Inc SAS Campus Drive Cary North Carolina 27513

1st print ing October 1999

S AS reg a nd a ll ot her SA S I ns t it u t e I nc p rodu ct or s ervice na m es a re regis t ered t ra dema rkst d k f SA S I t it t I i t h U S A d t h t i reg i di t U SA

Page 47: Statespace Model

8132019 Statespace Model

httpslidepdfcomreaderfullstatespace-model 4752

Chapter 18 Example

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Selected Statespace Form and Preliminary Estimates

State Vector

x(TT) y(TT) x(T+1T) y(T+1T) y(T+2T)

Estimate of Transition Matrix

0 0 1 0 0

0 0 0 1 0

-084718 0026794 1711715 -005019 0

0 0 0 0 1

-019785 0334274 -018174 -123557 1787475

Input Matrix for Innovation

1 0

0 11925887 -000124

0050496 1299793

0142421 1361696

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Selected Statespace Form and Preliminary Estimates

Variance Matrix for Innovation

0035274 -000734

-000734 0097569

1043SAS OnlineDoc 991522 Version 8

8132019 Statespace Model

httpslidepdfcomreaderfullstatespace-model 4852

Part 2 General Information

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Selected Statespace Form and Fitted Model

State Vector

x(TT) y(TT) x(T+1T) y(T+1T) y(T+2T)

Estimate of Transition Matrix

0 0 1 0 0

0 0 0 1 0

-086192 0030609 1724235 -005483 0

0 0 0 0 1

-034839 0292124 -009435 -109823 1671418

Input Matrix for Innovation

1 0

0 1192442 -000416

0015621 1258495

008058 1353204

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Selected Statespace Form and Fitted Model

Variance Matrix for Innovation

0035579 -000728

-000728 0095577

Parameter Estimates

Standard

Parameter Estimate Error t Value

F(31) -086192 0072961 -1181

F(32) 0030609 0026167 117

F(33) 1724235 0061599 2799

F(34) -005483 0030169 -182

F(51) -034839 0135253 -258

F(52) 0292124 0046299 631

F(53) -009435 0096527 -098F(54) -109823 0109525 -1003

F(55) 1671418 0083737 1996

G(31) 1924420 0058162 3309

G(32) -000416 0035255 -012

G(41) 0015621 0095771 016

G(42) 1258495 0055742 2258

G(51) 0080580 0151622 053

G(52) 1353204 0091388 1481

SAS OnlineDoc 991522 Version 8 1044

8132019 Statespace Model

httpslidepdfcomreaderfullstatespace-model 4952

Chapter 18 Example

The two series are believed to have a transfer function relation with the gas rate

(variable X) as the input and the CO2

concentration (variable Y) as the output Since

the parameter estimates shown in Output 1811 support this kind of model the model

is reestimated with the feedback parameters restricted to 0 The following statements

fit the transfer function (no feedback) model

title3 rsquoTransfer Function Modelrsquo

proc statespace data=seriesj printout=none

var x y

restrict f(32)=0 f(34)=0

g(32)=0 g(41)=0 g(51)=0

run

The last 2 pages of the output are shown in Output 1812

Output 1812 STATESPACE Output for Transfer Function Model

Gas Furnace Data

Box amp Jenkins Series J

Transfer Function Model

The STATESPACE Procedure

Selected Statespace Form and Fitted Model

State Vector

x(TT) y(TT) x(T+1T) y(T+1T) y(T+2T)

Estimate of Transition Matrix

0 0 1 0 0

0 0 0 1 0

-068882 0 1598717 0 0

0 0 0 0 1

-035944 0284179 -00963 -107313 1650047

Input Matrix for Innovation

1 0

0 1

1923446 0

0 1260856

0 1346332

1045SAS OnlineDoc 991522 Version 8

8132019 Statespace Model

httpslidepdfcomreaderfullstatespace-model 5052

Part 2 General Information

Gas Furnace Data

Box amp Jenkins Series J

Transfer Function Model

The STATESPACE Procedure

Selected Statespace Form and Fitted Model

Variance Matrix for Innovation

0036995 -00072

-00072 0095712

Parameter Estimates

Standard

Parameter Estimate Error t Value

F(31) -068882 0050549 -1363

F(33) 1598717 0050924 3139

F(51) -035944 0229044 -157

F(52) 0284179 0096944 293

F(53) -009630 0140876 -068

F(54) -107313 0250385 -429

F(55) 1650047 0188533 875G(31) 1923446 0056328 3415

G(42) 1260856 0056464 2233

G(52) 1346332 0091086 1478

SAS OnlineDoc 991522 Version 8 1046

8132019 Statespace Model

httpslidepdfcomreaderfullstatespace-model 5152

Chapter 18 References

References

Akaike H (1974) Markovian Representation of Stochastic Processes and Its Ap-

plication to the Analysis of Autoregressive Moving Average Processes Annals

of the Institute of Statistical Mathematics 26 363-387

Akaike H (1976) Canonical Correlations Analysis of Time Series and the Use of

an Information Criterion in Advances and Case Studies in System Identification

eds R Mehra and DG Lainiotis New York Academic Press

Anderson TW (1971) The Statistical Analysis of Time Series New York John

Wiley amp Sons

Ansley CF and Newbold P (1979) Multivariate Partial Autocorrelations Pro-

ceedings of the Business and Economic Statistics Section American Statistical

Association 349-353

Box GEP and Jenkins G (1976) Time Series Analysis Forecasting and Control

San Francisco Holden-Day

Brockwell PJ and Davis RA (1991) Time Series Theory and Methods 2nd Edi-

tion Springer-Verlag

Hannan EJ (1970) Multiple Time Series New York John Wiley amp Sons

Hannan EJ (1976) The Identification and Parameterization of ARMAX and State

Space Forms Econometrica 44 713-722

Harvey AC (1981a) The Econometric Analysis of Time Series New York John

Wiley amp Sons

Harvey AC (1981b) Time Series Models New York John Wiley amp Sons

Jones RH (1974) Identification and Autoregressive Spectrum Estimation IEEE Transactions on Automatic Control AC-19 894-897

Pham-Dinh-Tuan (1978) On the Fitting of Multivariate Processes of the

Autoregressive-Moving Average Type Biometrika 65 99-107

Priestley MB (1980) System Identification Kalman Filtering and Stochastic Con-

trol in Directions in Time Series eds DR Brillinger and GC Tiao Institute

of Mathematical Statistics

Whittle P (1963) On the Fitting of Multivariate Autoregressions and the Approxi-

mate Canonical Factorization of a Spectral Density Matrix Biometrika 50 129-

134

1047SAS OnlineDoc 991522 Version 8

8132019 Statespace Model

httpslidepdfcomreaderfullstatespace-model 5252

The correct bibliographic citation for this manual is as follows SAS Institute Inc SAS

ET S U serrsquos Gui de Version 8 Cary NC SAS Institute Inc 1999 1546 pp

SASET S Userrsquos Guide Version 8

Copyright copy 1999 by S AS I ns t it u t e I nc Ca ry N C U S A

ISBN 1ndash58025ndash489ndash6

All r ight s res erved P r int ed in t he U n it ed St a t es of A merica N o pa rt of t his p u blica t ionma y b e reprodu ced s t ored in a ret r ieva l s ys t em or t ra ns mit t ed in a ny form or b y a nymeans electronic mechanical photocopying or otherwise without the prior writtenpermission of the publisher SAS Institute Inc

US Government Restricted Rights Notice Use du plicat ion or disclosure of the

software by the government is subject to restrictions as set forth in FAR 52227ndash19Commercial Computer Softw ar e-Restricted Rights (J une 1987)

SAS Institute Inc SAS Campus Drive Cary North Carolina 27513

1st print ing October 1999

S AS reg a nd a ll ot her SA S I ns t it u t e I nc p rodu ct or s ervice na m es a re regis t ered t ra dema rkst d k f SA S I t it t I i t h U S A d t h t i reg i di t U SA

Page 48: Statespace Model

8132019 Statespace Model

httpslidepdfcomreaderfullstatespace-model 4852

Part 2 General Information

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Selected Statespace Form and Fitted Model

State Vector

x(TT) y(TT) x(T+1T) y(T+1T) y(T+2T)

Estimate of Transition Matrix

0 0 1 0 0

0 0 0 1 0

-086192 0030609 1724235 -005483 0

0 0 0 0 1

-034839 0292124 -009435 -109823 1671418

Input Matrix for Innovation

1 0

0 1192442 -000416

0015621 1258495

008058 1353204

Gas Furnace Data

Box amp Jenkins Series J

Automatically Selected Model

The STATESPACE Procedure

Selected Statespace Form and Fitted Model

Variance Matrix for Innovation

0035579 -000728

-000728 0095577

Parameter Estimates

Standard

Parameter Estimate Error t Value

F(31) -086192 0072961 -1181

F(32) 0030609 0026167 117

F(33) 1724235 0061599 2799

F(34) -005483 0030169 -182

F(51) -034839 0135253 -258

F(52) 0292124 0046299 631

F(53) -009435 0096527 -098F(54) -109823 0109525 -1003

F(55) 1671418 0083737 1996

G(31) 1924420 0058162 3309

G(32) -000416 0035255 -012

G(41) 0015621 0095771 016

G(42) 1258495 0055742 2258

G(51) 0080580 0151622 053

G(52) 1353204 0091388 1481

SAS OnlineDoc 991522 Version 8 1044

8132019 Statespace Model

httpslidepdfcomreaderfullstatespace-model 4952

Chapter 18 Example

The two series are believed to have a transfer function relation with the gas rate

(variable X) as the input and the CO2

concentration (variable Y) as the output Since

the parameter estimates shown in Output 1811 support this kind of model the model

is reestimated with the feedback parameters restricted to 0 The following statements

fit the transfer function (no feedback) model

title3 rsquoTransfer Function Modelrsquo

proc statespace data=seriesj printout=none

var x y

restrict f(32)=0 f(34)=0

g(32)=0 g(41)=0 g(51)=0

run

The last 2 pages of the output are shown in Output 1812

Output 1812 STATESPACE Output for Transfer Function Model

Gas Furnace Data

Box amp Jenkins Series J

Transfer Function Model

The STATESPACE Procedure

Selected Statespace Form and Fitted Model

State Vector

x(TT) y(TT) x(T+1T) y(T+1T) y(T+2T)

Estimate of Transition Matrix

0 0 1 0 0

0 0 0 1 0

-068882 0 1598717 0 0

0 0 0 0 1

-035944 0284179 -00963 -107313 1650047

Input Matrix for Innovation

1 0

0 1

1923446 0

0 1260856

0 1346332

1045SAS OnlineDoc 991522 Version 8

8132019 Statespace Model

httpslidepdfcomreaderfullstatespace-model 5052

Part 2 General Information

Gas Furnace Data

Box amp Jenkins Series J

Transfer Function Model

The STATESPACE Procedure

Selected Statespace Form and Fitted Model

Variance Matrix for Innovation

0036995 -00072

-00072 0095712

Parameter Estimates

Standard

Parameter Estimate Error t Value

F(31) -068882 0050549 -1363

F(33) 1598717 0050924 3139

F(51) -035944 0229044 -157

F(52) 0284179 0096944 293

F(53) -009630 0140876 -068

F(54) -107313 0250385 -429

F(55) 1650047 0188533 875G(31) 1923446 0056328 3415

G(42) 1260856 0056464 2233

G(52) 1346332 0091086 1478

SAS OnlineDoc 991522 Version 8 1046

8132019 Statespace Model

httpslidepdfcomreaderfullstatespace-model 5152

Chapter 18 References

References

Akaike H (1974) Markovian Representation of Stochastic Processes and Its Ap-

plication to the Analysis of Autoregressive Moving Average Processes Annals

of the Institute of Statistical Mathematics 26 363-387

Akaike H (1976) Canonical Correlations Analysis of Time Series and the Use of

an Information Criterion in Advances and Case Studies in System Identification

eds R Mehra and DG Lainiotis New York Academic Press

Anderson TW (1971) The Statistical Analysis of Time Series New York John

Wiley amp Sons

Ansley CF and Newbold P (1979) Multivariate Partial Autocorrelations Pro-

ceedings of the Business and Economic Statistics Section American Statistical

Association 349-353

Box GEP and Jenkins G (1976) Time Series Analysis Forecasting and Control

San Francisco Holden-Day

Brockwell PJ and Davis RA (1991) Time Series Theory and Methods 2nd Edi-

tion Springer-Verlag

Hannan EJ (1970) Multiple Time Series New York John Wiley amp Sons

Hannan EJ (1976) The Identification and Parameterization of ARMAX and State

Space Forms Econometrica 44 713-722

Harvey AC (1981a) The Econometric Analysis of Time Series New York John

Wiley amp Sons

Harvey AC (1981b) Time Series Models New York John Wiley amp Sons

Jones RH (1974) Identification and Autoregressive Spectrum Estimation IEEE Transactions on Automatic Control AC-19 894-897

Pham-Dinh-Tuan (1978) On the Fitting of Multivariate Processes of the

Autoregressive-Moving Average Type Biometrika 65 99-107

Priestley MB (1980) System Identification Kalman Filtering and Stochastic Con-

trol in Directions in Time Series eds DR Brillinger and GC Tiao Institute

of Mathematical Statistics

Whittle P (1963) On the Fitting of Multivariate Autoregressions and the Approxi-

mate Canonical Factorization of a Spectral Density Matrix Biometrika 50 129-

134

1047SAS OnlineDoc 991522 Version 8

8132019 Statespace Model

httpslidepdfcomreaderfullstatespace-model 5252

The correct bibliographic citation for this manual is as follows SAS Institute Inc SAS

ET S U serrsquos Gui de Version 8 Cary NC SAS Institute Inc 1999 1546 pp

SASET S Userrsquos Guide Version 8

Copyright copy 1999 by S AS I ns t it u t e I nc Ca ry N C U S A

ISBN 1ndash58025ndash489ndash6

All r ight s res erved P r int ed in t he U n it ed St a t es of A merica N o pa rt of t his p u blica t ionma y b e reprodu ced s t ored in a ret r ieva l s ys t em or t ra ns mit t ed in a ny form or b y a nymeans electronic mechanical photocopying or otherwise without the prior writtenpermission of the publisher SAS Institute Inc

US Government Restricted Rights Notice Use du plicat ion or disclosure of the

software by the government is subject to restrictions as set forth in FAR 52227ndash19Commercial Computer Softw ar e-Restricted Rights (J une 1987)

SAS Institute Inc SAS Campus Drive Cary North Carolina 27513

1st print ing October 1999

S AS reg a nd a ll ot her SA S I ns t it u t e I nc p rodu ct or s ervice na m es a re regis t ered t ra dema rkst d k f SA S I t it t I i t h U S A d t h t i reg i di t U SA

Page 49: Statespace Model

8132019 Statespace Model

httpslidepdfcomreaderfullstatespace-model 4952

Chapter 18 Example

The two series are believed to have a transfer function relation with the gas rate

(variable X) as the input and the CO2

concentration (variable Y) as the output Since

the parameter estimates shown in Output 1811 support this kind of model the model

is reestimated with the feedback parameters restricted to 0 The following statements

fit the transfer function (no feedback) model

title3 rsquoTransfer Function Modelrsquo

proc statespace data=seriesj printout=none

var x y

restrict f(32)=0 f(34)=0

g(32)=0 g(41)=0 g(51)=0

run

The last 2 pages of the output are shown in Output 1812

Output 1812 STATESPACE Output for Transfer Function Model

Gas Furnace Data

Box amp Jenkins Series J

Transfer Function Model

The STATESPACE Procedure

Selected Statespace Form and Fitted Model

State Vector

x(TT) y(TT) x(T+1T) y(T+1T) y(T+2T)

Estimate of Transition Matrix

0 0 1 0 0

0 0 0 1 0

-068882 0 1598717 0 0

0 0 0 0 1

-035944 0284179 -00963 -107313 1650047

Input Matrix for Innovation

1 0

0 1

1923446 0

0 1260856

0 1346332

1045SAS OnlineDoc 991522 Version 8

8132019 Statespace Model

httpslidepdfcomreaderfullstatespace-model 5052

Part 2 General Information

Gas Furnace Data

Box amp Jenkins Series J

Transfer Function Model

The STATESPACE Procedure

Selected Statespace Form and Fitted Model

Variance Matrix for Innovation

0036995 -00072

-00072 0095712

Parameter Estimates

Standard

Parameter Estimate Error t Value

F(31) -068882 0050549 -1363

F(33) 1598717 0050924 3139

F(51) -035944 0229044 -157

F(52) 0284179 0096944 293

F(53) -009630 0140876 -068

F(54) -107313 0250385 -429

F(55) 1650047 0188533 875G(31) 1923446 0056328 3415

G(42) 1260856 0056464 2233

G(52) 1346332 0091086 1478

SAS OnlineDoc 991522 Version 8 1046

8132019 Statespace Model

httpslidepdfcomreaderfullstatespace-model 5152

Chapter 18 References

References

Akaike H (1974) Markovian Representation of Stochastic Processes and Its Ap-

plication to the Analysis of Autoregressive Moving Average Processes Annals

of the Institute of Statistical Mathematics 26 363-387

Akaike H (1976) Canonical Correlations Analysis of Time Series and the Use of

an Information Criterion in Advances and Case Studies in System Identification

eds R Mehra and DG Lainiotis New York Academic Press

Anderson TW (1971) The Statistical Analysis of Time Series New York John

Wiley amp Sons

Ansley CF and Newbold P (1979) Multivariate Partial Autocorrelations Pro-

ceedings of the Business and Economic Statistics Section American Statistical

Association 349-353

Box GEP and Jenkins G (1976) Time Series Analysis Forecasting and Control

San Francisco Holden-Day

Brockwell PJ and Davis RA (1991) Time Series Theory and Methods 2nd Edi-

tion Springer-Verlag

Hannan EJ (1970) Multiple Time Series New York John Wiley amp Sons

Hannan EJ (1976) The Identification and Parameterization of ARMAX and State

Space Forms Econometrica 44 713-722

Harvey AC (1981a) The Econometric Analysis of Time Series New York John

Wiley amp Sons

Harvey AC (1981b) Time Series Models New York John Wiley amp Sons

Jones RH (1974) Identification and Autoregressive Spectrum Estimation IEEE Transactions on Automatic Control AC-19 894-897

Pham-Dinh-Tuan (1978) On the Fitting of Multivariate Processes of the

Autoregressive-Moving Average Type Biometrika 65 99-107

Priestley MB (1980) System Identification Kalman Filtering and Stochastic Con-

trol in Directions in Time Series eds DR Brillinger and GC Tiao Institute

of Mathematical Statistics

Whittle P (1963) On the Fitting of Multivariate Autoregressions and the Approxi-

mate Canonical Factorization of a Spectral Density Matrix Biometrika 50 129-

134

1047SAS OnlineDoc 991522 Version 8

8132019 Statespace Model

httpslidepdfcomreaderfullstatespace-model 5252

The correct bibliographic citation for this manual is as follows SAS Institute Inc SAS

ET S U serrsquos Gui de Version 8 Cary NC SAS Institute Inc 1999 1546 pp

SASET S Userrsquos Guide Version 8

Copyright copy 1999 by S AS I ns t it u t e I nc Ca ry N C U S A

ISBN 1ndash58025ndash489ndash6

All r ight s res erved P r int ed in t he U n it ed St a t es of A merica N o pa rt of t his p u blica t ionma y b e reprodu ced s t ored in a ret r ieva l s ys t em or t ra ns mit t ed in a ny form or b y a nymeans electronic mechanical photocopying or otherwise without the prior writtenpermission of the publisher SAS Institute Inc

US Government Restricted Rights Notice Use du plicat ion or disclosure of the

software by the government is subject to restrictions as set forth in FAR 52227ndash19Commercial Computer Softw ar e-Restricted Rights (J une 1987)

SAS Institute Inc SAS Campus Drive Cary North Carolina 27513

1st print ing October 1999

S AS reg a nd a ll ot her SA S I ns t it u t e I nc p rodu ct or s ervice na m es a re regis t ered t ra dema rkst d k f SA S I t it t I i t h U S A d t h t i reg i di t U SA

Page 50: Statespace Model

8132019 Statespace Model

httpslidepdfcomreaderfullstatespace-model 5052

Part 2 General Information

Gas Furnace Data

Box amp Jenkins Series J

Transfer Function Model

The STATESPACE Procedure

Selected Statespace Form and Fitted Model

Variance Matrix for Innovation

0036995 -00072

-00072 0095712

Parameter Estimates

Standard

Parameter Estimate Error t Value

F(31) -068882 0050549 -1363

F(33) 1598717 0050924 3139

F(51) -035944 0229044 -157

F(52) 0284179 0096944 293

F(53) -009630 0140876 -068

F(54) -107313 0250385 -429

F(55) 1650047 0188533 875G(31) 1923446 0056328 3415

G(42) 1260856 0056464 2233

G(52) 1346332 0091086 1478

SAS OnlineDoc 991522 Version 8 1046

8132019 Statespace Model

httpslidepdfcomreaderfullstatespace-model 5152

Chapter 18 References

References

Akaike H (1974) Markovian Representation of Stochastic Processes and Its Ap-

plication to the Analysis of Autoregressive Moving Average Processes Annals

of the Institute of Statistical Mathematics 26 363-387

Akaike H (1976) Canonical Correlations Analysis of Time Series and the Use of

an Information Criterion in Advances and Case Studies in System Identification

eds R Mehra and DG Lainiotis New York Academic Press

Anderson TW (1971) The Statistical Analysis of Time Series New York John

Wiley amp Sons

Ansley CF and Newbold P (1979) Multivariate Partial Autocorrelations Pro-

ceedings of the Business and Economic Statistics Section American Statistical

Association 349-353

Box GEP and Jenkins G (1976) Time Series Analysis Forecasting and Control

San Francisco Holden-Day

Brockwell PJ and Davis RA (1991) Time Series Theory and Methods 2nd Edi-

tion Springer-Verlag

Hannan EJ (1970) Multiple Time Series New York John Wiley amp Sons

Hannan EJ (1976) The Identification and Parameterization of ARMAX and State

Space Forms Econometrica 44 713-722

Harvey AC (1981a) The Econometric Analysis of Time Series New York John

Wiley amp Sons

Harvey AC (1981b) Time Series Models New York John Wiley amp Sons

Jones RH (1974) Identification and Autoregressive Spectrum Estimation IEEE Transactions on Automatic Control AC-19 894-897

Pham-Dinh-Tuan (1978) On the Fitting of Multivariate Processes of the

Autoregressive-Moving Average Type Biometrika 65 99-107

Priestley MB (1980) System Identification Kalman Filtering and Stochastic Con-

trol in Directions in Time Series eds DR Brillinger and GC Tiao Institute

of Mathematical Statistics

Whittle P (1963) On the Fitting of Multivariate Autoregressions and the Approxi-

mate Canonical Factorization of a Spectral Density Matrix Biometrika 50 129-

134

1047SAS OnlineDoc 991522 Version 8

8132019 Statespace Model

httpslidepdfcomreaderfullstatespace-model 5252

The correct bibliographic citation for this manual is as follows SAS Institute Inc SAS

ET S U serrsquos Gui de Version 8 Cary NC SAS Institute Inc 1999 1546 pp

SASET S Userrsquos Guide Version 8

Copyright copy 1999 by S AS I ns t it u t e I nc Ca ry N C U S A

ISBN 1ndash58025ndash489ndash6

All r ight s res erved P r int ed in t he U n it ed St a t es of A merica N o pa rt of t his p u blica t ionma y b e reprodu ced s t ored in a ret r ieva l s ys t em or t ra ns mit t ed in a ny form or b y a nymeans electronic mechanical photocopying or otherwise without the prior writtenpermission of the publisher SAS Institute Inc

US Government Restricted Rights Notice Use du plicat ion or disclosure of the

software by the government is subject to restrictions as set forth in FAR 52227ndash19Commercial Computer Softw ar e-Restricted Rights (J une 1987)

SAS Institute Inc SAS Campus Drive Cary North Carolina 27513

1st print ing October 1999

S AS reg a nd a ll ot her SA S I ns t it u t e I nc p rodu ct or s ervice na m es a re regis t ered t ra dema rkst d k f SA S I t it t I i t h U S A d t h t i reg i di t U SA

Page 51: Statespace Model

8132019 Statespace Model

httpslidepdfcomreaderfullstatespace-model 5152

Chapter 18 References

References

Akaike H (1974) Markovian Representation of Stochastic Processes and Its Ap-

plication to the Analysis of Autoregressive Moving Average Processes Annals

of the Institute of Statistical Mathematics 26 363-387

Akaike H (1976) Canonical Correlations Analysis of Time Series and the Use of

an Information Criterion in Advances and Case Studies in System Identification

eds R Mehra and DG Lainiotis New York Academic Press

Anderson TW (1971) The Statistical Analysis of Time Series New York John

Wiley amp Sons

Ansley CF and Newbold P (1979) Multivariate Partial Autocorrelations Pro-

ceedings of the Business and Economic Statistics Section American Statistical

Association 349-353

Box GEP and Jenkins G (1976) Time Series Analysis Forecasting and Control

San Francisco Holden-Day

Brockwell PJ and Davis RA (1991) Time Series Theory and Methods 2nd Edi-

tion Springer-Verlag

Hannan EJ (1970) Multiple Time Series New York John Wiley amp Sons

Hannan EJ (1976) The Identification and Parameterization of ARMAX and State

Space Forms Econometrica 44 713-722

Harvey AC (1981a) The Econometric Analysis of Time Series New York John

Wiley amp Sons

Harvey AC (1981b) Time Series Models New York John Wiley amp Sons

Jones RH (1974) Identification and Autoregressive Spectrum Estimation IEEE Transactions on Automatic Control AC-19 894-897

Pham-Dinh-Tuan (1978) On the Fitting of Multivariate Processes of the

Autoregressive-Moving Average Type Biometrika 65 99-107

Priestley MB (1980) System Identification Kalman Filtering and Stochastic Con-

trol in Directions in Time Series eds DR Brillinger and GC Tiao Institute

of Mathematical Statistics

Whittle P (1963) On the Fitting of Multivariate Autoregressions and the Approxi-

mate Canonical Factorization of a Spectral Density Matrix Biometrika 50 129-

134

1047SAS OnlineDoc 991522 Version 8

8132019 Statespace Model

httpslidepdfcomreaderfullstatespace-model 5252

The correct bibliographic citation for this manual is as follows SAS Institute Inc SAS

ET S U serrsquos Gui de Version 8 Cary NC SAS Institute Inc 1999 1546 pp

SASET S Userrsquos Guide Version 8

Copyright copy 1999 by S AS I ns t it u t e I nc Ca ry N C U S A

ISBN 1ndash58025ndash489ndash6

All r ight s res erved P r int ed in t he U n it ed St a t es of A merica N o pa rt of t his p u blica t ionma y b e reprodu ced s t ored in a ret r ieva l s ys t em or t ra ns mit t ed in a ny form or b y a nymeans electronic mechanical photocopying or otherwise without the prior writtenpermission of the publisher SAS Institute Inc

US Government Restricted Rights Notice Use du plicat ion or disclosure of the

software by the government is subject to restrictions as set forth in FAR 52227ndash19Commercial Computer Softw ar e-Restricted Rights (J une 1987)

SAS Institute Inc SAS Campus Drive Cary North Carolina 27513

1st print ing October 1999

S AS reg a nd a ll ot her SA S I ns t it u t e I nc p rodu ct or s ervice na m es a re regis t ered t ra dema rkst d k f SA S I t it t I i t h U S A d t h t i reg i di t U SA

Page 52: Statespace Model

8132019 Statespace Model

httpslidepdfcomreaderfullstatespace-model 5252

The correct bibliographic citation for this manual is as follows SAS Institute Inc SAS

ET S U serrsquos Gui de Version 8 Cary NC SAS Institute Inc 1999 1546 pp

SASET S Userrsquos Guide Version 8

Copyright copy 1999 by S AS I ns t it u t e I nc Ca ry N C U S A

ISBN 1ndash58025ndash489ndash6

All r ight s res erved P r int ed in t he U n it ed St a t es of A merica N o pa rt of t his p u blica t ionma y b e reprodu ced s t ored in a ret r ieva l s ys t em or t ra ns mit t ed in a ny form or b y a nymeans electronic mechanical photocopying or otherwise without the prior writtenpermission of the publisher SAS Institute Inc

US Government Restricted Rights Notice Use du plicat ion or disclosure of the

software by the government is subject to restrictions as set forth in FAR 52227ndash19Commercial Computer Softw ar e-Restricted Rights (J une 1987)

SAS Institute Inc SAS Campus Drive Cary North Carolina 27513

1st print ing October 1999

S AS reg a nd a ll ot her SA S I ns t it u t e I nc p rodu ct or s ervice na m es a re regis t ered t ra dema rkst d k f SA S I t it t I i t h U S A d t h t i reg i di t U SA