dsci 5340: predictive modeling and business forecasting spring 2013 – dr. nick evangelopoulos

DSCI 5340: Predictive Modeling and Business Forecasting

Spring 2013 – Dr. Nick Evangelopoulos

Lecture 8: Estimation & Diagnostic Checking in

Box-Jenkins Models (Ch. 10)

Material based on:Bowerman-O’Connell-Koehler, Brooks/Cole

DSCI 5340FORECASTING

Page 443-445Ex 9.5,Ex 9.6Ex 9.7

Homework in Textbook


EX 9.5 Page 443

Assume time origin is 122.

y122 = 15.9265 is given.

Y122hat = 16.1880 from page 427

Y122 – Y122hat = a122hat

= 15.9265-16.1880 = -.2615


EX 9.5 43 Y123hat

y123hat = y122hat + a123hat - 1hat*a122hat

Note that at time origin 122, we do know y122.

Note that we can compute a122hat since we know y122.

a123hat is zero since we do not know y123.


y123(hat) = 15.9265 + 0 - (-.3534)(-.2615) =15.83409


EX 9.5 Page 443 Y124hat


Note that at time origin 122, we do not know y123 – so we need to use y123hat.

Note that we cannot compute a123hat or a124 since we are at time origin 122.


y124(hat) = 15.83409 + 0 - (-.3534)(0) =15.83409


EX 9.6 Page 443

SAC cutoffs after lag 2.

SPAC dies down.

Second order moving average


EX 9.7 Page 445

Part a: Close call – first difference would be better.


EX 9.7 Page 445 part b

SAC dies down and SPAC cuts off after lag of 2.

AR(2) is best fit.


EX 9.7 Page 445 part c.

SPAC seems to die down slower than SAC.

Assuming SAC cuts off, we can say that SAC cuts off after lag 1.

Hence, MA(1).


Questions

1. What is the name of the function that identifies the order of an autoregressive B-J model?

SPAC – Sample Partial Autocorrelation function

2. What is the name of the function that identifies the order of a moving average B-J model?

SAC – Sample Autocorrelation function


Questions

3. What happens to the mean and variance of an AR(1) model if the 1 coefficient is equal to 1? Greater than 1?

Mean is undefined for

1 value equal to 1 and has

a sign opposite to that of

the constant

for a 1 value > 1.

Variance is undefined

for value =1.

1

11

1)(

)1(

t

ttt

yE

ARayy

1

)1()var(

1

21

2

ty


Questions

4. Is the model yt = yt-1 + at stationary or nonstationary? Why?

Note that yt = yt-1 + at implies

yt = (yt-2 + at-1) + at and this implies

yt = (yt-3 + at-2) + at-1 + at

So yt = at + at-1 + at-2 + …

The variance of yt is the sum of approximately infinite variances from each independent noise term. Each yt has higher variance then the one before it.

yt is not stationary. But zt = yt –yt-1 is stationary.


13

The condition for stationarity of a general AR(p) model is that the roots of all lie outside the unit circle.

A stationary AR(p) model is required for it to have an MA() representation.

Example 1: Is yt = yt-1 + ut stationary?

The characteristic root is 1, so it is a unit root process (so non-stationary)

Example 2: Is yt = 3yt-1 - 0.25yt-2 + 0.75yt-3 +ut stationary?

The characteristic roots are 1, 2/3, and 2. Since only one of these lies outside the unit circle, the process is non-stationary.

The Stationary Condition for an AR Model

1 01 22 z z zp

p...


Invertibility of a MA process

zt can be expressed as an infinite series of past z-observations. However, if the coefficients (weights) do not decline fast enough then the model will be unstable.

Invertibility implies that recent observations count more heavily than distant past observations.


Conditions for Stationarity and Invertibility


Point Estimates – Preliminary & Final

Computer Packages ( such as MINITAB and SAS) supply default preliminary estimates for time series models. These estimates must meet stationarity and invertibility conditions.

The easiest preliminary point estimate that almost always satisfies these conditions is the value of .1.

We can usually do better. For example, for an AR(1), the first order autocorrelation is equal to the autoregressive coefficient


Preliminary Estimate for constant term)in an AR(1)


Iterative Search for Coefficient Estimates


Box and Jenkins (1970) were the first to approach the task of estimating an ARMA model in a systematic manner. There are 3 steps to their approach:

1. Identification

2. Estimation

3. Model diagnostic checking

Step 1:

- Involves determining the order of the model.

- Use of graphical procedures

- A better procedure is now available

Building ARMA Models – The Box Jenkins Approach


Step 2:

- Estimation of the parameters

- Can be done using least squares or maximum likelihood depending on the model.

Step 3:

- Model checking

Box and Jenkins suggest 2 methods:

- deliberate overfitting

- residual diagnostics

Building ARMA Models – The Box Jenkins Approach (cont’d)


Diagnostic Checking Using Autocorrelations of Residuals


Chi-Square Distribution Associated with Ljung-Box Statistic


Ljung-Box Values in SAS printout

Large p-values under Pr > ChiSq indicate that the model is adequate.


MINITAB Example of Ljung-Box Statistic

Large p-values indicate no more terms need to be added to model.


RSAC and RSPAC should not have significant spikes


Prediction Intervals

np is the number of parameters being estimated in the model.


Page 480Ex 10.1 through 10.6

Homework in Textbook

dsci 5340: predictive modeling and business forecasting spring 2013 – dr. nick evangelopoulos

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