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Outer Product Analysis (OPA) studying the relations among sets of variables measured on the same individuals. Douglas N. Rutledge. Some publications on Outer Product Analysis. Infrared spectroscopy and outer product analysis for quantification of fat, nitrogen, and moisture of cocoa powder - PowerPoint PPT Presentation

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Outer Product Analysis (OPA)

studying the relations among sets of variables measured on the same individuals

Douglas N. Rutledge

Some publications on Outer Product Analysis

Infrared spectroscopy and outer product analysis for quantification of fat, nitrogen, and moisture of cocoa powder

A. Vesela, A. S. Barros, A. Synytsya, I. Delgadillo, J. Copıkova, M. A. Coimbra

Analytica Chimica Acta 601 (2007) 77–86

Multi-way analysis of outer product arrays using PARAFACD. N. Rutledge, D. Jouan-Rimbaud BouveresseChemometrics and Intelligent Laboratory Systems 85 (2007) 170–178

Image processing of outer-product matrices – a new way to classify samples. Examples using visible/NIR/MIR spectral data

B. Jaillais, V. Morrin, G. DowneyChemometrics and Intelligent Laboratory Systems xx (2006) xxx–xxx

Some publications on Outer Product Analysis

Variability of cork from Portugese Quercus suber studied by solid state 13C-NMR and FTIR spectroscopies

M.H. Lopes, A.S. Barros, C. Pascoal Neto, D. Rutledge, I. Delgadillo, A. M. GilBiopolymers (Biospectroscopy) 62 (5) (2001) 268–277

Outer Product Analysis of electronic nose and visible spectra:

application to the measurement of peach fruit characteristicsC. di Natale, M. Zude-Sasse, A. Macagnano, R. Paolesse, B. Herold, A.

D'AmicoAnalytica Chimica Acta 459 (2002) 107–117

Determination of the degree of methylesterification of pectic polysaccharides by FT-IR using an outer product PLS1 regression

A.S. Barros, I. Mafra, D. Ferreira, S. Cardoso, A. Reis, J.A. Lopes de Silva, I. Delgadillo, D.N. Rutledge, M.A. Coimbra

Carbohydrate Polymers 50 (2002) 85–94

Some publications on Outer Product Analysis

Enhanced multivariate analysis by correlation scaling and

fusion of LC/MS and 1H NMR dataJ. Forshed, R. Stolt, H. Idborg, S. P. JacobssonChemometrics and Intelligent Laboratory Systems 85 (2007) 179–185

Outer-product analysis (OPA) using PCA to study the influence of temperature on NIR spectra of water

B. Jaillais, R. Pinto, A.S. Barros, D.N. RutledgeVibrational Spectroscopy 39 (2005) 50–58

Outer-product analysis (OPA) using PLS regression to study the retrogradation of starch

B. Jaillais, M.A. Ottenhof, I.A. Farhat, D.N. RutledgeVibrational Spectroscopy 40 (2006) 10–19

Principal Components Analysis (PCA)

Calculate the covariance matrix, C, of the original data matrix, X

Covariance Matrix : C

p

p

Cij = cov(i,j)

1 2

1 1 2 21, 2 12

1 2 ,( , )1

n

i ii i

x x

x x x x

Cov x x sn

Calculate the matrix of individual covariances between variables of one data set, X

xiT

xi

Ci = xiT . xi

1 p

1

p

p

p

Mutual weighting of each signal by the other:• if intensities simultaneously high in the two domains, the product is higher;• if intensities simultaneously low in the two domains, the product is lower;• if one intensity high and the other low, the product tends to an intermediate value

For n samples, one gets n Outer Product matrices

Group them together one under the other in the form of a cube of individual matrices of covariances among variables

1

.

.

n

1

p1 p

1

.

n

1

p

1 p

Calculate all the individual covariance matricesof a single matrix, X

A cube of symmetrical matrices

Calculate the mean of the individual covariance matrices to have a :matrix of mean covariances

1

.

.

n

1

p1 p

1

.

n

1

p

1 p

1

p 1 p

Decomposition of the column-mean matrix by SVD Principal Components Analysis

Decomposition of the « Mean » OP matrix by SVD≡ Principal Components Analysis

SVD applied to the initial data matrix, X

S : diagonal matrix of singular values V : loadings matrix U*S : scores matrix

20 40 60 80 100 120 140 160

5

10

15

20

25

30

X(n,p) = U(n,r) S(r,r) VT(r,p)

SVD applied the covariance matrix, XTXor column-means of the Outer Product cube

S2 : diagonal matrix of eigenvaluesV : loadings matrix X*V : scores matrix

20 40 60 80 100 120 140 160

20

40

60

80

100

120

140

160

XTX(p,p) = V(p,r) S2(r,r) VT

(r,p)

Lignin-starch mixtures by TD-NMR

Application of « Mean » Outer Product Analysis to real data (1)

50 100 150 200 2500

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

D.N. Rutledge, Food Control, (2001) 12(7), 437-445

50

100

150

200

250

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

50 100 150 200 250

50

100

150

200

250

50 100 150 200 2500

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

Column-means of Outer Products

0 10 20 30 40 50 60-0.6

-0.4

-0.2

0

0.2

0.4

0.6

0.8

50 100 150 200 250

-0.2

-0.1

0

0.1

0.2

0.3

Decomposition of the matrix by SVD(Principal Components Analysis)

X*VScores on PC2, PC3 & PC4

VLoadings on PC2, PC3 & PC4

Retrogradation of starch by X-ray diffraction

Application of « Mean » Outer Product Analysis to real data (2)

Diffraction Rayons X

0 50 100 150 200 250 300 350-100

0

100

200

300

400

500

B. Jaillais, M.A. Ottenhof, I.A. Farhat, D.N. Rutledge, Vib. Spec. (2006), 40, 10–19.

Column-means of Outer Products

50 100 150 200 250 300

50

100

150

200

250

300

50 100 150 200 250 300

0

50

100

150

200

250

300

350

400

450

50

100

150

200

250

300

0 50

100

150

200

250

300

350

400

450

50 100 150 200 250 300

-0.1

-0.05

0

0.05

0.1

0.15

1 2 3 4 5 6 7 8 9-5

-4

-3

-2

-1

0

1

2

3

4

5

Decomposition of the matrix by SVDPrincipal Components Analysis

X*VScores on PC2

VLoadings on PC2

Unfold the cube to form a matrix

1

.

.

n

1

p 1 p

1

.

n

1

p

1 p

« Unfold » Outer Product Analysis

Analyse the unfolded individual covariance matrices

n-PLS, n-PCA (ANOVA) …

p x p

n

1

Different data unfolding schemes

X

3 x PCA

n

qp

X1

np x

q

X3

p

n x

q

X2

p

n x

p

1 2 3 4 5 6

x 104

5

10

15

20

25

30

35

40

45

50

55

Lignin-starch mixtures by TD-NMRunfolded OP matrix (X1)

Application of unfolded OP to real data (1)

n

p x p

50 100 150 200 250

50

100

150

200

250

0 10 20 30 40 50 60-10

-8

-6

-4

-2

0

2

4

6

8

Decompose the unfolded OP matrix (X1) by SVD(Unfold-PCA)

U*SScores of X1 on PC2, PC3 & PC4

VRefolded Loadings of X1

on PC2, PC3 & PC4

50 100 150 200 250

50

100

150

200

250

50 100 150 200 250

50

100

150

200

250

p

p

50 100 150 200 250

-4

-3

-2

-1

0

1

2

3

Decompose the unfolded OP matrices (X1 & X2) by SVD(Unfold-PCA)

U*SScores of X1 on PC2, PC3 & PC4

U*SScores of X2 on PC2, PC3 & PC4

0 10 20 30 40 50 60-10

-8

-6

-4

-2

0

2

4

6

8

0 10 20 30 40 50 60-0.6

-0.4

-0.2

0

0.2

0.4

0.6

0.8

50 100 150 200 250

-0.2

-0.1

0

0.1

0.2

0.3

Decomposition of the matrix by SVD(Column-mean PCA)

X*VScores on PC2, PC3 & PC4

VLoadings on PC2, PC3 & PC4

50 100 150 200 250

-1

-0.5

0

0.5

1

1.5

2

2.5

3

5 10 15 20 25 30 35 40 45 50 55

-2

-1

0

1

2

3

4

Decompose the unfolded OP matrices (X1 & X2) by SVDUnfold-PCA

U*SScores of X1 on PC4

U*SScores of X2 on PC4

50 100 150 200 250

5

10

15

20

25

30

35

40

45

50

55

50 100 150 200 250

-1

-0.5

0

0.5

1

1.5

2

2.5

3

Decompose the unfolded OP matrix (X2) by SVDUnfold-PCA

U*SScores of X2 (X3) on PC4

VRefolded Loadings of X2 (X3) on PC4

p

n

1 2 3 4 5 6 7 8 9 10 11

x 104

1

2

3

4

5

6

7

8

9

Starch retrogradation by XRDunfolded OP matrix (X1)

Application of unfolded OP to real data (2)

n

p x p

50 100 150 200 250 300

50

100

150

200

250

300

1 2 3 4 5 6 7 8 9-150

-100

-50

0

50

100

150

Decompose the unfolded OP matrix (X1) by SVDUnfold-PCA

U*SScores of X1 on PC2

VRefolded Loadings of X1 on PC2

p

p

50 100 150 200 250 300-20

-15

-10

-5

0

5

10

15

20

1 2 3 4 5 6 7 8 9-150

-100

-50

0

50

100

150

Decompose the unfolded OP matrices (X1 & X2) by SVDUnfold-PCA

U*SScores of X1 on PC2

U*SScores of X2 on PC2

50 100 150 200 250 300

1

2

3

4

5

6

7

8

9

50 100 150 200 250 300-20

-15

-10

-5

0

5

10

15

20

Decompose the unfolded OP matrix (X2) by SVDUnfold-PCA

U*SScores of X2 on PC2

VRefolded Loadings of X2 on PC2

p

n

Group them together, one under the other, in the form of a cube of individual matrices of covariances among variables

1

.

.

n

1

p 1 p

1

.

n

1

p

1 p

« Multi-way » Outer Product Analysis

Decomposition of the cube PARAFAC

PARAFAC – Parallel Factor Analysis

= + +…

F is the number of Factors used in the PARAFAC model.

This model minimises the sum of squared residuals.

xijk =

F

f=1

aifbjfckf + eijk

3-way data X (n,q,p) :

nq

k

n

q

p

n

q

p

1

1 1 F1

R. Bro, Chemometrics and Intelligent Laboratory Systems, (1997), 38, 149-171

Cube of individual covariances matrices

Loadings on the 1° mode (samples) Time

1 2 3 4 5 6 7 8 9-200

-100

0

100

200

300

400

500

Sample

1, 2

1 2 3 4 5 6 7 8 90

5

10

15

20

25

PARAFAC applied to OP cube

Starch retrogradation by XRD

Loadings on the 2° mode (XRD)

Loadings on the 3° mode (XRD)

50 100 150 200 250 300-0.08

-0.06

-0.04

-0.02

0

0.02

0.04

0.06

0.08

0.1

0.12

Variable

1, 2

50 100 150 200 250 300-0.08

-0.06

-0.04

-0.02

0

0.02

0.04

0.06

0.08

0.1

0.12

Variable

1, 2

Starch Data : PARAFAC Model

Comparaison PARAFAC / SVD

OP-PARAFAC SVD on XTX= PCA

50 100 150 200 250 300-0.08

-0.06

-0.04

-0.02

0

0.02

0.04

0.06

0.08

0.1

0.12

Variable

1, 2

0 50 100 150 200 250 300 350-0.15

-0.1

-0.05

0

0.05

0.1

0.15

0.2

1 2 3 4 5 6 7 8 9-500

0

500

1000

1500

2000

2500

3000

1 2 3 4 5 6 7 8 9-200

-100

0

100

200

300

400

500

Sample

1, 2

Calculate the matrix of individual covariancesbetween variables of 2 different matrices X & Y

xi

yi

Ci = xiT . yi

1 q

1

p

1,1 1, q

p, q

=

p

n

qSignal 1

Signal 2

n

n

q

p

Visualisation of the Outer Product cube

For n samples, one gets n Outer Product matrices

Group them together in the form of a “cube”

Calculate the column-mean of the individual covariance matrices to give the matrix of covariances between the 2 groups of variables

Apply SVD

1

.

.

n

1

p1 q

1

.

n

1

p

1 q

Calculate the matrix of covariances of 2 matrices X & Y

1

p1 q

n OP (p, q) matrices 1 “cube” (n, p, q) 1 mean matrix (p, q)

Analyse the links between 2 tables of data, X & Y

Singular Value Decomposition of the matrix of covariances between the 2 groups of variables (1/n)XTY

That decomposition of the matrix (1/n)XTY corresponds to looking for successive pairs of variables (th = Xah , uh = Ybh ) where :

- covariance between th et uh maximal,- axes ah orthogonal- axes bh orthogonal

Decompose the « Mean » OP matrix by SVD≡ Tucker Analysis

L. Tucker, Psychometrika, (1958), 23, 111-136

20 40 60 80 100 120 140 160

20

40

60

80

100

120

140

160

SVD applied to the covariance matrix, XTYor column-means of the Outer Product cube

XTY = VX S VYT

S : diagonal matrix of singular valuesVX et VY : X & Y loadings matricesX*VX : scores of XY*VY

T : scores of Y

Application of « Mean » Outer Product Analysis to real data (3)

Complexation between TPP & Cu

1000 2000 3000 4000 5000 60000

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1Spectres RMN

550 600 650 700

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

2

2.2Spectres UVTD-NMR Vis

D.N. Rutledge, A.S. Barros, F. Gaudard, Mag. Res. in Chemistry, 35 (1997), 13–21

Column-means of Outer Products

20

40

60

80

100

120

140

160

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9 1

20 40 60 80 100 120 140 160

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

2

2.2

20 40 60 80 100 120 140 160

20

40

60

80

100

120

140

160

[uRMN, sRMN_Vis, vVis] = svd (meanRMN_Vis,'econ');

50 100 150

-0.2

-0.15

-0.1

-0.05

50 100 150

-0.2

-0.1

0

50 100 150

-0.2

-0.1

0

0.1

50 100 150

-0.2

-0.1

0

0.1

50 100 150

-0.12

-0.1

-0.08

-0.06

-0.04

-0.02

50 100 150-0.1

-0.05

0

0.05

0.1

0.15

50 100 150

-0.1

-0.05

0

0.05

0.1

0.15

50 100 150-0.1

-0.05

0

0.05

0.1

uRMN vVis

SVD on matrix of column-means of Outer Product (Tucker Analysis)

0 10 20 30 40-0.5

-0.4

-0.3

-0.2

-0.1

0

0 10 20 30 40-10

-5

0

5

0 10 20 30 40-20

-10

0

10

20

30

0 10 20 30 40-200

-100

0

100

200

0 10 20 30 40-6

-5

-4

-3

-2

0 10 20 30 40-1.5

-1

-0.5

0

0.5

1

0 10 20 30 40-0.6

-0.4

-0.2

0

0.2

0 10 20 30 40-0.2

-0.15

-0.1

-0.05

0

0.05

SVD on matrix of column-means of Outer Product (Tucker Analysis)

sRMN = RMN x uRMN / (uRMN' x uRMN); sVis = Vis x vVis / (vVis' x vVis);

sRMN sVis

Application of unfolded OP to real data (3)

0.5 1 1.5 2 2.5

x 104

5

10

15

20

25

30

Complexation between TPP & Cuunfolded OP matrix (X1)

p x q

n

20 40 60 80 100 120 140 160

20

40

60

80

100

120

140

1600 5 10 15 20 25 30 35

-100

-80

-60

-40

-20

0

20

40

Decompose the unfolded OP matrix (X1) by SVDUnfold-PCA

U*SScores of X1 on PC1 & PC2

VLoadings of X1 on PC1 & PC2

20 40 60 80 100 120 140 160

20

40

60

80

100

120

140

160

q

p

0 20 40 60 80 100 120 140 160-80

-70

-60

-50

-40

-30

-20

-10

0

10

0 5 10 15 20 25 30 35-100

-80

-60

-40

-20

0

20

40

Decompose the unfolded OP matrices (X1 & X2) by SVDUnfold-PCA

U*SScores of X1 on PC1 & PC2

U*SScores of X2 on PC1 & PC2

20 40 60 80 100 120 140 160

5

10

15

20

25

30

0 20 40 60 80 100 120 140 160-80

-70

-60

-50

-40

-30

-20

-10

0

10

Decompose the unfolded OP matrix (X2) by SVDUnfold-PCA

U*SScores of X2 on PC1 & PC2

VLoadings of X2 on PC1 & PC2

20 40 60 80 100 120 140 160

5

10

15

20

25

30

p

n

20 40 60 80 100 120 140 160-40

-35

-30

-25

-20

-15

-10

-5

0

5

10

0 5 10 15 20 25 30 35-100

-80

-60

-40

-20

0

20

40

Decompose the unfolded OP matrices (X1 & X3) by SVDUnfold-PCA

U*SScores of X1 on PC1 & PC2

U*SScores of X3 on PC1 & PC2

20 40 60 80 100 120 140 160

5

10

15

20

25

30

20 40 60 80 100 120 140 160-40

-35

-30

-25

-20

-15

-10

-5

0

5

10

Decompose the unfolded OP matrix (X3) by SVDUnfold-PCA

U*SScores of X3 on PC1 & PC2

VLoadings of X3 on PC1 & PC2

q

n20 40 60 80 100 120 140 160

5

10

15

20

25

30

For n samples, one gets n Outer Product matrices

Group them together, one under the other, in the form of a cube of individual matrices of covariances among variables

1

.

.

n

1

p 1 q

1

.

n

1

p

1 q

Decomposition of the cube PARAFAC

« Multi-way » Outer Product Analysis

Cube of individual covariances matrices

0 5 10 15 20 25 30 35-20

0

20

40

60

80

100

120

N° éch.

Load

ings

sur

le m

ode

1

facteur 1

facteur 2

0 5 10 15 20 25 30 350

50

100

150

200

250

300

350

« Loadings » on the 1° mode (samples) Concentrations

Cu

TPPCu + TPP

Complexation between TPP & Cu

PARAFAC applied to OP cube of real data (2)

Complexation between TPP & Cu

PARAFAC applied to OP cube of real data (2)

20 40 60 80 100 120 140 160

0.05

0.1

0.15

0.2

0.25

20 40 60 80 100 120 140 160

0.02

0.04

0.06

0.08

0.1

0.12

0.14

Loadings on the 2° mode (TD-NMR) Loadings on the 3° mode (Vis)

Fructose solutions

1200 1400 1600 1800 2000 2200 2400

0

0.5

1

1.5

2

2.5

3

3.5

4

4.5Spectres IR

1000 2000 3000 4000 50000

0.5

1

1.5

2

2.5

3

3.5

4

Spectres RMN

PARAFAC applied to OP cube of real data (3)

D.N. Rutledge, A.S. Barros, R. Giangiacomo, Magnetic Resonance in Food Science—A View to the Future, RSC, 2001, pp. 179–192

Cube of individual covariances matrices

PARAFAC model

Loadings on the 1° mode

0 5 10 15 20 25-5

0

5

10

15

20

25

30

facteur 1

facteur 2

0 5 10 15 20 250

10

20

30

40

50

60

N° de l'échantillon

Con

c. e

n fr

ucto

se

Concentrations

PARAFAC model

Loadings on the 2° mode (TD-NMR)

0 1000 2000 3000 4000 5000 60000

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

Longueurs d'onde

facteur 1

facteur 2

1200 1400 1600 1800 2000 2200 24000

0.02

0.04

0.06

0.08

0.1

0.12

0.14

0.16

Longueur d'onde

facteur 1

facteur 2

Loadings on the 3° mode (NIR)

0 5 10 15 20 25 30 35 400

0.5

1

1.5

2

2.5

0 20 40 60 80 100 120 140 1600.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

1.1

0 50 100 150 200 250 300 350-100

0

100

200

300

400

500

Log(TD-NMR) MIR XRD

PARAFAC on 4-D OP hypercube MIR NMR XRD (1)

(9 x 157 x 40 x 341)

Starch retrogradation

Loadings on the 1° mode (Samples)

1 2 3 4 5 6 7 8 9-100

-50

0

50

100

150

200

250

300

Sample number

Load

ings

on

the

first

mod

e

factor 1

factor 2

0 20 40 60 80 100 120 140 160-0.05

0

0.05

0.1

0.15

0.2

0.25

0.3

data1

data2

Loadings on the 2° mode (MIR)

0 5 10 15 20 25 30 35 400

0.05

0.1

0.15

0.2

0.25

0.3

0.35

data1

data2

Loadings on the 3° mode (NMR)

0 5 10 15 20 25 30 35 40-0.08

-0.06

-0.04

-0.02

0

0.02

0.04

0.06

0.08

0.1

0.12

Load

ings

on

the

four

th m

ode

factor 1

factor 2

Loadings on the 4° mode (XRD)

Comparison of 2D-Correlation Spectroscopyand unfold OP PCA

38 NIR spectra of water; acquired in the region 1300-1600 nm; from 6 to 80 ºC

-0.60

-0.30

0.00

0.30

0.60

0.90

1.20

1300 1350 1400 1450 1500 1550 1600

Wavelength (nm)

Abso

rban

ce

80ºC 6ºC

V.H. Segtnan et al., Anal. Chem. (2001), 73, 31-53B. Jaillais et al., Vib. Spec., (2005), 39, 1, 50-58

2D-COS Sync vs PC1 Loadings

1493

1412-2.0E-04

-1.0E-04

0.0E+00

1.0E-04

2.0E-04

3.0E-04

1300 1350 1400 1450 1500 1550 1600

Wavelength (nm)

Sync

hron

ous

Corr

elat

ion

1412

1491

2D-COS Async vs PC2 Loadings

1446

1428

1404

-6.0E-06

-3.0E-06

0.0E+00

3.0E-06

6.0E-06

1300 1350 1400 1450 1500 1550 1600

Wavelength (nm)

Asyn

chro

nous

Cor

rela

tion

ss 2D-COS vs. Loadings of PCAon transposed row-normalised, column-centred spectra

-2.5E-04

-1.5E-04

-5.0E-05

5.0E-05

1.5E-04

2.5E-04

0 10 20 30 40 50 60 70 80

Sample temperature (ºC)

Sync

hron

ous

corr

elat

ion

6ºC 80ºC

PC1Sync

ss 2D-COS vs Loadings of PCAon transposed row-normalised, column-centred spectra

-4.0E-06

-2.0E-06

0.0E+00

2.0E-06

4.0E-06

0 10 20 30 40 50 60 70 80

Sample temperature (ºC)

Asy

nchr

onou

s co

rrel

atio

n

6ºC80ºC

(6,38) (54,80)(80,6)

PC2ASync

Unfold PCT-OP-PCA (or PLS) algorithmfor huge X & Y

Step Computation Comments

1 X,Y input of X and Y matrices

2 [TX, PX] PCA(X) full rank PCA on X

3 [TY, PY] PCA(Y) full rank PCA on Y

4 K = OP(TX, TY) unfolded outer product

between TX and TY

5 [T, PPCT] = PCA(K) PCA (or PLS etc.) on K

T = scores in the X-space & PC-space

PPCT = loadings in PC-space

6 for a=1:n

refold PPCT(a)

P(a) = PY PPCT(a) PTX

end for rebuild PC loadings in X-space

A.S. Barros & D.N. Rutledge, Chemom. Intell. Lab. Syst., (2004) 73 245– 255A.S. Barros & D.N. Rutledge, Chemom. Intell. Lab. Syst., (2005), 78, 125–137

X

PCA

X1

T1

P1

X2

T2

P2

Xq

Tq

Pq

T1 T2 ... T1

PCA/PLS

TPCT

PTPCT

PX1 PX2 PXq...

Segmented PCT-PCA (or PLS)

TPCT TX=

OPA vs. 2D-COS, PCA and Tucker Analysis

• Why use the mean ! ?(PCA & TA are in a sense compromises)

• Not limited to two data sets

• Cube analysable by unfolding or by multi-way methods

• Multi-way methods extract Factors

• Unfold-OPA can reveal relations between variablesnot limited to two matricesno need to sort samples (unlike 2D-COS)

• No memory problem with (segmented) PCT-OPA

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