FAQ/ManovaCov - CBU statistics Wiki

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Comparison of handling a varying covaruate (within subject) using SPSS MANOVA and SPSS MIXED

We illustrate using the data below with one time factor df() with three time points wach with a different covariate value cv() 

data list list
  / sub group dv1 dv2 dv3 cv1 cv2 cv3.
begin data.
1 1  3  4  7  3 1 2
2 1  6  8 12  9 3 1
3 1  7 13 11 11 8 4
4 1  0  3  6  6 2 1
5 2  5  6 11  7 8 3
6 2 10 12 18 15 9 5
7 2 10 15 15 14 8 6
8 2  5  7 11  9 2 8 
end data.

Using MANOVA 

manova dv1 dv2 dv3 by group(1,2) with cv1 cv2 cv3
  /wsfactors trial (3)
  /wsdesign trial
  /design

we obtain

Tests of Significance for T1 using UNIQUE sums of squares

Source

SS

DF

MS

F

Sig of F

WITHIN+RESIDUAL

33.81

5

6.76

REGRESSION

173.11

1

173.11

25.60

.004

GROUP

5.09

1

5.09

.75

.425

  • - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - Regression analysis for WITHIN+RESIDUAL error term --- Individual Univariate .9500 confidence intervals Dependent variable .. T4

COVARIATE

B

Beta

Std. Err.

t-Value

Sig of t

Lower -95%

CL-Upper

T4

1.398

1.054

0.276

5.06

.004

0.6878

2.108

Tests involving 'TRIAL' Within-Subject Effect.

  • AVERAGED Tests of Significance for dv using UNIQUE sums of square

Source

SS

DF

MS

F

Sig of F

WITHIN+RESIDUAL

24.51

11

2.23

REGRESSION

.33

1

.33

.15

.709

TRIAL

45.94

2

22.97

10.31

.003

GROUP BY TRIAL

3.27

2

1.63

.73

.502

We then need to reformat the data into 'long format' using the varstocases syntax and then fit the repeated measures data with varying covariate as a mixed model. 

compute cvone=cv1.

varstocases
 /make dv from dv1 dv2 dv3
 /make cv from cv1 cv2 cv3
 /index = trial.
        
mixed dv by group trial with cv
  /fixed= group trial group*trial cv
  /repeated= trial | subject(sub) covtype(cs).

Source

Num df

Denom DF

F

Sig of F

Intercept

1

12.80

27.75

.000

group

1

5.881

2.177

.191

trial

2

10.858

12.708

.001

group by trial

2

10.287

0.704

.517

cv

1

12.24

0.081

.780

The results using the two approaches are similar except for the group main effect which although non-significant in both has a lower significance level for the MANOVA.