What happens if you use mean-centered versions of your covariates?

Here's a demo that may help.

* Get some data for 2x2x(2) model with repeated measures on the last factor.

GET FILE='C:\SPSSdata\survey_sample.sav'.

* Let A and B be between-Ss factors.

COMPUTE A = wrkstat EQ 1.

COMPUTE B = marital EQ 1.

FORMATS A B (F1).

DESCRIPTIVES A B.

* Use the first 2 education variables (educ paeduc) for the repeated

measures.

* Use age as the covariate.

COMPUTE X = age.

SELECT IF NMISS(A,B,X,educ,paeduc) EQ 0.

AGGREGATE

/OUTFILE=* MODE=ADDVARIABLES

/BREAK=

/X_mean=MEAN(X).

COMPUTE Xmc = X - X_mean.

GLM educ paeduc BY A B WITH X

/WSFACTOR=Time 2 Polynomial

/EMMEANS=TABLES(Time) WITH(X=MEAN)COMPARE ADJ(LSD)

/EMMEANS=TABLES(Time) WITH(X=0)COMPARE ADJ(LSD)

/WSDESIGN=Time

/DESIGN=X A B A*B.

GLM educ paeduc BY A B WITH Xmc

/WSFACTOR=C 2 Polynomial

/EMMEANS=TABLES(C) WITH(Xmc=MEAN)COMPARE ADJ(LSD)

/WSDESIGN=C

/DESIGN=Xmc A B A*B.

msherman wrote

> Dear List: I am conducting a 2 X 2 X (2) repeated measures (Treatment vs

> Control, Younger vs Older, Time 1 vs Time 2) analysis with 2 covariates

> (Body Mass Index, duration of illness). Upon examining the repeated

> measures effect (Tests of within-subjects effects) and the pairwise

> comparisons results I expected the stats to mirror image each other. That

> is, if the main effect for Time was not significant then I also expected

> to see that the pairwise comparison between Time 1 and Time 2 to be not

> significant. What I find is that the overall Time effect is not

> significant but the time pairwise comparison output shows that the two

> times periods are statistically different. The pairwise comparison stats

> come out under the Estimated Marginal Means evaluated at the means of the

> two covariates. Shouldn't the results be the same? What am I missing.

>

> Martin F. Sherman, Ph.D.

> Professor of Psychology

> Director of Masters Program: Thesis Track

> Loyola University Maryland

> 4501 North Charles Street

> 222 B Beatty Hall

> Baltimore, MD 21210

>

>

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