Re: Within Ss contrasts vs.pairwise comparison inconsistency

Posted by Bruce Weaver on Jul 30, 2018; 5:42pm
URL: http://spssx-discussion.165.s1.nabble.com/Within-Ss-contrasts-vs-pairwise-comparison-inconsistency-tp5736399p5736403.html

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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Bruce Weaver
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