Within Ss contrasts vs.pairwise comparison inconsistency

Posted by msherman on Jul 30, 2018; 3:44pm
URL: http://spssx-discussion.165.s1.nabble.com/Within-Ss-contrasts-vs-pairwise-comparison-inconsistency-tp5736399.html

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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