Repeated measures: Linear trend significant but not within-subjs test

Posted by Allan Lundy, PhD on Mar 31, 2011; 12:09am
URL: http://spssx-discussion.165.s1.nabble.com/Repeated-measures-Linear-trend-significant-but-not-within-subjs-test-tp4272571.html


Dear Listers,
I am running analyses of a number of different outcome measures to assess treatment over 7 months, with testing following each treatment.  N= 23.  No predictor variables other than the effect of time.  The client expects to see steady improvement over time, hence I am using GLM Repeated Measures with polynomial contrasts, and expecting significant linear effects.  On a few outcome measures, I am getting a significant linear trend (e.g., p= .034) but non-sig tests of within-subjects effect (e.g., p= .069).  Also, nowhere near sig multivariate tests, (p= .368), though I have read that that is irrelevant in such a simple analysis.  I assume that it would be conservative to trust the least significant result, but has anyone seen a discussion of when and why this occurs?  (I would have assumed that the within-subjects effect was an omnibus test, hence nothing under it would be significant.)  Has anyone seen an argument to the effect that the linear trend can be honestly significant even if the overall effect is not?

Thanks for your attention!
Allan

Allan Lundy, PhD
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