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st: Re: Contrasts and repeated measures


From   David Airey <david.airey@Vanderbilt.Edu>
To   Statalist <statalist@hsphsun2.harvard.edu>
Subject   st: Re: Contrasts and repeated measures
Date   Thu, 24 Sep 2009 23:03:58 -0500

.

It might be easier to use the amusingly written advice on this page:

http://www.uvm.edu/~dhowell/StatPages/More_Stuff/RepMeasMultComp/RepMeasMultComp.html

even though SPSS is the software used.

You say you have 4 treatments, with subjects nested in treatments (I'm assuming, but you don't make this clear), and each subject is repeatedly measured (5 times), and you want to test for treatment differences at two time points.

Ignoring polynomial contrasts for trend analysis, which might get you out of lots of post-hoc comparisons, you could use paired t-tests (if interested in time 1 versus time 2 in treatment 1 for example) and or independent sample t-tests (if interested in treatment 1 vs treatment 2 at time 1 for example), and separately compute a critical statistic for the family of tests as suggested.

One point is that with repeated measures designs, comparisons of the repeated measures may benefit from just using the error associated with the data used in the comparison rather than the model error.

The -test- command following anova for a split plot like "anova y b / subject|b w b*w /, repeated(w)" or more complicated ANOVA may not be using the error term you really want. You may have to pick that up separately from another anova on a data subset if its not left behind in the -ereturn list-, in a manner similar to described in another FAQ (http://www.stata.com/support/faqs/stat/singledof.html).

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