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re: st: posthoc testing after ANOVA


From   David Airey <[email protected]>
To   [email protected]
Subject   re: st: posthoc testing after ANOVA
Date   Sat, 3 Jan 2004 01:46:50 -0600

I have a question related to the FAQ. Can one form _any_ linear combination of any of the cell means from any cell means model ANOVA?
Having read a bit more it seems to be so. And I'm almost getting the hang of posthoc testing via Stata's testanova framework.

Below I think I'm being silly and animal doesn't need to be in the cell means table which makes things easier to follow from the online examples.



For instance, if I have a split plot design,

within-subject factor
---------------------
stimulus1 stimulus2

drug animal1 cell 1 2
animal2 3 4

control animal3 5 6
animal4 7 8


This anova is: anova y drug/animal|drug stimulus drug*stimulus/, repeated(stimulus).

I was interested in the following comparison of cells after finding a drug*stimulus interaction significant:

((2-1)+(4-3)) / 2 = ((6-5)+(8-7)) / 2, average of differences between stim1 and stim2 by group

This test would be equivalent to transforming the data to difference scores (stimulus2-stimulus1), and then testing for a group difference between drug and control groups. It's easy to do the latter, but I wanted to know if test could do it directly and if the comparison I want is an allowable test.
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