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Re: st: Re: Interpretation of contrasts


From   David Airey <david.airey@vanderbilt.edu>
To   statalist@hsphsun2.harvard.edu
Subject   Re: st: Re: Interpretation of contrasts
Date   Wed, 13 Feb 2008 10:05:25 -0600

.

I don't have the answer you asked for, but another approach. It sounds like you have one between subject factor (3 levels) and one within subject factor (5 levels) and you have a significant interaction to examine further. One way to follow up an interaction is to example the simple effects, that is, to look either within a given stress level at each repeated measure, which reduces to a oneway repeated measures model, or to look at a given time period between stress levels, which reduces to a oneway anova. It sounds like you want to do the latter, in which case just do "oneway" if time is equal to the last period. I've read that most packages use the error term from a simple oneway anova for testing the between subject simple effects following an interaction in your design, although the pooled term is more often used for testing within subject simple effects at a given between subject level following interaction. This is due to what violations are more often expected in behavioral research (Maxwell and Delaney, Designing experiments and analyzing data, 2nd edition, LEA publishers).

-Dave

On Feb 13, 2008, at 7:10 AM, Janet Hill wrote:


I have a repeated measures anova with 5 repeated time periods and the response measured at 3 stress levels and I am estimating the model as:
wsanova resp time,id(spec) between(stress) eps
test, showorder shows there to be 54 elements in the design matrix.

My problem is that I want to look at time*stress interactions at the last time period. I have constructed a test matrix for the necessary constraints but I get the following error message:
. test, test(c6)
( 1) time[5]*stress[1] - time[5]*stress[2] = 0
( 2) time[5]*stress[1] - time[5]*stress[3] = 0
( 3) time[5]*stress[2] - time[5]*stress[3] = 0
Constraint 1 dropped
Constraint 2 dropped
Constraint 3 dropped

. test, test(c6) mtest(sidak)
_mtest: 0 valid tests

I can carry out this tests at other time periods.

My question is how can I carry out this particular test.

Thank you,
Janet





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David C. Airey, Ph.D.
Pharmacology Research Assistant Professor
Center for Human Genetics Research Member

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