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Re: st: Re: Interpretation of contrasts


From   Janet Hill <janethill73@yahoo.co.uk>
To   statalist@hsphsun2.harvard.edu
Subject   Re: st: Re: Interpretation of contrasts
Date   Thu, 14 Feb 2008 08:42:28 +0000 (GMT)

Dave,
Thank you for your advice and the reference. I was wary of reducing the analysis to a oneway model but I will now pursue that approach.
Janet

----- Original Message ----
From: David Airey <david.airey@vanderbilt.edu>
To: statalist@hsphsun2.harvard.edu
Sent: Wednesday, 13 February, 2008 4:05:25 PM
Subject: Re: st: Re: Interpretation of contrasts

..

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

Department of Pharmacology
School of Medicine
Vanderbilt University
Rm 8158A Bldg MR3
465 21st Avenue South
Nashville, TN 37232-8548

TEL   (615) 936-1510
FAX   (615) 936-3747
EMAIL david.airey@vanderbilt.edu
URL   http://people.vanderbilt.edu/~david.c.airey/dca_cv.pdf
URL   http://www.vanderbilt.edu/pharmacology



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