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re: st: powercalculation for four-sample trial with repeated measures


From   David Airey <david.airey@Vanderbilt.Edu>
To   statalist@hsphsun2.harvard.edu
Subject   re: st: powercalculation for four-sample trial with repeated measures
Date   Thu, 23 Jun 2005 11:34:36 -0500

Hello Stata-users,
My name is Maren from Hamburg. I am doing an evaluation about different
curricula in medical education. I have four different treatment groups
(students) to compare and longitudinal data for 7 different measuring times.
Since we have small sample size I would like to calculate the power for our
tests.
Does anybody know how to calculate power (or samplesize) for a four- sample
comparison with repeated measures?
I found that:

sampsi mean1 mean2, sd1() sd2() method(change) pre(x) Post(y) r1(z) n1()
n2()

works for two-sample comparison with repeated measures but not for a
four-sample trial.
The ADOs: simpower and ssizebi are brilliant for a four-sample comparison,
but do not offer a method for repeated measures.

If anybody solved this problem before, I would be glad for help.
I think on this list it has been said that the most flexible choice is the follow Al Feiveson's advice and use simulation. Do a search from within Stata for power simulation. There is an FAQ and Stata Journal article on this too. Since your data set is small, you might try ANOVA or MANOVA methods for repeated measures. I would also try xtgee and xtreg and xtmixed out of curiosity!

-Dave
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