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

[David Airey <david.airey@Vanderbilt.Edu>]

> 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
*
*   For searches and help try:
*   http://www.stata.com/support/faqs/res/findit.html
*   http://www.stata.com/support/statalist/faq
*   http://www.ats.ucla.edu/stat/stata/