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st: Why does -anova- calculate repeated effects (F) twice?


From   Michael Ingre <Michael.Ingre@ipm.ki.se>
To   "statalist@hsphsun2.harvard.edu" <statalist@hsphsun2.harvard.edu>
Subject   st: Why does -anova- calculate repeated effects (F) twice?
Date   Fri, 06 Feb 2004 12:22:43 +0100

Dear listers

I have now made several repeated -anova- and noticed some odd behaviour in
the procedure. When -anova- is run with the repeated option, it first
calculates the standard anova table (including F), then for each effect with
repeated variables it calculates the epsilon coefficients. But then, it
seems like Stata starts calculating the effects (with F) once again. Is this
really necessary?

It is my understanding that once the epsilon coefficients are calculated you
only need to run the F-test once again with the corrected degrees of
freedom. There is no need to recalculate F. This is also how it is explained
in the Stata manual p51.

Given the time it takes to compute F in repeated designs (sometimes minutes
and even hours as has been discussed lately on this list) and the apparently
very fast calculation of epsilon (Stata displays it almost immediately) it
seems to me that the speed of -anova- with repeated measures could almost be
doubled if the second calculation of F were to be omitted.

Am I right or am I wrong?

Thanks

Michael
----------------- 
PhD-student 
Department of Psychology
Stockholm University &
National Institute for
Psychosocial Medicine
 








 



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