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st: Post estimation power estimations.


From   Lars Folkestad <lfolkestad@health.sdu.dk>
To   "statalist@hsphsun2.harvard.edu" <statalist@hsphsun2.harvard.edu>
Subject   st: Post estimation power estimations.
Date   Sat, 15 Oct 2011 17:45:55 +0200

Evnin' list.

I have a question for you: when you do a ttest and you have no significant difference, you can estimate the group size needed for the between group differences to be significant, using SD and the differences in mean. I know this is a statistical no no - but lets save that discussion for another chain of emails.

My problem is that the between group difference is significant. When i correct for a variable, that shouldent interfere the between group difference dissaperes. 

What i've done:
Xi: regress var1 var2 i.group
_group_2 p=0.45
Indicating that the Var2 adjusted var1 between group difference is no longer significant. I believe This is a power issue.

Can i aply the same logic as in ttest exemple, and if so - how to do that in Stata? 

Mvh
Lars Folkestad


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