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st: fpower


From   "CHIDLOW Agnieszka" <A.Chidlow@staffs.ac.uk>
To   <statalist@hsphsun2.harvard.edu>
Subject   st: fpower
Date   Mon, 22 Jun 2009 00:25:48 +0100

Dear Phil,
 
Thank you very much for your e-mail and suggestions regarding fpower and anove (you are absolutely right, thanks for that). 
 
If I can ask, why would you run "simpower nm year, reps(2000)" using 2000?
 
Once again, many thanks
Aggie

There certainly are educated guesses involved in any power analysis.
In the case of -fpower- the main educated guess is concerning the
delta().  After that -fpower- computes power based on the noncentral
F-distribution.  So there are indeed calculations going on.  The
-simpower- program generates random samples with given means and
standard deviations and computes power by counting the number of
significant results.  I usually suggest at least 1,000 iterations.

As for Agnieszka power question, I would recommend running the
following: "simpower nm year, reps(2000)"

I'm a little confused by the statement that nm is 2 groups.  Does this
mean that there is a categorical response variable?  If so, then
-anova- is the wrong tool to
use.


--
Phil Ender
Statistical Consulting Group
UCLA  Academic Technology Services
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