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RE: st: sampsi and power


From   Cameron McIntosh <cnm100@hotmail.com>
To   STATA LIST <statalist@hsphsun2.harvard.edu>
Subject   RE: st: sampsi and power
Date   Mon, 10 Aug 2009 15:41:24 -0400

What does it tell you about sample size if you try and estimate it for 80% power?
http://www.ats.ucla.edu/stat/stata/dae/proportionpow.htm
Cam

----------------------------------------
> Date: Mon, 10 Aug 2009 10:50:39 -0700
> From: ovaldia@yahoo.com
> Subject: st: sampsi and power
> To: statalist@hsphsun2.harvard.edu
>
> Dear all,
>
> With this question I will demonstrate my basic misunderstanding of power analyses.
>
> A journal reviewer has asked for a power calculation of a chi-square 2x2 table from a submitted manuscript. Although I am aware of the futility of doing power analyses after the fact, and will most likely not include it in the manuscript, I was intrigued by the results. The odds ratio from the table is significant 3.284 (95%CI=1.145 to 9.419). Plugging the observed percentages and sample sizes in -sampsi- I get power=0.624. I was expecting the power to be higher. Although I suspect that this is correct, I am having a hard time explaining why I do not get 80% or better?
>
> Thank you,
> Ricardo.
>
> Ricardo Ovaldia, MS
> Statistician
> Oklahoma City, OK
>
>
>
>
>
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