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Re: st: sampsi and power


From   sjsamuels@gmail.com
To   statalist@hsphsun2.harvard.edu
Subject   Re: st: sampsi and power
Date   Tue, 11 Aug 2009 12:12:54 -0400

-
There is no surprise here. In fact, if you had seen p exactly equal to
0.05, the post-hoc power would have been 50%.  You can work this out
for yourself from the derivations for power calculations contained in
most introductory texts.

-Steve


On Mon, Aug 10, 2009 at 3:41 PM, Cameron McIntosh<cnm100@hotmail.com> wrote:
> 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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-- 
Steven Samuels
sjsamuels@gmail.com
18 Cantine's Island
Saugerties NY 12477
USA
845-246-0774

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