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st: sampsi and power

From   Ricardo Ovaldia <>
Subject   st: sampsi and power
Date   Mon, 10 Aug 2009 10:50:39 -0700 (PDT)

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 Ovaldia, MS
Oklahoma City, OK

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