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Re: st: RE: RE: -powercal-


From   Ronán Conroy <rconroy@rcsi.ie>
To   statalist@hsphsun2.harvard.edu
Subject   Re: st: RE: RE: -powercal-
Date   Fri, 9 Feb 2007 18:14:06 +0000

On 9 Feabh 2007, at 17:41, Mak, Timothy wrote:

My hypothesis is that the proportion of attendance for the control group
is 0.5, and the proportion for the experimental group is 0.8. How do you
translate that into a z-test?
This is not how you set a statistical power (unless I have misunderstood they way you have phrased this). Note that the power calculation must be set to the minimum effect size that is clinically significant (has 'real world' significance) not to the size that you expect the effect to be.

The rationale is that a study should have a reasonable power (90% or 95% – no-one believes 80% is reasonable!) to detect the smallest effect size that is clinically significant. You may believe that the effect is quite large, but if you base your power calculation on this large effect size, you have a significant chance of failing to detect an effect which, although small, would still be clinically important.



=========
Ronán Conroy
Royal College of Surgeons in Ireland
rconroy@rcsi.ie
+353 (0) 1 402 2431
+353 (0) 87 799 97 95
http://www.flickr.com/photos/ronanconroy




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