Re: st: RE: RE: -powercal-

[Ron�n Conroy <rconroy@rcsi.ie>]

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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