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st: Power calculation and sample sizes


From   "Vieira, Cecilia" <CVIEIRA3@PARTNERS.ORG>
To   statalist@hsphsun2.harvard.edu
Subject   st: Power calculation and sample sizes
Date   Thu, 16 Aug 2007 23:49:47 -0400

Dear All,

I would like to determine the sample size for a RCT. Control and intervention
groups will have the same size.
The intention of the intervention is to increase a certain preventive measure
among patients. 
We will measure prevalence of the preventive measure at baseline for both groups
and expect to find that 20% of all the patients (control and intervention) adopt
the measure before the experiment.
We would like to be able to detect a change from 20% to 40% in the intervention
group, while we expect that the control group will change their adoption rates
from 20% to 25%.

My first plan was to simulate some samples, starting with some hypothetical
sample size, then run a logistic model multiple times, and calculate power as a
proportion of times when the expected coeficients were significant. Then repeat
the simulations for different sample sizes until I could had an idea of sample
size versus power.

I would like to hear from you if there is a simple command in stata that gives
me the result more easily.

Thank you.

Cecilia


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