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Re: st: Power calculation and sample sizes

From   Jeph Herrin <>
Subject   Re: st: Power calculation and sample sizes
Date   Fri, 17 Aug 2007 09:38:05 -0400

You might find this helpful


Vieira, Cecilia wrote:
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.


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