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Re: st: sample size

From   Roger Newson <[email protected]>
To   [email protected]
Subject   Re: st: sample size
Date   Thu, 06 Jan 2005 14:32:25 +0000

Kazi Mizanur Rahman wrote:

The baseline proportion of a an event was 0.9.
We assume that after providing an intervention in one group for a particular
time period we get the proportion as 0.5. We also have a comparison group
that receives no intervention. After that particular time period the
proportion becomes 0.8 in the comparison group. How to calculate the sample
size with 80% power and alpha .05 ?

Another possibility is to use the -powercal- package, downloadable from SSC, which can be used to do power calculations for all generalized linear models, including the logistic. Information on the methods and formulas used to do this can be found in Newson (2004). If Kazi does not have access to the latest Stata Journal in a hurry, then a draft of this can be downloaded in preprint form from my website at

either by using a browser or by typing in Stata

findit powergen

and downloading the PDF as an ancillary file. (In the longer run, of course, Kazi might like to subscribe to The Stata Journal, in which a lot of other useful things can be found.)

I hope this helps.



Newson R. Generalized power calculations for generalized linear models and more. The Stata Journal 2004; 4(4): 379-401.

Roger Newson
Lecturer in Medical Statistics
Department of Public Health Sciences
King's College London
5th Floor, Capital House
42 Weston Street
London SE1 3QD
United Kingdom

Tel: 020 7848 6648 International +44 20 7848 6648
Fax: 020 7848 6620 International +44 20 7848 6620
or 020 7848 6605 International +44 20 7848 6605
Email: [email protected]

Opinions expressed are those of the author, not the institution.

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