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st: RE: calculation of sample size


From   "Yulia Marchenko" <ymarchenko@stata.com>
To   <statalist@hsphsun2.harvard.edu>
Subject   st: RE: calculation of sample size
Date   Fri, 8 Oct 2004 16:55:16 -0500

Aijing Shang wrote:

>Dear all,
>Recently when I calculate power using sampsi, a strange thing happened. I
>want to compare two proportions, one is 0.001, another is 0.002. As what I
>know, the bigger the sample size is, the more power it is. However, the
>results sampsi gave are reversed. See the results following.

>. sampsi 0.001 0.002, n1(10) n2(10) alpha(0.05)
>Estimated power:
         power =   0.9999
>. sampsi 0.001 0.002, n1(100) n2(100) alpha(0.05)
>Estimated power:
         power =   0.3762
>. sampsi 0.001 0.002, n1(1000) n2(1000) alpha(0.05)
>Estimated power:
         power =   0.0250
>Can anybody tell me what is wrong? Thank you very much.

In addition to my previous answer:

We've spent some more time looking carefully at the -sampsi- command today.
In a future update -sampsi- will display a warning message in the case when
the approximation to normality of the test statistic for proportions is
doubtful.

--Yulia
ymarchenko@stata.com


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