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st: calculation of sample size


From   "Aijing Shang" <shang@ispm.unibe.ch>
To   <statalist@hsphsun2.harvard.edu>
Subject   st: calculation of sample size
Date   Fri, 8 Oct 2004 14:50:55 +0200

Dear all,
Recently when I calculate power using sampsi, a strange thing happened. I
want to compare two propotions, 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 for two-sample comparison of proportions

Test Ho: p1 = p2, where p1 is the proportion in population 1
                    and p2 is the proportion in population 2
Assumptions:

         alpha =   0.0500  (two-sided)
            p1 =   0.0010
            p2 =   0.0020
sample size n1 =       10
            n2 =       10
         n2/n1 =     1.00

Estimated power:

         power =   0.9999

. sampsi 0.001 0.002, n1(100) n2(100) alpha(0.05)

Estimated power for two-sample comparison of proportions

Test Ho: p1 = p2, where p1 is the proportion in population 1
                    and p2 is the proportion in population 2
Assumptions:

         alpha =   0.0500  (two-sided)
            p1 =   0.0010
            p2 =   0.0020
sample size n1 =      100
            n2 =      100
         n2/n1 =     1.00

Estimated power:

         power =   0.3762

. sampsi 0.001 0.002, n1(1000) n2(1000) alpha(0.05)

Estimated power for two-sample comparison of proportions

Test Ho: p1 = p2, where p1 is the proportion in population 1
                    and p2 is the proportion in population 2
Assumptions:

         alpha =   0.0500  (two-sided)
            p1 =   0.0010
            p2 =   0.0020
sample size n1 =     1000
            n2 =     1000
         n2/n1 =     1.00

Estimated power:

         power =   0.0250

Can anybody tell me what is wrong? Thank you very much.

Aijing Shang


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