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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