Sample Size Estimation


Comparison of two proportions



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Comparison of two proportions:
Next, we estimate the sample size required to compare two proportions.
To compare two proportions p0 and p1, use the formula:

N = 16 p(1 - p) / (p0 - p1)2 where p = (p0 + p1)/2.


For example, if p0 = .30 and p1 = .10, then p = .20, so the required sample size per group is 64.


As with the comparison of two means, the multiplier of 16 can be changed. The same values as in the previous table apply here too.
Notes:

An upper limit on the required sample size occurs when p = .50; then the formula becomes:



N= 4 / (p0 - p1)2 . This is very conservative and works best when the proportions are centered around .50.
When the proportions are less than .05, use: N = 4 / (√p0 - √p1)2


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