Abstract
Estimates of sample size requirements for 2 × 2 tables, derived from three approximate formulae, were evaluated by comparison wlth exact calculations. Although different, the estimates derived from all three formulae were judged adequate for most practical purposes, with the most accurate results being obtained from Cohen's formula. The strategy for exact determination of the minimum N required to provide specified power for tests of significance of differences in proportions in two treatment groups is described.
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