This article evaluates power and Type I error probabilities for a modified Fisher exact test for homogeneity of response probabilities in 2 × 2 tables. The original Fisher exact test and the Pearson chi-square are employed as standards for comparison. The continuity-corrected Fisher statistic is shown to be considerably more powerful than the uncorrected Fisher exact test, while it is less often nonconservatively biased than the Pearson chi-square.

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.