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Abstract I suggest for memorization an equation for calculating approximate sample size requirements intended only for a specific set of values (80 per cent power for a two‐tailed alpha = 0.05 test) which seems to occur often in biopharmaceutical research. After presenting the formula in terms of variance estimate s 2 and effect size d , I derive a few alternative forms and then discuss the accuracy of the approximation and other properties as well as examples of its use.
Robert Lehr (1992) studied this question.