A clinical trial with an interim analysis

Abstract We consider a fixed‐sample parallel‐group clinical trial with an interim analysis that tests H o : μ x = μ y against H 1 : μ x < μ y . If we do not reject at the interim analysis, then the probability of making a type I error by rejecting H o in favour of H 2 : μ x > μ y and the power at the final analysis are not appreciably affected by performing the interim analysis for certain relevant critical regions.