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We consider a fixed-sample parallel-group clinical trial with an interim analysis that tests H0:mu x = mu y against H1:mu x less than mu y. If we do not reject at the interim analysis, then the probability of making a type I error by rejecting H0 in favour of H2:mu x greater than mu y and the power at the final analysis are not appreciably affected by performing the interim analysis for certain relevant critical regions.
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Chi et al. (1988) studied this question.
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