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SUMMARY Lan & DeMets (1983) devised a method of constructing discrete group sequential boundaries by using the type I error spending rate function. It is extended so as to generate asymmetric as well as symmetric two-sided boundaries for clinical trials. The design aspect of this procedure is explored in terms of the maximum sample size needed to achieve a desired level of power and the expected stopping times under both null and alternative hypotheses. Finally, these properties are employed in search of appropriate type I error spending rate functions for differing situations.
Kim et al. (Thu,) studied this question.