In recent years, many researchers have been interested in performing futility analyses at interim points during clinical trials in order to terminate futile trials. To implement such an analysis, the researcher must decide what percentage of the total number of patients must have completed the trial before the interim analysis is done. This percentage is usually between 20 and 80. We examined the relationship between the percentage chosen and the probability of stopping the trial, given a parameter value associated with the treatment effect. The results of this examination will help the researcher plan when to do a futility analysis.
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