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Internal Pilot Studies for Determining the Sample Size without Unblinding

If no reliable information about the variance of the key response is available at beginning of a clinical trial, the use of data from the first few patients entered in the trial (‘internal pilot’) may be appropriate to estimate the variance and thus to recalculate the required sample size. We investigate the influence on existing methods of variance estimation via the Expectation-Maximization (EM) algorithm without unveiling of the treatment status. In addition, we investigate the use of a formula derived by Guenther to estimate the preplanned sample size. This simple modification could improve somewhat the existing methods for the same problem. There is no effect of the EM algorithm on the type I error rate of the existing procedures, and the effect on the type II error rate is negligible as long as the number of patients in the internal pilot is not too small. Thus, to recalculate the sample size it is not necessary to unveil the treatment status in an internal pilot with a moderate number of patients.