Sample Size Calculations for Randomized Controlled Trials
Sample size Calculations for Randomized Controlled TrialsJanet WittesINTRODUCTIONMost informed consent documents for Randomized con-trolled Trials implicitly or explicitly promise the prospectiveparticipant that the trial has a reasonable chance of answer-ing a medically important question. The medical literature,however, is replete with descriptions of Trials that providedequivocal answers to the questions they addressed. Papersdescribing the results of such studies may clearly imply that thetrial required a much larger Sample size to adequately addressthe questions it posed. Hidden in file drawers, undoubtedly, aredata from other Trials whose results never saw the light ofday some, perhaps, victims of inadequate Sample size . Al-though many inadequate-sized studies are performed in a sin-gle institution with patients who happen to be available, someare multicenter Trials designed with overly optimistic assump-tions about the effectiveness of therapy, too high an estimate ofthe event rate in the control group, or unrealistic assumptionsabout follow-up and this review, I discuss statistical considerations in thechoice of sam
rate and the desired power. For test statistics with well-known distributional properties, one may use a standard formula for sample size. Controlled trials often involve deviations from assumptions such that the test statistic has more complicated behavior than a simple formula allows. Loss to follow-up, incomplete compliance with therapy,
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