PEDIATRICS Vol. 83 No. 4 April 1989, pp. A30
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HOW DID YOU COME TO CHOOSE THE VALUE OF N?

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The set of possible results [in a clinical trial] on which the power depends is not an objective quantity but is created by the experimenter's intentions. Did he plan to draw a sample of n, or did he intend to sample until he tired and just happened to arrive at a sample of size n, or did he use some other "stopping rule"? Such questions, which are intuitively quite irrelevant as evidence, are crucial to evaluating the outcome space of the trial and hence the power of the test.