The main purpose of performing interim analyses in a clinical trial is to stop the trial once a convincing treatment difference can be demonstrated. The major problem with repeated significance tests during inclusion and/or follow-up in a clinical trial is that the more often one analyses accumulating data, the greater the probability of obtaining a statistically significant result even when there is in reality no treatment difference. To allow for repeated testing, one must therefore choose a more stringent nominal significance level as a criterion for stopping the trial. This paper presents two simple strategies for undertaking interim analyses.

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