Wanneer is er genoeg bewijs?

Conflict of interest: none declared. Financial support: none declared.

• There are ethical and economic reasons to design and conduct medical research as efficiently as possible.

• Clinical trials often take a long time because of the number of patients that must be included and the follow-up time in the study. Many data already become available during that time.

• By making interim analyses of these data it is possible to decide whether a final conclusion can be made. This may save time and may lead to a reduction in the number of patients.

• The statistical methodology of interim analyses is more complex than a fixed sample analysis and must be planned carefully in advance.

• The most important methods are sequential analysis and group sequential analysis. Both methods are aimed at a low risk of a false-positive conclusion.

• In this article we explain the application of these methods as well as considerations in the design of a study and interpretation of the results.