Planned interim analysis and its role in cancer clinical trials.

Although interim analyses in cancer clinical trials are commonplace, clinical trials are usually designed with the implicit assumption that data analysis will occur only after the trial is completed. The design of randomized trials with planned interim analyses, "group sequential trials," is described and examples are given. A method to redesign trials in which unplanned interim analyses have been undertaken is described. Planned interim analysis should be considered whenever a cancer clinical trial is designed.