Interim Analysis
A pre-planned analysis of clinical trial data conducted before the study is complete. Interim analyses allow early detection of large treatment effects or safety concerns. They must be conducted with strict statistical controls to avoid inflating the false positive rate from multiple data looks.
Technical Context
Interim analyses are performed by an independent statistical centre with unblinded data access, reviewed by the DSMB. Statistical approaches to maintain overall Type I error include: O'Brien-Fleming boundaries (very conservative early, allowing significance at final analysis close to conventional p<0.05), Lan-DeMets alpha-spending function (flexible timing of analyses), and Haybittle-Peto rule (requiring p<0.001 at interim, preserving p<0.05 at final analysis). For cardiovascular outcomes trials (CVOTs), interim analyses may occur after 50% and 75% of target events have accumulated. The SELECT trial's interim analysis showed such strong benefit that the DSMB recommended stopping the trial early — semaglutide 2.4mg met its primary cardiovascular endpoint before the planned final analysis date.