Group sequential design

A group sequential design analyzes accumulating data at prespecified interim analyses and may stop early for efficacy or futility.

An interim analysis examines data before a study reaches its planned maximum sample size.

A sequential stopping boundary specifies values of an accumulating test statistic that trigger early stopping for efficacy, futility, or harm.

At information levels $I_1<\cdots <I_J$, standardized efficient test statistics have unit variances, means proportional to $\sqrt{I_j}$, and correlations $\mathrm{Corr}(Z_j,Z_k)=\sqrt{I_j/I_k}$ for $j\le k$.