Past exam of the mathematics course of the University of Cambridge / 2021 / iii / Paper 207 / 3 / b / i / Solution

A group sequential design can stop early for efficacy or futility, reducing expected sample size, cost, and time and limiting exposure to an inferior treatment. Its disadvantages include greater design and operational complexity, a potentially larger maximum sample size, and the need for adjusted stopping boundaries and inference to preserve the type I error after repeated looks.