 Solution

A group sequential design can reduce the expected sample size by stopping at an interim analysis once efficacy or futility is sufficiently clear. It does not generally reduce the prespecified maximum sample size: repeated opportunities to reject inflate the Type I error, so valid sequential stopping boundaries usually require a modest increase in maximum information relative to a fixed-sample design with the same power. The benefit is a smaller expected sample size under alternatives that often cross an early boundary, and sometimes under the null through early futility stopping.