= 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.
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