  Recently, methodology was presented to facilitate the incorporation of
interim analyses in stepped-wedge (SW) cluster randomised trials (CRTs). Here,
we extend this previous discussion. We detail how the stopping boundaries,
allocation sequences, and per-cluster per-period sample size of a group
sequential SW-CRT can be optimised. We then describe methods by which point
estimates, p-values, and confidence intervals, which account for the sequential
nature of the design, can be calculated. We demonstrate that optimal sequential
designs can reduce the expected required number of measurements under the null
hypothesis, compared to the classical design, by up to 30%, with no cost to the
maximal possible required number of measurements. Furthermore, the adjusted
analysis procedure almost universally reduces the average bias in the point
estimate, and consistently provides a confidence interval with coverage close
to the nominal level. In contrast, the coverage of a naive 95% confidence
interval is observed to range between 92 and 98%. Methodology is now readily
available for the efficient design and analysis of group sequential SW-CRTs. In
scenarios in which there are substantial ethical or financial reasons to
terminate a SW-CRT as soon as possible, trialists should strongly consider a
group sequential approach.
