Sample size calculations for stepped-wedge cluster randomised trials with unequal cluster sizes

Background The current methodology for sample size calculation for steppedwedge cluster randomised trials (SW-CRTs) is based on the assumption of the clusters being of equal size. However, as is often the case in CRTs, the clusters in SW-CRTs are likely to vary in size which in CRTs of other designs leads to a reduction in power. The effect of an imbalance in cluster sizes on SW-CRTs was not known, nor what an appropriate adjustment to the sample size should be. Trials 2016, Volume 17 Suppl 1 Page 4 of 6 Methods We proposed three adjusted design effects (DEs) for use in the calculation of the sample size for SW-CRTs with varying degrees of imbalance in cluster size, based on those suggested for use in CRTs with unequal cluster sizes. A simulation study was conducted which investigated the effect of unequal cluster sizes on the power of SW-CRTs, when the sample size was calculated using both the standard method and the three proposed adjusted DEs. Results An imbalance in cluster size was not found to significantly affect the power of a SW-CRT, and the proposed adjusted DEs generally resulted in trials that were severely over-powered. Conclusions We recommend that the standard method of sample size calculation for SW-CRTs be used when any imbalance in cluster size is expected to be small. When there is likely to be a large imbalance in cluster size it is recommended that simulations be used to determine if additional clusters are needed.