Improving the feasibility of stepped-wedge cluster randomised trials: a mixed methods enquiry

Stepped-wedge cluster randomised trials are methodologically complex, and can be logistically challenging, due to the staggered implementation of the intervention. These trials may therefore be more likely than other trials, to encounter issues that make them unable to successfully meet their objectives. Consequently, feasibility studies might be particularly useful for stepped-wedge cluster randomised trials. However, it has not been known what specific aspects of this trial design affect its feasibility, nor to what extent the feasibility of these trials has been investigated. The overarching aim of this thesis, was to improve the feasibility of stepped-wedge cluster randomised trials, by: understanding how the feasibility of these trials has been investigated; identifying the aspects of this trial design that affect its feasibility; investigating a solution to an identified issue; and making recommendations to improve the feasibility of trials of this design. This was achieved through a mixed methods enquiry. Feasibility studies, designed to inform stepped-wedge cluster randomised trials, were identified. The objectives of these studies were examined and the quality of reporting of those that had been published was assessed. The objectives of these studies were rarely specific to the chosen study design. Aspects of the stepped-wedge cluster randomised trial design that could affect its feasibility, were identified from individuals with an involvement, or interest, in trials of this design, through an online questionnaire and interview study. The process for stepped-wedge cluster randomised trials, of balancing prognostic covariates during the randomisation, was identified as a cause for concern for those involved in trials of this design. The use of covariate constrained randomisation for stepped-wedge cluster randomised trials, was therefore investigated using a simulation study. Based on the findings of this thesis, recommendations were made to improve the feasibility of future stepped-wedge cluster randomised trials.</div

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