Prevalence and Methodological Characteristics of Subgroup Analyses in Stepped Wedge Cluster Randomised Trials: Protocol for a Systematic Review

Background: The stepped wedge cluster randomised trial is an increasingly common trial design. The design can be useful for informing real-world clinical decision-making, including decisions about the effectiveness of interventions in particular subgroups. However, there is little existing guidance about how to perform subgroup analyses in the stepped wedge design. We aim to determine the prevalence of subgroup analyses and describe statistical methods used to perform them in stepped wedge cluster randomised trials.Methods: We will conduct a systematic review following the methodology recommended in the Cochrane Handbook for Systematic Reviews of Interventions. We report this protocol according to the PRISMA-P checklist. The protocol has been registered in the Open Science Framework. We will search for terms related to ‘stepped wedge’. Sources will be PubMed, Embase, PsycINFO, Web of Science, CINAHL, Cochrane Library, and Current Controlled Trials Register up to 16 October 2020. Studies will be eligible if they are written in English, involve human participants and are primary or secondary reports of planned or completed stepped wedge cluster randomised trials. Two reviewers will first screen the titles and abstracts, then full texts, to select studies that should be included in the review. Disagreements will be solved by consensus through discussion with a third reviewer. We will extract data related to study characteristics including presence or absence of subgroup analyses, characteristics of subgroup variables examined, statistical methods used to perform subgroup analyses, and adherence to the most consistently recommendations suggested for subgroup analyses in general including in clinical trials. We will perform a qualitative synthesis of the extracted data.Discussion: This protocol offers a reproducible and transparent procedure for a systematic review of the literature. It will provide a portrait of the frequency and types of subgroup analyses performed in stepped wedge cluster randomised trials. These results will inform the development of recommendations for subgroup analyses in such trials.Systematic review registration: This protocol has been registered on Open Science Framework, Registration ID: https://osf.io/2kwrz.