June 9, 2022
IMPACT members Fan Li and Monica Talijaard are among the authors responding to the need for guidance in selecting the most appropriate power and sample size calculations software to use when designing a stepped wedge cluster randomized trial. The authors provide practical advice for investigators planning stepped wedge trials using any of the available sample size calculators. They review main sample size considerations, summarize features supported by available calculators, and discuss the four main calculators, identifying their strengths and limitations. They illustrate how to use these calculators to compute power for continuous and binary outcomes and compare results. Implications of the calculator used are discussed and recommendations for implementing sample size or power calculations using available calculators are made.
Abstract:
Recent years have seen a surge of interest in stepped-wedge cluster randomized trials (SW-CRTs). SW-CRTs include several design variations and methodology is rapidly developing. Accordingly, a variety of power and sample size calculation software for SW-CRTs has been developed. However, each calculator may support only a selected set of design features and may not be appropriate for all scenarios. Currently, there is no resource to assist researchers in selecting the most appropriate calculator for planning their trials. In this paper, we review and classify 18 existing calculators that can be implemented in major platforms, such as R, SAS, Stata, Microsoft Excel, PASS and nQuery. After reviewing the main sample size considerations for SW-CRTs, we summarize the features supported by the available calculators, including the types of designs, outcomes, correlation structures and treatment effects; whether incomplete designs, cluster-size variation or secular trends are accommodated; and the analytical approach used. We then discuss in more detail four main calculators and identify their strengths and limitations. We illustrate how to use these four calculators to compute power for two real SW-CRTs with a continuous and binary outcome and compare the results. We show that the choice of calculator can make a substantial difference in the calculated power and explain these differences. Finally, we make recommendations for implementing sample size or power calculations using the available calculators. An R Shiny app is available for users to select the calculator that meets their requirements (https://douyang.shinyapps.io/swcrtcalculator/).