• Willem Woertman, Esther de Hoop, Mirjam Moerbeek, Sytse U Zuidema, Debby L Gerritsen, and Steven Teerenstra.
• Department for Health Evidence, Radboud University Medical Centre, 133, PO Box 9101, 6500 HB Nijmegen, The Netherlands.
• J Clin Epidemiol. 2013 Jul 1;66(7):752-8.

ObjectiveThe stepped wedge design is increasingly being used in cluster randomized trials (CRTs). However, there is not much information available about the design and analysis strategies for these kinds of trials. Approaches to sample size and power calculations have been provided, but a simple sample size formula is lacking. Therefore, our aim is to provide a sample size formula for cluster randomized stepped wedge designs.Study Design And SettingWe derived a design effect (sample size correction factor) that can be used to estimate the required sample size for stepped wedge designs. Furthermore, we compared the required sample size for the stepped wedge design with a parallel group and analysis of covariance (ANCOVA) design.ResultsOur formula corrects for clustering as well as for the design. Apart from the cluster size and intracluster correlation, the design effect depends on choices of the number of steps, the number of baseline measurements, and the number of measurements between steps. The stepped wedge design requires a substantial smaller sample size than a parallel group and ANCOVA design.ConclusionFor CRTs, the stepped wedge design is far more efficient than the parallel group and ANCOVA design in terms of sample size.Copyright © 2013 Elsevier Inc. All rights reserved.

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