Stepped wedge designs could reduce the required sample size in cluster randomized trials

J Clin Epidemiol. 2013 Jul;66(7):752-8. doi: 10.1016/j.jclinepi.2013.01.009. Epub 2013 Mar 22.


Objective: The 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 setting: We 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.

Results: Our 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.

Conclusion: For CRTs, the stepped wedge design is far more efficient than the parallel group and ANCOVA design in terms of sample size.

Publication types

  • Comparative Study

MeSH terms

  • Analysis of Variance
  • Binomial Distribution
  • Cluster Analysis
  • Computer Simulation
  • Cross-Over Studies
  • Depression / diagnosis*
  • Depression / therapy*
  • Epidemiologic Research Design
  • Humans
  • Mathematical Computing
  • Randomized Controlled Trials as Topic / methods*
  • Randomized Controlled Trials as Topic / standards
  • Reproducibility of Results
  • Sample Size*