Skip to main page content
U.S. flag

An official website of the United States government

Dot gov

The .gov means it’s official.
Federal government websites often end in .gov or .mil. Before sharing sensitive information, make sure you’re on a federal government site.

Https

The site is secure.
The https:// ensures that you are connecting to the official website and that any information you provide is encrypted and transmitted securely.

Access keys NCBI Homepage MyNCBI Homepage Main Content Main Navigation
. 2016 Jan:69:137-46.
doi: 10.1016/j.jclinepi.2015.08.015. Epub 2015 Sep 5.

Sample size calculations for stepped wedge and cluster randomised trials: a unified approach

Karla Hemming  1 , Monica Taljaard  2
Affiliations

Sample size calculations for stepped wedge and cluster randomised trials: a unified approach

Karla Hemming et al. J Clin Epidemiol. 2016 Jan.

Abstract

Objectives: To clarify and illustrate sample size calculations for the cross-sectional stepped wedge cluster randomized trial (SW-CRT) and to present a simple approach for comparing the efficiencies of competing designs within a unified framework.

Study design and setting: We summarize design effects for the SW-CRT, the parallel cluster randomized trial (CRT), and the parallel cluster randomized trial with before and after observations (CRT-BA), assuming cross-sectional samples are selected over time. We present new formulas that enable trialists to determine the required cluster size for a given number of clusters. We illustrate by example how to implement the presented design effects and give practical guidance on the design of stepped wedge studies.

Results: For a fixed total cluster size, the choice of study design that provides the greatest power depends on the intracluster correlation coefficient (ICC) and the cluster size. When the ICC is small, the CRT tends to be more efficient; when the ICC is large, the SW-CRT tends to be more efficient and can serve as an alternative design when the CRT is an infeasible design.

Conclusion: Our unified approach allows trialists to easily compare the efficiencies of three competing designs to inform the decision about the most efficient design in a given scenario.

Keywords: Cluster randomized trial; Efficiency; Power; Sample size; Stepped wedge; Study design.

PubMed Disclaimer

Figures

Fig. 1
Fig. 1
Schematic illustration of design of the conventional parallel CRT, the CRT-BA, and the SW-CRT (with five steps). CRT, cluster randomized trial; CRT-BA, cluster randomized trial with before and after observations; SW-CRT, stepped wedge cluster randomized trial.
Fig. 2
Fig. 2
Comparative efficiency of the conventional parallel CRT, the CRT-BA, and the SW-CRT (for fixed cluster sizes). CRT, cluster randomized trial; CRT-BA, cluster randomized trial with before and after observations; SW-CRT, stepped wedge cluster randomized trial.
See this image and copyright information in PMC

References

    1. Campbell M.J., Donner A., Klar N. Developments in cluster randomized trials and statistics in medicine. Stat Med. 2007;26:2–19. - PubMed
    1. Campbell M.K., Piaggio G., Elbourne D.R., Altman D.G. Consort 2010 statement: extension to cluster randomised trials. BMJ. 2012;345:e5661. - PubMed
    1. Teerenstra S., Eldridge S., Graff M., de H.E., Borm G.F. A simple sample size formula for analysis of covariance in cluster randomized trials. Stat Med. 2012;31:2169–2178. - PubMed
    1. Brown C.A., Lilford R.J. The stepped wedge trial design: a systematic review. BMC Med Res Methodol. 2006;6:54. - PMC - PubMed
    1. Mdege N.D., Man M.S., Taylor Nee Brown C.A., Torgerson D.J. Systematic review of stepped wedge cluster randomized trials shows that design is particularly used to evaluate interventions during routine implementation. J Clin Epidemiol. 2011;64:936–948. - PubMed

Publication types

MeSH terms