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Pragmatic Clinical Trials Unit

Cluster Randomised Trials

A cluster randomised trial (CRT) is a randomised controlled trial in which pre-existing groups, called clusters, of individuals are randomly allocated to treatment arms. For example, clusters may be clinical practices or schools where the individuals are patients and school children, respectively. CRTs can be used when individual randomisation to treatment arms is not possible or the intervention is naturally applied to a whole cluster. A cluster randomised design is associated with a loss in statistical power and additional complexity in design, conduct and analysis. Extensions to the cluster randomised design include cluster randomised crossover trials and stepped wedge trials. We provide a course 'A Practical Guide to Cluster Randomised Trials' based on a book written by Sandra Eldridge and Sally Kerry. We also organise a meeting 'Current Developments in Cluster Randomised and Stepped Wedge Designs' for talks and discussion about new perspectives for the design, analysis and reporting of cluster randomised and stepped wedge trials.  

A website designed to support those conducting cluster randomised trials and stepped wedge design:


Specific topics of our work include: 

  • Design 
  • Sample size calculation 
  • Recruitment and consent 
  • Use and reporting of covariates 
  • Analysis 

We have also conducted several systematic reviews of CRTs.

Sandra Eldridge describes CRTs and their usefulness at an international conference.

References to our published papers :

  • Eldridge, S., & Kerry, S. (2012). A Practical Guide to Cluster Randomised Trials in Health Services Research. John Wiley & Sons.
  • Hooper, R., & Bourke, L. (2015). Cluster randomised trials with repeated cross sections: alternatives to parallel group designs. BMJ, 350.
  • Arnup, S. J., Forbes, A. B., Kahan, B. C., Morgan, K. E., McDonald, S., & McKenzie, J. E. (2014). The use of the cluster randomized crossover design in clinical trials: protocol for a systematic review. Systematic Reviews, 3(1), 1–6.
  • Weijer, C., Grimshaw, J. M., Eccles, M. P., McRae, A. D., White, A., Brehaut, J. C., & Taljaard, M. (2012). The Ottawa Statement on the Ethical Design and Conduct of Cluster Randomized Trials. PLoS Medicine, 9(11), e1001346.
  • Eldridge, S. M., Ukoumunne, O. C., & Carlin, J. B. (2009). The Intra-Cluster Correlation Coefficient in Cluster Randomized Trials: A Review of Definitions. International Statistical Review, 77(3), 378–394. Sample size calculations for cluster randomised trials, with a focus on ordinal outcomes. (PhD thesis)
  • Rutterford, C., Copas, A., & Eldridge, S. (2015). Methods for sample size determination in cluster randomized trials. International Journal of Epidemiology, 44(3), 1051–67.
  • Rutterford, C., Taljaard, M., Dixon, S., Copas, A., & Eldridge, S. (2015). Reporting and methodological quality of sample size calculations in cluster randomized trials could be improved: a review. Journal of Clinical Epidemiology, 68(6), 716–23.
  • Teerenstra, S., Eldridge, S., Graff, M., de Hoop, E., & Borm, G. F. (2012). A simple sample size formula for analysis of covariance in cluster randomized trials. Statistics in Medicine, 31(20), 2169–2178.
  • Eldridge, S. M., Costelloe, C. E., Kahan, B. C., Lancaster, G. A., & Kerry, S. M. (2015). How big should the pilot study for my cluster randomised trial be? Statistical Methods in Medical Research.
  • Eldridge, S., Kerry, S., & Torgerson, D. J. (2009). Bias in identifying and recruiting participants in cluster randomised trials: what can be done? BMJ, 339.
  • Diaz-Ordaz, K., Slowther, A.-M., Potter, R., & Eldridge, S. (2013). Consent processes in clusterrandomised trials in residential facilities for older adults: a systematic review of reporting practices and proposed guidelines. BMJ Open , 3 (7 ). Choosing covariates in the analysis of cluster randomised trials. (PhD thesis)
  • Wright, N., Ivers, N., Eldridge, S., Taljaard, M., & Bremner, S. (2015). A review of the use of covariates in cluster randomised trials uncovers marked discrepancies between guidance and practice. Journal of Clinical Epidemiology, 68(6), 603–609.
  • Diaz-Ordaz, K., Kenward, M. G., Cohen, A., Coleman, C. L., & Eldridge, S. (2014). Are missing data adequately handled in cluster randomised trials? A systematic review and guidelines. Clinical Trials (London, England), 11(5), 590–600.
  • Froud, R., Eldridge, S., Diaz Ordaz, K., Marinho, V. C. C., & Donner, A. (2012). Quality of cluster randomized controlled trials in oral health: a systematic review of reports published between 2005 and 2009. Community Dentistry and Oral Epidemiology, 40, 3–14. 0528.2011.00660.x
  • Diaz-Ordaz, K., Froud, R., Sheehan, B., & Eldridge, S. (2013). A systematic review of cluster randomised trials in residential facilities for older people suggests how to improve quality. BMC Medical Research Methodology, 13(1), 1–10.
  • Eldridge, S., Ashby, D., Bennett, C., Wakelin, M., & Feder, G. (2008). Internal and external validity of cluster randomised trials: systematic review of recent trials. BMJ, 336(7649), 876–880.




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