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Ethical implications of excessive cluster sizes in cluster randomised trials
  1. Karla Hemming1,
  2. Monica Taljaard2,
  3. Gordon Forbes3,
  4. Sandra M Eldridge3,
  5. Charles Weijer4
  1. 1 Institute of Applied Health Research, University of Birmingham, Birmingham, UK
  2. 2 Department of Clinical Epidemiology, Ottawa Hospital Research Institute, Ottawa, Canada
  3. 3 Primary Care and Public Health, Queen Mary University of London, London, UK
  4. 4 Rotman Institute of Philosophy, Western University, London, UK
  1. Correspondence to Dr Karla Hemming, School of Health and Population Sciences, University of Birmingham, Birmingham b152tt, UK; k.hemming{at}


The cluster randomised trial (CRT) is commonly used in healthcare research. It is the gold-standard study design for evaluating healthcare policy interventions. A key characteristic of this design is that as more participants are included, in a fixed number of clusters, the increase in achievable power will level off. CRTs with cluster sizes that exceed the point of levelling-off will have excessive numbers of participants, even if they do not achieve nominal levels of power. Excessively large cluster sizes may have ethical implications due to exposing trial participants unnecessarily to the burdens of both participating in the trial and the potential risks of harm associated with the intervention. We explore these issues through the use of two case studies. Where data are routinely collected, available at minimum cost and the intervention poses low risk, the ethical implications of excessively large cluster sizes are likely to be low (case study 1). However, to maximise the social benefit of the study, identification of excessive cluster sizes can allow for prespecified and fully powered secondary analyses. In the second case study, while there is no burden through trial participation (because the outcome data are routinely collected and non-identifiable), the intervention might be considered to pose some indirect risk to patients and risks to the healthcare workers. In this case study it is therefore important that the inclusion of excessively large cluster sizes is justifiable on other grounds (perhaps to show sustainability). In any randomised controlled trial, including evaluations of health policy interventions, it is important to minimise the burdens and risks to participants. Funders, researchers and research ethics committees should be aware of the ethical issues of excessively large cluster sizes in cluster trials.

  • cluster trials
  • health services research
  • evaluation methodology

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  • Contributors KH is guarantor, conceived the idea, led the writing of the manuscript and produced the figures. KH and MT identified the case studies. CW wrote the ethical parts of the paper. SME and GF made important contributions to all aspects of the paper and development of ideas. All authors contributed to writing, drafting and editing the paper.

  • Funding Joe Bloggs is funded by the NIHR CLAHRC West Midl.

  • Competing interests None declared.

  • Provenance and peer review Not commissioned; externally peer reviewed.

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