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  1. Sandra Eldridge, professor of biostatistics1,
  2. Sally Kerry, senior lecturer in medical statistics2,
  3. David J Torgerson, director 3
  1. 1Centre for Health Sciences, Barts and the London School of Medicine and Dentistry, Queen Mary, University of London, London E1 2AT
  2. 2Department of Community Health Sciences, St George’s, University of London, London SW17 0RE
  3. 3York Trials Unit, Department of Health Sciences, University of York, York YO10 5DD
  1. Correspondence to: D J Torgerson djt6{at}york.ac.uk
  • Accepted 13 July 2009

Blinded recruitment of participants presents particular challenges for cluster trials, but careful design can minimise the risk of selection bias

Concealment of allocation is regarded as crucial for individually randomised controlled trials. The process ensures that the people selecting participants for randomisation do not know their allocation and avoids selective recruitment. In cluster randomised trials, groups (or clusters) of participants are randomised rather than individuals, yet data are collected on individual participants. Selective recruitment of individual participants can occur in these trials if the people recruiting participants know the participants’ allocation, even when allocation of clusters has been adequately concealed.

Summary points

  • Poor design of cluster trials risks bias in selection of participants

  • Ideally participants should be identified before the cluster is randomised

  • When this is not possible recruitment should be by someone masked to the cluster allocation

  • Statistical solutions to selection bias are less satisfactory than design solutions

  • Cluster trials need to report cluster sizes to enable the readers to ascertain any differences in recruitment between treatment groups

Two recent reviews found that up to 40% of cluster trials published in major medical journals may be biased.1 2 Yet articles and books on cluster randomised trials have tended to focus on statistical issues rather than the need to ensure unbiased sampling of individuals within the cluster.3 4

Cluster randomised trials are well established in education, where their use is relatively straightforward.5 6 Children are usually identified from school or class registers with little or no problem of selection bias being introduced after randomisation, and no consent is required from the children or their parents. Many healthcare trials can be designed along similar lines, with potential participants being all those who belong to a cluster (box 1).7 We describe how bias may occur when individual participants are identified …

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