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J Eval Clin Pract. 2012 Aug;18(4):878-86. doi: 10.1111/j.1365-2753.2011.01700.x. Epub 2011 Jun 20.

Bias in recruitment to cluster randomized trials: a review of recent publications.

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York Trials Unit, Department of Health Sciences, University of York, York, UK.



To assess recruitment bias and the techniques employed to counter this problem in a recent selection of published cluster randomized trials.


Review of 24 cluster trials published in 2008 in four leading medical journals.


Studies were assessed by four reviewers to identify if an alternative design could have been employed using individual randomization. Data were also extracted on the randomization procedure and the likelihood of this introducing bias to the selection of participants into the study.


Of the 24 trials, eight could have used individual randomization as an alternative to cluster allocation. Seven studies could have recruited participants prior to cluster randomization but did not. In eight studies where recruitment bias was possible, more than half (five) demonstrated some evidence of differential recruitment rates.


Many cluster trials published in leading medical journals are not clear in their justification for the design. We also found significant proportions of cluster trials used suboptimal designs that increase their risk of introducing selection bias. Better design of cluster trials is possible and should be adopted.

[Indexed for MEDLINE]

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