Intended for healthcare professionals

Letters Diabetes education

Selection bias in cluster trial

BMJ 2008; 336 doi: (Published 13 March 2008) Cite this as: BMJ 2008;336:573
  1. David J Torgerson, director
  1. 1York Trials Unit, University of York, York YO10 5DD
  1. djt6{at}

The large cluster randomised controlled trial by Davies et al suffers from recruitment bias as a result of poor allocation concealment,1 which is crucial in both individually randomised trials and cluster trials. Allocation was not concealed from the people doing the …

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