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Education And Debate

Analysis of a trial randomised in clusters

BMJ 1998; 316 doi: (Published 03 January 1998) Cite this as: BMJ 1998;316:54
  1. Sally M Kerry, lecturer in medical statisticsa,
  2. J Martin Bland, professor of medical statisticsb
  1. a Division of General Practice and Primary Care, St George's Hospital Medical School, London SW17 0RE
  2. b Department of Public Health Sciences, St George's Hospital Medical School, London SW17 0RE
  1. Correspondence to: Mrs Kerry

    A cluster randomised study is one where a group of subjects are randomised to the same treatment together—for example, when women in some districts are offered breast cancer screening and compared with women in other districts, or when the patients of general practitioners who have been given special training are compared with the patients of those who have not.1

    Several techniques exist for analysing the data from such studies, but the essence of them is that the experimental unit (district or general practitioner) is the unit of analysis.2 A simple approach is to construct a summary statistic for each cluster and then analyse these summary values. The idea is similar to the analysis of repeated measurements on the same subject, where we construct a single summary …

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