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Education And Debate

CONSORT statement: extension to cluster randomised trials

BMJ 2004; 328 doi: (Published 18 March 2004) Cite this as: BMJ 2004;328:702
  1. Marion K Campbell, programme director (,
  2. Diana R Elbourne, professor of health care evaluation2,
  3. Douglas G Altman, professor of statistics in medicine3
  1. 1Health Services Research Unit, University of Aberdeen, Aberdeen, AB25 2ZD
  2. 2Medical Statistics Unit, London School of Hygiene and Tropical Medicine, London
  3. 3Centre for Statistics in Medicine, Oxford
  1. Correspondence to: M K Campbell
  • Accepted 10 December 2003

Reports of cluster randomised trials require additional information to allow readers to interpret them accurately

The effective reporting of randomised controlled trials has received useful attention in recent years. Many journals now require that reports conform to the guidelines in the Consolidated Standards of Reporting Trials (CONSORT) statement, first published in 19961 and revised in 2001.2 The statement includes a checklist of items that should be included in the trial report. These items are evidence based whenever possible and are regularly reviewed.3 The statement also recommends including a flow diagram to show the flow of participants from group assignment through to the final analysis.

The CONSORT statement focused on reporting parallel group randomised trials in which individual participants are randomly assigned to study groups. However, in some situations it is preferable to randomly assign groups of individuals (such as families or medical practices) rather than individuals. Reasons include the threat of contamination of some interventions (such as dietary interventions) if individual randomisation is used.4 5 Also, in certain settings randomisation by group may be the only feasible method of conducting a trial.6 Trials with this design are variously known as field trials, community based trials, place based trials, or (as in this paper) cluster randomised trials.7 In an earlier discussion paper we considered the implications of the CONSORT statement for the reporting of cluster randomised trials.8 Here we present updated guidance, based on the 2001 revision of the CONSORT statement.2

Methodological issues in cluster randomised trials

Compared with individually randomised trials, cluster randomised trials are more complex to design, require more participants to obtain equivalent statistical power, and require more complex analysis. The methodological issues in cluster randomised trials have been widely discussed.7 9 In brief, observations on individuals in the same cluster tend to be correlated …

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