The Consolidated Standards of Reporting Trials (CONSORT) statement was developed to improve the reporting of randomised controlled trials. It was initially published in 1996 and focused on the reporting of parallel group randomised controlled trials.

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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 trialsCompared 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 (nonindependent), and so the effective sample size is less than the total number of individual participants.The reduction in effective sample size depends on average cluster size and the degree of correlation within clusters, known as the intracluster (or intraclass) correlation coefficient ( ). The intracluster correlation coefficient is the proportion of the total variance of the outcome that can be explained by the variation between clusters. To retain power, the sample size should be multiplied by 1+(m − 1) , called the design effect, where m is the average cluster size. Hayes and Bennett describe a related coefficient of variation, k, between clusters 10 and Connelly considers an economic approach.11 Software is available to adjust for the intracluster correlation coefficient in an analysis. 12The conduct of cluster randomised controlled trials may also differ from that of trials that randomise individuals. For instance, clusters are usually randomised all at once (or in batches) rather than one at a time. After randomisation, individuals in the clusters may be approached for consent, which raises the possibility of post-randomisation selection bias, 4 or they may not, which raises ethical concerns. 14An expanded explanation of the methods of cluster randomised trials is available on the CONSORT website (www. consort_statement.org). Quality of reporting of cluster trialsSurveys of published ...

Background: A commonly reported problem with the conduct of multicentre randomised controlled trials (RCTs) is that recruitment is often slower or more difficult than expected, with many trials failing to reach their planned sample size within the timescale and funding originally envisaged. The aim of this study was to explore factors that may have been associated with good and poor recruitment in a cohort of multicentre trials funded by two public bodies: the UK Medical Research Council (MRC) and the Health Technology Assessment (HTA) Programme.

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