Cluster randomised trials in the medical literature: two bibliometric surveys.
BACKGROUND
Several reviews of published cluster randomised trials have reported that about half did not take clustering into account in the analysis, which was thus incorrect and potentially misleading. In this paper I ask whether cluster randomised trials are increasing in both number and quality of reporting.


METHODS
Computer search for papers on cluster randomised trials since 1980, hand search of trial reports published in selected volumes of the British Medical Journal over 20 years.


RESULTS
There has been a large increase in the numbers of methodological papers and of trial reports using the term 'cluster random' in recent years, with about equal numbers of each type of paper. The British Medical Journal contained more such reports than any other journal. In this journal there was a corresponding increase over time in the number of trials where subjects were randomised in clusters. In 2003 all reports showed awareness of the need to allow for clustering in the analysis. In 1993 and before clustering was ignored in most such trials.


CONCLUSIONS
Cluster trials are becoming more frequent and reporting is of higher quality. Perhaps statistician pressure works.
Department of Health Sciences, University of York, York YO10 5DD, United Kingdom. mb55@york.ac.uk
J Martin
JM
Bland
D015982:Bias; D015706:Bibliometrics; D016000:Cluster Analysis; D006801:Humans; D010506:Periodicals as Topic / Q000592:standards / Q000706:statistics & numerical data*; D016032:Randomized Controlled Trials as Topic / Q000379:methods* / Q000592:standards