• Steve McDonald, Margaret Westby, Mike Clarke, Carol Lefebvre, and Cochrane Centres' Working Group on 50 Years of Randomized Trials.
• UK Cochrane Centre, NHS Research & Development Programme, Oxford OX2 7LG, UK. steve.mcdonald@med.monash.edu.au
• Int J Epidemiol. 2002 Feb 1;31(1):125-7.

BackgroundRandomized trials are important for controlling selection biases, and where sufficient numbers of participants are involved, have the potential to yield reliable estimates of treatment effects.MethodsWe investigated trends in the number and size of randomized trials reported in general health care journals from 1948 to 1997. From the handsearching of 18 general health care journals we collected data on the number of reports of randomized trials in each journal per year, and the number of participants in each trial.ResultsA total of 5503 reports of trials were identified in 18 general health care journals. More than a third appeared in the British Medical Journal. The peak period for trial reports was the mid 1980s, with more in 1986 than any other year (242). By the mid 1990s the number per year had declined by a third. Trials with fewer than 100 participants accounted for most of the reports (69%). In spite of the overall decline in the number of trial reports, those involving 100 participants or more continued to increase throughout the period studied.ConclusionsThe continued increase in the number of larger trials reported is encouraging, especially if it represents an increase in the size of trials more generally. Further research is needed to determine whether the trends over time identified here are reflective more of trends in the actual conduct of, rather than simply the reporting, of randomized trials.

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