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BMJ. 1996 March 23; 312(7033): 742–744.
PMCID: PMC2350472

Blinding and exclusions after allocation in randomised controlled trials: survey of published parallel group trials in obstetrics and gynaecology.


OBJECTIVE: To assess the methodological quality of approaches to blind ing and to handling of exclusions as reported in randomised trials from one medical specialty. DESIGN: Survey of published, parallel group randomised controlled trials. DATA SOURCES: A random sample of 110 reports in which allocation was described as randomised from 1990 and 1991 volumes of four journals of obstetrics and gynaecology. MAIN OUTCOME MEASURES: The adequacy of the descriptions of double blinding and exclusions after randomisation. RESULTS: Through 31 trials reported being double blind, about twice as many could have been. Of the 31 trials only eight (26%) provided information on the protection of the allocation schedule and only five (16%) provided some written assurance of successful implementation of double blinding. Of 38 trials in which the authors provided sufficient information for readers to infer that no exclusions after randomisation had occurred, six (16%) reported adequate allocation concealment and none stated that an intention to treat analysis had been performed. That compared with 14 (27%) and six (12%), respectively, for the 52 trials that reported exclusions. CONCLUSIONS: Investigators could have double blinded more often. When they did double blind, they reported poorly and rarely evaluated it. Paradoxically, trials that reported exclusions seemed generally of a higher methodological standard than those that had no apparent exclusions. Exclusions from analysis may have been made in some of the trials in which no exclusions were reported. Editors and readers of reports of randomised trials should understand that flawed reporting of exclusions may often provide a misleading impression of the quality of the trial.

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