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

BMJ 1996; 312 doi: https://doi.org/10.1136/bmj.312.7033.742 (Published 23 March 1996) Cite this as: BMJ 1996;312:742
  1. Kenneth F Schulz, assistant directora,
  2. David A Grimes, professorb,
  3. Douglas G Altman, headc,
  4. Richard J Hayes, headd
  1. a Division of Sexually Transmitted Diseases Prevention, Centers for Disease Control and Prevention, Atlanta, GA 30333, USA
  2. b Department of Obstetrics, Gynecology, and Reproductive Sciences, University of California, San Francisco, CA 94110, USA
  3. c Medical Statistics Laboratory, Imperial Cancer Research Fund Medical Statistics Group, Centre for Statistics in Medicine, Oxford OX3 7LF
  4. d Tropical Health Epidemiology Unit, London School of Hygiene and Tropical Medicine, London WC1E 9HT
  1. Correspondence to: Dr Schulz.
  • Accepted 4 January 1996

Abstract

Objective: To assess the methodological quality of approaches to blinding 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 the 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: Though 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 implemention 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.

Key messages

  • Key messages

  • Poor reporting of methods used for double blinding raises concerns about how effective blinding was in many of the studies

  • Some investigators may have excluded some patients from the analysis but not reported these exclusions in their published paper

  • Given the apparent failure to report exclusions, some of the more biased trials may be mistaken as unbiased and vice versa

  • Investigators must pay more attention to executing and reporting their approaches to blinding and the handling of exclusions

Footnotes

  • Funding Division of Sexually Transmitted Diseases Prevention, Centers for Disease Control.

  • Conflict of interest None.

  • Accepted 4 January 1996
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