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Research Methods & Reporting

The impact of outcome reporting bias in randomised controlled trials on a cohort of systematic reviews

BMJ 2010; 340 doi: (Published 15 February 2010) Cite this as: BMJ 2010;340:c365
  1. Jamie J Kirkham, research associate1,
  2. Kerry M Dwan, research assistant1,
  3. Douglas G Altman, director, professor2,
  4. Carrol Gamble, senior lecturer1,
  5. Susanna Dodd, lecturer1,
  6. Rebecca Smyth, research associate3,
  7. Paula R Williamson, director, professor1
  1. 1Centre for Medical Statistics and Health Evaluation, University of Liverpool, Liverpool L69 3GS
  2. 2Centre for Statistics in Medicine, University of Oxford, Oxford OX2 6UD
  3. 3Population, Community and Behavioural Sciences, University of Liverpool, Liverpool, L69 3GB
  1. Correspondence to: P R Williamson prw{at}
  • Accepted 26 October 2009


Objective To examine the prevalence of outcome reporting bias—the selection for publication of a subset of the original recorded outcome variables on the basis of the results—and its impact on Cochrane reviews.

Design A nine point classification system for missing outcome data in randomised trials was developed and applied to the trials assessed in a large, unselected cohort of Cochrane systematic reviews. Researchers who conducted the trials were contacted and the reason sought for the non-reporting of data. A sensitivity analysis was undertaken to assess the impact of outcome reporting bias on reviews that included a single meta-analysis of the review primary outcome.

Results More than half (157/283 (55%)) the reviews did not include full data for the review primary outcome of interest from all eligible trials. The median amount of review outcome data missing for any reason was 10%, whereas 50% or more of the potential data were missing in 70 (25%) reviews. It was clear from the publications for 155 (6%) of the 2486 assessable trials that the researchers had measured and analysed the review primary outcome but did not report or only partially reported the results. For reports that did not mention the review primary outcome, our classification regarding the presence of outcome reporting bias was shown to have a sensitivity of 88% (95% CI 65% to 100%) and specificity of 80% (95% CI 69% to 90%) on the basis of responses from 62 trialists. A third of Cochrane reviews (96/283 (34%)) contained at least one trial with high suspicion of outcome reporting bias for the review primary outcome. In a sensitivity analysis undertaken for 81 reviews with a single meta-analysis of the primary outcome of interest, the treatment effect estimate was reduced by 20% or more in 19 (23%). Of the 42 meta-analyses with a statistically significant result only, eight (19%) became non-significant after adjustment for outcome reporting bias and 11 (26%) would have overestimated the treatment effect by 20% or more.

Conclusions Outcome reporting bias is an under-recognised problem that affects the conclusions in a substantial proportion of Cochrane reviews. Individuals conducting systematic reviews need to address explicitly the issue of missing outcome data for their review to be considered a reliable source of evidence. Extra care is required during data extraction, reviewers should identify when a trial reports that an outcome was measured but no results were reported or events observed, and contact with trialists should be encouraged.


  • The authors are grateful to the many Cochrane reviewers who collaborated in some way to make this research possible. Their input includes defining the review primary outcome of interest, forwarding trial reports from their reviews, and establishing the outcome reporting bias classification for particular trials within their reviews. We thank the Cochrane Steering Group, and Cochrane statisticians and clinicians from the University of Liverpool who have provided expert knowledge when further assistance was required. We also acknowledge the trialists who answered specific queries about the reporting of outcomes in their trials. Finally, we thank Steve Taylor for undertaking some of the assessments and Chris Braithwaite for designing the study database.

  • Contributors: PRW, DGA, and CG designed the study protocol and developed the classification system for assessing outcome reporting bias in trials. The study case report form was designed by JJK, SD, and PRW. JJK contacted all review authors to confirm the chosen review primary outcome for use in the study. PRW and SD identified and agreed on the review primary outcome when no contact with the review authors was achieved. Assessments of outcome reporting bias (including obtaining trial reports and completing the outcome reporting bias classifications and justifications with the guidance from the systematic reviewers) were completed by JJK, SD, and KMD. All assessment justifications were checked by PRW. All the data were entered into the database by JJK. JJK and RS contacted trialists to verify if the review primary outcome was measured and/or analysed when outcome reporting bias was suspected. JJK and KMD undertook the data analysis under the supervision of PRW. JJK prepared the initial manuscript. JJK, PRW, DGA, and KMD were all involved in the substantial revision of this manuscript. All authors commented on the final manuscript before submission. PRW is the guarantor for the project.

  • Funding: The Outcome Reporting Bias In Trials (ORBIT) project was funded by the Medical Research Council (grant number G0500952). The funders had no role in the study design, data collection and analysis, decision to publish, or preparation of this manuscript.

  • Competing interests: PRW, CG, DGA, KMD, and RS are members of the Cochrane Collaboration. JJK and SD declare no competing interests.

  • Ethical approval: No ethics committee opinion was required for this study.

  • Provenance and peer review: Not commissioned; externally peer reviewed.

  • Accepted 26 October 2009
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