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General medicine
Catalogue of bias: selective outcome reporting bias
  1. Elizabeth T Thomas,
  2. Carl Heneghan
  1. Centre for Evidence-Based Medicine, Nuffield Department of Primary Care Health Sciences, University of Oxford, Oxford, UK
  1. Correspondence to Dr Elizabeth T Thomas, Centre for Evidence-Based Medicine, Nuffield Department of Primary Care Health Sciences, University of Oxford, Oxford OX2 6GG, UK; elizabeth.thomas{at}

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Clinical trials reduce uncertainties about the benefits and harms of an intervention. Outcomes of interest should be specified before the trial starts (a priori), and clinically relevant to patient care.

Selective reporting of prespecified outcomes based on the nature and direction of the analysed results occurs among a large proportion of published clinical trials. Selective outcome reporting bias can potentially compromise the validity of a trial and any subsequent meta-analyses.1

Selective outcome reporting can occur in different ways:

  1. Omitting outcomes which are deemed to be unfavourable or not statistically significant.

  2. Adding new outcomes based on collected data to favour statistical significance.

  3. Including only a subset of the analysed data in the published study.

  4. Failing to report data that was analysed in the trial (such as adverse effects).

  5. Changing outcomes of interest (from primary outcomes to secondary outcomes if they do not yield significant results or the desired direction and magnitude of effect).

Detecting outcome reporting bias can be difficult. When the clinical trial protocol or the trial registration outcomes of interest differ from the outcomes in the published paper, it constitutes selective outcome reporting bias.


  • A cohort study of trials and their protocols conducted in Denmark between 1994 and 1995 found that in 62% of trials (63 out of 102), the authors had changed, introduced or omitted at least one of their primary outcomes.2

  • A similar review of a cohort of Canadian studies found that 88% of randomised trials (42 out of 48) failed to report at least one prespecified outcome.3

  • A systematic review including 27 studies analysed the registered and published outcomes of trials across a range of specialties including critical care, sports medicine, gastroenterology and emergency medicine.4 The authors found that the median proportion of trials that selectively reported the primary outcome was 31% …

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  • Twitter @_ETThomas, @carlheneghan

  • Contributors Both authors contributed equally to manuscript drafting and revision.

  • Funding The authors have not declared a specific grant for this research from any funding agency in the public, commercial or not-for-profit sectors.

  • Competing interests None declared.

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