Research Methods & Reporting

Out of sight but not out of mind: how to search for unpublished clinical trial evidence

BMJ 2012; 344 doi: (Published 03 January 2012) Cite this as: BMJ 2012;344:d8013
  1. An-Wen Chan, assistant professor and Phelan scientist
  1. 1Women’s College Research Institute, University of Toronto, Toronto, Ontario, Canada
  1. Corrrespondence to: A-W Chan anwen.chan{at}

A key challenge in conducting systematic reviews is to identify the existence and results of unpublished trials, and unreported methods and outcomes within published trials. An-Wen Chan provides guidance for reviewers on adopting a comprehensive strategy to search beyond the published literature

Summary points

  • The validity of systematic reviews relies on the identification of all relevant evidence

  • Systematic reviewers should search for unpublished information on the methods and results of published and unpublished clinical trials

  • The potential sources of unpublished information on clinical trials have expanded over recent years

  • Recognition of the strengths and limitations of these key information sources can help to identify areas for further emphasis and improvement

Systematic reviews of randomised trials play a key role in guiding patient care and health policy. Their validity depends to a large extent on reviewers’ ability to retrieve relevant information from all existing trials. Unfortunately, about half of clinical trials remain unpublished after receiving ethics approval—particularly those with statistically non-significant findings.1 Even when published, most journal articles do not report all of the outcome data or key methodological information.2 3 The overall result is that the published literature tends to overestimate the efficacy and underestimate the harms of a given intervention, while providing insufficient information for readers to evaluate the risk of bias.

It is thus important that systematic reviewers adopt a comprehensive strategy to search beyond the published literature. The optimal systematic review would have complete information about every trial—the full protocol, final study report, raw dataset, and any journal publications and regulatory submissions.4 The eligibility and risk of bias for each trial could then be evaluated, regardless of its publication status.

There are several potential sources of unpublished information on trial methods and results (table). These sources can help to identify the existence and results of …

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