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Understanding why evidence from randomised clinical trials may not be retrieved from Medline: comparison of indexed and non-indexed records

BMJ 2012; 344 doi: (Published 03 January 2012) Cite this as: BMJ 2012;344:d7501
  1. L Susan Wieland, research associate12,
  2. Karen A Robinson, assistant professor34,
  3. Kay Dickersin, professor and director2
  1. 1University of Maryland School of Medicine, Center for Integrative Medicine, Baltimore, MD 21201, USA
  2. 2US Cochrane Center, Johns Hopkins Bloomberg School of Public Health, Baltimore, MD, USA
  3. 3Department of Medicine, School of Medicine, Johns Hopkins University, Baltimore, MD, USA
  4. 4Department of Epidemiology, Johns Hopkins Bloomberg School of Public Health, Baltimore, MD, USA
  1. Correspondence to: L Susan Wieland lswieland{at}
  • Accepted 7 November 2011


Objective To explore why reports that seem to describe randomised controlled trials are sometimes not indexed (“tagged”) with RCT (randomised controlled trial) [pt] (publication type) in Medline.

Design Cross sectional study.

Setting The Cochrane Collaboration and US National Library of Medicine worked together to identify and retag records of randomised controlled trials with RCT [pt], 1994 to 2006.

Data source Published reports entered into Medline in 2005.

Main outcome measures Type of trial information presented (for example, main results, design, and methods), trial design, and other Medline indexing terms applied.

Results 572/591 (97%) untagged records and 578/594 (97%) tagged records contained information from randomised controlled trials. Type of trial information and design differed between untagged and tagged reports. Fewer than half (234/572, 41%, 95% confidence interval 37% to 45%) of untagged reports but most tagged reports (526/578, 91%, 89% to 93%) described the main results of the trial. Untagged reports were more likely than tagged reports to contain information on design and methods, baseline characteristics, long term follow-up, and secondary analyses. Untagged reports of main results were more likely than tagged reports to be from trials using a crossover design (36% v 10%, difference 25%, 95% confidence interval 19% to 32%). The Medical Subject Heading “Randomized Controlled Trials” was the most common clinical trial term applied to untagged reports, although more than half of untagged reports had no indexing related to trials.

Conclusion Based on the results for 2005, at least 3000 records describing randomised controlled trials but not indexed using RCT [pt] may have been entered into Medline between 2006 and 2011. Researchers and healthcare decision makers relying on using RCT [pt] may be missing important evidence in their searches, particularly for design and methods, baseline characteristics, long term follow-up, and secondary data analyses.


  • Part of this material was presented in poster form at the 16th Cochrane Colloquium, Freiburg, Germany, 2008 (Wieland S, Dickersin K. Why were they missed? Randomized controlled trials (RCTs) identified through the Medline Retagging Project but not the US National Library of Medicine (NLM)) and the Sixth International Congress on Peer Review and Biomedical Publication, Vancouver, Canada, 2009 (Wieland S, Dickersin K. Understanding why the US National Library of Medicine (NLM) fails to properly index the publication type [pt] of a number of randomized controlled trials (RCTs)). We thank Carol Lefebvre, senior information specialist at the UK Cochrane Centre, for her comments on an earlier version of this manuscript.

  • Contributors: LSW is the guarantor of the study, had full access to all the data in the study, and takes responsibility for the integrity of the data and the accuracy of the data analysis. LSW contributed to the study conception and design, acquisition of data, analysis and interpretation of data, and drafting the article. KD and KR contributed to the study design, analysis and interpretation of data, and drafting the article.

  • Funding: This study received no funding.

  • Competing interests: All authors have completed the ICMJE uniform disclosure form at (available on request from the corresponding author) and declare: no financial relationships with any organizations that might have an interest in the submitted work in the previous three years; no other relationships or activities that could appear to have influenced the submitted work.

  • Ethical approval: Not required.

  • Data sharing: No additional data available.

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