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Searching for unpublished data for Cochrane reviews: cross sectional study

BMJ 2013; 346 doi: (Published 23 April 2013) Cite this as: BMJ 2013;346:f2231
  1. Jeppe Bennekou Schroll, PhD student1,
  2. Lisa Bero, professor2,
  3. Peter C Gøtzsche, professor1
  1. 1The Nordic Cochrane Centre, Rigshospitalet, Dept 7810, Blegdamsvej 9, DK-2100 Copenhagen Ø, Denmark
  2. 2Clinical Pharmacy and Health Policy Studies, University of California San Francisco, San Francisco, CA, USA
  1. Correspondence to: J B Schroll js{at}
  • Accepted 26 March 2013


Objective To describe the experiences of authors of Cochrane reviews in searching for, getting access to, and using unpublished data.

Design Cross sectional study.

Setting Cochrane reviews.

Participants 2184 corresponding authors of Cochrane reviews as of May 2012.

Main outcome measure Frequencies of responses to open ended and closed questions in an online survey.

Results Of 5915 authors contacted by email, 2184 replied (36.9% response rate). Of those, 1656 (75.8%) had searched for unpublished data. In 913 cases (55.1% of 1656), new data were obtained and we received details about these data for 794 data sources. The most common data source was “trialists/investigators,” accounting for 73.9% (n=587) of the 794 data sources. Most of the data were used in the review (82.0%, 651/794) and in 53.4% (424/794) of cases data were provided in less than a month. Summary data were most common, provided by 50.8% (403/794) of the data sources, whereas 20.5% (163/794) provided individual patient data. In only 6.3% (50/794) of cases were data reported to have been obtained from the manufacturers, and this group waited longer and had to make more contacts to get the data. The data from manufacturers were less likely to be for individual patients and less likely to be used in the review. Data from regulatory agencies accounted for 3.0% (24/794) of the obtained data.

Conclusions Most authors of Cochrane reviews who searched for unpublished data received useful information, primarily from trialists. Our response rate was low and the authors who did not respond were probably less likely to have searched for unpublished data. Manufacturers and regulatory agencies were uncommon sources of unpublished data.


  • Contributors: PCG and LB conceived the study. The protocol was drafted by JBS; LB and PCG contributed. JBS created the online survey, sent out the invitations, and tabulated the data. All authors analysed the data. JBS drafted the manuscript; PCG and LB contributed. All authors had full access to all the data in the study. JBS is guarantor and takes responsibility for the integrity of the data and the accuracy of the data analysis. All authors have approved the final manuscript.

  • Funding: This study was funded by the Cochrane Collaboration Methods Innovation Fund. The funder had no influence on the study design, interpretation of data, or the decision to publish the results.

  • Competing interests: All authors have completed the ICMJE uniform disclosure form at (available on request from the corresponding author) and declare: no support from any organisation for the submitted work; no financial relationships with any organisations 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: This study was certified as exempt from human subjects review by the University of California human research protection programme (reference No 037504).

  • Data sharing: Anonymised datasets are available on request from the corresponding author at

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