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Multiplicity of data in trial reports and the reliability of meta-analyses: empirical study

BMJ 2011; 343 doi: (Published 30 August 2011) Cite this as: BMJ 2011;343:d4829
  1. Britta Tendal, research fellow1,
  2. Eveline Nüesch, research fellow23,
  3. Julian P T Higgins, senior statistician4,
  4. Peter Jüni, head of division23,
  5. Peter C Gøtzsche, professor and director1
  1. 1Nordic Cochrane Centre, Rigshospitalet, Blegdamsvej 9, DK-2100, Copenhagen Ø, Denmark
  2. 2Institute of Social and Preventive Medicine, University of Bern, Switzerland
  3. 3CTU Bern, Bern University Hospital, Switzerland
  4. 4MRC Biostatistics Unit, Cambridge, UK
  1. Correspondence to: B Tendal bt{at}
  • Accepted 7 June 2011


Objectives To examine the extent of multiplicity of data in trial reports and to assess the impact of multiplicity on meta-analysis results.

Design Empirical study on a cohort of Cochrane systematic reviews.

Data sources All Cochrane systematic reviews published from issue 3 in 2006 to issue 2 in 2007 that presented a result as a standardised mean difference (SMD). We retrieved trial reports contributing to the first SMD result in each review, and downloaded review protocols. We used these SMDs to identify a specific outcome for each meta-analysis from its protocol.

Review methods Reviews were eligible if SMD results were based on two to ten randomised trials and if protocols described the outcome. We excluded reviews if they only presented results of subgroup analyses. Based on review protocols and index outcomes, two observers independently extracted the data necessary to calculate SMDs from the original trial reports for any intervention group, time point, or outcome measure compatible with the protocol. From the extracted data, we used Monte Carlo simulations to calculate all possible SMDs for every meta-analysis.

Results We identified 19 eligible meta-analyses (including 83 trials). Published review protocols often lacked information about which data to choose. Twenty-four (29%) trials reported data for multiple intervention groups, 30 (36%) reported data for multiple time points, and 29 (35%) reported the index outcome measured on multiple scales. In 18 meta-analyses, we found multiplicity of data in at least one trial report; the median difference between the smallest and largest SMD results within a meta-analysis was 0.40 standard deviation units (range 0.04 to 0.91).

Conclusions Multiplicity of data can affect the findings of systematic reviews and meta-analyses. To reduce the risk of bias, reviews and meta-analyses should comply with prespecified protocols that clearly identify time points, intervention groups, and scales of interest.


  • Contributors: All authors had full access to all the data in the study and take responsibility for the integrity of the data and the accuracy of the data analysis. BT and EN contributed to the study equally. BT, EN, and PCG contributed to the study concept and design. BT and EN contributed to the acquisition of data and drafted the manuscript. JPTH, EN, and BT contributed to the analysis and interpretation of data. All the authors critically reviewed the manuscript for publication. PCG provided administrative, technical, and material support, and was the study supervisor and guarantor.

  • Funding: This study was part of a PhD (BT) funded by IMK Charitable Fund. The funding source had no role in the design and conduct of the study; data collection, management, analysis, and interpretation; preparation, review, and approval of the manuscript; or the decision to submit the paper for publication.

  • Competing interests: All authors have completed the ICMJE uniform disclosure form at (available on request from the corresponding author) and declare: this study is part of a PhD funded by IMK Charitable Fund; no financial relationships with any organisations that might have an interest in the submitted work in the previous 3 years; no other relationships or activities that could appear to have influenced the submitted work.

  • Data sharing: No additional data available.

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