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. 2011 Aug 30:343:d4829.
doi: 10.1136/bmj.d4829.

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

Britta Tendal  1 , Eveline Nüesch, Julian P T Higgins, Peter Jüni, Peter C Gøtzsche
Affiliations

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

Britta Tendal et al. BMJ. .

Abstract

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.

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Conflict of interest statement

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.

Figures

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Fig 1 Flowchart for selection of meta-analyses
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Fig 2 Monte Carlo distributions of possible pooled SMDs in each meta-analysis. Dots=number of trials included. Open dots=trials without multiplicity of data. Filled dots=trials with multiplicity of data. Stars=published pooled SMDs. Meta-analyses are ordered according to the number of trials included. Negative SMDs on y axis indicate experimental intervention has more beneficial effect than control intervention
See this image and copyright information in PMC

References

    1. Bender R, Bunce C, Clarke M, Gates S, Lange S, Pace NL, et al. Attention should be given to multiplicity issues in systematic reviews. J Clin Epidemiol 2008;61:857-65. - PubMed
    1. Tendal B, Higgins JP, Juni P, Hrobjartsson A, Trelle S, Nuesch E, et al. Disagreements in meta-analyses using outcomes measured on continuous or rating scales: observer agreement study. BMJ 2009;339:b3128. - PMC - PubMed
    1. Reichenbach S, Sterchi R, Scherer M, Trelle S, Burgi E, Burgi U, et al. Meta-analysis: chondroitin for osteoarthritis of the knee or hip. Ann Intern Med 2007;146:580-90. - PubMed
    1. Abbass AA, Hancock JT, Henderson J, Kisely S. Short-term psychodynamic psychotherapies for common mental disorders. Cochrane Database Syst Rev 2006;4:CD004687. - PubMed
    1. Afolabi BB, Lesi FE, Merah NA. Regional versus general anaesthesia for caesarean section. Cochrane Database Syst Rev 2006;4:CD004350. - PubMed

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