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Br Med J (Clin Res Ed). 1987 September 12; 295(6599): 654–656.
PMCID: PMC1257776

Reference bias in reports of drug trials.


Articles published before 1985 describing double blind trials of two or more non-steroidal anti-inflammatory drugs in rheumatoid arthritis were examined to see whether there was any bias in the references they cited. Althogether 244 articles meeting the criteria were found through a Medline search and through examining the reference lists of the articles retrieved. The drugs compared in the studies were classified as new or as control drugs and the outcome of the trial as positive or not positive. The reference lists of all papers with references to other trials on the new drug were then examined for reference bias. Positive bias was judged to have occurred if the reference list contained a higher proportion of references with a positive outcome for that drug than among all the articles assumed to have been available to the authors (those published more than two years earlier than the index article). Altogether 133 of the 244 articles were excluded for various reasons--for example, 44 because of multiple publication and 19 because they had no references. Among the 111 articles analysed bias was not possible in the references of 35 (because all the references gave the same outcome); 10 had a neutral selection of references, 22 a negative selection, and 44 a positive selection--a significant positive bias. This bias was not caused by better scientific standing of the cited articles over the uncited ones. Thus retrieving literature by scanning reference lists may produce a biased sample of articles, and reference bias may also render the conclusions of an article less reliable.

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Selected References

These references are in PubMed. This may not be the complete list of references from this article.
  • Poynard T, Conn HO. The retrieval of randomized clinical trials in liver disease from the medical literature. A comparison of MEDLARS and manual methods. Control Clin Trials. 1985 Dec;6(4):271–279. [PubMed]
  • Dickersin K, Hewitt P, Mutch L, Chalmers I, Chalmers TC. Perusing the literature: comparison of MEDLINE searching with a perinatal trials database. Control Clin Trials. 1985 Dec;6(4):306–317. [PubMed]
  • Sackett DL. Bias in analytic research. J Chronic Dis. 1979;32(1-2):51–63. [PubMed]

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