Duplicate multiple publication bias

In 1987, G0tzsche82 found that among 244 reports of trials comparing non-steroidal anti-inflammatory drugs in rheumatoid arthritis 44 (18%) were redundant, multiple publications, which overlapped substantially with an already published article.83 Twenty trials were published twice, 10 trials three times and one trial times.84 The production of multiple publications from single studies can lead to bias in a number of ways.85 Most importantly, studies with significant results are more likely to lead to multiple publications and presentations,45 which makes it more likely that they will be located and included in a meta-analysis. The inclusion of duplicated data may therefore lead to overestimation of treatment effects, as recently demonstrated for trials of the efficacy of ondansetron to prevent postoperative nausea and vomiting86 (Figure 3.6). It is not always obvious that

Figure 3.6 The inclusion of duplicated data may lead to overestimation of treatment effects: Tramer et al.86 found that of 19 trials that compared prophylactic intravenous ondansetron, data from three large multicentre trials had been duplicated in six further reports. In the 16 reports which were not duplicated, the number needed to treat (NNT) to prevent one episode of vomiting was 9-5 compared to 3-9 in the three reports that were subject to duplication. When all 19 original reports were combined, the NNT was 6-4; when original and duplicate reports were combined, a biased NNT of 4-9 was obtained.

Figure 3.6 The inclusion of duplicated data may lead to overestimation of treatment effects: Tramer et al.86 found that of 19 trials that compared prophylactic intravenous ondansetron, data from three large multicentre trials had been duplicated in six further reports. In the 16 reports which were not duplicated, the number needed to treat (NNT) to prevent one episode of vomiting was 9-5 compared to 3-9 in the three reports that were subject to duplication. When all 19 original reports were combined, the NNT was 6-4; when original and duplicate reports were combined, a biased NNT of 4-9 was obtained.

multiple publications come from a single study, and one set of study participants may thus be included in an analysis twice. Huston and Moher85 and, more recently, Johansen and G0tzsche87 vividly described the difficulties and frustration caused by redundancy and the "disaggregation" of medical research when results from a multicentre trial are presented in several publications. Indeed, it may be extremely difficult - if not impossible - for reviewers to determine whether two papers represent duplicate publications of one trial or two separate trials, since examples exist where two articles reporting the same trial do not share a single common author.84,86

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