It is not always obvious that multiple publications come from a single study, and one set of study participants may be included in an analysis twice. Most importantly, studies with significant results are more likely to lead to multiple publications and presentations (Easterbrook 1991), which makes it more likely that they will be located and included in a meta-analysis. The production of multiple publications from single studies can lead to bias in a number of ways (Huston 1996).
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PUBLICATION BIAS IN COMPREHENSIVE META ANALYSIS TRIAL
Twenty trials were published twice, ten trials three times and one trial four times (Gøtzsche 1989). In 1989, Gøtzsche 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 a previously published article. These biases have received much less consideration than publication bias, but their consequences could be of equal importance. Indeed, among published studies, the probability of identifying relevant studies for meta-analysis is also influenced by their results.
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While publication bias has long been recognized and much discussed, other factors can contribute to biased inclusion of studies in meta-analyses.
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The selective reporting of some outcomes but not others, depending on the nature and direction of the results The publication of research findings in a particular language, depending on the nature and direction of the results The citation or non-citation of research findings, depending on the nature and direction of the results The publication of research findings in journals with different ease of access or levels of indexing in standard databases, depending on the nature and direction of results. The multiple or singular publication of research findings, depending on the nature and direction of the results The rapid or delayed publication of research findings, depending on the nature and direction of the results The publication or non-publication of research findings, depending on the nature and direction of the results The table below summarizes some different types of reporting biases. The contribution made to the totality of the evidence in systematic reviews by studies with non-significant results is as important as that from studies with statistically significant results. Statistically significant, ‘positive’ results that indicate that an intervention works are more likely to be published, more likely to be published rapidly, more likely to be published in English, more likely to be published more than once, more likely to be published in high impact journals and, related to the last point, more likely to be cited by others. Reporting biases arise when the dissemination of research findings is influenced by the nature and direction of results. It has long been recognized that only a proportion of research projects ultimately reach publication in an indexed journal and thus become easily identifiable for systematic reviews.
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The dissemination of research findings is not a division into published or unpublished, but a continuum ranging from the sharing of draft papers among colleagues, through presentations at meetings and published abstracts, to papers in journals that are indexed in the major bibliographic databases (Smith 1999). We refer to Chapter 10 of the Cochrane Handbook.