The dissemination of research findings is not a division into published or unpublished, but a continuum ranging from the sharing of draft papers among colleagues, through presentations at meetings and published abstracts, to papers in journals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). 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.
Reporting biases arise when the dissemination of research findings is influenced by the nature and direction of 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. 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.
Table 10.1.a summarizes some different types of reporting biases. We consider these in more detail in Section 10.2, highlighting in particular the evidence supporting the presence of each bias. We discuss approaches for avoiding reporting biases in Cochrane reviews in Section 10.3, and address funnel plots and statistical methods for detecting potential biases in Section 10.4 . Although for the purpose of discussing these biases we will sometimes denote statistically significant (P<0.05) results as ‘positive’ results and statistically non-significant or null results as ‘negative’ results, such labels should not be used by Cochrane reviews authors.