We are currently in the process of updating this chapter and we appreciate your patience whilst this is being completed.
Systematic reviews are considered to provide the highest level of evidence, and authors seek to include all high-quality studies that address the question under review. However, many researchers have shown that studies with significant positive results are easier to locate than those with non-significant, or negative, results, because the latter often fail to get published.
This is known as publication bias, and it has further been shown that studies with positive results are not only more likely to be published, they are also more likely to be published rapidly, in English language journals, and be cited more by other authors.4 One reason this might occur is that scientists are less inclined to submit negative results for publication, but it may also reflect an attitude amongst journal editors that positive results make better articles. Thankfully, published manuscripts studying publication bias do not themselves appear to be victim to publication bias!2
Publication bias can by minimised by thorough literature searching, including inclusion of the grey literature (see previous section). However, if a negative study has never been published or committed to paper, it will never be found, and some attempt should therefore be made to quantify how big a problem publication bias is. One method by which publication bias can be ascertained is by the construction of a funnel plot. This is described in more detail in the Section 1B chapter “Comparison of survival rates; heterogeneity; funnel plots; the role of Bayes’ theorem”.
- Easterbrook P, Berlin J, Gopalan R, Matthews D. Publication bias in clinical research. Lancet, 1992;337:867–872.
- Dubben H, Beck-Bornholdt. Systematic review of publication bias in studies of publication bias. BMJ 2005;331:433.
© Helen Barratt 2009, Saran Shantikumar 2018