Empirical assessment of effect of publication bias on meta-analyses

Objective: To assess the effect of publication bias on the results and conclusions of systematic reviews and meta-­analyses. Design: Analysis of published meta­-analyses by trim and fill method. Studies: 48 reviews in Cochrane Database of Systematic Reviews that considered a binary endpoint and contained 10 or more individual studies. Main outcome measures: Number of reviews with missing studies and effect on conclusions of meta-­analyses. Results: The trim and fill fixed effects analysis method estimated that 26 (54%) of reviews had missing studies and in 10 the number missing was significant. The corresponding figures with a random effects model were 23 (48%) and eight. In four cases, statistical inferences regarding the effect of the intervention were changed after the overall estimate for publication bias was adjusted for. Conclusions: Publication or related biases were common within the sample of meta­analyses assessed. In most cases these biases did not affect the conclusions. Nevertheless, researchers should check routinely whether conclusions of systematic reviews are robust to possible non­random selection mechanisms.