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 meta-analyses 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 non-random selection mechanisms.