“Assessing Treatment Effects and Publication Bias across Different Specialties in Medicine: a Large Empirical Study of the Cochrane Database of Systematic Reviews”, 2020-10-09 (; backlinks; similar):
Publication bias is a persisting problem in meta-analyses for evidence based medicine. As a consequence small studies with large treatment effects are more likely to be reported than studies with a null result which causes asymmetry.
Here, we investigated treatment effects from 57,186 studies from 1922–972019, and overall 99,129 meta-analyses and 5,557 large meta-analyses from the Cochrane Database of Systematic Reviews. Altogether 19% (95%-CI 18%–20%) of the meta-analyses demonstrated evidence for asymmetry, but only 3.9% (95%-CI 3.4%–4.4%) showed evidence for publication bias after further assessment of funnel plots. Adjusting treatment effects resulted in overall less evidence for efficacy, and treatment effects in some medical specialties or published in prestigious journals were more likely to be statistically-significant.
These results suggest that asymmetry from exaggerated effects from small studies causes greater concern than publication bias.