Meta-analyses, like all reviews, are potentially subject to publication bias (i.e., the file-drawer problem). Studies generating smaller effect sizes are less likely to find significance. If studies with non-significant findings are more likely to go unpublished, then the published effect sizes available for meta-analysis may be biased toward overestimating the mean effect size (Rosenthal and DiMatteo, 2001). For all mean effect sizes that were significantly different from zero, we therefore calculated Rosenthal’s (1979) fail-safe N, which estimates the number of additional null findings (from hypothetical unpublished studies) necessary to render the mean effect non-significant. The fail-safe N serves as an indicator of the likelihood that the observed effect is not due to publication bias.
