Publication Bias
Publication bias was assessed using the Begg and Mazumdar29 rank correlation and Egger et al30 tests. Results showed a significant rank correlation test (P < .001, one tailed) and Egger test (P < .001, one tailed), indicating evidence of possible publication bias. In order to assess the potential effect of such bias on study results, an Orwin’s fail-safe N was calculated, indicating that 7 103 studies reporting a zero effect would be needed to reduce the observed effect to 0.20. The large number of “null” studies needed to eliminate the observed effects suggest that any possible publication bias had a minimal to nonexistent influence on the current results.