3.4. Evaluation of publication bias
The classic fail-safe N and Orwin's fail-safe N were adopted to demonstrate the publication bias for the 108 selected studies. As suggested by the data in Table 3, the classic fail-safe N test determined that a total of 4144 studies with null results
would be needed in order to nullify the effect size. Moreover, the results of Orwin's fail-safe N test (see Table 4) show that the