t in case of inclusion could
render the overall significant effect size nonsignificant (Durlak & Lipsey, 1991). Metaanalytic
findings are considered to be robust if the fail-safe number exceeds the critical
value obtained with Rosenthal’s (1995) formula of 5k + 10 in which k is the number of
studies used in the meta-analysis. If the fail-safe number falls below this critical value,
a publication bias or file drawer problem may exist