Heterogeneity was assessed based on calculation of the I² statistic (percentage variability of estimated effect that can be attributed to the heterogeneity of the effects) and the null statistic test. Galbraith graphs displayed the degree of heterogeneity. In studies where great heterogeneity was detected, a sensitivity analysis was performed to determine its source. Funnel plots and the Egger test were used to assess risk of bias of the accepted statistical significance level of 5% (p = 0.05).