Statistical significance was assumed in case of p < 0.05. Heterogeneity of effect sizes was evaluated by the I2 statistic. An alpha error p < 0.05 and/or I2 of at least 50% were taken as indicators of substantial heterogeneity of outcomes. In this case, we examined the effect of removing single studies with special study designs on the results and on the heterogeneity. If meta-analyses were not possible, the results of individual studies are presented. Meta-analyses were performed using Review Manager, version 5 (The Cochrane Collaboration, Oxford, England) for all calculations.