Data synthesis and analysis
We calculated the standardized summary mean differences and the relative
risks (RR), along with the corresponding 95% CI, by pooling the
study-specific estimates using fixed and random-effects models. The
standardized mean difference indicated the mean change per SD allowing
for the comparison of scores in different scales. The presence and the
degree of heterogeneity were assessed with I2 (ranging
from 0% to 100%). When high heterogeneity was detected, it was further
investigated through subgroup analysis. We further assessed possible
small study effects (an indication of publication bias) by visual
inspection of funnel plots and Egger test. All analyses were performed
using Stata (version 14; StataCorp, College Station, TX, USA).