2.1.6 Statistical analysis
In this study, since the outcome indicators were continuous variables, the effect sizes were combined using the standardized mean difference (SMD) with 95% confidence intervals and the random effects model was used to pool effect size. A significant difference between the experimental and control groups was indicated by a P-value < 0.05. To evaluate heterogeneity, the I-square (I2) was used, and values of I2 ≤ 50% indicated that the included studies were homogeneous. If I2> 50%, it indicated that the included studies existed heterogeneity, and subgroup analyses were conducted to explore the sources of heterogeneity. The combined effect sizes were then combined using random effects models.