2.6. Data synthesis and statistical analysis
The fixed-effects model and the random-effects model are based on different assumptions. The results of meta-analysis using fixed-effect models are limited to specific populations. As we cannot expect these studies included in the research to show the same widespread impact; the fact that the studies were conducted under different conditions (i.e., geography, dosage, measurement, etc.) could indispensably cause differences among the results. Thus, in the identification of effect sizes during the present meta-analytical processes, analyses were conducted according to the random effects model31. We computed standardized mean differences (SMD) with 95% confidence intervals(95%CI)27 to estimate effect sizes for continuous outcomes. Besides,we use stratified analysis to explore subgroup analysis. Data were further analyzed by using Stata 12.0, Comprehensive Meta-Analysis 3.0, and Review Manager 5.4.