2.4. Data analysis
2.4.1. Calculating the effect size
The following meta-analysis steps recommended by Borenstein, Hedges, Higgins, and Rothstein (2009) were employed in this study: (a) determine the effect sizes of each article, (b) determine the weighted mean effect size across articles, (c)
calculate the confidence interval for the average effect size, and (d) determine whether the effect size of any particular group was influenced by a moderator variable based on a heterogeneity analysis (Q). Two formulae were used to calculate the effect sizes of the studies. Cohen's d formula (Cohen, 1988) was used to determine the effect size for the experimental research with random assignment and without a pretest: