Introduction to calculating the overall effect size of an intervention in meta-analyses in educational sciences
When several comparison group studies relate to the same intervention in the school environment, the synthesis of their results makes it possible to evaluate the effect of this intervention and to meet the expectations of practitioners, but also to guide the future research. It is for this purpose that quantitative summaries or meta-analyses are carried out. For each of the selected studies, an effect size is calculated; when the variable to be explained is a posttest score, we use Hedges’ g (standardized difference of the means of the two groups). The overall effect size is then estimated by following in most cases the random effects model. Finally, to determine the interest of the intervention, the results of the meta-analysis are interpreted according to several grids. A worked example is provided to help the understanding of the analytical models implemented in a meta-analysis in simple cases.