John Hattie developed a way of synthesizing various influences in different meta-analyses according to their effect size (Cohen’s d). He ranked influences that are related to learning outcomes from very positive effects to very negative effects. Hattie found that the average effect size of all the interventions he studied was 0.40. Therefore he decided to judge the success of influences relative to this number, in order to find an answer to the question “What works best in education?”