John Hattie has studied in a meta-study which factors lead to visible success in learning. He has taken into account studies from Marzano in his investigations. As a measure of the value of teaching-learning methods, John Hattie identifies the effect size.

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 ‘hinge point’, in order to find an answer to the question “What works best in education?”

An effect size of "1" indicates that a particular approach to teaching or technique advanced the learning of the students in the study by one standard deviation above the mean. So an effect size of "1" is very good indeed.