The Times Educational Supplement described Hattie's meta-study as "teaching's holy grail".
[2] Hattie compared the effect sizes of influences on learning outcomes - in particular by using Cohen's d as a measure.
The question is which strategies and innovations work best and where to concentrate efforts in order to improve student achievement.
[3][4][5] In 2014, Rolf Schulmeister and Jörn Loviscach pointed out "considerable issues in terms of the selection of studies and the methods employed" in the meta analysis.
[6] Amongst others, they criticize the use of questionable or wrongly-categorized studies and misleading or nonsensical statistics.