Visible Learning

Hattie's synthesis of 800 meta-analyses was a genuine landmark in making educational research usable by practitioners, and the effect-size framework has shaped professional development worldwide. The methodological criticisms from statisticians are real and serious — aggregating heterogeneous meta-analyses is legitimately contested — and the near-exclusive focus on test scores is a meaningful limitation. Worth reading for educators, but pair it with the critical literature rather than treat it as gospel.