ABSTRACT
Effect size is the basis of much evidence-based education policymaking. In particular, it is assumed to measure the educational effectiveness of interventions. Policy is being driven by the influential work of John Hattie, the Education Endowment Foundation, and others, which is grounded in this assumption. This article demonstrates the assumption is false and notes that, when criticized, proponents either attempt to inoculate themselves by listing (without checking) assumptions or use the specious reasoning that, however flawed their argument, no-one has disproved their conclusions.
