Princesses are bigger than elephants : effect size as a category error in evidence based education.

Much of the evidential basis for recent policy decisions is grounded in effect size: the standardised mean difference in outcome scores between a study's intervention and comparison groups. This is interpreted as measuring educational influence, importance or effectiveness of the intervention. This article shows this is a category error at two levels. At the individual study level, the intervention plays only a partial role in effect size, so treating effect size as a measure of the intervention is a mistake. At the meta‐analytic level, the assumptions needed for a valid comparison of the relative effectiveness of interventions on the basis of relative effect size are absurd. While effect size continues to have a role in research design, as a measure of the clarity of a study, policy makers should recognise the lack of a valid role for it in practical decision‐making