For example, a report by the Organisation for Economic Co-operation and Development (OECD) on the most recent Programme for International Student Assessment (PISA) study focuses on both 'excellence' and 'equity' (OECD, 2016a). There is broad agreement on suitable measures of excellence. The most commonly used measure for this purpose is mean test scores, sometimes supplemented by the percentage of children who have reached a specific threshold of proficiency. There is much less agreement on what equality is and how best to measure it.