Every three years, the OECD tests the skills and knowledge of 15-year-olds around the world, using the results to rank participating education systems. In 2015, more than half a million teenagers — representing 28m students in 72 countries — took the test, known as the Programme for International Student Assessment, or Pisa.
The first Pisa test was performed in 2000, but since then it has grown to become a global school league table, carefully watched by governments, policymakers and journalists alike. Its influence provokes fear in education ministries, and criticism by academics.
If a country does poorly — or even just not as well as in previous years — that can be a career-limiting result for an education minister. In a 2014 open letter to the OECD, academics accused Pisa of stifling innovation, encouraging rote learning and being too narrow.