This is based on the OECD's Programme for International Student Assessment (PISA) survey in which scores range from level 1 to level 5 (highest level).
See põhineb OECD rahvusvahelise õpitulemuste hindamise programmi (PISA) vaatlusel, kus taset hinnatakse hindega 1 kuni 5 (kõrgeim tase).