Mean score calculated for the results of the Programme for International Student Assessment (PISA) in the specific subject and grade, as reported by the Local Education Group (LEG) of the evaluated country. PISA is an international study that was launched by the OECD in 1997. It aims to evaluate education systems worldwide every three years by assessing 15-year-olds' competencies in the key subjects: reading, mathematics and science. For further details please refer to this web site: http://www.oecd.org/pisa/. Pupils who were 15 year old participated in this test, some of them were at grade 8 and some at grade 10.