At the school level, the analyses consider the relationship between the average socio-economic status of 15-year-old students in the school and the scores of the 15-year-olds attending that school. At the country level, the socio-economic status of students, both on average and its distribution within the country, can be related to average performance at the school-system level. These associations partly reflect the advantages in resources that relatively high socio-economic status confers.