The Programme for International Student Assessment (PISA) International Socio-Economic Index of Occupational Status (ISEI) was derived from students’ responses on parental occupation. The index captures the attributes of occupations that convert parents’ education into income. The index was derived by the optimal scaling of occupation groups to maximise the indirect effect of education on income through occupation and to minimise the direct effect of education on income, net of occupation (both effects being net of age). For more information on the methodology, see Ganzeboom et al. (1992). The PISA International Socio-Economic Index of Occupational Status is based on either the father or mother’s occupations, whichever is the higher.