In all OECD countries, the performance of 15-year-olds in mathematics is strongly associated with the location of their school (in rural or urban areas) and with their socio-economic background. These levels of socio-economic inequity have remained the same for the last decade in the majority of countries. The ESCS parity index refers to the ratio of the value for the bottom quartile over the value for the top quartile of the ESCS index. Location parity is measured using the PISA definition of rural and urban areas (see the Definitions section at the end of this chapter).