The relationship between the socioeconomic and cultural level of families and the performance in mathematical skills of students aged 15 years represents a measure of the capacity of education systems to reduce social inequalities. A weaker relationship indicates that the education system has a better capacity for moderating the effect that the social and family environment of students has on their school performance.

The ESCS indicates the economic, social and cultural status of the families of students assessed in PISA (Programme for International Student Assessment). Taken into account to calculate this index are the professional occupation and education level of the parents, as well as the resources available in the home (e.g. number of books in the home). Four categories of economic, social and cultural status are used corresponding to four quartiles. Category ESCS 1 corresponds to 25% of families with the lowest socioeconomic levels; Category ESCS 2, to the 25% of families with socioeconomic and cultural levels lower than the average value but higher than category ESCS 1; category ESCS 3 represents the 25% of families with socioeconomic and cultural levels higher than the average value but lower than category ESCS 4, and category ESCS 4, the 25% of families with the highest socioeconomic and cultural levels.