See Boxes 11.2.1,11.2.2 and 11.2.3 in Chapter 2 for a description of how PISA defines socio-economically disadvantaged and advantaged schools, public and private schools, and urban and rural schools. The index of equity in resource allocation (material) is the percentage of the variation on the index of shortage of educational material explained by the PISA index of economic, social and cultural status of the school multiplied by a negative or positive sign, depending on the sign of the relationship. A value of zero indicates that there is no difference between socio-economically advantaged and disadvantaged schools in how concerned principals are about the educational material at school, and positive values (higher equity) indicate that principals of socio-economically advantaged schools are more concerned than principals of disadvantaged schools.