Over the last 20 years, education survey data have been increasingly analysed with multilevel models. Indeed, since simple linear regression models without taking into account the potential effects that may arise from the way in which students are assigned to schools or to classes within schools, they may provide an incomplete or misleading representation of efficiency in education systems. In some countries, for instance, the socioeconomic background of a student may partly determine the type of school that he or she attends and there may be little variation in the socio-economic background of students within each school. In other countries or systems, schools may draw on students from a wide range of socio-economic backgrounds, but within the school, the socio-economic background of the student impacts the type of class he or she is allocated to and, as a result, the within-school variance is affected. A linear regression model that does not take into account the hierarchical structure of the data will thus not differentiate between these two systems.