Globally, enrolment in secondary education has expanded dramatically over the past decades. This expansion is also reflected in PISA data, particularly for low- and middle-income countries. Between 2003 and 2015, Indonesia added more than 1.1 million students, Turkey and Brazil more than 400 000 students, and Mexico more than 300 000 students, to the total population of 15-year-olds eligible to participate in PISA.

This welcome expansion in education opportunities makes it more difficult to interpret how mean scores in PISA have changed over time. Indeed, increases in coverage can lead to an underestimation of the real improvements that education systems have achieved. Household surveys often show that children from poor households, ethnic minorities or rural areas face a greater risk of not attending or completing lower secondary education. Typically, as populations that had previously been excluded gain access to higher levels of schooling, a larger proportion of low-performing students will be included in PISA samples.