The Programme for International Student Assessment (PISA) index of the use of school resources was derived from the frequency with which students reported using the following resources in their school: the school library; calculators; the Internet; and laboratories. Students responded on a five-point scale with the following categories: ‘never or hardly ever’, ‘a few times a year’, ‘about once a month’, ‘several times a month’ and ‘several times a week’. The index was derived using the WARM estimator.