A two-stage sampling design was used to select nationally representative samples of students at Year 4 and at Year 8. The first stage involved sampling schools; the second stage involved sampling students within schools. A stratified random sampling approach was taken to select 100 schools at Year 4 and 100 schools at Year 8. A maximum of 25 students were randomly selected from each school to form national samples at Year 4 and Year 8. The Ministry of Education July 2017 school returns for Year 3 and Year 7 were used to inform the selection of Year 4 and Year 8 schools in 2018.