Technical Notes: A.2 International requirements for sampling, data collection, and response rates
To provide valid estimates of student achievement and characteristics, the sample of students for each assessment is selected in a way that represents the full target population in each jurisdiction. The international desired population or full target population in each jurisdiction is carefully defined for each study. This international desired population includes all students in the target grade or age range.
All three assessments require a minimum of 150 schools to participate and have minimum student sample sizes: PIRLS and PISA require a minimum of 4,500 students, and TIMSS requires a minimum of 4,000 students per grade. To realize these target sample sizes for a nationally representative sample of students, samples are drawn with two substitute schools identified for each sampled school. Substitute schools can replace a sampled school if the sampled school refuses to participate. The specific procedures for drawing the sample and for using substitute schools differ by study and even by administration: see the appropriate assessment's technical manual for specific details.
Each jurisdiction collects its own data, following international guidelines and specifications. International guidelines and specifications require that testing for each assessment occur within a set time period. They define response rate targets for schools (typically 85 percent) and students (typically 85 or 80 percent) as well as minimum response rates for results to be reported. International guidelines and specifications also define maximum rates of exclusion.
Schools and students excluded from the national defined target population are referred to as the excluded population. Exclusions can occur at the school level, with entire schools being excluded (e.g., schools that are extremely small or remote), or within schools, with specific students (e.g., functionally or intellectually disabled students) or entire classrooms excluded (e.g., classrooms for non-native language speakers). See the appropriate technical manual for each study's specific policy on exclusion and student accommodations.
To ensure that testing procedures are carried out in a consistent manner, international quality monitors visit a sample of schools in every jurisdiction.