Answer: TIMSS selects a random sample of learners that represents the full population of learners in each target grade to provide valid estimates of learner achievement and characteristics. The sample must consist of a minimum of 150 schools per grade, with a minimum of 4,000 learners assessed per grade. The school response rate target is 85 percent for all countries. In order for a country’s data to be included in the international database, a minimum participation rate of 50 percent of schools from the original sample of schools is required.  In addition, there is a response rate target for classrooms of 95 percent, and a target learner response rate of 85 percent, from both original and substitute schools. Once the 50 percent minimum participation rate of original school sampling is reached, countries may use substitute schools to increase the response rate.  Substitute schools may be used when an original sampled school refuses to participate. Substitute schools are chosen from the two schools neighbouring the sampled school in the frame, and these schools must have similar demographic characteristics to the sampled school. The IEA applied explicit and implicit stratified random sampling techniques for South Africa by using the province, school type (public and independent) and language of learning and teaching (Afrikaans, English and dual medium) as stratification variables. A further random selection process of classes is applied for each sampled school. The sampling strategy changed slightly in 2019, and the language of learning and teaching was not included as a stratification variable. At the second stage of sampling, an intact class is randomly selected in each sampled school.