How does PIRLS select a representative sample of students?
• To provide valid estimates of student achievement and characteristics, PIRLS selects a sample of students that represents the full population of students in the fourth grade. This population was defined internationally as all students enrolled in the grade corresponding to the fourth year of formal schooling, beginning with the first year of schooling as defined by the International Standard Classification of Education (ISCED), Level 1. The sample design employed by the PIRLS 2006 assessment is generally referred to as a three-stage stratified cluster sample. Schools were selected at the first stage with probability proportional to size (PPS), size being the estimated number of students enrolled in the target grade. The second-stage sampling units were classrooms within sampled schools. Countries were required to randomly select a minimum of one eligible classroom per target grade per school from a list of eligible classrooms prepared for each target grade. The third-stage sampling u
