How does TIMSS select a representative sample of students?
• To provide valid estimates of student achievement and characteristics, TIMSS selects a sample of students that represents the full population of students in the fourth and eighth grades. This population was defined internationally as (1) 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; and (2) all students enrolled in the grade corresponding to the eighth year of schooling, again beginning with ISCED Level 1. The sample design employed by the TIMSS 2007 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 on
