What if countries select only their best students to participate? Won they look better than the rest?
Sampling of schools and students is carefully planned and monitored. Students in each country are selected from a national probability sample of all students in the particular grade or of a particular age. The rules of participation require that countries submit a sampling plan for international approval. Subsequent international quality control procedures ensure that the approved sampling plan is implemented properly. Once a sample of schools is selected and schools agree to participate, they are asked to provide a list of students of a particular age within the school or a list of a particular kind of classes (for example, 8th grade math classes or 4th grade classrooms). From those lists, a group or whole class of students is then randomly selected for assessment. Each study establishes a set of guidelines for excluding individual students from assessment—typically, if a student has a verifiable mental or physical disability, he or she can be excluded from assessment. However, all st
