Every sampled school and student received a base weight equal to the reciprocal of its probability of selection. Base weights can vary because selection probabilities can vary. For example, in most states larger schools have a higher chance of selection than smaller schools; and consequently, selected smaller schools in these states have larger base weights than the selected larger schools. Computation of a school base weight varies by whether the school was an originally selected school (i.e., not a new or substitute school), a new school, or a substitute school.
Computation of a student base weight reflects its overall probability of selection accounting for school and student sampling and its assignment to the mathematics or reading assessment.