The NAEP 2008 long-term assessment (LTT) sample design yielded a nationally representative sample of public school students in each age group (9, 13, and 17) through a three-stage approach: selection of primary sampling units (PSUs), selection of schools within PSUs, and selection of students within schools. The 2008 sampling plan was designed to assess 13,250 students for each of the LTT public school age-specific samples.
These students were allocated among three different tests. The operational tests were in reading and mathematics, and a pilot test in mathematics was also conducted.
The public school samples were drawn from a sampling frame that only included the sampled LTT primary sampling unit (PSUs). From the stratified frame of public schools for each grade associated with the age range, a systematic random sample of age-eligible schools was drawn with probability proportional to a measure of size based on the estimated age-specific enrollment of the school. The number of age-eligible students was estimated by applying population-level percentages of age-eligible students within each grade to estimated grade enrollments for each grade, and aggregating to an age-eligible total for the school.
Each selected school was asked to provide a roster of all of its students who were age-eligible. A fixed sample of 62 students was selected from these rosters, with the exception of smaller schools with total age-eligible counts less than the sample size. The combination of using the number of age-eligible students as a measure of size and a fixed sample size of students is intended to give a self-weighting sample (each student has an equal probability of selection). In practice, differences between the estimated age-eligible enrollment for the school and the actual size of the finalized roster introduce slightly differing student probabilities across schools.