The sample design for the 2005 NAEP study in twelfth-grade public schools (the "gamma" sample) achieved a nationally representative sample of public school students in the twelfth grade through the use of a two-stage approach: selection of schools within strata and selection of students within schools. The first-stage sample of schools was selected with probability proportional to a measure of size based on the estimated grade-specific enrollment in the schools.
The general target for the twelfth-grade public school sample was 48,450 assessed students allocated among nine different tests. The three operational study tests were in mathematics, reading, and science. There was also a science bridge assessment to bridge to the 2000 NAEP science study. Five pilot tests were also included. Target sample sizes were adjusted for expected school and student response and eligibility.
As in past assessments, Black and Hispanic students were oversampled at a moderate rate. Schools in a high race/ethnicity stratum (schools with 15 percent or more Black and Hispanic students) were sampled at twice the rate as those in a low race/ethnicity stratum to implement oversampling of races/ethnicities in the twelfth-grade public school sample.
From the stratified frame of twelfth-grade public schools, a systematic random sample of grade-eligible schools was drawn with probability proportional to a measure of size based on the estimated grade-specific enrollment of the school. High race/ethnicity schools were sampled at twice the rate.
Each participating selected school in the twelfth grade public school sample provided a list of eligible enrolled students from which a systematic, equal probability sample of students was drawn. As many as 144 students were taken with certainty if the number of twelfth-graders was less than or equal to 144, and a sample of 135 was taken in larger schools.