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​The eighth-grade private school sample for the national science assessment was designed to produce nationally representative samples of students enrolled in eighth grade in private schools in the United States. ​The target sample size for the private school science sample was 2,200 assessed students. Prior to sampling, the target sample sizes were adjusted upward to offset expected school and student attrition due to nonresponse and ineligibility.

Samples were selected using a two-stage probability-based design that involved selection of schools within strata and selection of students within schools. The first-stage samples of schools were selected with probability proportional to a measure of size based on estimated grade-specific enrollment in the schools.

The sampling of students at the second-stage involved two steps: (1) sampling of students in the targeted grade (eighth) from each sampled school and (2) assignment of assessment subject (science) to the sampled students.​