The first stage of selection for all national main samples—except the grade 8 samples—was the selection of primary sampling units (PSUs). A PSU is a geographic area comprising an individual county or a group of contiguous counties. The entire United States (the 50 states and the District of Columbia) was divided into 1,027 PSUs. Each PSU met a minimum size requirement and was contained entirely within one of four geographic regions, specifically defined for NAEP, known as NAEP regions. These regions were used in stratifying the PSUs and ensured that each region was adequately represented in the various assessments.
The primary sampling units (PSUs) used in the national main assessment of 2000 were selected from the PSU sampling frame designed and used for the assessments from 1994 through 2000. The basic PSU sample design for this NAEP period is a stratified probability sample with one PSU selected per stratum with probability proportional to population. The population size used was the from the 1990 Census. A PSU can be of a Consolidated metropolitan statistical area (CMSA), a metropolitan statistical area (MSA), a New England County Metropolitan Area (NECMA), a county, or a group of contiguous counties in the United States (including Alaska, Hawaii, and the District of Columbia). A total of 94 PSUs per survey year were selected. Some large PSUs were sampled with certainty and are common to all five survey years.
The sampling frame used from 1994–2000 was constructed by partitioning all the counties in the United States into 1,027 non-overlapping groups following these specifications.
Each 1990 CMSA, and MSA that was not part of a CMSA, was considered a separate PSU. In New England, NECMAs were the metropolitan PSU unit (In New England a MSA may be part of county; but a NECMA may consist only of whole counties).
Non-MSA PSUs were made to consist only of non-MSA counties. Whenever possible, each non-MSA PSU contained geographically contiguous counties with a minimum 1990 total population of 60,000 persons in the Northeast and Southeast regions and 45,000 persons in the Central and West regions.
Region boundaries were not crossed in the definition of a PSU not even in the case of MSAs. If an MSA straddled regions, it was split into two PSUs, each falling entirely within regional boundaries.
The three pages that link from this page describe
how the 2000 national main PSUs were stratified;
how the PSUs were selected; and
how PSUs were formed and selected in six sparsely populated states in the grade 8 public school sample
to improve the efficiency of the field operations.