NAEP Technical DocumentationDefinition of Primary Sampling Unit Strata

Primary sampling units (PSUs) on the PSU frame were grouped into eight major strata by crossing NAEP region and metropolitan statistical area (MSA) status. Each of the four NAEP regions (Northeast, Southeast, Central, and West) contained about one-fourth of the U.S. population at the time the frame was constructed. These regions, as listed below, were defined primarily by state.

The Northeast region included Connecticut, Delaware, District of Columbia, Maine, Maryland, Massachusetts, New Hampshire, New Jersey, New York, Pennsylvania, Rhode Island, Vermont, and Virginia.
The Southeast region included Alabama, Arkansas, Florida, Georgia, Kentucky, Louisiana, Mississippi, North Carolina, South Carolina, Tennessee, Virginia, and West Virginia.
The Central region included Illinois, Indiana, Iowa, Kansas, Michigan, Minnesota, Missouri, Nebraska, North Dakota, Ohio, South Dakota, and Wisconsin.
The West region included Alaska, Arizona, California, Colorado, Hawaii, Idaho, Montana, Nevada, New Mexico, Oklahoma, Oregon, Texas, Utah, Washington, and Wyoming.

Note that those counties and independent cities in Virginia that are part of the Washington, DC-MD-VA MSA are included in the Northeast region. The remainder of Virginia is included in the Southeast region.

The 22 largest PSUs on the sampling frame were included in the PSU sample with certainty (guaranteed to be in the sample). The inclusion of these PSUs in the sample with certainty provided an approximately optimum, cost efficient sample of schools and students when samples were drawn within them at the required national sampling rate.

The remaining smaller PSUs were not guaranteed to be selected for the sample. These PSUs were grouped into a number of noncertainty strata within the eight major strata. In all, there were 72 noncertainty strata (see the number of noncertainty strata in each major stratum by NAEP region). Further stratification was achieved by ordering the noncertainty PSUs within each major stratum according to certain socioeconomic characteristics discussed below.

The noncertainty strata were defined so that the aggregate of the measures of size of the PSUs in a stratum was approximately equal for each stratum. The size measure used was the population from the 1990 Census. The characteristics, available for all PSUs, that were used to define strata were the percent minority population, the percentage change in total population since 1980, the per capita income, the percent of persons age 25 or over with college degrees, the percent of persons age 25 or over who completed high school, and the civilian unemployment rate. Up to four of these characteristics were used in any one major stratum. For each major stratum, the characteristics used were chosen by modeling NAEP PSU-level mean reading proficiency scores for the assessment years 1988 through 1992 and were the best predictors of PSU-level mean reading proficiency scores. An adjustment was made to rescale 1988 data to match 1989, 1990, and 1992. (NAEP did not conduct an assessment in 1991.) PSU-level mean reading scores were not needed for all PSUs in stratum. The scores were modeled from characteristics that were available for all PSUs, so that those characteristics could be used to form efficient strata for estimating proficiency scores.

Last updated 08 May 2008 (MH)

Printer-friendly Version