NAEP Technical DocumentationStratification of Public Schools for the 2010 Assessment

Prior to stratification, the public school sampling frame was divided into grade-specific files, one each for fourth, eighth, and twelfth grades. For each grade-specific frame file, separate implicit stratification schemes were used to sort schools into certainty primary sampling units (PSUs) and noncertainty PSUs. In all cases, the implicit stratification was achieved via a "serpentine sort."

For certainty PSUs, the schools were hierarchically sorted by

• census region;
• urbanization classification (urban-centric locale);
• race/ethnicity strata (less than 15 percent Black and Hispanic students, 15 percent or more Black and Hispanic students); and
• estimated grade enrollment.

If there were less than two expected sampled schools for a particular urbanization classification cell (nested within the Census region), the cell was collapsed with a neighboring urbanization classification cell. If the expected sampled schools exceeded four, then the race/ethnicity strata were defined based on the percentage of Blacks, Hispanics, and American Indians/Alaska Natives. It was defined so that there were at least two expected sampled schools for each racial/ethnic stratum. If the urbanization classification stratum had an expected sample size less than four, no race/ethnicity strata were generated, and the final sort variable was percent Blacks, Hispanics, and American Indians/Alaska Natives rather than estimated grade enrollment.

Schools in noncertainty PSUs were hierarchically sorted by

• PSU stratum;
• urbanization classification (urban-centric locale); and
• percent race/ethnicity (Blacks, Hispanics, American Indians/Alaska Natives).

The collapsing of cells within the noncertainty PSUs was implemented in a fashion similar to that described for certainty PSUs.

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