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NAEP Technical DocumentationStratification of Public Schools for the 2012 Economics Assessment

Prior to stratification, the public school sampling frame was sub-setted to twelfth grade. 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);
• percent race/ethnicity (Blacks, Hispanics, American Indians/Alaska Natives); and
• estimated grade enrollment.

If there were fewer 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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