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NAEP Technical DocumentationStratification of Public Schools for the 2006 Assessment 

Separate implicit stratification schemes were used to sort schools in 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,
• race/ethnicity status (less than 15 percent Black and Hispanic students, 15 percent or more Black and Hispanic students), and
• median income.

Median income was obtained from the SF3 2000 Census data file for the ZIP Code tabulation area in which the school was located.

Schools in noncertainty PSUs were hierarchically sorted by

• PSU stratum,
• urbanization classification,
• race/ethnicity status, and
• median income. 

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