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NAEP Technical DocumentationStratification of Private 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."

Within each certainty PSU, the schools were hierarchically sorted by:

• Census region,
• private school type (Roman Catholic, Lutheran, Conservative Christian, Other private, and Unknown affiliation), and
• estimated grade enrollment.

Private school affiliation was unknown for nonrespondents to the National Center for Education Statistics (NCES) Private School Universe Survey (PSS).

Schools in noncertainty PSUs were hierarchically sorted by:

• PSU stratum,
• private school type, and
• estimated grade enrollment. 

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