​​​​NAEP Technical DocumentationStratification of Private Schools for the 2022 Long-Term Trend Assessment

For the private school sampling frame file at each age (9 or 13), schools were explicitly stratified by private school affiliation (Catholic, non-Catholic, and unknown affiliation). Private school affiliation was unknown for nonrespondents to the NCES Private School Universe Survey (PSS). Within private school type, 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;
  
• urbanization classification (four categories based on urban-centric locale); and
• estimated age-specific enrollment.

Schools in noncertainty PSUs were hierarchically sorted by

• PSU stratum;
  
• urbanization classification (four categories based on urban-centric locale); and
• estimated age-specific enrollment.

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