NAEP Technical DocumentationStratification of Private Schools for the 2008 Arts 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),
• race/ethnicity strata, and
• estimated eighth-grade enrollment.

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

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 equaled or 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 eighth-grade enrollment.

Schools in noncertainty PSUs were hierarchically sorted by

• PSU stratum,
• private school type, 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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