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:
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: