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
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