The purpose of stratification is to increase the efficiency of the school samples by reducing (or eliminating) variability in the sample size for important school reporting groups within each jurisdiction. NAEP school sampling utilizes two types of stratification: explicit and implicit.
In any NAEP assessment, from the viewpoint of the national main assessment for fourth and eighth grades, the jurisdictions used in the state assessment are explicit strata. Implicit stratification occurs within the jurisdictions.
In 2003, for private schools, there was explicit stratification by private school type, as well as implicit stratification.