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 (region, type of location, etc.) within each jurisdiction. NAEP school sampling utilizes two types of stratification: explicit and implicit.
Explicit stratification assigns each school to a mutually exclusive and exhaustive stratum, and samples are drawn with an independent random start from each stratum. The explicit stratification variable was jurisdiction (usually state).
Under implicit stratification, schools are ordered within an explicit stratum and sampled systematically using this ordering. This stratification reduces the variability of the sample size around the optimal sample size for important reporting groups within the explicit stratum. Implicit stratification leaves some variability in the sample size. In the 2007 state assessment sampling process, all stratification within a jurisdiction is implicit.