NAEP school sampling utilizes two types of stratification: explicit and implicit. From the viewpoint of the national assessment for fourth and eighth grades, the jurisdictions used in state assessment stratification of schools are explicit strata. Implicit stratification also occurred within the jurisdictions. For the twelfth-grade public school sample, all stratification is implicit. For private schools, there was explicit stratification by private schools type, as well as implicit stratification.
Explicit stratification controls the sample size completely for a subgroup. This reduces variance (the proportion in the subgroup in the sample exactly matches that of the population). Implicit stratification controls the sample size for subgroups, but not completely, by sorting the file via the implicit stratification variables, and taking a systematic sample. The sample proportion for a subgroup will be close to that of the population. The variability of the sample sizes in the subgroups will be reduced considerably by systematic sampling, but will not be reduced to zero as in explicit stratification. However, most of the beneficial effect of stratification is accomplished.