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Substitutes were pre-selected for the fourth- and eighth-grade science school samples, using both of the nearest neighbors on the frame. The last sort ordering in the implicit stratification was by grade enrollment, which led to the expectation that the two schools nearest to the sampled school on the frame (the nearest neighbors) had grade enrollment values very close to that of the sampled school. Higher-level stratum boundaries were not crossed: only schools in the same study type, geography, and urbanicity stratum as the sampled school were eligible to be substitutes.
Schools were disqualified as potential substitutes if selected in the NAEP State school sample or the twelfth-grade public school sample, or assigned as a substitute for another national science school (earlier in the sort ordering) or for a twelfth-grade public school. Schools assigned as substitutes for eighth-grade schools were disqualified as potential substitutes for fourth-grade schools.
If both nearest neighbors were still eligible to be substitutes, the one with a closer grade enrollment was chosen. If both nearest neighbors were equally distant from the sampled school in their grade enrollment (an uncommon occurrence), one of the two was randomly selected.