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For each combination of jurisdiction and grade with neither available nor usable achievement data, median household income served as the implicit school stratification variable.
A school's median household income resulted from matching its ZIP code against a geographic-based median household income file derived by Donnelly Marketing Analytical Services from 1990 Census data. Any school still missing an income value after the match received the average median household income calculated for the three-digit ZIP code prefix or county of its location.