NAEP Technical DocumentationMissing Stratification Variables

Schools with missing stratification variables had their data imputed as follows:

Schools with missing estimated grade enrollment had their estimated grade enrollment set to 20. Charter school status and Trial Urban District Assessment (TUDA) identification were not missing for those schools for which these variables were used in stratification. Schools missing the type of location variable were assigned the modal value of type of location for schools in the same five-digit ZIP Code or the same city. The mean was imputed at the five-digit ZIP Code level only if all schools were missing ethnicity at the district level, and only at the three-digit ZIP Code level if the five-digit ZIP Code mean was missing as well.

Schools with missing or questionable values in minority enrollment data—those in which the summation of the ethnicity percentages did not fall in the range 97 through 103, indicating a gross error—were assigned the average minority enrollment within their school district, five-digit ZIP Code, or three-digit ZIP Code prefix.

Schools with missing achievement data in jurisdictions and grades for which achievement data were used in stratification were assigned the mean achievement data value within their urbanization and minority classification. The achievement data were imputed only for those schools in jurisdictions and grades in which achievement data were used for stratification.

Schools missing median household income were assigned the mean value of median household income for the three-digit ZIP Code prefix or county in which they were located. In some cases, imputation was not possible at the three-digit ZIP Code level, and needed to be done at the county level. There were 227 DoDDS and territory schools with missing median income values at the end of this process. They could not be imputed and were left missing.

Last updated 24 October 2008 (DB)

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