NAEP Technical DocumentationStratification of Schools in the 2011 State Assessment


			Stratification Variables

The purpose of school stratification is to increase the efficiency and ensure the representativeness of the school samples in terms of important school-level characteristics, such as geography (e.g., states and TUDA districts), urbanicity, and race/ethnicity classification. NAEP school sampling utilizes two types of stratification: explicit and implicit.

Explicit stratification partitions the sampling frame into mutually exclusive groupings called strata. The systematic samples selected from these strata are independent, meaning that each is selected with its own unique random start. The explicit school strata for the 2011 NAEP state assessments were usually states. If a state contained Trial Urban District Assessment (TUDA) districts, the explicit strata were each individual TUDA district and the balance of the state. In 2011, there were 21 participating TUDA districts in the NAEP state assessment program. They are listed below. The ones in bold are those introduced in 2011.

Albuquerque Public Schools, New Mexico;
Atlanta Public Schools, Georgia;
Austin Independent School District, Texas;
Baltimore City Public Schools, Maryland;
Boston Public Schools, Massachusetts;
Charlotte-Mecklenburg Schools, North Carolina;
Chicago Public Schools, Illinois;
Cleveland Metropolitan School District, Ohio;
Dallas Independent School District, Texas;
Detroit Public Schools, Michigan;
District of Columbia Public Schools, District of Columbia;
Fresno Unified School District, California;
Hillsborough County Public Schools, Florida;
Houston Independent School District, Texas;
Jefferson County Public Schools (Louisville) , Kentucky;
Los Angeles Unified School District, California;
Miami-Dade County Public Schools, Florida;
Milwaukee Public Schools, Wisconsin;
New York City Department of Education, New York;
School District of Philadelphia, Pennsylvania; and
San Diego Unified School District, California.

Implicit stratification involves sorting the sampling frame, as opposed to grouping the frame. For NAEP, schools are sorted by key school characteristics within explicit strata and sampled systematically using this ordering. This type of stratification ensures the representativeness of the school samples with respect to the key school characteristics. The implicit school stratification variables for the 2011 state assessments included urbanicity, race/ethnicity classification, and achievement score/median income. Further details about these variables can be found here.

Last updated 12 July 2012 (JL)

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