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To facilitate comparisons between residential segregation and school segregation, we aggregate schools into metropolitan regions, and then separately into the central city and suburban portions of those regions. Therefore our results do not measure segregation within particular school districts, but more broadly the disparities between schools in the same geographic area regardless of district boundaries.

We recognize that schools are segregated not only by race but also by class. To take class segregation into account, we take advantage of the NCES reports of student eligibility for free or reduced-price lunches. To be eligible, a family must fall below 185% of the poverty line - around $32,000 for a family of four in 1999-2000. Though some states still do not provide information on the school lunch program, this is the best available indicator of the income level of students in public schools.

How We Measure Segregation

The standard measure of segregation is the Index of Dissimilarity (D), which captures the degree to which two groups are evenly spread among schools in a given city. Evenness is defined with respect to the racial composition of the city as a whole. The index ranges from 0 to 100, giving the percentage of children in one group who would have to attend a different school to achieve racial balance - one where every school replicates the group composition of the city. A value of 60 or above is considered very high. For example, a D score of 60 for black-white segregation would mean that 60% of either group must move to a different school for the two groups to become equally distributed. Values of 40 to 50 are usually considered moderate levels of segregation, while values of 30 or less are considered low.

This report also refers to information on neighborhood segregation. For this purpose we use data from the U.S. Census of 1990 and 2000 specifically for the under-18 population. Neighborhood segregation is measured at the level of census tracts, areas that usually have 3000-5000 residents.

Standards for evaluating change in dissimilarity scores

In our analysis, we interpret change either up or down on the following criteria:
  • Change of 10 points and above - Very significant change
  • Change of 5-10 points - Moderate change
  • Below 5 points - Small change or no real change at all

Exposure and Isolation Indices

Another widely used measure of segregation reported here is a class of Exposure Indices (P*) that refer to the racial/ethnic composition of the school that the average member of a given group attends. Exposure of a group to itself is called the Index of Isolation, while exposure of one group to other groups is called the Index of Exposure. Both range from 0 to 100. For example, an Isolation score of 78.1 for whites means that the average white attends a school that is 78.1 % white. An Exposure score of 9.4 for white-black exposure indicates that the average white attends a school that is 9.4% black.

Even if segregation (measured by the Index of Dissimilarity) remains the same over time, growth in a minority population will tend to leave it more isolated - that is, attending schools where they are already over represented.

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