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How Do We Measure Segregation?

The Mumford Center is providing information on segregation at the level of census tracts, areas that typically have 3000-5000 residents. For more specific details on measurement issues, see our Updates and Technical Notes webpage: http://www.s4.brown.edu/cen2000/technote.html. For data on individual metropolitan regions, or their city or suburban portions, see: http://www.s4.brown.edu/cen2000/data.html.

Index of Dissimilarity

The standard measure of segregation is the Index of Dissimilarity (D), which captures the degree to which two groups are evenly spread among census tracts 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 one group who would have to move to achieve an even residential pattern - one where every tract 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 means that 60% of either group must move to a different tract 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.

Demographers typically interpret change either up or down in the following way:
  • Change of 10 points and above in one decade - Very significant change
  • Change of 5-10 points in one decade - Moderate change
  • Below 5 points in one decade - Small change or no real change at all

Change can be cumulative, and small changes in a single decade - if they are repeated over two or three decades - can constitute a significant trend.

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 a tract where the average member of a given group lives. 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 80.2 for whites means that the average white lives in a neighborhood that is 80.2% white. An Exposure score of 6.7 for white-black exposure indicates that the average white lives in a neighborhood that is 6.7% 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, leaving group members in neighborhoods where they are a larger share of the population.

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