Residential Segregation:  What it is And How We Measure It

 

            If we examine metropolitan areas, we find different types of neighborhoods.  Some of them contain large homes with prosperous populations.  Others are neighborhoods of modest or even run-down homes.  In most suburban rings, there are areas where people live in manufactured housing and others with magnificent homes.  One of the most important ways in which neighborhoods differ is in their racial composition.  In the older metropolises of the Midwest and Northeast, there are many central city neighborhoods where most residents are African Americans but, in the suburban ring, you will likely find neighborhoods in which most residents are whites.

 

The Index of Dissimilarity

 

The measures of segregation reported on this website describe whether two or more groups tend to live in the same neighborhoods or different neighborhoods.  The most commonly used measure of neighborhood segregation is the index of dissimilarity.  This is a measure of the evenness with which two groups are distributed across the component geographic areas that make up a larger area.  For purposes of census taking, metropolises are divided into census tracts that contain, on average, about 4,000 residents.  We could consider a metropolitan area such as Los Angeles and determine the evenness with which whites and blacks are distributed across census tracts.

 

One extreme possibility would be an American Apartheid situation in which all blacks lived in exclusively black census tracts while all whites lived in all-white census tracts.  Of course this does not occur but this would be the maximum residential segregation of blacks from whites. If there were such an apartheid situation, the index of dissimilarity would take on its peak value of 100.  Another extreme example would be a situation in which blacks and whites were randomly assigned to their census tracts of residence.  This never happens but, if it did, the index of dissimilarity would equal 0 meaning that blacks and whites were evenly distributed across census tracts.

 

In metropolitan Los Angeles in 2000, the index of dissimilarity comparing the distribution of blacks and whites across census tracts was 69 indicating a moderately high degree of residential segregation.  This value reports that either 69 percent of the white or 69 percent of the black population would have to move from one census tract to another to produce a completely even distribution of the two races across census tract; that is, an index of dissimilarity of 0.

 

Isolation and Exposure Measures

 

The index of dissimilarity measures the evenness with which two groups are distributed across the component geographic parts of a large area.  But we could also consider different groups and determine how geographically isolated they are.  For example, we can readily determine the percentage of residents who are white in the census tract of the typical white in metropolitan Los Angeles.  Similarly, we could determine the percentage of residents black in the census tract of the typical black in metropolitan Los Angeles.  These are known as isolation measures since they tell us how isolated a groups is.  If whites tend to live in almost all white census tracts, this measure will take on a high value. 

 

Analyzing racial data from the census, we can consider a typical white and determine the percent black in his or her neighborhood.  Similarly, we can determine the percent that are Asian or Hispanic in the neighborhood of the typical white.  We could also ascertain the percent white or the percent African American in the neighborhood of the typical Asian.  These are exposure or interaction measures of segregation since they tell us how different racial groups are “exposed” to each other.  The census, of course, does not determine how often neighbors visit each other but it does determine whether racial groups tend to live with each other in the same neighborhoods or tend to isolate themselves.

 

The index of dissimilarity is statistically independent of the size of the two racial groups used in its composition.  It is not, however, independent of the geographic units used in the computation.  Indexes of dissimilarity computed from geographic units with large populations – census tracts – are numerically smaller than indexes computed from geographic units with small populations – city blocks.

 

The isolation and exposure or interaction measures are influenced by the evenness with which races are distributed across neighborhoods and by the relative size of the groups.

 

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