How to Calculate, Interpret, and Understand the Index of Dissimilarity

The index of dissimilarity is an indicator of just how much one group is spatially segregated from another group. It is used frequently by population geographers and demographers. The index of dissimilarity is the standard measure of segregation and is often used in studies of black and white residential patterns in urban areas, for example. But it may be used to compare the amount of spatial segregation or spatial dissimilarity between two population (or ethnic/racial/immigrant) groups. The index ranges from 0 to 100 (when reported as a percentage), with 0 meaning no segregation or spatial disparity, and 100 being complete segregation between the two groups with no spatial intermingling.

Instructions

Difficulty: Moderately Easy

Things You’ll Need:

• A computer with spreadsheet or database software, such as Excel.

1. Open your area and regional data in the spreadsheet or database software program (such as Excel). There should basically be three columns of data to start out with: one column for the name of the particular case, and then two remaining columns of numerical data for each case: one variable for the base phenomenon (total population in our example) and one variable for the activity phenomenon being measured (ethnic group population in our example).
2. The actual index of dissimilarity for a geographic area is based on the following conventions:
   
   I.D. = 1/2Σ ABS[(b/B)-(w/W)]
   
   The following is an example of variables used in the formula if comparing white and black population:
   b = the black population of the kth area, e.g. census tract
   B = the total black population of the large geographic entity for which the index of dissimilarity is being calculated.
   w = the white population of the kth area
   W = the total white population of the large geographic entity for which the index of dissimilarity is being calculated.
   ABS is syntax for Absolute Value of the calculations within the brackets.
3. You can set up this formula in the spreadsheet software to calculate the index. The calculation will produce an index of dissimilarity value as a percentage ranging from 0 to 100. In our example comparing the amount of spatial segregation between the white and black population in a particular geographic area (such as a metropolitan area), if we calculate an index of dissimilarity value of 63% then this means that 63% of the black population would have to move in order for the white and black population to be spatially integrated.