Index of Dissimilarity
Also known as: dissimilarity index, Duncan index, D index, segregation index
The index of dissimilarity, often called the Duncan segregation index, measures how unevenly two groups — such as two racial or occupational groups — are distributed across a set of units like neighborhoods, schools, or occupations. It ranges from 0, when both groups have identical distributions across units, to 1, when the units are completely segregated, and has the intuitive interpretation of the share of one group that would have to relocate to achieve an even distribution.
Key highlights
- Intuitive interpretation as the share of a group that must relocate to achieve evenness.
- Simple to compute from readily available cross-tabulated counts.
- Bounded on [0,1], enabling clear comparison across cities, periods, or domains.
- The long-standing standard measure of the evenness dimension, ensuring comparability with a vast literature.
Intuition
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How it works
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When to use it
Use the index of dissimilarity to measure the evenness with which two groups are spread across units — residential segregation across neighborhoods, school segregation, occupational sex or race segregation. It is the standard measure of the evenness dimension and is easy to compute and communicate. It is appropriate only for comparing exactly two groups (multigroup settings need entropy-based measures), and it does not capture other dimensions of segregation such as isolation, clustering, or concentration. It is sensitive to the number and size of units (the checkerboard and modifiable-areal-unit problems) and ignores the spatial arrangement of units, so it should be reported alongside complementary measures.
Strengths & limitations
- Intuitive interpretation as the share of a group that must relocate to achieve evenness.
- Simple to compute from readily available cross-tabulated counts.
- Bounded on [0,1], enabling clear comparison across cities, periods, or domains.
- The long-standing standard measure of the evenness dimension, ensuring comparability with a vast literature.
- Defined for only two groups at a time; multigroup segregation requires entropy or other indices.
- Captures only evenness, ignoring isolation, exposure, clustering, and concentration.
- Aspatial: it ignores where units sit relative to one another, so two very different spatial patterns can score identically.
- Sensitive to the number and size of units (modifiable areal unit problem) and biased upward when units are small or counts sparse.
Common pitfalls
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Applications
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Frequently asked
What does a dissimilarity index of, say, 0.5 mean?
It means that 50 percent of one group would have to move to different units for both groups to be evenly distributed — that is, for every unit to have the same group composition as the whole area. Conventionally, values below 0.3 are considered low segregation, 0.3–0.6 moderate, and above 0.6 high, though these thresholds are rules of thumb rather than hard cutoffs.
Why can the index only compare two groups?
The formula contrasts two distributional profiles by summing their absolute differences. With three or more groups there is no single pair of profiles to compare, and averaging pairwise dissimilarities loses a clean interpretation. Multigroup segregation is better measured with entropy-based indices such as Theil's information index, which decompose total segregation across all groups simultaneously.
What is the small-unit (checkerboard) bias?
When units are small and contain few people, random variation alone makes them look more segregated, biasing the index upward. Additionally, because the index ignores spatial arrangement (the checkerboard problem), rearranging segregated units into clustered versus dispersed patterns leaves D unchanged. Both issues mean D should be interpreted cautiously and paired with spatial or clustering measures.
Sources
- 1.Duncan, O. D., & Duncan, B. (1955). A methodological analysis of segregation indexes. American Sociological Review, 20(2), 210–217.
- 2.Massey, D. S., & Denton, N. A. (1988). The dimensions of residential segregation. Social Forces, 67(2), 281–315.
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Cite this page
ScholarGate. (2026, June 22). Index of Dissimilarity. ScholarGate. https://scholargate.app/sociology/index-of-dissimilarity