Disparity Index

A disparity index is a quantitative measure that summarizes the difference in an outcome, such as income, health, education, or arrest rates, between social groups or across a population, often by comparing one group’s rate with a reference group or with the overall average. Common forms include rate ratios, rate differences, and summary measures of inequality. Corrado Gini’s coefficient provided an early model of summarizing distributional inequality in a single figure, and Otis Dudley Duncan and Beverly Duncan developed the index of dissimilarity to measure residential segregation between groups. Douglas Massey and Nancy Denton extended segregation measurement to dimensions such as exposure, concentration, and clustering. Anthony Atkinson and Amartya Sen advanced theories of inequality measurement that link indices to value judgments, and Thomas Piketty’s work relies on income and wealth shares to track disparities. In public health, Michael Marmot’s research uses the slope index and relative index of inequality to capture social gradients. Critics note that indices can obscure causes, depend on the choice of reference group, and hide variation within groups. A disparity index remains central to research on inequality, segregation, health, education, and the evaluation of policy.