Mathematical Association of America (MAA)
The Gini index is a summary statistic that measures how fairly a resource is distributed in a population; income is a primary example. In addition to a self-contained presentation of the Gini index, we give two equivalent ways to interpret this summary statistic: first in terms of the percentile level of the person who earns the average dollar, and second in terms of how the lower of two randomly chosen incomes compares, on average, to mean income.
"FARRIS, Frank A. "The Gini Index and Measures of Equitability,"American Mathematical Monthly, December 2010: 851-864."