" The spread is often measured by the Gini coefficient, named after Corrado Gini, an Italian economist who worked in the first half of the twentieth century. Gini’s coefficient, or simply the Gini, is a number that lies between 0 (perfect equality—everyone has the same) and 1 (perfect inequality, with one person having everything). It measures how far people are apart on average. (If you really want know the details, it is the average difference in income between all pairs of people divided by twice the average income. If there are two of us, and you have everything, the difference between us is twice the mean, and the Gini is 1. If we both have the same, the difference between us is 0, and so is the Gini.) "

― Angus Deaton , The Great Escape: Health, Wealth, and the Origins of Inequality