The GINI Index is a calculated value between 0 and 1 that is intended to represent the degree of inequality within a given area. The index can be calculated for an array of indicators but is most commonly used to measure income inequality. On the chart above the GINI Index is representing the inequality in household incomes within each of the 10 core counties of the Atlanta Metro region. An index value of 0 denotes perfect equality, meaning all households in the county would have the same income. An index value of 1 denotes maximum inequality by which a single household has all of the income in the county. As you can see, the range of GINI Index values for the counties on the chart range from 0.4 for Henry County to 0.54 for Fulton County, with the state average being 0.48. To put this in perspective, among the US States and Commonwealths, Wyoming has the lowest GINI Index at 0.42 and Puerto Rico has the highest at 0.54.
It should be clear from the chart that the median household income of the counties, itself, does not impact the GINI Index. Wealthier counties with few poor residents will exhibit similar levels of inequality as poorer counties with few wealthy residents. For example, Clayton and Fayette Counties with equally low levels of inequality have median household incomes which are significantly different.
Data Source: US Census, American Community Survey (ACS), 2011-15, 5-year estimate