From the National Priorities Project (http://nationalpriorities.org/):
by Al Shaw
ProPublica, Nov. 4, 2011, 11:36 a.m.
ProPublica has prepared a tool that graphically shows income inequality in the 818 largest counties in the U.S. To check out income inequality in your own area, use the search box at http://www.propublica.org/article/income-inequality-near-you to find your county by address or ZIP code.
The tool uses what is known as the Gini index, a statistical measure that ranges from zero, which would describe a community in which every citizen has precisely the same income, to one, which would describe the opposite extreme, in which one person receives literally all of the income.
For example, Dauphin County (Harrisburg) ranks .421 on the Gini index, with about 29% of the most populous counties having greater income equality, while Allegheny County (Pittsburgh) weighs in at .488, with 90% having greater equality, and Philadelphia weighs in at .505, with 95% having greater equality.