Economics
One number, invented in 1912, still measures how unequal a country's income is
Italian statistician Corrado Gini devised a way to compress an entire income distribution into a single score between 0 and 1: 0 means everyone earns the same, 1 means one person earns everything. Plot the gap between a country's actual income distribution and perfect equality, and the Gini coefficient is essentially the size of that gap. It's now the standard yardstick economists use to compare inequality between countries and across time.