Statistician Walker Harrison of the excellently named site Perplex.city recently studied the density of sidewalk gum stains across several NYC neighborhoods, then found that it correlates to the average income of that neighborhood.
[…] nice neighborhoods, like those aforementioned clusters of extreme wealth in the Upper East Side, have fewer gum stains on their streets. Poorer neighborhoods, like the ones north of 96th Street, are heavily gummed. […] The two have a correlation coefficient of -0.78, which scores as a strong, negative linear relationship. The trendline overlaid on the graph has a slope of -0.0000465, which means that on average every additional gumspot comes with about a $20,000 drop in per capita income.
Check out the full article for more info.