Priceonomics takes the association of fixie bikes to hipsters, and creates the Fixie Bike Index. After starting with New York, they branch out to national numbers.
In short, fixed gear bikes = hipsters, and New York boroughs that have more fixies per capita should have more hipsters per capita. We sampled our data to see the number of used bikes for sale per capita in each borough with the term "fixie" or "fixed gear" in the product title to create the Fixie Index.
I don't know about these numbers. I lived in Modesto for a year and don't remember people riding bikes — or hipsters, and riding your bike in Los Angeles kind of sucks.