Krulwich spells out Brockmann’s method in the NRP article, “A ‘Whom Do You Hang With’ Map of America”:
“Look at the center of this (U.S.) map, at the little red dot that marks Kansas City,” Krulwich writes. “Technically, Kansas City is at the edge of Missouri, but here on this map it’s in the upper middle section of a bigger space with strong blue borders. We don't have a name for this bigger space yet, but soon we will.”
“I would call it, for the moment, ‘The Part of America Kansas City Hangs With’ because that’s what this map is saying. It’s a new, intriguing way to see our country. This one was built by tracking dollar-bill circulation. There’ve been similar maps built from phone call data. The idea here is to show America not as 50 states, but as regions where people do stuff together. In other words: a ‘Whom Do You Hang With?’ map.”
According to Krulwich, the idea started when Brockmann was visiting an American friend, and they got talking about population mobility. Brockmann knew Americans move around a lot, but he wondered how to capture where they go, who they talk to.
His friend said, “Have you ever heard of Where’s George?” Brockmann hadn’t. It’s a website that tracks the movement of dollar bills. Thousands of people participate. All you do is take a bill out of your wallet, type the denomination, serial number, the date and your zip code onto the Where’s George? site, and then, with a pen or a stamp, deface the bill with the words “WheresGeorge.com.”
Krulwich’s article continues: “Going back to Missouri for a moment…Notice there’s a softer blue line running between Kansas City and St. Louis. That’s because each city pulls money to itself, creating an absence of movement (and therefore a border) between them… In this part of the United States, money sloshes around a territory marked by Missouri, Kansas, Oklahoma and Arkansas, with the Mississippi river forming a border on the eastern edge.”