You didn’t miss it, because there was no methodology to speak of. Colin Woodard isn’t a sociologist or historian, he is a reporter for the Maine Sunday Telegram who wrote a book premised on the idea that the arc of 300-400 years of culture and history of the various regions of the US are cleanly defined entirely by the initial groups that settled them. Every region has a stupid, manufactured brand (it’s not the West Coast and New England, it’s the “Left Coast” and “Yankeedom”). What little data that is referenced is cherry picked to support the narratives. It’s also hilariously patronizing (spoiler: The reporter from Maine concludes that New England and the West Coast are the epitome of all that is clean and good, and everyone else are barbarians and unwashed rubes).

It is closer to a Buzzfeed quiz explaining how your astrological sign dictates your Hogwarts House than anything remotely resembling academic rigor.