They use statistical clustering based on commute patterns. Once there's enough overlap, two metro areas get merged into one. That's the case for Baltimore and DC, for instance.

Undisciplined

mostly_harmless

A dozen completely random things I learned this week:

mallardshead

Yeah, how you carve up the geography matters a lot.  I always wondered if there was some underlying order to it.  Certainly what you decide counts as NYC is not intuitive, as the whole Eastern seaboard feels like one giant city.