Page 37250 of the 2010 OMB regs regarding merging MSAs:
Two adjacent CBSAs will merge to form one CBSA if the central county or counties (as a group) of one CBSA qualify as outlying to the central county or counties (as a group) of the other CBSA…
A county qualifies as an outlying county of a CBSA if it meets the following commuting requirements:
(a) At least 25 percent of the workers living in the county work in the central county or counties of the CBSA; or
(b) At least 25 percent of the employment in the county is accounted for by workers who reside in the central county or counties of the CBSA.
A county may be included in only one CBSA. If a county qualifies as a central county of one CBSA and as outlying in another, it falls within the CBSA in which it is a central county.
For Durham vis-a-vis Wake, it seems like criterion A is just missed, while criterion B is met.
Elsewhere in the document is evidence of a 2010 letter-writing campaign by Triad partisans! “Thirteen commenters expressed concern about the current delineations of the Greensboro-High Point, Winston-Salem, and Burlington…” Little do they know that, even though Alamance currently sends more commuters to Guilford, Orange is just a few percentage points behind.
Anyhow, I’m going to tweet at @ncdemography to see if they want to clear this up, since apparently both Triangle and Triad really care about this.
BTW, Wikipedia really does have the most complete dataset of old Census counts, apparently because a grad student didn’t have enough data entry work to do:
http://creatingdata.us/datasets/US-cities/