Counts how many reference records share each exact mail_key (box + ZIP).
This is what makes a shared box interpretable rather than merely true, and
it is not an optional refinement — measured over the 3,687,435-row unified
BMF, the key separates two populations that want opposite treatment:
Arguments
- mail_keys
Character vector of mail keys (reference
mail_key), as built bynp_mail_key(). Empty strings are dropped, so records that name no box do not all pile onto one entry.
Details
A box shared by two or three filers is the signal worth having: an organization and its foundation, a parent and its chapter, a small group of charities run out of one accountant's office. 30,097 of the 36,910 shared boxes hold exactly two EINs and 3,829 hold three.
A box shared by hundreds is a service bureau and is evidence of nothing. 678 boxes hold ten or more EINs, and the most crowded —
PO BOX 653067in ZIP 75265 — holds 3,626. Treating that as common administration would link 3,626 unrelated organizations to each other.
So read mail_key as evidence only below a threshold, the same way
np_addr_freq()'s share_min gates an address. Overall: 496,830 distinct
mail keys, 7.43% of them shared, and 20.79% of box-holding EINs on a shared
box.