What is ZIP + 4?

Ever wondered about the ZIP + 4 codes that you're encouraged (or, if you're in the direct-mail business, required) to use? The US Postal Service began using ZIP + 4 in 1983. A typical ZIP + 4 code -- say, 01254-9012 if you're somewhere in Richmond, Massachusetts -- is used to pinpoint a highly focused geographic segment within the five-digit delivery area.

How many households does a ZIP + 4 code represent? The claim, according to Wikipedia is that the code can be resolved to, "a city block, a group of apartments, [or] an individual high-volume receiver of mail."

I did a little experiment on my street. I live in a quiet suburban neighborhood, about eighteen miles northeast of a major metropolitan downtown area. It's typical of thousands of other suburban enclaves: houses on lots ranging from 1/3 acre to a half-acre, upper-middle-class, good school system. You know: the 2005 equivalent of Ward Cleaver's crib.

Using the Post Office ZIP-code lookup tool, I set out on a mission to figure out how many households a typical "ZIP + 4" code represents -- at least in my neighborhood. Is your heart racing yet? Here are the results of all of the actual houses on my street with the "+4" extended code:

11566 MISSIONHILL DR 1805 11569 MISSIONHILL DR 1806 11570 MISSIONHILL DR 1805 11574 MISSIONHILL DR 1805 11575 MISSIONHILL DR 1806 11578 MISSIONHILL DR 1805 11581 MISSIONHILL DR 1806 11582 MISSIONHILL DR 1805 11586 MISSIONHILL DR 1805 11587 MISSIONHILL DR 1806 11593 MISSIONHILL DR 1806 11594 MISSIONHILL DR 1805 11599 MISSIONHILL DR 1806 11602 MISSIONHILL DR 1844 11605 MISSIONHILL DR 1843 11611 MISSIONHILL DR 1843 11617 MISSIONHILL DR 1843 11624 MISSIONHILL DR 1844 11629 MISSIONHILL DR 1843 11630 MISSIONHILL DR 1844 11636 MISSIONHILL DR 1844

These translate to:

1805 7 1806 6 1843 4 1844 4

So, at least in my little 'hood, a ZIP + 4 code translates to between four and seven households.

Why do I mention this? Because as marketers target us, they often use data that's aggregated at the ZIP + 4 level. And you can tell that this level is highly granular: it doesn't take very sophisticated analysis for summarized ZIP + 4 household data to be disassembled into individual households.

And your ZIP + 4 code isn't private. Anyone can look it up if they know your address. Marketers that share information about households at this level aren't seeking your permission, from what I can tell. It's an interesting little game and one that isn't well-publicized.