Zip5 codes are allocated by geography and population. See very large rural tracts of land in AZ or NM that have a large zip5 code area. Zip-9 probably could be associated with one household. Zips are actually a pain in the ass for purposes other than mail delivery because they change from time to time, are not cleanly allocated to geography or logical features on a map. They can be discontinuous and overlapping. Many companies employee proprietary means of assigning geographic identifiers that are not dependent on zip, population size or anything other than geo.
To get a rough idea of how often a ZIP-9 narrows things down to a single street address, I took a look at the sales tax rate and boundary files made available for states that are in the Streamlined Sales Tax Agreement [1].
12 of those states, Arkansas, Georgia, Iowa, Kansas, Nebraska, North Carolina, Ohio, Oklahoma, South Dakota, Tennessee, Vermont, and Washington, use address-based tax rates. (The other 12 either just have one rate for the whole state, or just go by ZIP).
For each ZIP-9 in the address files for the 12 address-based states, I found the lowest and the highest street number for that ZIP-9. I then counted how many of the ZIP-9s had the lowest street number the same as the highest street number.
There were a total of 9,311,327 distinct ZIP-9 values.
2,415,305 of them had a low to high range that only included one number.
That's about 26% of the ZIP-9's having a unique street number. Note that this does not necessarily mean a single household, because I'm not looking at the full address. Apartment buildings, for instance, will in many cases show up in that 26%.
Excellent analysis. That’s like 2% or less of households in the US? So even at the Zip9 level not ideal for identifying specific households but problematic if you’re one of those 2M Zip9s.
