(I apologize for the duplicate - I accidentally posted this as a reply to an old thread - reposting here.)
Using the US Geocoder with the Zip9 geocoder, regardless of the zip level that is returned (5, 7, 9) a 15 character CB level census code is returned. Is it a correct assumption that these CBs represent the CB that contains the centroid of the zip 5 or 7 or 9 that the address falls within?
Mainly I am asking about zip 9 - does a zip 9 fall entirely within one census block, such that I can assume that the address has been geocoded to the correct CB when the geolevel is zip9? Or does a zip 9 boundary cross streets, and therefore probably CBs, in which case I shouldn't assume anything other than the CB is the that of the centroid of the correct zip9 that contains the address, and the address may or may not be in that exact CB?
I received a response from Wendy Chow who answered my question, confirming that maybe 1/3 of the time the zip 9 includes both sides of the street, and therefore, the potential that multiple CBs are represented in that zip 9, and therefore the CB returned is not 100% accurate at the CB level. It is the centroid of the zip 9.
Here is Wendy's response, taken from two separate emails we exchanged. It answered by question thoroughly.
There will be a high correlation of addresses of a given ZIP9 residing within the same Census Block as the ZIP9 centroid, but given the generalization of the geographies, and the small footprint of each Census Block, it is possible for an address geocoded at the ZIP9 level to reside within a neighboring Census Block. ZIP9’s can be associated with 1.) the Even side of a road, 2.) the Odd side of a road, or 3.) BOTH sides of the road. Based on the feed with get from the USPS (for ‘S’ type records, i.e. Street Delivery), we see that these three options are split just about evenly. Thus, about a third of the time we see ZIP9’s associated with Both sides of the road.