Why is zip code X missing? Or what is a ZCTA, and how is it different from a zip code?
A ZIP code is a linear postal route. You can think of a linear postal route as a postman driving up and down streets, delivering mail.
To estimate the demographics of people who live in ZIP codes, the US Census Bureau turns linear postal routes into polygons. And they do this by creating ZCTAs, which are "generalized areal representations of US Postal Service ZIP Code service areas." You can think of a ZIP code as the path an usher takes to escort guests to their exact seats, while ZCTAs represent the clustered seating sections within the theater.
Here’s a fun animation about how the Census Bureau creates ZCTAs.
Examples of why a zip would be missing
Sometimes, the US Census Bureau can't create a ZCTAs for a zip code, because no one lives in that zip. For example, the Empire State building has its own zip of 10118 but no ZCTA as no one lives there. Other examples of zips include:
- only PO Boxes zips,
- only business zips,
- IRS buildings,
- post offices,
- other large mail stops like military installations, hospitals, or office buildings, and
- zips that are new, and the US Census Bureau hasn't built a ZCTA for it yet .
Other times, there aren't enough residents living in a ZCTA for the US Census Bureau to collect enough surveys to produce reliable estimates.
We use the terms ZCTA, zip code or zip interchangeably, because most of our clients 1. don’t know what a ZCTA is and 2. the difference between a zip code and ZCTA doesn’t impact how they need to use the data. That said, theoretical problems could arise with using ZCTAs if you are doing a mass mailing project. If you are using this data to send US postal mail, please contact us so we can make sure you have the best data for your use case.
What should you do if you still need demographics for a zip that doesn't have a ZCTA?
You can get this handy zip to ZCTA crosswalk added to your Demographics By Zip Code list or your Income by Zip Code list for no extra charge and by contacting us. We don't include it in every report, because the majority of our clients don't need it and find it confusing. Here's an example of how this crosswalk works if you do need it.
Let's pretend that you need to join demographics to a list of addresses. One of your addresses has a zip of 10118 (the Empire State Building), and there's no corresponding ZCTA for this zip. But using this handy crosswalk, you can see that zip 10118 is inside the polygon ZCTA of 10001.
And then you can associate the demographics for ZCTA 10001 with your address in zip10118.
Got more zip/ZCTA questions? Contact us. We're here to help!