What are Zip Code Tabulation Areas?

What’s the difference between Zip Code Tabulation Areas and Zip Codes?

Zip Code Tabulation Areas are a geographic entity made by the Census to approximate Zip Codes.

As we’ve discussed previously, Zip Codes were designed by the U.S. Postal Service for efficient mail delivery, not for statistical data collection. Technically speaking, a Zip Code is not a geographic boundary, but a collection of delivery routes. Since most people think of them as a type of geography, like a neighborhood, third-party companies have created maps of Zip Codes based on information from the USPS. PolicyMap uses Zip Code boundaries from Maponics.

So why do we need these Zip Code Tabulation Areas?

Since people want Census data for Zip Codes, the Census Bureau created Zip Code Tabulation Areas (ZCTAs)—geographies that were designed to correspond with USPS Zip Codes, but with fewer geographical quirks, and are only changed once every ten years. They have the same five-digit names as the Zip Codes they represent.

The Census is able to create data at these ZCTAs for the decennial census, and the American Community Survey.

Z.C.T.A.?

We pronounce it “ZIC-tuh”.

How similar are ZCTAs to Zip Codes?

They’re similar… but not the same. In many cases, such as our own 19103, they’re very similar:

More than half of all ZCTAs share more than 90% of their area with their associated Zip Code. Over 70% of ZCTAs share at least 80% of their area with their Zip Codes.

There are some instances where the ZCTA is quite different than its corresponding Zip Code. 11% share less than half their area with their Zip Code. Sometimes, the ZCTA is a small piece of the Zip Code, sometimes the Zip Code is a small piece of the ZCTA, and sometimes they’re just in different places:

Does every Zip Code have a ZCTA?

No. Some Zip Codes are special, such as those for large buildings surrounded by another Zip Code. These usually don’t get their own ZCTA. And since Zip Codes change based on postal routes, they’re often added and removed throughout the decade, with no corresponding change to the ZCTA.

So PolicyMap is now showing Census American Community Survey data at these ZCTAs?

Yep.

Didn’t PolicyMap used to have ACS data for Zip Codes?

Sort of. Since the Census does not release data at the Zip Code geography, we used the other smallest geography available, block groups, to create data for Zip Codes.

Why didn’t you just use ZCTAs before?

The ZCTAs drawn up in 2000 had a bad reputation. They didn’t look much like Zip Codes. If they had been on PolicyMap, people would have been confused by their dissimilarity to the Zip Codes. So we constructed data for the Zip Codes people were familiar with.

However, the 2010 ZCTAs are much better. We continued using our Zip Code data methodology for a few years, but now we’ve decided it’s time to move to ZCTAs.

Can I still use Zip Codes on PolicyMap?

Yes. Zip Code boundaries will be kept updated, so if you search for a Zip Code on the map, you’ll be taken to the area that best represents the current USPS Zip Code. Also, we still have data that’s collected according to Zip Codes, like IRS tax data and Valassis vacancy data.

What happens if I search for a Zip Code on PolicyMap?

You’ll get two options in the suggestions menu: Zip Code and Zip Code Tabulation Area.

On the Maps page, even if you have ACS data loaded, you can click on the Zip Code, and it will take you to the Zip Code, but the data underneath will be for ZCTAs (or, if you’re zoomed in enough, census tracts or block groups).

What about in Tables and Reports? Should I use ZCTAs or Zip Codes?

In Tables, if you’re looking at ACS data, you should use ZCTAs. If you click on the Zip Code, it will either add together block groups (if it’s a count), or give you an N/A value (if it’s a percent, median, or average).

If you’re looking at another dataset that uses Zip Codes, like IRS tax data, you should use Zip Code.

As a friendly helper, if either a Zip or ZCTA is loaded in tables, it will let you know if a similar version of the other boundary is available:

In Reports, if you’re making a Community Profile or Rental Housing report, it’s usually a good idea to use a ZCTA. That way, it’ll use the Census data for that ZCTA, giving you the most accurate numbers. If you use a Zip Code, it will use whichever block groups or census tracts best approximate the Zip Code. It’s a little like trying to fit a square peg in a round hole.

To remind you to use the ZCTA, when you try to load a Community Profile or Rental Housing report for a Zip Code, it will give you an alert to ask if you’d like to use the ZCTA instead.

However, on occasion, you may decide that the ZCTA is so different from the Zip Code that you’d rather make the report for the Zip Code. According to our analysis, only 13 percent of Zip Codes are so different than their corresponding ZCTA that you’ll be better off using the Zip Code for your report.

The Home Sale report and the HMDA report use Zip Code data, so you should continue to use Zip Codes for them.

I’m annoyed. You took away Zip Code data. I know what a Zip Code is. I don’t know what a ZCTA is.

Zip Codes are tough. The old methodology we used to make Zip Code ACS data was good, but it wasn’t perfect. ZCTAs are pretty close to Zip Codes, and their data is official from the Census. The boundaries might not always be as accurate, but the data within them will be better.

And we’d never discard Zip Codes entirely. Mr. Zip and I go way back:

Usually when you have a blog post on this topic, you include a 60s musical video with catchy songs all about Zip Codes. Will you continue to do that?

Absolutely.