Monday, February 18, 2013

Your Missing Package: When Address Correction Fails

Amazon address correction is wrong for large parts of Chicago. This leads to late and missing packages. This handy map shows areas most affected by address correction failure. To avoid delivery problems always use your full ZIP+4 when placing online orders. You can find the full ZIP+4 for your address via the USPS ZIP Code (TM) Lookup Tool.

I don't mean to pick on Amazon -- this problem has happened with several other retailers. I used Amazon because it was easy to cross-check their address verification with USPS. If you are an online retailer, make sure you have a working address correction system. If Amazon can get it wrong, what makes you think yours works? Bad address correction is costing you customers.

The Problem

Have your Amazon packages ever been late or missing?
Have you ever gotten a "notice left" email but no notice?
Did USPS confirm delivery but there was no package?
Do you only use a 5 digit ZIP code when filling out your address?

You may be a victim of address correction failure. And you are not alone.

Here is how to check:

First, go to "manage addresses" and look at your address on Amazon.
Now, go to the USPS ZIP Code (TM) Lookup Tool and check your address.

If the full 9 digit ZIP Codes do not match, there is a problem. If you live in Chicago, I made a heat map of where verification failures are most likely to occur.

Address Verification Failures

Mailers validate your address prior to shipment to save money on shipping costs. The address validation step is called Delivery Point Validation (DPV), and it requires a complete mailing address including a full 9 digit ZIP Code. Since few people know their full ZIP Codes, a suite of software called Coding Accuracy Support System (CASS) will correct an address into one that can be checked via DPV. The correction step can fail, and "correct" your address to a different building. To find out why, its time for a quick lesson on DPV, CASS, and ZIP Codes.

Note: I am not an expert on mailing, this information is what I have learned from judicious searching. It may be wrong. If I am, please correct me.


Mailers use DPV to ensure an address is deliverable before passing the mail to USPS. In return, they receive discounted postage rates for reducing the work USPS has to do. From The History of Worksharing Discounts and CASS Certified™ Software:

In 1983, the United States Postal Service (USPS) implemented a program that provided mailers a postage discount for sharing the work to prepare the mail for processing. This allowed the USPS to provide more cost-efficient mail processing based on the advance work performed by the mailer in providing high-quality addresses for their mail.

People are notoriously bad typers and spellers, and tend to omit information. Before a delivery point is verified, an address has to go through a Coding Accuracy Support System (CASS) check. The CASS software will fix an address to one that can be validated by DPV. From the Wikipedia page:

The input of:
Produces the output of:
REDMOND WA 98052-8300

CASS software has to be certified by the USPS and has to undergo certification testing every two years. The caveat is that CASS validation only checks address matching, not the accuracy of the matched address. From the USPS:

However, CASS processing does not measure the accuracy of ZIP + 4, delivery point, 5-digit ZIP, or carrier route codes in a mailer’s address file.

If the mailer's ZIP+4 database is wrong, CASS can't fix it.

Why do ZIP+4 Codes matter?

In a city, a ZIP+4 will determine the building or even the floor or group of apartments a piece of mail goes to. From the USPS website (emphasis mine):

The ZIP+4 Code was introduced in 1983. The extra four numbers allow mail to be sorted to a specific group of streets or to a high-rise building. In 1991, two more numbers were added so that mail could be sorted directly to a residence or business. Today, the use of ZIP Codes extends far beyond the mailing industry, and they are a fundamental component in the nation’s 911 emergency system.

If the ZIP+4 code is wrong, your mail goes to the wrong building. Your mailman might not catch this. Mail with electronic mailing information (i.e. pretty much all packages from online retailers) is automatically sorted and binned by machines. On busy urban routes the mailman doesn't know everyone and they aren't going to check every single piece of mail. They're going to take machine sorted mail bin, deposit it at the address they always do, and move on. If you're lucky, you may get a redelivery notice.

... but Amazon ships via UPS/Fedex?

UPS and FedEx may do hand-off to USPS for final delivery. This is a part of USPS work-share programs that UPS calls a mailing innovation.

The Address Verification Failure Map

The following map shows differences between ZIP+4 Codes returned by USPS and ZIP+4 Codes corrected by Amazon for 1,857 addresses in the City of Chicago. Green markers mean a match, blue markers represent ZIP+4 Codes from USPS, and yellow markers represents ZIP+4 codes from Amazon. A red connecting line associates the USPS and Amazon results for the same address.

There are correction mistakes throughout the City, with the most mistakes in the Loop and the area immediately to the north and northwest. This correlates pretty well with the number of large apartments and condos, and hence specificity of ZIP+4 codes.

I chose Chicago addresses because thats where I live. The addresses were a random sampling from the City of Chicago business license holders. The City of Chicago has an excellent open data site at This research would not have been possible without it.

I sampled 2000 addresses out of a possible 381677. Of these, 143 (~7%) addresses were not found -- that is, either the USPS or Amazon had a failure in obtaining a ZIP+4 for the address. There were 519 (~26%) addresses with a different ZIP+4 between USPS and Amazon, and 1338 (67%) addresses with the same ZIP+4.

I am making available the addresses used to generate this map.

File Metadata Description
zip_diffs.txt41KB, textZIP+4 Differences
zip_equals.txt100KB, textZIP+4 Matches
zip_fails.txt11KB, textFailure to get ZIP+4 for an address

My verification scripts would select the first suggested address or the automatically corrected address (assuming no address was suggested) given by Amazon. For some streets, the suggested address was very far from the initial input. No human would have selected it, so the most egregious correction errors would likely have been caught. The places where the yellow and blue marker are close together are the most dangerous -- it is likely only a +4 digit difference which most users (like myself) would never notice.

To map ZIP+4 addresses to latitude/longitude and to create the map, I used the MapQuest API. MapQuest may seem like an odd choice, but it had great documentation and examples, and it was the first service I could find with support for mapping a ZIP+4 to latitude/longitude.


I recently moved to Chicago with only what I could fit in my car, which meant I had to buy a lot of household items. I do most of my shopping online since I hate the crowds, salesmen, and poor selection at brick and mortar stores. This means I buy a lot of stuff on Amazon.

I first became suspicious when I received the following email:

Fool me once, shame on you.
It is impossible to leave an unattended package at my address. The building has 24/7 front desk staff and a dedicated package receiving room. I dutifully filled out the re-delivery form, and received my package a few days later. I thought nothing of it until I received this second email:

Fool me twice, shame on me.
Around the same time my fiancee had several packages (not from Amazon, but other vendors) never arrive, despite USPS confirming delivery. Something was wrong, it was time to investigate.

The addresses on the re-delivered package labels, order confirmation, and all seemed correct. The front desk staff hadn't noticed any delivery attempts, and no packages had been left for me.

I was stumped and considered just not shopping online, until I had a thought: USPS re-delivery worked, but original delivery sent it to a mystery address. Was there a difference between the USPS address and the address?

Sure enough, there was. The ZIP+4 code had the wrong +4 digits. Searching online for the ZIP+4 Code from USPS results only in matches with my building's address. Searching for the ZIP+4 Code from Amazon results only in matches from buildings a few numbers down, with no front desk staff.

Mystery solved.

I immediately emailed Amazon with the problem. This was in mid January. As of February 18th, my address is still corrected to the wrong ZIP+4 Code.

A Bigger Problem

Did I just live at the wrong address, and this was an isolated case, or if there was a more systematic address correction problem?

That is why I made the map. Turns out some areas are more affected than others, and that my address is not the only one. I hope that by exposing this publicly I can help others avoid the hassle and headaches of online ordering. 


Major vendors, including Amazon, get address correction wrong. In my sample of Chicago business addresses, 26% had a ZIP+4 that did not match the one returned by USPS.

If you are an online retailer, please check your CASS and DPV software. Don't just assume it works, but write some scripts to test it yourself. Your customers will thank you. If your customers complain about missing packages, check that their address corrects properly.

If you buy things online, memorize your ZIP+4 Code and use the full code where you can. If you live in an urban area, and the vendor only accepts a 5 digit ZIP, shop somewhere else because you may never get what you bought.

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