ZIP Codes shouldn't be used for collecting statistics.

It is a sad fact that the primary statistical collection unit used by businesses in the United States today is the ZIP Code. Unfortunately, they're collecting a lot of mostly meaningless data that can't be compared over time.

ZIP Codes are used by the U.S Postal Service to balance mail delivery workload. They are useless for collecting statistics because ZIP codes change every day.

In my little corner of the world, representing about 1% of the population of the United states, in one year alone, the U.S. Postal Service split six ZIP codes into twelve.

Not only that, there were thousands of minor changes -- that is, one segment of street had its ZIP code reassigned to a neighboring ZIP code so that one letter carrier has the same workload as his/her neighbor.

I'm sure there are a lot of salesmen wondering why sales numbers for their territory suddenly dropped in half. I'm sure their bosses wonder that too.

There is no way you can collect meaningful statistics using collection units that writhe around like amoebae!

In an attempt to remedy this, the US Census Bureau has devised a system of "Zip Code Tabulation Areas", abbreviated "ZCTA", that approximate the Zip codes of the United States as of the 2000 Census. You can now get summary Census data by ZIP code. Unfortunately for this idea, people will continue to report that they live in the Postal Service's ZIP codes. I'm just waiting for the 2010 Census, when people will report the population of a split ZIP Code has dropped in half.

A ZIP-code is a postal code used by the US Postal Service. It is a 5-digit number that identifies a specific geographic delivery area. It was established in 1963 to replace the aging and limited Zone system. Hence, ZIP stands for "Zone Improvement Plan".

Considering 80521 -- a zip code for Colorado State University:

  • The first three digits identify which central postal sorting station. So the 805 zone includes both post offices in Ft. Collins, plus a number of smaller surrounding towns.
  • The second two digits, 21, designate an associate post office, post office branch, or post office station. In this case, 21 is assigned to the North Fort Collins Station. All post offices are assigned at least one unique 5-digit code, and often many more.

ZIP Codes can represent an area within a state (an area that may or may not cross county boundaries), an area that crosses state boundaries (an unusual condition), or a single building or company that has a very high mail volume. For example, the autoresponder@whitehouse.gov reports that the President and his/her family have the ZIP-code 20500. Smokey the Bear is the only person(?) in Zip Code 20252. The entire Sears Tower in Chicago is at 60684. In general, Zip codes start at 00000 in the Eastern US, and enter the 90000s as they go west. Hawaii and Alaska have Zip codes in the 90000 rage.

ZIP+4 is an enhanced code consisting of the 5-digit ZIP Code and four additional digits that identify a specific range of delivery addresses. The first two additional digits designate the sector (a geographic area) and the last two digits designate the segment (a building, floor, etc.).

ZIP Code and Zip+4 are USPS trademarks.

The POSTNET barcode found printed on nearly all mail contains at minimum the ZIP code, and at maximum your ZIP+4 code, and the last two digits of your house number. A mod-10 checkdigit is at the end.

A fair amount of this is sourced direct from USPS documents, so it is a little dry.

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