ZIP Code
A ZIP Code (an acronym for Zone Improvement Plan[1]) is a system of postal codes used by the United States Postal Service (USPS). The term ZIP was chosen to suggest that the mail travels more efficiently and quickly[2] (zipping along) when senders use the code in the postal address.
Introduced on July 1, 1963, the basic format comprised five digits.[3] In 1983, an extended code was introduced named ZIP+4 ; it included the five digits of the ZIP Code, followed by a hyphen and four digits that designated a more specific location.
ZIP Code is a trademark and ZIP+4 is a registered trademark of the United States Postal Service, which also registered ZIP Code as a service mark until 1997.[4]
Structure and allocation[edit]
Scope and international mail[edit]
ZIP Codes designate delivery points within the United States (and its territories). There are generally no ZIP Codes for deliveries to other countries, except for the independent countries of the Federated States of Micronesia, the Republic of the Marshall Islands, and the Republic of Palau, each of which is integrated into the U.S. postal system under a Compact of Free Association.[19] Another exception is ZIP Codes used for overseas stations of U.S. armed forces.[20]
Mail to U.S. diplomatic missions overseas is addressed as if it were addressed to a street address in Washington, D.C. The four-digit diplomatic pouch number is used as a building number, while the city in which the embassy or consulate is located is combined with the word "Place" to form a fictional street name. Each mission uses a ZIP+4 Code consisting of 20521 and the diplomatic pouch number.
For example, the U.S. Embassy in India has this address in India's postal system:[21]
Other uses[edit]
Delivery services[edit]
Delivery services other than the USPS, such as FedEx, United Parcel Service, and DHL, require a ZIP Code for optimal internal routing of a package.[31]
Statistics[edit]
As of October 2019, there are 41,702 ZIP Codes in the United States.[32] Due to convenience, ZIP Codes are used not only for tracking of mail, but also commonly for gathering geographical statistics in the United States by some researchers.[33][34] ZIP Codes are not created for statistical analysis, thus use for statistical analysis is heavily criticized for numerous reasons and advised against as a cartographic practice.[34][35] As ZIP Codes are not polygons, but collections of mail routes and points, they are unsuitable for publication or distribution of most data.[36] Polygons for ZIP Codes are not released by the USPS and instead interpolated by 3rd party vendors.[33] These interpolations introduce topological errors and are not standard between vendors.[33] The USPS often discontinues, splits, or otherwise modifies ZIP Codes, making continuous space-time analysis challenging, leading to issues with both the modifiable areal unit problem (MAUP) and modifiable temporal unit problem (MTUP).[33][35] As the ZIP Codes are postal routing numbers, individuals and organizations without concrete spatial locations may be given their own number, making it impossible to associate demographic data with them.[33] Demographic data is not consistent between ZIP Codes, and no effort is made to ensure they are proper enumeration units for analysis.[33] As ZIP Codes are not made with the same considerations as other enumeration units, and is not possible without committing the ecological fallacy.[33][34] This again becomes an issue with the MAUP. They have been found not to have significant correlations with health indicators, which can lead to poor conclusions.[34] Despite these issues, ZIP Codes remain popular among researchers in fields such as public health due to their convenience, public familiarity with them, ability to anonymize subject addresses through aggregation, and possible ignorance of more appropriate enumeration units on the part of researchers.[33][35]
In an attempt to satisfy demand " by data users for statistical data by ZIP Code area", the U.S. Census Bureau calculates approximate boundaries of ZIP Code areas, which it calls ZIP Code Tabulation Areas (ZCTAs).[36][33] Statistical census data is then provided for these approximate areas. The geographic data provided for these areas includes the latitude and longitude of the center-point of the ZCTAs. ZIP Codes are inherently discrete or point-based data, as they are assigned only at the point of delivery, not for the spaces in between the delivery points. The United States Census Bureau then interpolates this discrete data set to create polygons by attempting to match ZIP Code extents with Census blocks.[33] The resulting aerial units represent the approximate extent of the ZIP Code which are combined with to use for mapping and data presentation. The process of creating ZCTAs and their use for statistical analysis are heavily criticized in the literature. First, the creation of ZCTAs from Census blocks encounters issues when a Census block straddles multiple ZIP Codes.[33] Addressing this is another instance of the MAUP, and the solution of dividing aggregate units between ZIP Codes causes some individuals to fall into ZCTAs that do not match their ZIP Code.[33] The creation of these units is therefore committing the ecological fallacy by attempting to disaggregate aggregate data. As ZIP Codes are not continuous, not everyone in the United States has one, and there are ZIP Codes for non-populated or geographic areas, resulting in there not being one ZCTA for every ZIP Code.[33] ZCTAs are not updated as frequently as the USPS updates ZIP Codes, resulting in further temporal analysis issues when ZIP Codes change during a study period.[33] Datasets providing a similar approximate geographic extent to ZCTA are commercially available. Despite these issues, ZCTAs are still very popular with researchers in fields like epidemiology, and among government agencies, with some states employing them to publish and distribute public health data during the COVID-19 pandemic.[35]
Marketing[edit]
The data is often used in direct mail marketing campaigns in a process called ZIP-code marketing. Point-of-sale cashiers sometimes ask consumers for their home ZIP Codes. Besides providing purchasing-pattern data useful in determining the location of new business establishments, retailers can use directories to correlate this ZIP Code with the name on a credit card to obtain a consumer's full address and telephone number. ZIP-Coded data are also used in analyzing geographic factors in risk, an insurance and banking industry practice pejoratively known as redlining. This can cause problems, e.g., expensive insurance, for people living near a town with a high crime rate and sharing its ZIP Code, while they live in a relatively crime-free town. (See Elkins Park, Pennsylvania, above.)
California outlawed this practice in 2011.[37]
Legislative districts[edit]
ZIP Codes may not currently be used to identify existing legislative districts. Although the website of the United States House of Representatives has a "Find Your Representative" feature that looks up congressional districts based on ZIP Codes alone, it often returns multiple districts corresponding to a single ZIP Code.[38][39] This is because different parts of one ZIP Code can be in different districts.[40] One proposal to eliminate the possibility of extreme partisan gerrymandering calls on using ZIP Codes as the basic units for redistricting.[41]
Internet[edit]
A 1978 proposal for a nationwide system of community networks suggested using ZIP Codes for routing.[42]
ZIP Code data is an integral part of dealer / store locator software on many websites, especially brick-and-click websites. This software processes a user-input ZIP Code and returns a list of store or business locations, usually in the order of increasing distance from the center of the input ZIP Code. As the ZIP Code system is confined to the U.S. Postal network, websites that require ZIP Codes cannot register customers outside the United States. Many sites will purchase postal code data of other countries or make allowances in cases where the ZIP Code is not recognized.
ZIP Codes are regularly used on the Internet to provide a location in situations where an exact address is not necessary (or desirable) but the user's municipality or general location is needed. Examples (in addition to the store locator example listed above) include weather forecasts, television listings, local news, and online dating (most general-purpose sites, by default, search within a specified radius of a given ZIP Code, based on other users' entered ZIP Codes).