DOI: 10.3886/icpsr39431.v1
title: National ZIP Code Crosswalk, [United States], 1990-2020
title: Version 1

description: ZIP Codes are administrative codes generated by the United States Postal Service (USPS) that refer to the geographic area covered by a specific set of mail delivery routes. The U.S. Census Bureau calculates and distributes aggregated social, economic, and demographic information for the population associated with "ZIP Code Tabulation Areas" (ZCTAs), which are roughly analogous to ZIP Codes and serve as identifiers for specific neighborhoods and communities. These aggregated census data, however, are unable to account for changes in ZIP Code boundaries that occur between decennial censuses, leading to measurement error and missing data problems for scholars who attempt to use the aggregated ZCTA data. The purpose of this crosswalk file is to allow researchers to overcome this limitation, enabling them to appropriately link spatial reference information (ZIP Codes) with characteristics of the populations to which they refer.
Most ZIP Codes do not change boundaries in a decade, but a large enough percentage do as to create a problem with missing or mis-specified data. Boundary changes typically involve one or more of the following three processes, although a small number of cases do not conform to these typologies: (1) two or more existing ZIP Codes are combined to create a single surviving ZIP Code, (2) an existing ZIP Code is divided into multiple resulting ZIP Codes, and (3) boundaries between two or more existing ZIP Codes are altered.
Each of these types of changes alters the geographic area that a ZIP Code refers to, and as such, the spatial unit identified by the ZIP Code includes a different population, with a different array of characteristics. By linking the spatial units associated with ZIP Codes as these boundary changes are enacted, the research team can both prevent the loss of observations due to missing data, and more accurately measure social, demographic, and economic characteristics associated with each ZIP Code.

This data set identifies changes in ZIP Code boundaries between 1990 and 2020, and provides numeric codes that cluster the ZIP Codes into the smallest geographic unit, or group of ZIP Codes, that are consistent across a decade: 1990 - 2000, 2000 - 2010, and 2010 - 2020. This "crosswalk" covers the contiguous United States, Alaska, Hawaii, and the District of Columbia. Since much administrative data is available with ZIP Code as the smallest identifiable geography, ZIP Codes are often used to embed observations from administrative data (patients, businesses, survey respondents, etc.) within their social, demographic, and economic contexts. However, ZIP Code boundaries change over time, resulting in measurement error (matching observations to the wrong contextual unit) or missing data (due to an observation reporting a ZIP Code that did not exist at the beginning of the observational period). These data were collected, and the crosswalk created, in an attempt to resolve these data quality issues.
description: The<em> National ZIP Code Crosswalk (1990-2020)</em> was created to identify changes in the U.S. Postal Service ZIP Code boundaries and mitigate data quality issues created by these changes.
description: The researchers reviewed each issue of the <em>U.S. Postal Bulletin</em> published between January 1, 1990, and December 31, 2020 (a total of 781 issues) and identified all instances in which ZIP Code boundaries were changed. They also identified ZIP Code changes indicating that a particular ZIP Code was designated exclusively for Post Office Boxes. Each of these changes was then recorded in an Excel spreadsheet, organized by state and in the chronological order in which the announcements appeared.
description: The data include ZIP codes, FIPS codes, state names, ZIP code cluster codes with specific dates of transition, and the year of transition.
description: These data are a complete census of all ZIP Code boundary changes that were published in the <em>U.S. Postal Bulletin</em> between 1990-2020.
description: Datasets: 
DS0: Study-Level Files
DS1: National ZIP Code Crosswalk Public-Use Dataset 1990-2000
DS2: National ZIP Code Crosswalk Public-Use Dataset 2000-2010
DS3: National ZIP Code Crosswalk Public-Use Dataset 2010-2020
description: ZIP Code boundary changes published in the <em>U.S. Postal Bulletin</em> between 1990-2020. Smallest Geographic Unit: ZIP Code
description: The <em>U.S. Postal Bulletin</em> is a biweekly administrative publication produced by the U.S. Postal Service which identifies and publishes ZIP Code administrative changes that are of interest to postal workers. Some of these changes do not affect the shape of ZIP Code boundaries (such as reassigning of a ZIP Code to a new administrative office). In other cases, some changes that do affect boundaries are announced and later retracted, while in other cases a ZIP Code may experience multiple boundary changes over the course of a decade. In this dataset, the research team only included changes that affect the <em>boundaries</em> of a ZIP Code, and what has been identified as a final geographic cluster, accounting for all changes that went into effect. This is meant to allow for researchers to identify the specific geographic area that a ZIP Code may refer to over the course of a decade.
The research team also identified two specific instances that suggest that the reported ZIP Code for an observation may not actually correspond to a specific geographic catchment area. The first of these are announcements where a ZIP Code is created solely for the use of post office boxes. The second is an announcement that a ZIP Code is adding delivery services. However, as only the changes that occurred during the three observational decades were able to be identified, these lists should be considered partial. At best, they provide an incomplete list of "other than geographic" ZIP Codes, and while they will allow the identification of observations that do not correspond to a specific geographic area, they should be used with caution. 
A self-published version of this study is also available.