What Is A Census TractWhy does CHARTS display data by census tract and not ZIP code?Key Facts
Census tracts are small, relatively permanent statistical subdivisions of a county or statistically equivalent entity delineated by local participants as part of the U.S. Census Bureau’s Participant Statistical Areas Program. The U.S. Census Bureau delineated census tracts where no local participant existed or where a local or tribal government declined to participate. The primary purpose of census tracts is to provide a stable set of geographic units for the presentation of decennial census data. This is the first decennial census for which the entire United States is covered by census tracts. For the 1990 census, some counties had census tracts and others had block numbering areas (BNAs). For Census 2000, all BNAs were replaced by census tracts, which may or may not represent the same areas. Census tracts in the United States, Puerto Rico, and the Virgin Islands of the United States generally have between 1,500 and 8,000 people, with an optimum size of 4,000 people. For American Samoa, the Northern Mariana Islands, and Guam, the optimum size is 2,500 people. Counties and statistically equivalent entities with fewer than 1,500 people have a single census tract. Census tracts on American Indian reservations, off-reservation trust lands, and special places must contain a minimum of 1,000 people. (Special places include correctional institutions, military installations, college campuses, workers’ dormitories, hospitals, nursing homes, and group homes.) When first delineated, census tracts are designed to be relatively homogeneous with respect to population characteristics, economic status, and living conditions. The spatial size of census tracts varies widely depending on the density of settlement. Census tract boundaries are delineated with the intention of being maintained over many decades so that statistical comparisons can be made from decennial census to decennial census. However, physical changes in street patterns caused by highway construction, new developments, and so forth, may require occasional boundary revisions. In addition, census tracts occasionally are split due to population growth or combined as a result of substantial population decline. Census tracts are identified by a four-digit basic number and may have a two-digit numeric suffix; for example, 6059.02. The decimal point separating the four-digit basic tract number from the two-digit suffix is shown in the printed reports and on census maps. In computer-readable files, the decimal point is implied. Many census tracts do not have a suffix; in such cases, the suffix field is either left blank or is zero-filled. Leading zeros in a census tract number (for example, 002502) are shown only in computer-readable files. Census tract suffixes may range from .01 to .98. For the 1990 census, the .99 suffix was reserved for census tracts/block numbering areas (BNAs) that contained only crews-of-vessels population; for Census 2000, the crews-of-vessels population is included with the related census tract. Census tract numbers range from 1 to 9999 and are unique within a county or statistically equivalent entity. The U.S. Census Bureau reserves the basic census tract numbers 9400 to 9499 for census tracts delineated within or to encompass American Indian reservations and offreservation trust lands that exist in multiple states or counties (see TRIBAL CENSUS TRACTS below). The number 0000 in computer-readable files identifies a census tract delineated to provide complete coverage of water area in territorial seas and the Great Lakes. Tribal Census Tract Tribal census tracts are small, relatively permanent statistical subdivisions of a federally recognized American Indian reservation and/or off-reservation trust land. The optimum size for a tribal census tract is 2,500 people; it must contain a minimum of 1,000 people. Where a federally recognized American Indian reservation or off-reservation trust land crosses county or state lines, the same tribal census tract number may be assigned on both sides of the state/county boundary. The U.S. Census Bureau uses the census tract numbers 9400 to 9499 for tribal census tracts that cross state/county boundaries and are within or encompassing American Indian reservations and off-reservation trust land. The difference between a tribal census tract and a nontribal census tract is in the hierarchical presentation of the data. A tribal census tract is part of the American Indian hierarchy; that is, the tribal census tract is within a federally recognized American Indian reservation and/or off-reservation trust land. Why does CHARTS display data by census tract and not ZIP code?The Florida CHARTS website includes a GIS-based mapping function that will allow users to see public health data mapped at the census tract level for certain indicators. The decision to use census tracts and not zip codes for the online mapping function was based on several factors.First, census tracts are the most detailed (and lowest) level of geography available for a majority of the data provided by the U.S. Census. Therefore, the public health data provided by the Florida Department of Health can be easily compared to data from the U.S. Census for research and analysis of smaller areas. Second, zip code boundaries are established to organize the distribution of the U.S. Mail. According to the U.S. Census and U.S. Postal Service, A ZIP (Zone Improvement Plan) Code is a five-, seven-, nine-, or eleven-digit code assigned by the U.S. Postal Service to a section of a street, a collection of streets, an establishment, structure, or group of post office boxes, for the delivery of mail1. It is important to note that despite the geographic derivation of most ZIP Codes, the codes themselves are not geographic regions, but simply categories for grouping mailing addresses. ZIP Codes can overlap, be subsets of each other, or be artificial constructs with no geographic area. In order to simplify these areas for census data collection, the U.S. Census established ZIP Code Tabulation Areas. “ZIP Code Tabulation Areas were developed to overcome the difficulties in precisely defining the land area covered by each ZIP Code®. Each one is built by aggregating the Census 2000 blocks, whose addresses use a given ZIP Code, into a ZCTA which gets that ZIP Code assigned as its ZCTA code. Defining the extent of an area is necessary in order to accurately tabulate census data for that area.”2 Third, the establishment and alteration of census tracts happens less often, and when it does happen, is more structured. It is almost exclusively based on population change. Zip codes are established as needed, based mainly on ease of mail distribution. 1Source: http://factfinder.census.gov/home/en/epss/glossary_z.html 2Source: http://www.census.gov/geo/ZCTA/zcta.html |