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A census tract, census area, or census district is a geographic region defined for the purpose of taking a census. Usually these coincide with the limits of cities, towns or other administrative areas and several tracts commonly exist within a county. In unincorporated areas of the United States these are often arbitrary, except for coinciding with political lines.
Census tracts represent the smallest territorial unit for which population data are available in many countries. In the United States, census tracts are subdivided into block groups and census blocks. In Canada they are divided into dissemination areas. In the U.S., Census tracts are "Designed to be relatively homogeneous units with respect to population characteristics, economic status, and living conditions, census tracts average about 4,000 inhabitants."U.S. Census Bureau definition
United States 
The concept of the census tract was first developed in the United States. In 1906, Dr. Walter Laidlaw originated the concept of permanent, small geographic areas as a framework for studying change from one decennial census to another in neighborhoods within New York City. For the 1910 Census, eight cities—New York, Baltimore, Boston, Chicago, Cleveland, Philadelphia, Pittsburgh, and St. Louis—delineated census tracts (then termed ‘‘districts’’) for the first time. No additional jurisdictions delineated census tracts until just prior to the 1930 Census, when an additional ten cities chose to do so. The increased interest in census tracts for the 1930 Census is attributed to the promotional efforts of Howard Whipple Green, who was a statistician in Cleveland, Ohio, and later the chairman of the American Statistical Association’s Committee on Census Enumeration Areas. For more than twenty-five years, Mr. Green strongly encouraged local citizens, via committees, to establish census tracts and other census statistical geographic areas. The committees created by local citizens were known as Census Tract Committees, later called Census Statistical Areas Committees.
After 1930, the Census Bureau saw the need to standardize the delineation, review, and updating of census tracts and published the first set of census tract criteria in 1934. The goal of the criteria has remained unchanged; that is, to assure comparability and data reliability through the standardization of the population thresholds for census tracts, as well as requiring that their boundaries follow specific types of geographic features that do not change frequently. The Census Bureau began publishing census tract data as part of its standard tabulations beginning with the 1940 Census. Prior to that time, census tract data were published as special tabulations.
For the 1940 Census, the Census Bureau began publishing census block data for all cities with 50,000 or more people. Census block numbers were assigned, where possible, by census tract, but for those cities that had not yet delineated census tracts, ‘‘block areas’’ (called ‘‘block numbering areas’’ [BNAs] in later censuses) were created to assign census block numbers. Starting with the 1960 Census, the Census Bureau assumed a greater role in promoting and coordinating the delineation, review, and update of census tracts. For the 1980 Census, criteria for BNAs were changed to make them more comparable in size and shape to census tracts. For the 1990 Census, all counties contained either census tracts or BNAs.
Census 2000 was the first decade in which census tracts were defined in all counties. In addition, the Census Bureau increased the number of geographic areas whose boundaries could be used as census tract boundaries. It also allowed tribal governments of federally recognized American Indian tribes with a reservation and/or off-reservation trust lands to delineate tracts without regard to State and/or county boundaries, provided the tribe had a 1990 Census population of at least 1,000.
United Kingdom 
British census tracts were first developed in the city of Oxford. The Inter-University Census Tract Committee was formed in 1955 and Oxford was divided into 48 tracts with an average population of 2,645 each. The Registrar General, however, opted for enumeration districts containing less than 1,000 people on average, rather than adopting census tracts. While tracts composed of enumeration districts were later developed, these were not extensively used. Census tracts have, however, been constructed and used by British demographers. The Office for National Statistics now uses enumeration districts only for the collection of data, with output areas used as the base unit in census releases.
- "Census Tracts and Block Numbering Areas". U. S. Census Bureau. 2000-04-19. Retrieved 2007-12-05.
- Domínguez-Berjón, Felícitas; Borrell, Carme; López, Rosario; Pastor, Vicente (2005). "Mortality and socioeconomic deprivation in census tracts of an urban setting in Southern Europe". Journal of Urban Health 82 (2): 225–236. doi:10.1093/jurban/jti047.
- Krieger, Nancy (2006). "A century of census tracts: Health & the body politic (1906–2006)". Journal of Urban Health 83 (3): 355–361. doi:10.1007/s11524-006-9040-y.
- "Census Tract Program for the 2010 Decennial Census—Final Criteria". Federal Register. U. S. Census Bureau. 2008-05-14. Retrieved 2008-10-14.
- Longley, Paul; Clarke, Graham (1996). GIS for Business and Service Planning. New York: John Wiley & Sons. pp. 80–81. ISBN 0-470-23510-1.
- Robson, Brian Turnbull (1969). Urban Analysis: A Study of City Structure with Special Reference to Sunderland. Cambridge: Cambridge University Press. p. 44. ISBN 0-521-07272-7.
- Exeter, Daniel J.; Boyle, Paul; Feng, Zhiqiang; Flowerdew, Robin; Schierloh, Nick (2005). "The creation of 'Consistent Areas Through Time' (CATTs) in Scotland, 1981–2001". Population Trends 119: 28–36.
- "Tracts – Information page". Social and Spatial Inequalities Group, Department of Geography, University of Sheffield. Retrieved 2009-10-25.
- "Beginners' guide to UK geography: Census geography". Office for National Statistics. 2007-10-30. Retrieved 2009-10-25.
- Chapter 10: Census Tracts and Block Numbering Areas, U.S. Census Bureau, Geographic Areas Reference Manual (PDF)
- gCensus: Browse Tract Numbers, Block Group Numbers, and Block Numbers