[2] Sometimes these coincide with the limits of cities, towns or other administrative areas[2] and several tracts commonly exist within a county.
Census tracts represent the smallest territorial entity for which population data are available in many countries.
In the U.S., census tracts are "designed to be relatively homogeneous units with respect to population characteristics, economic status, and living conditions" and "average about 4,000 inhabitants".
In 1999 in France, in order to prepare for the dissemination of the 1999 French population census, INSEE (Institut national de la statistique et des études économiques) developed a system for dividing the country into units of equal size, known as IRIS2000, but now known simply as 'IRIS'.
Since 1999, IRIS has represented the fundamental unit for dissemination of infra-municipal data in France and its overseas departments and regions.
[7] In the Republic of Ireland, the Central Statistics Office (CSO), established in 1949, organises the census.
[9] Beginning from the 2011 census, the CSO website has made available "Small Area Population Statistics" (SAPS) for each "small area", a subdivision of an electoral division (ED) defined by Ordnance Survey Ireland constrained by natural landscape features.
Each of these borders another to form a network covering the whole country including inlets and coasts, and extending out to the 200 mile economic zone.
[15] The Registrar General, however, opted for enumeration districts containing less than 1,000 people on average, rather than adopting census tracts.
[17] 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.
In 1906, Walter Laidlaw originated the concept of permanent, small geographic areas as a framework for studying change from one decennial United States Census to another in neighborhoods within New York City.
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.