Census Tracts

Tracts are regions of U.S. census geography that originated as analogs of urban neighborhoods. They retain some of that purpose today, and for that reason, tract limits are partially defined by population characteristics in addition to political boundaries and population. Tracts are preferable to other regions because the tabulations are more complete than those of smaller areas and are more easily compared than wards, zip codes, or other administrative regions not defined by population.

First proposed in 1906 by the demographer Walter Laidlaw, census tracts were intended to be a permanent framework to simplify tracking demographic changes. Although the census bureau agreed in principle to provide tract-level tabulations for eight cities in 1910, there was no immediate interest outside New York City. Demand had grown enough by 1940 so that the census bureau adopted the tract as an official tabulation area, including them in decennial tabulations, and took control over the definition of their boundaries. Tract coverage was extended into the suburbs following World War II, and most metropolitan areas were covered by 1980 and the entire nation, in 1990. Although tracts have always been intended to provide stable census geography, it has proven impossible to retain stability against the background of a highly fluid population. For this reason, efforts have been made to keep them comparable from one enumeration to the next by preferring changes that simplify reaggregation, especially by splitting a high-population tract or merging multiple low-population tracts.

Most changes in tract geography result from the population redistribution; however, some changes result from changes in the criteria defining tracts, and there have been two general overhauls of the system since 1940. Some basic tract criteria have been constant since 1910; a particularly important criterion is that every attempt should be made so that they are compact and follow visible and identifiable features but do not cross state or county boundaries.

The Census Bureau redefined tracts nationwide in 1960 to standardize the populations of existing tracts and to impose a uniform decimal numbering system. For the second reorganization in 2010, the bureau established new standards meant to address the problems of low-population tracts resulting from extending the system across the country. The newer criteria define a tract as having either a minimum population of 1,200 persons (down from 1,500) or encompassing a minimum of 480 housing units. Persistent low-population areas, including parks, airports, or other unoccupied land, may now be designated as belonging to a “special land use area” so long as it exceeds a minimum size of 1 mi.2 (square mile) of urban land or 10 mi.2 outside an urban region.