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## Tracts and Block Numbering Areas

View larger imageCensus tract-level data showing the Change in population in Ohio between the 1990 and 2000 Censuses of Population Housing.Photo courtesy of the Ohio Department of Development

The first recorded instances of the delineation of small geographic entities based on population, topography, and housing characteristics were the sanitary districts ― nested within city wards ― of a special vital statistics study associated with the 1890 census.  The Census Office, predecessor of the Census Bureau, worked with local officials in a number of cities to delineate a network of small geographic areas.

These sanitary districts were then used to analyze and compare the effect of population, topography, and housing on the mortality rate of the inhabitants.  The delineation of these sanitary districts was an important step in the evolution of geographic statistical entities.  This may have been the first instance of federal and local cooperation in designing a set of small geographic units based on population and housing characteristics. 

In 1906, Dr. Walter Laidlaw, director of the Population Research Bureau of the New York Federation of Churches, published an article putting forth the idea of delineating and using small geographic areas as a method of studying neighborhoods in New York city. Dr. Laidlaw had been studying neighborhoods by using the 1900 census data for assembly districts (subdivisions of New York city's boroughs) together with information from other sources.

In 1905, the state of New York changed the boundaries of the assembly districts, thereby altering the geographic framework and impairing the usefulness of all his information.  In search of a solution, Dr. Laidlaw proposed a scheme that did away with both ward and assembly districts as data tabulation units.  Instead of these, he suggested the delineation of permanent small areas that would retain their boundaries from census to census.  His plan was to subdivide each square mile of New York City into quarter sections of about 160 acres. 

Prior to the census of 1910, Dr. Laidlaw divided the city of New York into 40-acre tracts called "districts."  In 1909, he persuaded the Census Office to adopt the concept, and to extend the plan of tract tabulations to the seven other cities have a population of 500,000 or more (Baltimore, Boston, Chicago, Cleveland, Philadelphia, Pittsburgh, and St. Louis).  While the tabulation was done by tract for these cities, the data were not used anywhere but New York City and St. Louis. 

The 1920 census data were also tabulated by tract for these cities, and Dr. Laidlaw published the figures for New York.  Others followed:  Chicago and Cleveland purchased and published their census tract data.  By the end of the decade, 18 cities (the same eight from 1910, along with Los Angeles, Columbus, Nashville, Berkeley, Syracuse, and Yonkers) were reviewing or delineating census tracts for the 1930 census. 

This increased interest in census tracts was largely due to the promotional efforts of Howard Whipple Green, a statistical consultant working in Cleveland, Ohio.  Having experienced data problems similar to those faced by Dr. Laidlaw, he found that census tracts were a solution. 

In 1931, the American Statistical Association appointed Mr. Green chairman of its newly formed Committee on Census Enumeration Areas.  Along with this appointment came the unofficial assignment to promote the delineation of census tracts in large cities throughout the country.  Over the next 25 years, he worked hard at this task, contacting interested people in other cities, encouraging the formation of local committees, and publicizing uses for census tract data in a newsletter.  In 1955, upon Mr. Green's retirement, the Census Bureau assumed the functions of promoting and coordinating the delineation of census tracts.

After the 1930 census, 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 over time; 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.

For the 1940 census, the Census Bureau adopted the census tract as an official geographic entity to be included in data tables of the standard publications of the decennial census.  This relieved the census tract committees of the need to purchase the data tabulations and to fund their publication.  In 1940, the Census Bureau also began publishing census block data for all cities with 50,000 or more inhabitants. Census block numbers were assigned, where possible, by census tract, but for those cities that had not yet delineated census tracts, ''block areas'' were created to assign census block numbers.  The block areas were renamed block numbering areas (BNAs) in 1960.

In 1970 and 1980 there was an increase in the number of jurisdictions receiving data by census block under the BNA program. Additionally, beginning with the 1980 census, the BNA criteria were changed to make BNAs more comparable to census tracts. 

For the 1990 census, census tracts and BNAs together covered the entire nation, and all counties contained either census tracts or BNAs.  For Census 2000, the BNA concept was retired and census tracts were defined nationwide.

The 2010 Census marks the 100th anniversary of census tracts.  A century of census tract use has shown that census tracts continue to be an important geography for the tabulation of decennial census data and other statistical programs, and that census tract boundary continuity and the resulting census tract data comparability over time are of considerable significance to data users.  

For more information, see:

Census Tract Manual: 1934 Edition  Download PDF  [861KB PDF]Census Tract Manual: 1937 Edition Download PDF  [647KB PDF]Census Tract Manual: 1947 Edition  Download PDF  [2.96MB PDF]

Delivering What Users Want: the Evolution of Census Bureau Small Area Data [1 MB PDF] - Michael S. Snow, U.S. Census Bureau, September 2011.

Increasing demand for small area data has driven the expansion of the decennial census since the late nineteenth century. Responding to public health officials' need for data on relatively homogeneous units, the Census Bureau began tabulating data on subdivisions of a few cities in the 1890s. When social workers and business organizations joined public health officers in asking the Census Bureau for such data, the agency agreed to publish data based on their delineation of census tracts. To meet growing demand from marketers and government planners, the Census Bureau added data on census blocks in 1940 and later on other geographic entities aggregated from blocks. Since the 1970s, the need for small area data for legislative redistricting pushed the agency to extend nationwide the areas for which it provided small area data. The most recent evolution has arisen out of calls for more timely data. The American Community Survey in 2010 replaced the decennial long-form and began delivering small area data more than once a decade.

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