The Decennial Census is actually two Censuses taken concurrently—the Census of Population (1790-present) counts numbers of persons and selected social & economic characteristics, the Census of Housing (1940-present) counts numbers of residential units and selected physical & financial characteristics. The Constitution states:
"The actual Enumeration shall be made within three Years after the first Meeting of the Congress of the United States, and within every subsequent Term of ten Years, in such Manner as they shall by Law direct."
The purposes of the Census are:
Census data are also used for a great variety of purposes outside of government. Businesses, social agencies, and non-profit organizations all use Census data.
Prior to the middle of the 20th century, the Census was simple and straightforward, reflecting the limited role the government played in everyday life.
Over time, an increase in government activity led to a need for more information to formulate policy. This resulted in adoption of the long form, a sample of the population, as it would be too expensive to poll every single person. With the use of sampling techniques, analysts have been able to study in great detail a smaller portion of the population from which they can infer characteristics of the population in general.
At the same time, the need for an exact count of the population has remained. The result has been that there are two sets of Census data—a broad look at the entire population on a narrow range of questions and a closer look at a sample of the population on a deep range of questions. Census practices have varied over time, but for the 2000 Census, five out of six housing units in the U.S. received the short form, while one in six received the long form.
The 2010 census will only include a short form; more detailed data are now being derived from the American Community Survey (ACS), with data available 1995-present. ACS data is available from American FactFinder, the Census Bureau's main data portal.