The Decennial Census:

The Decennial census is a constitutionally mandated count of the population of the U.S. which was first conducted by the government in 1790 and has been done every ten years since. The count is used for apportioning Congressional seats, as well as many other purposes, such as demographic research, allocating funding, and informing public policy.

Historical Changes:

The Census has changed greatly over time. Though the first Census in 1790 did little more than count population, over the years it has grown to include many other types of information. Many questions and terminology (e.g. racial categories) have changed over time as well.

The Long Form and the American Community Survey:

Over time Congress became concerned that there were too many questions. In 1940 the Long Form was created as a way of finding out more detailed information (e.g. questions on income and educational attainment) from a sample of the population. The long form of the Census was sent to 1 in 6 households. The 2000 Census was the last time the long form was used. Since 2000, the American Community Survey (ACS) has replaced the long form. The ACS is an ongoing survey of a sample population that will provide us with detailed and more current information. ACS data comes in 1-year, 3-year, or 5-year estimates.

Blocks are statistical areas bounded by visible features, such as streets, roads, streams, and railroad tracks, and by nonvisible boundaries, such as selected property lines and city, township, school district, and county limits and short line-of-sight extensions of streets and roads

Block groups are statistical divisions of census tracts, and generally contain between 600 and 3,000 people. They are a collection of census blocks within a census tract, sharing the same first digit of their four-digit identifying numbers.

Census tracts generally contain between 1,000 and 8,000 people with an optimum size of 4,000 people. Census tract boundaries are delineated with the intention of being stable over many decades, so they generally follow relatively permanent visible features. However, they may follow governmental unit boundaries and other invisible features in some instances; the boundary of a state or county is always a census tract boundary. Block Numbering Areas (BNAs) are now called census tracts.

Places are concentrations of population such as cities, that have legally prescribed boundaries, powers, and functions. Other population centers without legally defined corporate limits or corporate powers are defined by the Census Bureau in cooperation with state officials and local data users. These are called Census-Designated Places (CDPs) and are identified in data tables by the acronym CDP following the place name.

Minor Civil Divisions (MCDs)/Census County Divisions (CCDs) MCDs are legally defined county subdivisions, such as towns and townships. In 21 states where MCDs do not exist or are not adequate for reporting subcounty statistics, the Census Bureau, in cooperation with state and local officials, delineates county subdivisions known as Census County Divisions.

Counties and equivalent areas are the primary divisions of most states, Puerto Rico, and the Island Areas. They include counties in 48 states; parishes in Louisiana; boroughs and census areas in Alaska; municipios in Puerto Rico; independent cities in Maryland, Missouri, Nevada, and Virginia; and other entities in the Island Areas.