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The Census Bureau's Population Estimates Program (PEP) produces July 1 estimates for years after the last published decennial census (1990), as well as for past decades. Existing data series such as births, deaths, Federal tax returns, medicare enrollment, and immigration, are used to update the decennial census base counts. PEP estimates are used in Federal funding allocations, in setting the levels of national surveys, and in monitoring recent demographic changes. A methodology reference accompanies most of our population estimates offerings.
There is not a distinct dichotomy between population estimates and population projections, but there are some differences in time reference and derivation. Estimates usually are for the past, while projections typically are for future dates. Estimates generally use existing symptomatic data, for example, (births, deaths, migration), collected from various sources. Projections must assume future trends for fertility, mortality, and other demographic processes. At the Census Bureau, the population projections use the latest available estimates as starting points. In our current product offerings the user may see both an estimate and a projection available for the same reference date, which may not agree because they were produced at different times. In such cases, estimates are the preferred data.
With each new issue of July 1 estimates, PEP revises estimates for years back to the last census. Previously released estimates become superseded. Revisions to estimates are usually due to input data updates, changes in methodology, or legal boundary changes. The frequency of estimates and availability of demographic detail vary by geographic level.
The legal requirement for the Census Bureau to produce subnational population estimates is given in Title 13 of the U.S. Code. Title 13 states that: "During the intervals between each census of population required under section 141 of this title, the Secretary, to the extent feasible, shall annually produce and publish for each State, county, and local unit of general purpose government of fifty thousand or more, current data on total population and population characteristics and, to the extent feasible, shall biennially produce and publish for units of general purpose government current data on total population." The reason for producing estimates is given in Section 183 of Title 13: "Except as provided in subsection (b), for the purpose of administering any law of the United States in which population or other population characteristics are used to determine the amount of benefit received by State, county, or local units of general purpose government, the Secretary shall transmit to the President for use by the appropriate departments and agencies of the executive branch the data most recently produced and published under this title."
In other words, the Census Bureau produces subnational estimates for use in the allocation of funds to state, county and local governments. For this reason, the Census Bureau produces population estimates for general-purpose functioning governments. These governments have elected officials who can provide services and raise revenue. In addition to states and counties, incorporated places and minor civil divisions also serve as general-purpose functioning governmental units.
In addition to the Nation, the 50 states, and the District of Columbia, PEP produces estimates for the following geographic entities:
Counties are the primary legal divisions of most states. Most counties are functioning governmental units, whose powers and functions vary from state to state. In Louisiana, these primary divisions are known as parishes. In Alaska, the county equivalents consist of legally organized boroughs or "census areas" delineated for statistical purposes by the State of Alaska and the Census Bureau (since 1980). In four states (Maryland, Missouri, Nevada, and Virginia), one or more cities are independent of any county organization and thus constitute primary divisions of their states; the Census Bureau refers to these places as "independent cities" and treats them as the equivalents of counties for estimates purposes. The District of Columbia has no primary divisions and the entire area is considered to be the equivalent of a county and in Puerto Rico, municipios are the primary divisions and treated as county equivalents for estimates purposes. Legal changes to county boundaries or names are typically infrequent. Changes that have occurred since the 1990 Census are documented at: http://www.census.gov/popest/data/historical/1990s/boundary_changes/90s_boundary_changes.html. These notes also include information on changes to the areas described below.
Legally defined county subdivisions are referred to as minor civil divisions (MCDs.) MCDs are the primary divisions of a county. They comprise both governmentally functioning entities -- that is, those with elected officials who provide services and raise revenues -- and nonfunctioning entities that exist primarily for administrative purposes, such as election districts. Twenty-eight states and Puerto Rico have MCDs. However, the MCDs function as general purpose governmental units in all or part of only twenty states. Within these twenty states, PEP produces estimates for all governmentally functioning MCDs and for nonfunctioning MCDs in counties that contain at least one functioning MCD.
The legal powers and functions of MCDs vary from state to state. Most of the MCDs in twelve states (Connecticut, Maine, Massachusetts, Michigan, Minnesota, New Hampshire, New Jersey, New York, Pennsylvania, Rhode Island, Vermont, and Wisconsin) serve as general-purpose local governments. In the remaining eight states for which PEP produces MCD level estimates (Illinois, Indiana, Kansas, Missouri, Nebraska, North Dakota, Ohio, and South Dakota) the MCDs, for the most part, perform less of a governmental role and are less well known locally, even though they are active governmental units.
MCDs primarily are known as towns (in New England, New York, and Wisconsin), townships, and districts, but also include a variety of other entities. In Maine and New York, American Indian reservations are not part of any other MCD and therefore, the Census Bureau treats them as MCDs. PEP does not produce separate estimates for American Indian Reservations regardless of their MCD status. In some states, all or some incorporated places are subordinate to the MCDs in which they are located. Therefore, a place may be either independent of or dependent upon MCDs. In one state (Ohio), a multi-county place may be treated differently from county to county. The District of Columbia defined no MCDs for the 1990 census, so the District itself serves as the equivalent of an MCD for data presentation purposes. No functioning MCDs exist in Puerto Rico.
The legal designations, powers, and functions of incorporated places vary from state to state. Incorporated places include cities, towns (except in New England, New York, and Wisconsin where the Census Bureau recognizes towns as MCDs for census purposes), boroughs (except in Alaska, where the Census Bureau recognizes boroughs as equivalents of counties, and New York, where the Census Bureau recognizes the five boroughs that constitute New York City as MCDs) and villages. Incorporated places can cross both county and MCD boundaries. When this occurs, the place name is followed by the designation "pt" (which stands for part). The PEP produces estimates of the unincorporated "balance of county" area for counties that are not entirely composed of incorporated places. Another way to understand this is to think of the "balance of county" as the county population minus the county population resident within incorporated places.
Consolidated cities are a unit of government for which the functions of an incorporated place and its county or MCD have merged. The legal aspects of this action may result in both the primary incorporated place and the county or MCD continuing to exist as legal entities, even though the county or MCD performs few or no governmental functions. Where one or more other incorporated places within the consolidated government continue to function as separate governmental units, the primary incorporated place is referred to as a "consolidated city."
Estimates are not shown for consolidated cities. Rather, estimates are displayed for the consolidated city "remainder," which is the consolidated city minus the semi-independent incorporated places located within the consolidated city. Consolidated cities include: Butte-Silver Bow, MT; Athens-Clark County, GA, Augusta-Richmond County, GA, Columbus, GA; Indianapolis, IN; Jacksonville, FL; Milford, CT; and Nashville-Davidson, TN. Estimates also are produced for the semi-independent places which together with the "remainder record," sums to the entire territory of the consolidated city.
A more complete narrative treatment of these areas is found in the Geographic Areas Reference Manual: http://www.census.gov/geo/www/garm.html authored by the Census Bureau's Geography Division. This manual provides a comprehensive description of all the geographic entities recognized and reported in the Census Bureau's various Censuses and Surveys.