What is a population estimate?
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.
How are estimates different from projections?
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.
Revisions to estimates and geographic detail
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 Previously released estimates become superseded.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.
Why does the Census Bureau produce estimates?
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.