Population Estimates: Concepts
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
(2000), 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.
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.
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, the population estimates program revises estimates for years
back to the last census. Previously released estimates are superseded.
Revisions to estimates are usually due to revised or updated administrative input data, changes in
methodology, or legal boundary changes, especially for subcounty units of government, such as incorporated places. The frequency of estimates and
availability of demographic detail vary by geographic level.
Post-Census 2000 Change vs. 1990 to 2000 Change - Why Do the
Trends Differ?
Comparisons made between the census results for 1990 and 2000 can show
trends that vary widely from trends that appear in the post-2000 population
estimates. In addition to real change due to change in demographic events,
i.e. births, deaths, migration, there are additional factors contributing
to these differences:
Geographic Boundary Updates
In many cases a legal entity has had substantial changes to its boundaries
between 1990 and Census 2000. In these cases, a comparison of the 1990
census result with the Census 2000 count shows population change due not
only to demographic events (births, deaths and migration) but due also to
geographic events (annexations, deannexations, etc.).
In contrast, all boundary updates in the post-Census 2000 population
estimates are reflected in the 2000 population estimates base and not
the year in which the change occurred. Therefore the post-census
population estimates show population change caused by demographic
events only. A boundary change will make the estimates base for an
area larger or smaller, but it will not cause a population estimate
to show population growth over time. Population Division produces
tables that include both the Census 2000 count and the 2000 population
estimates base and indicate the amount of population change due to
boundary changes.
Changes in Census Coverage
Differences in census coverage between the 1990 and 2000 censuses for a
specific geographic area may distort the true demographic change during
the decade. For more information on estimates of census coverage, see
www.census.gov/dmd/www/EscapRep.html.
Geographic Terms and Definitions - for specific definitions of the types of areas estimated
and other geographic terms, please see:
geographic terms & definitions
For specific definitions of terms such as the population base, births,
deaths, immigration, group quarters population, and much more, see:
national terms and definitions
state and county terms & definitions |
