METHODOLOGY for ESTIMATES of STATE and COUNTY TOTAL POPULATION
(Used for 1990 to 1999 estimates.)
Source: U.S. Census Bureau
Revised 4/1/00
In the production of state population totals, states are treated
as a tabulation geography rather than an estimates geography.
This means the state estimates methodology is actually the
methodology for counties, and the state population estimates are
derived by summing the county estimates to the state level. For
population estimates purposes, the District of Columbia is treated as
a county equivalent.
Brief Summary of Methodology:
County population estimates are created by starting with the
most recent decennial census figure (April 1, 1990) and
updating that figure with information on births, deaths,
domestic migration (in/out flows with other counties in the U.S.),
and international migration (in/out flows with other countries)
that have between the census date and the date of the population
estimate. Birth and death data are obtained through vital statistics,
domestic migration is estimated through the address matching of
federal tax returns, and international migration data is supplied
by the Immigration and Naturalization Service.
Detailed Narrative:
The Census Bureau develops county estimates with a demographic
procedure called a "component change" method. A major assumption
underlying this approach is that the components that constitute
population change can be represented by administrative data
series in a statistical model. In order to build the model,
Census Bureau demographers estimated each component of population
change separately. For the population residing in households the
components of change are births, deaths, and net migration,
including net immigration from abroad. For the nonhousehold
population, change is represented by net change in the population
in group quarters. We have listed each of these components in
Appendix Tables 1-3 and cover them item by item in the
following text. Appendix Tables 1-3 show the derivation of a
July 1 population estimate for a typical county in a typical
estimate year. Note that this description of the population
estimates process is generic for a typical estimates year. The
process may not be applied exactly as described to an individual
county in a specific estimate year.
To develop county population estimates, we use a component change
procedure called the Tax Return method (formerly called the
Administrative Records method). Each component in our model is
represented with data that are symptomatic of some aspect of
population change. For example, birth certificate(s) are
symptomatic of additions to the population resulting from births,
so we use these data to estimate the birth component for a
county.
Except for the net migration component, we calculate the change
components for a July 1 county estimate from data items that are
extrapolated. We extrapolate because data needed for the
estimate year, covering the period from July 1 of the previous
year to June 30 of the current estimate year, are not always
available for counties at the time we develop the estimate. For
example, we could not obtain 1999 births for January through June
for most counties because the data for 1999 were not available at
the time we prepared the estimate.
When we do not have data for all counties for the current
estimate year, we develop an estimate of the data component
through one or more simplifying assumptions. In the simplest
case, we assume there is no change in the data between the
current estimate year and the prior estimate year. In other
cases, we may have data at the state level but not at the county
level. We assume that the distribution of data by county did not
change from the prior year. The county distribution is then
applied to the current state total data to estimate current year
data for counties. For example, we had total births for states
for 1993, but have total births for counties only for 1992. We
apply the 1992 county distribution of births within states to the
1993 state totals to estimate county totals for 1992. We always
examine the birth data for the previous year very carefully to ensure
that the current year estimate is reasonable. When we prepare
our initial population estimates, we use the same variant of the
component model for all states. A year later, we replace these
initial estimates with "revised" estimates based on the actual
data for all components of population change.
In the discussions below, line numbers refer to a hypothetical
county population estimate in a typical estimate year, shown in
Appendix Tables 1-3.
Explanation of Appendix Table 1. Estimate of the Population Under Age 65
Base Populations
Appendix 1--Line 1
Base total population--The base population for the estimate of
the population under age 65 is the revised county estimate for
the prior estimate year. Each year, the population estimate
represents population change from the prior year. The only year
in which this is not true is in the year of the decennial census.
In the decennial year, an estimate is prepared that represents
population change between the census date and July 1 of that
year. For example, the last decennial census was collected as of
April 1, 1990. A population estimate was prepared that
represented population change between April 1, 1990 and July 1,
1990. For official population estimates, the April 1, 1990
population is not adjusted for census undercount.
a. Census errata--Geographic and enumeration errors in the
1990 census for some areas have been identified and subsequent
corrections made. These errors may have an impact on population
estimates in some counties. Corrections to the 1990 census were
first incorporated into the estimates series with the release of
estimates for July 1, 1994.
b. Census modifications--The 1990 census population counts
were modified to handle two areas problematic for estimates
development: (1) We modified the census results so that persons
responding "other" to the 1990 census race question were
reclassified into one of the four major race classifications:
White, Black, Asian/Pacific Islander (API) or American
Indian/Alaska Native (AIAN); (2) The population age distribution
was also modified, owing to a misreporting problem. For more
information about these modifications, call the Census Bureau at
(301) 457-2422 and ask for a copy of the technical documentation
developed for the modified age/race/sex file (MARS).
c. Boundary changes--Boundary changes include new
incorporations, disincorporations, annexations, deannexations,
mergers and consolidations. In general, county population
estimates are consistent with boundaries current as of the
estimate date. Boundary changes occurring after January 1, 1990
were incorporated into the population estimates base beginning
with estimates for July 1, 1994.
Appendix 1--Line 2
Base group quarters population under age 65--This component is
primarily a combination of military personnel living in barracks,
college students living in dormitories and persons residing in
institutions. Inmates of correctional facilities, persons in
health care facilities and persons in Job Corps Centers are also
included in this category. We have excluded persons aged 65 and
over residing in nursing homes and other facilities from this
category since they are implicitly included in the estimate of
the 65+ population. The base group quarters population for the
current estimate year is the revised group quarters population
from the prior estimate year. In the first estimate year
following the decennial census, the base group quarters
population is the group quarters population as enumerated in the
decennial census.
Appendix 1--Line 3
Base total population aged 65 years and over--This component is
the revised estimate of the population aged 65 years and older
from the prior estimate year.
Appendix 1--Line 4
Household base population under age 65--After we subtract the
group quarters populations (Line 2) and the population aged 65
and over (Line 3) from the base population (Line 1) the remainder
is what we call the under 65 household population. The household
population under age 65 is also decremented by those persons aged
64 and over who will turn 65 (expressed as a factor) during the
estimates cycle. This process of estimating the population aged
64 years old for counties is achieved through the following
steps:
1. Develop a county-level ratio of the population aged 64
to the population 65 and older in the last census.
2. Develop a national-level ratio of the population aged 64
to the population 65 and older in the last census.
3. Develop a national-level ratio of the population aged 64
to the population 65 and older for each estimate year.
4. Update the county-specific ratios with the change in the
national level ratios (estimate year/census base)
5. Apply the county-specific ratios to the household
population under 65 in the estimates base.
Components of change for the household population under age 65
Appendix 1--Line 5
Estimated resident births, 7/1 (prior year) to 6/30 (estimate
year)--Resident births are recorded by residence of mother,
regardless of where the birth occurred; hence, a county need not
have a hospital to have resident births. If birth data are not
available by county for all states for the estimate year when the
county estimates are produced, we assume that prior year county
birth data approximate estimate year births.
Appendix 1--Line 6
Estimated resident deaths to the household population under 65,
7/1 (prior year) to 6/30 (estimate year)--We use death data
tabulated by the most recent residence of the decedent, not by
the place where death occurred. We tabulate NCHS (National
Center for Health Statistics) deaths to the population under 65
years, by race, and control these to state specific tabulations
provided by the FSCPE (Federal State Cooperative Program for
Population Estimates) agencies. We then rake the estimated
deaths so they sum to national death totals by race. If estimate
year death data are not available by county for all states when
the county estimates are produced, we assume that prior year
county death data approximate estimate year deaths. Our estimate
of deaths for the under 65 years of age population is shown on
Line 6.
Appendix 1--Line 7
Estimated net movement from abroad (Immigration), 7/1 (prior
year) to 6/30 (estimate year)--We estimate the number of foreign
immigrants who move into the county during the estimate interval.
The estimate is based on the national estimate of foreign
migration developed by the Census Bureau. Our estimate includes
emigration from the United States and the immigration of
refugees, legal immigrants, undocumented immigrants, net movement
from Puerto Rico, and federal and civilian citizen movement from
abroad. We allocate the national estimate of the number of
undocumented immigrants to states and counties by using the
distribution of the foreign born population who arrived between
1985 and 1990 and were enumerated as residents in the 1990
census. Legal immigrants and refugees are distributed to
counties on the basis of county of intended residence as reported
to the Immigration and Naturalization Service.
Appendix 1--Line 8
Estimated migration base--We develop the migration base by adding
one half of the following to the household base population under
65 years (Line 4): estimated resident births (Line 5), minus
estimated resident deaths under 65 years (Line 6), plus estimated
net immigration (Line 7). Only half of the additions/ deletions
to the population would have taken place by the midpoint of the
twelve months. A factor of one-half, or an "exposure factor"
must be entered into the equation. The population base at risk
of migrating is usually considered the base at the midpoint of
the period since the population at the beginning of the estimate
period has not yet experienced the births and deaths that are
reflected in the population at the end of the period. The
population at the end of the period includes inmigrants and
excludes outmigrants. The best compromise is to take the
population at the midpoint of the period. Estimated resident
births, estimated deaths to people under age 65, and net
immigration from abroad are assumed to have been evenly
distributed throughout the estimate interval and, therefore,
exposed to the potential for migration, on average, for one-half
of the period.
Appendix 1--Line 9
Estimated migration rate--We use individual Federal income tax
returns to measure the internal, or domestic, migration of the
population under 65 years of age, by matching the returns for
successive years for that age group. We determine the status of
the filer by noting the address, used as a proxy for place of
residence, on tax returns filed in the prior year and in the
estimate year. We categorize the filers in each county into: (1)
inmigrants (INS), (2) outmigrants (OUTS), and (3) nonmigrants
(NONMOVERS). We derive a net migration rate for each county,
based on the difference between the inmigration and outmigration
of tax filers and his or her dependents.
Appendix 1--Line 10
Estimated net domestic migration--Net migration is the product of
the migration base (Line 8) and the net migration rate (Line 9).
If this figure is preceded by a minus sign (-), then the figure
indicates net outmigration; otherwise, the figure represents net
inmigration.
Estimated populations
Appendix 1--Line 11
Estimated household population under age 65--The household base
population under age 65 (Line 4) is combined with the estimated
components of change for the household population under age 65 to
arrive at the estimated household population under age 65 in the
estimate year.
Appendix 1--Line 12
Estimated group quarters population under age 65--We assume that
military personnel living off base and those living on base in
family quarters are captured in the components of change of the
household population, described above. Military barracks
population figures and crews of naval vessels are obtained from
an annual survey of on-base housing facilities for unaccompanied
personnel. The survey is conducted by the Department of Defense
(DOD) in September each year. Certain states also collect
military barracks population for their states and those data are
used where appropriate. If Department of Defense survey results
are not available in the estimate year, the prior year results
are used as estimates for the current estimate year. College
students living in dormitories, inmates of correctional and
juvenile facilities and persons in health care facilities,
nursing homes and Job Corps Centers are also included in this
estimate. We exclude persons aged 65 and over residing in
nursing homes and persons in homes for the aged from this
estimate since they are implicitly included in the separate
estimate of the 65+ population. Data on college dormitory
populations relate generally to the fall of the preceding year.
Data on institutional populations relate to July 1 of the
estimate year or represent some type of average for the estimate
year. Data on the nonmilitary components of the group quarters
population are collected primarily through an annual group
quarters report submitted by state members of the Federal-State
Cooperative Program for Population Estimates. The group quarters
report is used to estimate the change in the group quarters
population between the prior year and the estimate year. If no
data are available for any component of the group quarters
population, we assume that no change has occurred. In the first
estimate year following a decennial census, the group quarters
report is used to estimate the change in group quarters
population between the census and the estimate year.
Appendix 1--Line 13
Estimated total population under age 65--The estimated total
population under age 65 is the estimated household population
under age 65 (Line 11) and the estimated group quarters
population under 65 (Line 12).
Explanation of Appendix Table 2. Estimate of the Population Aged 65 and Over
Base Populations
Appendix 2--Line 1
Base total population aged 65 and over--The base population for
the estimate of the population age 65 and over is the revised
household population estimate of the 65 and over population for
the prior estimate year. We use the county-level tabulations of
the number of medicare enrollees obtained from the Health Care
Financing Administration to assist us in developing the estimate.
The availability of this data allows us to develop a separate
estimate of change in the population over age 64. If the
Medicare enrollment data are not available at the time we prepare
the estimates, we use the change from the prior estimate year as
an estimate of change in the current estimate
Appendix 2--Line 2
Base group quarters population aged 65 and over--This component
is an estimate of the population aged 65 and over residing in
nursing homes, prisons and other group quarters facilities.
Appendix 2--Line 3
Estimated population turning 65 in current year--This component
is an estimate of the population who reached their 65th birthday
during the estimate year. They are in a sense the number of
people "born" into the 65 and older age group.
Appendix 2--Line 4
Household base population aged 65 and over--This component is
calculated by subtracting the group quarters population (Line 2)
and adding the population turning age 65 in the current year
(Line 3).
Components of change for the household population aged 65 and over
Appendix 2--Line 5
Estimated resident deaths to the household population aged 65 and
over, 7/1 (prior year) to 6/30 (estimate year)--Our estimate of
deaths for the population aged 65 and older is shown on Line 5.
See the explanation of under 65 deaths for more detail.
Appendix 2--Line 6
Estimated foreign migration 65+, 7/1 (prior year) to 6/30
(estimate year)--Same type of calculation as under 65 foreign
immigration (Appendix Table 1, Line 7)
Appendix 2--Line 7
Estimated migration base 65+--Same type of calculation as under
65 migration base (Appendix Table 1, Line 8)
Appendix 2--Line 8
Estimated migration rate 65+--The migration rate for the
population aged 65 and older is obtained by
MIGRO={MEDt-[MEDt-1+((AGEt-DEAOt)*MCOV)]}/MEDt-1
where MED is Medicare enrollees, Age is the Population turning 65
in the current year, DEAO is the Period deaths to the population
65 and over and MCOV is the Medicare Coverage (Medicare coverage
is defined as Medicare enrollees aged 65 and over in 1990 divided
by the Census population aged 65 and over).
Appendix 2--Line 9
Estimated net migration 65+--Same type of calculation as under 65
net migration (Appendix Table 1, Line 10)
Appendix 2--Line 10
Estimated household population aged 65 and over--The household
base population aged 65 and over (Line 4) is combined with the
estimated components of change for the household population aged
65 and over to arrive at the estimated household population aged
65 and over in the estimate year.
Appendix 2--Line 11
Estimated group quarters population aged 65 and over--Persons
aged 65 and over residing in nursing homes, correctional
facilities and other group quarters. See under 65 calculation.
Appendix 2--Line 12
Estimated total population aged 65 and over--The estimated total
population aged 65 and over is the sum of the estimated household
population aged 65 and over (Line 10) and the estimated group
quarters population 65 and over (Line 11).
-Explanation of Appendix Table 3 Final Total Population Estimate-
Appendix 3--Line 1
Estimated total population under 65--Copied from Appendix Table
1, Line 13
Appendix 3--Line 2
Rake factor for the population under age 65 (Line 2)--The rake
factor is used to ensure consistency between county estimates and
independent estimates for the entire population of the United
States. The factor is the national estimate of the total
population under age 65 divided by the sum of the estimated total
population under age 65 for all counties in the nation.
Appendix 3--Line 3
Final under age 65 estimate--The final under age 65 population is
the estimated total population multiplied by the rake factor.
Appendix 3--Line 4
Estimated total population aged 65 and over--Copied from Appendix
Table 2, Line 12.
Appendix 3--Line 5
Rake factor for the population aged 65 and over--The rake factor
is used to ensure consistency between county estimates and
independent estimates for the entire population of the United
States. The factor is the national estimate of the total
population aged 65 and over divided by the sum of the estimated
total population aged 65 and over for all counties in the nation.
Appendix 3--Line 6
Final estimate for the population aged 65 and over--The final 65
and older population is the estimated total population multiplied
by the rake factor.
Appendix 3--Line 7
Final total estimate--The final estimate is the sum of the under
65 estimate and the 65 and over estimate.
Final State Components of change
The components of change shown in Table 2 result from summing the
under and over 65 age segments of each (except births) county/
state component.
The net domestic migration shown also includes changes in group
quarters for both the under and over 65 population. The residual
shown is from the effect of the national proration procedure. It
is the difference between the implementation of the national
estimates model and the subnational model.
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APPENDIX TABLE 1
Derivation of 1996 Under 65 Population Estimate for a
Hypothetical County
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Value Derivation or Source
-------------------------------------------------------------------------
BASE POPULATIONS
1. Base population............ 93,401 Revised estimate from prior
year
2. Base group quarters
population under age 65.... 5,660 See text for detailed source
3. Base population age 65
years and over............. 4,021 See text for detailed source
4. Household base population
under age 65 years......... 83,705 (4)=(1)-(2)-(3)-[(.00362)x(3)]
COMPONENTS OF CHANGE FOR THE HOUSEHOLD POPULATION UNDER AGE 65
5. Estimated resident births:
7/1 (prior year) to
6/30 (estimate year)....... 1,924 See text for detailed source
6. Estimated resident deaths
to the household population
under age 65 years......... 157 See text for detailed source
7. Estimated immigration:
7/1 (prior year) to
6/30 (estimate year)....... 164 See text for detailed source
8. Estimated migration base ... 84,671 (8)=(4)+0.5 x [(5)-(6)+(7)]
9. Estimated migration rate.. -0.00943 See text for detailed source
10. Estimated net migration....... -798 (10)=(8) x (9)
11. Estimated household pop.
under age 65................ 84,838 (11)=(4)+(5)-(6)+(7)+(10)
12. Estimated group quarters
population under age 65..... 5,660 See text for detailed source
13. Estimated total population
under age 65.................90,498 (13)=(11)+(12)
__________________________________________________________________________
APPENDIX TABLE 2
Derivation of 1996 65 and Over Population Estimate for a
Hypothetical County
_________________________________________________________________
Value Derivation or Source
-----------------------------------------------------------------
BASE POPULATIONS
1. Base total population
aged 65 and over.............4,021 7/93 Population Estimate
2. Base group quarters
population aged 65 and over....642 See text for detailed source
3. Estimated population turning
65 in current year.............225 See text for detailed source
4. Household base population
aged 65 and over.............3,604 (4)=(1)-(2)+(3)
COMPONENTS OF CHANGE FOR THE HOUSEHOLD POPULATION AGED 65 AND OVER
5. Estimated resident deaths
to the household population
aged 65 and over...............168 See text for detailed source
6. Estimated foreign immigration:
7/1/95 to 6/30/96...............21 See text for detailed source
7. Estimated migration base.....3,531 (7)=(4)+0.5 x [(6)-(5)]
8. Estimated migration rate 0.0317236 See text for detailed source
9. Estimated net migration........112 (9)=(7) x (8)
10. Estimated household population
aged 65 and over.............3,569 (10)=(4)-(5)+(6)+(9)
11. Estimated 1994 group
quarters population............586 See text for detailed source
12. Estimated total population
aged 65 and over.............4,155 (12)=(11)+(10)
APPENDIX TABLE 3
Final Estimate for a Hypothetical County
_________________________________________________________________
Value Derivation or Source
-----------------------------------------------------------------
1. Estimated total population
under 65.........................90,498 Line 13 from Table 1
2. Rake factor for the
population under 65............1.000435 See text for explanation
3. Final estimate for the
population under 65..............90,537 (3)=(2) x (1)
4. Estimated total population
aged 65 and over..................4,155 Line 12 from Table 2
5. Rake factor for the
population aged 65 and over....1.001034 See text for explanation
6. Final estimate for the
population aged 65 and over.......4,159 (6)=(4) x (5)
7. Final total estimate.............94,696 (7)=(3)+(6)
For more information on the methodology used for creating estimates
of total population for states and counties, contact the Population
Estimates Branch, located within the Population Division
of the Census Bureau. (301) 457-2385