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. __________________________________________________________________ APPENDIX TABLE 1 Derivation of 1996 Under 65 Population Estimate for a Hypothetical County ________________________________________________________________________ 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