1. Overview
2. Cohort Component Method
3. Base Data on Population
4. Base Data on Mortality
5. Base Data on Fertility
6. Base Data on International Migration
7. Assumptions About the Population Projections
a. Projecting Mortality and Fertility
b. Projecting International Migration
8. Population Estimates and Projections for the United States
9. References
The U.S. Census Bureau has been preparing estimates and projections of the population of foreign countries for over 35 years. In 1985 the Census Bureau offered it's first comprehensive estimates and projections for over 200 countries and areas of the world. Since then, the Census Bureau has routinely updated countries as new data have become available.
The estimation and projection process involves data collection, data evaluation, parameter estimation, preparation of assumptions, and final projection of population for each country. Census Bureau demographers collect data from censuses, surveys, and vital statistics provided by National Statistics Offices, as well as data on international migration and refugee movements. Available data are evaluated, paying particular attention to internal and temporal consistency. Raw data are used to estimate fertility, mortality and migration parameters needed for population projection, a process that makes use of a variety of demographic techniques. Census Bureau demographers consider other information, such as public health efforts, socio-political circumstances, and historical events such as natural disasters and conflict in preparing the assumptions feeding into population projections. Finally, best available estimates and assumptions and the Census Bureau's Rural/Urban Projection (RUP) program are used to project population, by single years of age, for each calendar year beyond an initial, or base, year. Regional and world populations are obtained by projecting each country's population separately and then combining the results to derive aggregated totals.
The cohort-component method of population projection follows each group of people of the same age and gender throughout its lifetime, exposing it to assumed age- and sex-specific mortality, fertility, and migration.
An initial, or base year, population, disaggregated by age and sex, is exposed to estimated age-sex-specific chances of dying as determined by projected mortality levels and age patterns. Once deaths are estimated, they are subtracted from each age group, yielding the next older age group in the next time period.
Fertility rates are projected and applied to the female population of childbearing age to estimate the number of births every year. Each cohort of children born is also followed through time and survivors are calculated after exposure to mortality.
Finally, the component method takes into account in-migrants who are incorporated into the population and out-migrants who leave the population. Migrants are added to or subtracted from the population at each specific age. The whole procedure is repeated for each year of the projection period, resulting in the projected population by age and sex, as well as birth and death rates, rates of natural increase, rates of population growth, and other summary measures of fertility, mortality, and migration for each year.
For many developed countries, base data on population are taken from population registers or current official estimates prepared by the national statistical offices based on a census for an earlier year. For developing countries, the base population for a projection is based on census data. However, census enumerations are not perfect, and reported data on the population age and sex structure may be affected by age misreporting and by underenumeration of people of certain ages. If the projection starts with errors in the base year, such errors will be carried throughout the projection period and will have an impact on the projected number of births as well.
Consequently, before being accepted to serve as a base for the projections, a population must be evaluated, to detect errors, and adjusted, as necessary, to correct them. Various methods have been developed to detect age misreporting, including analysis of digit preference, age ratios, and sex ratios. Techniques have been developed for making any needed corrections. Depending on the country-specific data problems, mild smoothing or strong smoothing techniques may be recommended.1 The base population age and sex structures for most developing countries are at least slightly smoothed for the population ages 10 years and over.
Special attention is given to possible underenumeration of the youngest age groups, 0-4 years and 5-9 years, because errors in these ages may have a significant impact on the total projection and are frequently found. Suppose, for example, that children age 0-4 years were undercounted in the base population. In the projection, not only would the surviving cohorts of these children be smaller than they should be, but when the female cohorts reach reproductive ages, their number of births would also be underestimated. The completeness of enumeration of individuals in the youngest age groups is evaluated by checking for consistency between the number counted and the estimated levels of fertility and mortality during the 10-year period prior to the census date, as children of these ages represent the survivors of births during that period.
When vital registration data are available and complete, it is easy to construct life tables using microcomputer programs, and thereby to derive both a level and an age pattern of mortality suitable for the projection process. For most developing countries, however, it is necessary to estimate mortality some other way. Data on deaths may be available from vital statistics registers, as well as from surveys or censuses that include questions concerning deaths during a specific period of time, for example, deaths of any household members during the past year. If registered deaths can be evaluated and adjusted for errors, they can be used to obtain valuable information about the level and pattern of mortality.
Several techniques have been developed for estimating underregistration of deaths.2 Some of them are based on the assumption that the population is "stable." A stable population is one in which there has been no migration, and neither fertility nor mortality has changed in the past. Other techniques, developed more recently, do not require the assumption of stability. Some methods may be applied to estimate mortality during the first years of life.3 They are based on data on children ever born and children surviving, by age of mother.
Like mortality in infancy and childhood, mortality in adult ages can be estimated indirectly when reliable data are not available to measure it directly. Two principal techniques have been developed to estimate adult mortality based on information collected in censuses or surveys: the orphanhood technique, based on the number of people whose mother or father has died; and the widowhood technique, based on the number of people whose first spouse has died. Both provide an estimate of survivorship levels between two adult ages for a period of time prior to the year of data collection. However, these techniques are seldom used for the base mortality patterns of the projections because the reference period to which the estimated mortality pertains is not well defined.
As in the case of mortality, procedures for estimating fertility depend on the availability of data and on the level and type of detail of the information. For cases where vital registration is complete, fertility can be measured directly using classical procedures. Most developing countries, however, do not have reliable vital statistics, so techniques have been developed to measure fertility indirectly based on census or survey information.
Using the age structure of the population, the crude birth rate is sometimes estimated by the rejuvenation technique, in which the population at the youngest ages is "reverse survived" to determine the number of births from which they are survivors. This technique is attractive because it does not require the collection of data related specifically to fertility. However, the reliability of the estimate depends on the quality of both the census data on age and the survival ratios used for the rejuvenation.
Under certain circumstances, census data by age can be used to obtain not only a crude birth rate but age-specific fertility rates as well. This is done by using the own-children technique based on information on children and women by single years of age. This technique requires data linking individual children to their natural mothers.
Other techniques, such as the Rele technique, use census data by age to calculate the net reproduction rate or total fertility rate based on the relationship of children of specified ages to the number of women in childbearing ages.
Finally, and most importantly for many developing countries, many censuses and surveys include questions related specifically to fertility, for example, the number of children women have had and whether they had a birth in the year preceding the inquiry. Responses to such questions can be used to estimate fertility indirectly. Some techniques to do this include the P/F (Parity/Fertility) ratio developed by Brass, based on the average number of children ever born to women in 5-year age groups and women's age pattern of fertility derived from births in the year preceding the census or survey; and the Arriaga technique, which is similar to the P/F ratio technique but links data for more than one date. While the Brass P/F ratio method assumes constant fertility in the past, the Arriaga method does not. All of these methods can be used to estimate the age-specific fertility rates required for making component population projections.
6. Base Data on International Migration
Although migration is an important component of population change, it is not generally well recorded except in some European countries, such as Sweden and the Netherlands, that maintain complete and detailed population registers. Some countries collect information on arrivals and departures of passengers at the official borders of the national territory, but such data are seldom processed in a way that renders them useful for statistical purposes. Even in countries with otherwise excellent statistical systems, information on international migration is often unreliable. The primary source of information on immigration for purposes of population projections is census data on place of birth of the foreign-born population. To detect emigration as well, in order to calculate the net movement in or out of a country, it is necessary to find data for the countries in which the emigrants have settled (since they are the foreign immigrants of that country). In addition, special migration flows, such as refugee movement, are incorporated by considering reported numbers of refugees from the United Nations High Commissioner on Refugees, country sources, and media reports. Thus, most data on international migration are educated guesses, especially since not only total numbers but also age and sex distributions of the migrants are required for the projection process.
7. Assumptions About the Population Projections
Once levels of mortality, fertility, and migration have been determined for the base year of the projection, each component must be projected into the future. Although the procedure for doing this is mechanical, care must be taken in determining projected levels, trends, and patterns by age. Not only must the assumptions be appropriate for the particular country in question, but consistent assumptions must be made when projections are being carried out for more than one country.
An expected increase in contraceptive prevalence is implicit in the assumptions about future fertility declines for most developing countries. For many developed countries, future fertility levels are projected to experience only minor change, either slight decreases or slight increases.
In general, mortality is expected to continue to decline in most countries, as economic development occurs and health care improves. A particular exception relates to the impact that acquired immune deficiency syndrome (AIDS) will have on the mortality of some countries, where mortality levels in the next decade are expected to increase. (For a description of the method used to incorporate the impact of AIDS mortality on selected populations, see the Population Estimates and Projections including AIDS page.) While there is no single "right" way to make assumptions about the future, the following procedures are those recommended and generally used by the Census Bureau.
a. Projecting Mortality and Fertility
To project mortality and fertility, available data on past trends in life expectancy at birth and total fertility rate are considered. The trend in future levels of these indicators can be approximated using a logistic function.
Life expectancy at birth is projected by fitting a fixed-slope logistic curve to the most recent estimate of life expectancy at birth.4 If estimates of total fertility rates are available for more than one date in the past, a logistic function can be fitted to these data. The results of the logistic function must be carefully scrutinized, however, to ensure that they yield an acceptable future target for the individual country's circumstances.
In some instances, no data on past trends in fertility are available to which a logistic curve can be fitted. When trend data are not available for estimating future changes using a logistic function, the past experience of other countries serves as a guideline to determine the pace of future change.
Recent population and socioeconomic trends and policies of each country are taken into account to determine if the projected trends are plausible. For example, for mortality, information concerning programs of public health is considered in judging the results. For fertility, factors such as trends in age at marriage, the proportion of women using contraception, the strength of family planning programs, and any foreseen changes in women's educational attainment or in their labor force participation in the modern economic sector are considered.
The next task is to determine an age pattern of mortality and fertility for each of the projected values, since these patterns tend to vary as overall levels change. For each level of projected life expectancy at birth, a set of central death rates is estimated using an iterative interpolation process. The interpolation is logarithmic and uses a set of central death rates for the base year and a "limit" set of rates with very low mortality. Life tables constructed with the interpolated rates correspond to the life expectancies at birth projected previously. Age-specific fertility rates for each projected level of total fertility rate are interpolated between the set for the base year and an ultimate pattern of fertility derived at the International Programs Center using empirical data for countries with low fertility.
Once mortality and fertility have been tentatively projected for each country according to its particular circumstances, the estimates are compared with projected values for other countries in the same region and with those for other regions. Differences are evaluated to make sure they exist for valid reasons that can be explained by known peculiarities of the particular countries.
Finally, in recent years the Census Bureau has concluded that distinctive mortality assumptions must be made for selected countries because of the death risk due to AIDS. Using methodology that takes into account the effect of AIDS, country projections have been prepared that assess its impact on future populations in countries where the infection is significant (see Population Estimates and Projections including AIDS page).
b. Projecting International Migration
Assumptions about future migration are generally much more speculative than assumptions about fertility and mortality. International migration may occur as a result of changing economic conditions, political unrest, persecutions, famines, and other extreme conditions in the countries of origin. Thus, individuals may be attracted by economic opportunities perceived to be available in more industrialized societies or refugees may flee in large numbers.
Due to the unpredictability of conditions such as crop failure, emerging violence, and war, migration forecasts are subject to large errors. If migration is known to have a negligible impact on a country's current growth rate, future migration is often assumed to be nil. If a country's migration is known to be significant, the estimated number of migrants during the past is frequently held constant in projecting to the near future. The age and sex composition of international migrants depends on the situation in each country. If information is not available, model patterns by age and sex are sometimes used.
8. Population Estimates and Projections for the United States
The estimates and projections presented in the IDB take into account both the population count from Census 2000 as well as vital statistics and immigration data made available as of mid-2002. However, the current projections for the United States are interim projections, prepared in 2002 without undergoing a full and final Census Bureau review.
Links to the latest U.S. projections can be found at: http://www.census.gov/population/www/projections/popproj.html.
Arriaga, Eduardo E., and Associates. 1994. Population Analysis with Microcomputers. U.S. Census Bureau, International Programs Center: Washington, DC.
Population Estimates and Projections Guidelines. 2001. U.S. Census Bureau, International Programs Center: Washington, DC. Unpublished.
Notes:
1 Arriaga and Associates (1994: 39-42).
2 For example, the Preston-Coale technique, the growth balance technique developed by Brass, and the Bennett-Horiuchi technique.
3 For example, the Brass technique and modifications developed by Trussell, Sullivan, and Feeney; the Palloni-Heligman technique, and the Johnson technique.
4 The fixed-slope model of change in sex-specific life expectancy at birth in a population assumes that future change follows a fixed logistic curve from its initial level. The relationship is based on empirical research undertaken at the International Programs Center, U.S. Census Bureau, in 1998-1999.
Note: Data updated 3-27-2008 (Release notes).
Source: U.S. Census Bureau, Population Division
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