What are population projections?
Projections are estimates of the population for future dates. They illustrate plausible courses of future population change based on assumptions about future births, deaths, international migration, and domestic migration. Projected numbers are based on an estimated population consistent with the most recent decennial census as enumerated, projected forward using a variant of the cohort-component method.How are estimates different from projections?Several alternative series of projections are produced based on alternative assumptions for future fertility, life expectancy, net international migration, and (for state-level projections) state-to-state or domestic migration. For each of these components of population change--fertility, mortality and net migration--three different assumptions about the future are applied. The series using the middle assumption for each component--generally designated as the "middle series"--is the most commonly used. Population projections are produced by age, sex, race and Hispanic origin, and are released by the Census Bureau every two years.
While projections and estimates may appear similar, there are some distinct differences between the two measures. Estimates are for the past, while projections are based on assumptions about future demographic trends. Estimates generally use existing data collected from various sources, while projections must assume what demographic trends will be in the future. At the Census Bureau, starting points for population projections are the most recent estimates available at the time the projections are produced. In our current product offerings the user may see both an estimate and a projection available for the same date (e.g., July 1997), which may not agree because they were produced at different times. In such cases, estimates are the preferred data, unless the users' objective is to compare the number with others in the projected series.What is the cohort-component method?
In the cohort-component method, the components of population change (fertility, mortality, and net migration) are projected separately for each birth cohort (persons born in a given year). The base population is advanced each year by using projected survival rates and net international migration by single year of age, sex, race, and Hispanic origin. Each year, a new birth cohort is added to the population by applying the projected fertility rates by race and Hispanic origin to the January 1st female population. The components of change are individually applied to each of the eight resulting race/ethnic groups (Hispanic and non-Hispanic categories for each of the four races--White; Black; American Indian, Eskimo, and Aleut; and Asian and Pacific Islander) to project the next year's population. A the national level, a special procedure seeks to ensure that certain age-related biases, mainly under-enumeration and age misstatement, do not advance in age.What are household projections?For a more detailed explanation of this framework, see Population Working Paper 38.
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Household projections, made at the national level, are future estimates of the number and composition of households and families. These projections are based on current household estimates and population projections and assumptions about future household structure and family composition. Household projections are not intended as forecasts, but represent the results of assumptions about possible future trends in population change and household-type proportions.To get household projections, household estimates are projected forward using alternative marital status and household-type proportions by age of householders. Three alternative series are used to illustrate alternative patterns of future household change. Series 1, based on a time series model, is the preferred projection in light of past and possible future trends in household change. Series 2 reflects the consequences of projected change in the age/sex structure of the population only, since it does not assume any change in the proportion maintaining households for specific types by age and sex. Series 3 assumes changes in both the age/sex structure and the race/origin composition of the population, but also assumes no change in the proportions maintaining specific household types by these factors.
For more information, contact the Population Projections Branch at (301) 763-2428.
