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, net international migration, and domestic migration. Projected numbers are typically 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?In some cases, 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 periodically.
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. 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 female population. The components of change are individually applied to each of the race/ethnic groups to project the next year's population.For a more detailed explanation of methodologies used, see the Methodology section.
