Population projections are estimates of the population for future dates. They are typically based on an estimated population consistent with the most recent decennial census and are produced using the cohort-component method. Projections illustrate possible courses of population change based on assumptions about future births, deaths, net international migration, and domestic migration. In some cases, several 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.
While projections and estimates may appear similar, there are some distinct differences between the two measures. Estimates are for the past and present, while projections are based on assumptions about future demographic trends. Estimates generally use existing data collected from various sources, while projections make assumptions about what demographic trends will be in the future. Data users may find both an estimate and a projection available for the same date (e.g., July 2011), which may not agree. In such cases, estimates are the preferred data, unless the user's objective is to compare the number with others in the projected series.
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. Each year, a new birth cohort is added to the population by applying the projected fertility rates to the female population. For a more detailed explanation of methodologies used, see the Methodology section.
For assistance, please contact the Census Call Center at 1-800-923-8282 (toll free) or visit ask.census.gov for further information.