Population estimates are composed of three types: decennial census, postcensal estimates, and intercensal estimates.
The census of population (decennial census) has been held in the United States every 10 years since 1790. Since 1930, it has enumerated the resident population as of April 1 of the census year. The resident population includes all persons whose usual place of residence is in one of the 50 states or the District of Columbia, including Armed Forces personnel stationed in the United States. Census also provides estimates of the civilian and civilian non-institutionalized populations. The civilian population includes U.S. residents not in the active-duty military. The civilian non-institutionalized population is the civilian population not residing in institutions such as correctional institutions, juvenile facilities, skilled nursing facilities, and other long-term care living arrangements.
Data on sex, race, Hispanic origin, age, and marital status are collected from 100% of the enumerated population. More detailed data on education, housing, occupation, income, and other information are also collected from a representative sample of the population (about 17% of the total population). The questions on race on the 2000 and 2010 censuses were based on the Office of Management and Budget’s (OMB) 1997 Revisions to the Standards for the Classification of Federal Data on Race and Ethnicity. The 1997 Standards require a minimum set of categories to be used by federal agencies for identification of race (American Indian or Alaska Native, Asian, black or African American, Native Hawaiian or Other Pacific Islander, and white) and also require that federal data collection programs allow respondents to select one or more race categories when responding to a query on their racial identity. The 1997 Standards continue to call for use, when possible, of a separate question on Hispanic or Latino ethnicity and specify that the ethnicity question should appear before the question on race. Thus, persons of Hispanic origin may be of any race.
Postcensal population estimates are made for the years following a census, before the next decennial census has been taken. Each year, the Census Bureau's Population Estimates Program utilizes current data on births, deaths, and migration to calculate population change since the most recent decennial census and produce a time series of estimates of population, demographic components of change, and housing units. Each annual series is referred to as a vintage and the last year in the series is used to name the series. The annual time series of estimates begins with the most recent decennial census data and extends to the vintage year. For example, the Vintage 2012 postcensal series has estimates for July 1, 2010, July 1, 2011, and July 1, 2012.
Intercensal population estimates are estimates made for the years between two decennial censuses and are produced once the census at the end of the decade has been completed. They replace the postcensal estimates produced prior to the completion of the census at the end of the decade. Intercensal estimates reflect the population as of July 1 and are more accurate than postcensal estimates because they are based on both the census at the beginning and the census at the end of the decade. They are derived by adjusting the final postcensal estimates for the decade to correct for the error of closure (the difference between the estimated population at the end of the decade and the census count for that date). Rates that were calculated using postcensal population estimates are routinely revised when intercensal estimates become available.
Postcensal estimates become less accurate as the date of the estimates moves farther from the date of the census.
Some subgroups of the population (including some racial, ethnic, and age groups) are less likely than others to be completely enumerated in the decennial census. The undercounts of these groups decrease the denominators and result in overestimates of morbidity and mortality rates. The U.S. Census Bureau produces age-specific estimates of net census undercount for the total, white, and black populations. These estimates are used to weight the denominator populations for most of the national health surveys, including the National Health Interview Survey (NHIS), the National Health and Nutrition Examination Survey (NHANES), the National Survey of Family Growth (NSFG), and the National Health Care Surveys.
Bridged-race postcensal population estimates are available from: https://www.cdc.gov/nchs/nvss/bridged_race.htm.