Postcensal population estimates are estimates made for the years following a census, before the next census has been taken. National postcensal population estimates are derived annually by updating the resident population enumerated in the decennial census using a components of population change approach. Each annual series includes estimates for the current data year and revised estimates for the earlier years in the decade. The U.S. Census Bureau also produces postcensal estimates of the resident population for each state and county by using a component of population change method at the county level. An additional component of population change, net internal migration, is involved. The state population estimates are produced by summing all county populations within each state. The Census Bureau has annually produced a postcensal series of estimates of the July 1 resident population of the United States based on Census 2000 by applying the components of change methodology to the Modified Race Data Summary file. So that the race data for 2000-based postcensal estimates will be comparable with race data on vital records, the Census Bureau has applied the National Health Interview Survey (NHIS) bridging methodology to each 31-race-group postcensal series of population estimates to obtain bridged-race postcensal estimates (estimates for the four single-race categories: American Indian or Alaska Native, Asian or Pacific Islander, black, and white).Vital rates for 2000 were calculated using the bridged-race April 1, 2000, census counts, and vital rates for 2001 and beyond were calculated using bridged-race estimates of the July 1 population from the corresponding postcensal vintage. Intercensal population estimates are estimates made for the years between two censuses and are produced once the decennial census at the end of the decade has been completed. They replace the postcensal estimates that were produced prior to the completion of the census at the end of the decade. Intercensal estimates 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 and thus 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).
MethodologyThe following formula is used to derive the estimates for a given year from those for the previous year, starting with the decennial census enumerated resident population as the base: Resident population+ Births to U.S. resident women - Deaths to U.S. residents+ Net international migration. The postcensal estimates are consistent with official decennial census figures and do not reflect estimated decennial census under-enumeration. Estimates for the earlier years in a given series are revised to reflect changes in the components of change data sets (for example, births to U.S. resident women from a preliminary natality file are replaced with counts from a final natality file). To help users keep track of which postcensal estimate is being used, each annual series is referred to as a vintage and the last year in the series is used to name the series. For example, the Vintage 2001 postcensal series has estimates for July 1, 2000, and July 1, 2001, and the Vintage 2002 postcensal series has revised estimates for July 1, 2000, and July 1, 2001, as well as estimates for July 1, 2002. The estimates for July 1, 2000, and for July 1, 2001, from the Vintage 2001 and Vintage 2002 postcensal series, differ. Vital rates that were calculated using postcensal population estimates are routinely revised when intercensal estimates become available.