Neither the Census estimates nor the Biometrics estimates correct for under counting in the decennial census. Post enumeration surveys were conducted to analyze the degree of under counting, but the Census Bureau did not elect to change the official counts. To come up with the postcensal population estimates, the Census Bureau uses the decennial U.S. census modified count of the population and supplementary data to derive the estimated population for an area through computer modeling techniques. The supplementary information, which the Census Bureau calls components of change, includes births, deaths, domestic migration, and international migration. Births and deaths are counted from the birth and death certificates in every state.
Domestic migration is estimated from federal income tax and social security data.
International migration is estimated using data that accounts for legal immigration, federal citizen emigration, Puerto Rican migration, refugees and undocumented immigration. Other state specific data, such as school enrollment information, is also used to determine the state populations. Because the Census Bureau must wait until this supplementary information is compiled, the currency of the estimates depends on the degree of detail desired. In general the total population of the U.S. is made available before the state or county population estimates. Estimates for counties by age, sex, race and ethnicity take the longest.