Every ten years, the U.S. Census Bureau conducts its decennial census. In the non-decennial years, the Bureau produces and publishes estimates of the population for each state and county, as well as the nation as a whole. This series of estimates is part of the (no surprise here in its name) Estimates Program.
The most recently released estimate (the July 1, 2011 estimate) is the first based solely on the 2010 Census counts and what are referred to as the components of change. As estimates are produced each year between the decennial censuses, the entire estimate series is revised and updated. For each state and county, the Census Bureau each July releases annual estimates of the resident population by age, sex, race, and Hispanic origin. These estimates are produced largely through an analysis of three components of population change: births, deaths, and migration.
To estimate births, the Census Bureau utilizes birth certificate data collected by the National Center for Health Statistics (NCHS). NCHS provides final individual birth records for births occurring before January 1, 2010.
To estimate deaths, the Bureau utilizes death data collected by NCHS. NCHS provides final individual death records for all deaths occurring before January 1, 2010 by residence, age, sex, race, and Hispanic origin of each decedent, as well as the place and date each death occurred.
International migration estimate has several parts: immigration of the foreign born, emigration of the foreign born, net migration between the United States and Puerto Rico, net migration of natives to and from the United States, and net movement of the Armed Forces population to and from the United States.
The Census Bureau also estimates net domestic migration separately for two population universes – those living in household and group quarters and for those in each of two age groups (0 to 64 years and 65 years and older).
For the 0 to 64 year old household population, the Census Bureau uses Federal income tax returns supplied by the Internal Revenue Service (IRS) to track person-level data on filers and dependents aged 0 to 64 years. Two years of IRS tax returns are matched and the addresses are compared to identify the number of individuals (represented by exemptions) who moved from one county to another between tax filings. A process is used to account for non-tax filers.
For the 65 years and older household population, the Bureau uses annual Medicare enrollment data for each county from the Centers for Medicare and Medicaid. As with the IRS data, not all U.S. residents aged 65 and older receive, or are eligible to receive, Medicare benefits. Therefore, a separate process accounts for those not part of the Medicare process.
To estimate the net domestic migration of the group quarters population, data from the Census 2010 is used, in combination with data collected from a special survey of these facilities called the Group Quarters Report.
To see all of the estimates for the country you might want to visit the Census Bureau Population Estimates web page.
To see some great analysis of NYS counties you may want to read this piece by Cornell Program of Applied Demographics.
