Domestic migration in the U.S. has been measured using Decennial Census long form data since 1940. After Census 2000, the long form data collection was replaced by the American Community Survey (ACS), a monthly survey that collects social, economic, and household data that is released annually. One major difference between Census and ACS migration data is the time reference for the question. Decennial data asked where a person lived 5 years ago, while the ACS asks where a person lived 1-year ago. The time period was changed to reflect the ongoing data collection of the ACS, to allow an annual estimate of geographical mobility.
There are other Census data sources of yearly migration, most notably the Annual Social and Economic Supplement to the Current Population Survey (CPS ASEC). Other data exist that can proxy annual migration estimates, including administrative records from the Internal Revenue Service (IRS) and Medicare program. Another major source of administrative records data recording on-going residential change is the National Change of Address (NCOA) database from the United States Postal Service (USPS). However, much like the other administrative data, its utility in measuring US domestic migration is not well understood.