The U.S. Census Bureau released the first county-to-county migration flow estimates based on the American Community Survey (ACS) in April 2012. It used data collected from 2005 through 2009 that composed the first ACS 5-year dataset. The ACS program combines consecutive yearly datasets to increase the sample size and provide reliable estimates for smaller areas. Even though estimates are produced for 1-year and 3-year datasets, estimates are only published for geographic areas of at least 65,000 and 20,000 population, respectively. Only the 5-year datasets provide estimates for all counties and county equivalents in the U.S.
The 2006-2010 ACS County-to-County Migration Files included additional files crossed by certain characteristics: age, sex, race and Hispanic or Latino origin. For the 2007-2011 release, the characteristics Educational Attainment, and Household and Individual Income were used instead. The 2008-2012 release consists of Employment Status, Occupation, and Work Status. The values for Employment Status and Work Status are for the current residence (i.e., at the time the respondent was surveyed or the previous week for Employment Status.) The occupation is the one that the person worked the most hours for the previous week or at their last job within the prior 5 years if the person did not have a job the previous week.
Employment status is separated into those in the labor force and those not in the labor force. The labor force participants are further divided into employed civilian, unemployed, and in Armed Forces. Occupation is separated into six categories: Management, business, science, and arts occupations; service occupations; sales and office occupations; natural resources, construction, and maintenance occupations; production, transportation, and material moving occupations; and military specific occupations for those that could not be classified in an existing civilian occupation. Work status is separated into six categories: worked 50 to 52 weeks in the past 12 months and usually worked 35 or more hours per week; worked 50 to 52 weeks in the past 12 months and usually worked less than 35 hours per week; worked 1 to 49 weeks in the past 12 months and usually worked 35 or more hours per week; worked 1 to 49 weeks in the past 12 months and usually worked less than 35 hours per week; last worked 1 to 5 years ago; and last worked over 5 years ago or never worked.
Besides county-to-county migration flows, the product package also contains flow data for minor civil divisions (MCD) for a few select states2 in which they are commonly used as the primary substate aggregate unit in addition to or instead of counties.
In addition, there are files that contain the migration flow, migration counterflow, net migration, and gross migration between each county pair or county/mcd pair. These files are only available without characteristics.