The American Community Survey (ACS) and the Puerto Rico Community Survey (PRCS) ask respondents age 1 year and over whether they lived in the same residence 1 year ago. For people who lived in a different residence, the location of their previous residence is collected.
ACS uses a series of monthly samples to produce estimates. Estimates for geographies of population 65,000 or greater are published annually using these monthly samples.
Three years of monthly samples are needed to publish estimates for geographies of 20,000 or greater and five years for smaller geographies. The 5-year dataset is used for the county-to-county migration flows since many counties have a population less than 20,000. The first 5-year ACS dataset covers the years 2005 through 2009.
In addition to the county-to-county flow files, there are similar files that substitute minor civil divisions (mcd), in place of counties, for states where they serve as general-purpose local governments. These states are Connecticut, Maine, Massachusetts, Michigan, Minnesota, New Hampshire, New Jersey, New York, Pennsylvania, Rhode Island, Vermont, and Wisconsin.