Source: U.S. Bureau of the Census
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. The 5-year dataset is used for the county-to-county migration flows since many counties have a population less than 20,000.
U.S. Bureau of the Census, Net Migration Flow for Contra Costa County, CA [NETMIGNACS006013], retrieved from FRED, Federal Reserve Bank of St. Louis; https://fred.stlouisfed.org/series/NETMIGNACS006013, September 25, 2018.