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 state-to-county migration flows are created from tabulations of the current county of residence crossed by state of residence 1 year ago (including the District of Columbia and Puerto Rico). Tables of ACS 5-year state-to-county flows are available beginning in 2021.

Sample size and data quality measures, including coverage rates, allocation rates, and response rates, are available on the American Community Survey (ACS) Research & Methodology page.

Data are based on a sample and are subject to sampling variability. The degree of uncertainty for an estimate arising from sampling variability is represented through a margin of error. The tables on this page include the 90 percent margin of error. The margin of error can be interpreted roughly as providing a 90 percent probability that the interval defined by the estimate minus the margin of error and the estimate plus the margin of error (the lower and upper confidence bounds) contains the true value. In addition to sampling variability, the ACS estimates are subject to nonsampling error. For a discussion of nonsampling variability, see Accuracy of the Data. The effect of nonsampling error is not represented in these tables.