The IRS publishes year-to-year address changes as reported on individual income tax returns. Migration patterns are available for the inflow (the number of new tax filers who moved to a state) and outflow (the number of tax filers leaving a state). The IRS Migration of Tax Returns given here is the net loss or gain in the number of returns for a state and is derived by calculating the difference between inflows and outflows of returns for the year.
According to the IRS, "Migration data for the United States are based on year-to-year address changes reported on individual income tax returns filed with the IRS. They present migration patterns by State or by county for the entire United States and are available for inflows—the number of new residents who moved to a State or county and where they migrated from, and outflows—the number of residents leaving a State or county and where they went." SOI Tax Stats - Migration Data
The IRS Migration of Tax Returns, Ranking of the states from 1 to 50 for tax return migrations shows how the states compare concerning the net migration of income tax returns. States with the lower numbers refer to states with higher net in-migration compared to those with higher numbers. Thus Florida, which ranks 1, indicates that the largest number of people migrated and filed new tax returns there, versus New York which ranks 50, meaning that it has the lowest amount of migration and new tax returns.
For more information about methodology see the IRS Migration Data Users Guide.