The Internal Revenue Service (IRS) compiles tax return data each year. They match returns from one year to the next utilizing social security numbers. The newest data for 2008 was released in October 2009. The 2008 data should be interpreted as tax returns filed in the spring of 2008 (April 2008) for tax year 2007. The comparison is made between where the people filed in the current year versus their address when filing in the previous year (spring of 2007 for the 2006 tax year). The data are limited to tax filers, which tend to under-represent certain groups, such as new immigrants, the elderly, and the poor.
Drozd, David J., "Migration into and out of Nebraska based on IRS tax filing data: An analysis of new 2008 data versus historic annual data since 1989" (2009). Publications since 2000. 31.