As part of its annual review of intercensal population estimates, the U.S. Census Bureau determined that a downward adjustment should be made to the household survey population controls. This adjustment stemmed from revised estimates of net international migration for 2000 through 2003. In keeping with usual practice, the new controls were used in the survey starting with data for January 2004. Estimates for December 2003 and earlier months were not revised to reflect the new (lower) population controls.
...As a convenience to its data users, BLS created a research series that smoothes the level shifts in employment resulting from the January 2000, 2003, and 2004 population control adjustments.