Corrections and improvements made to the data. Each month, a preliminary estimate for the current month and a revised estimate for the immediately preceding month are released. After the initial revision, no further changes are made to that month until more complete source data become available in March, June, September, and December. The releases in March, June, September, and December contain revised estimates for the previous six months. The release in March also contains revisions for all months of the previous year in order to align the seasonally adjusted monthly data with annual totals. The release in June contains annual revisions, which reflect updated source data and changes in estimating methodologies.