The Foreign Trade Division applies adjustments for seasonal and working-day variations, and for price changes.
Goods are initially classified under the Harmonized System (HS) which describes and measures the characteristics of goods traded. Combining trade into approximately 140 export and 140 import end-use categories makes it possible to examine goods according to their principal uses. These categories are used as the basis for computing the seasonal and working-day adjusted data. These adjusted data are then summed to the six end-use aggregates for publication. These data are provided to the Bureau of Economic Analysis, from the U.S. Census Bureau, for use in the Balance of Payments and the National Income and Product Accounts.
The seasonal adjustment procedure is based on a model that estimates the monthly movements as percentages above or below the general level of each end-use commodity series (unlike other methods that redistribute the actual series values over the calendar year). Imports of petroleum and petroleum products are adjusted for the length of the month. Because of the extremely variable movements of the data series for aircraft, users studying data trends may wish to analyze aircraft separately from other trade.
Price Change Adjustments are done using the Fisher chain-weighted methodology. The deflators are primarily based upon the monthly price indexes published by the Bureau of Labor Statistics using techniques developed for the National Income and Product Accounts by the Bureau of Economic Analysis.