Imports: U.S. importers file tax documents with the U.S. Customs Service describing the type and value of imported goods. These reports are processed and tabulated to arrive at the overall level of U.S. imports. Inaccurate reports, delays in processing data, and smuggling can affect their value.

Exports: There is no tax on exports, so to collect information, the U.S. Department of Commerce developed a form called the Shippers’ Export Declaration (SED) form, which is filled out when goods are sent overseas. These are tallied to arrive at export totals.

The Bretton Woods Agreements Act of 1945 requires the publication of balance of payments information. The statistics are generally reliable although the collection process is often difficult, especially in case of data on travel, services, FDI and financial transactions.

Sometimes it is difficult to classify a good as an import or an export. Trade is usually tabulated on the basis of national origin rather than national ownership. If a product is shipped from the U.S. to Germany, it is considered a U.S. export and a German import. It makes no difference whether a foreign company owns the U.S. factory or if it is a U.S. firm in Germany that imports the product. If a U.S. company owns a plant in Brazil and sells a product to a Japanese company in Canada, the transaction is recorded as a Canadian import and a Brazilian export.

Source: www.newyorkfed.org