Data for the monthly compilation of the trade statistics are received on a flow basis from the U.S. Customs. This information is collected from approximately 15 million documents filed annually by importers and exporters.

The data sources for U.S. import statistics are Customs import documents, which importers or their agents are required to prepare and to file with Customs officials at the ports at which the merchandise is entered. On average about 550,000 of these documents are filed monthly. Customs ports throughout the United States transmit documents to the Bureau of the Census (Jeffersonville, Indiana processing center) on an almost daily basis. Informal entry documents, prepared for merchandise valued not over the exemption level of $1000 (except for certain commodities (shoe, textile, etc.) exempted at $250), are not transmitted to the Census Bureau. All formal import documents containing import items valued over $1000 are fully processed for inclusion in the monthly import statistics.