Box A - How to Interpret Shipment Value and Tonnage Data
The value and ton totals in the CFS represent the sum of separate shipments of a commodity as it moves through the production and consumption segments of the supply chain. Therefore the CFS totals are much larger than the value-added and final weight of materials used in products purchased by consumers and other end-users. Also, the total value of shipments is not directly comparable to the national Gross Domestic Product (GDP) even though both are ostensibly of similar size because GDP measures the value added or net output of production. The value of goods measured in the CFS includes the market value of goods used in production as well as final demand; hence the goods may be counted more than once in the production life cycle.
SOURCE: U.S. Department of Transportation, Bureau of Transportation Statistics, February 2004.