Measuring the Nations Freight Movements
Accurately measuring the magnitude of freight movement is a challenge. No single data source provides complete and timely information on all freight transportation modes for all goods and sectors of the economy.
The Commodity Flow Survey (CFS) is the primary source of national- and state-level data on domestic freight shipments by American businesses. As a shipper-based survey, the CFS collects information on how U.S. establishments transport raw materials and finished goods; the types of commodities shipped by mode of transportation; the value, weight, origin, and destinations of shipments; and the distance shipped. It covers establishments classified in the North American Industry Classification System (NAICS) as manufacturing, mining, and wholesale trade.
Produced as part of the Economic Census, the CFS allows analysis of the nations freight activities within the context of changes in the nations economy. The CFS data are helpful in market analysis of how businesses use competing transportation modes to move freight and facilitate production and trade activities. Although the CFS is the most comprehensive data source on nationwide freight movements, there are important data gaps in the coverage of certain industries and commodities and in the domestic movements of imports. Additional data must be used to fill gaps in CFS coverage.
To present a more complete national estimate of the overall freight moved on the nations transportation system in 2002, Bureau of Transportation Statistics (BTS) and the Federal Highway Administration (FHWA), Office of Freight Management and Operations have supplemented the CFS data with estimates from other sources on freight shipments that are not fully measured in the CFS. These additional estimates cover farm shipments to processing plants, crude petroleum pipeline shipments, waterborne imports and exports, and logs and wood in the rough. They also cover shipments by the service, retail, and construction sectors as well as municipal solid waste. The new composite national estimates provide the benchmark data for the FHWA Freight Analysis Framework II. Information on the methods and data sources used in developing these composite estimates will be available by summer 2006 at the agencies websites www.bts.gov and www.fhwa.dot.gov.
This report compares the final data from the 2002 CFS with data from the 1997 and 1993 CFS to show changes in modal shares, distance shipped, shipment sizes, and ton-miles generated on the national transportation network. It is important to note that most of the 1993 and 1997 freight data presented in this report are revised from those published in earlier BTS publications. They were adjusted to account for changes in industry coverage as a result of the transition to the NAICS code.
Coverage and Limitations of the CFS Freight Data
The 2002 CFS is the most recent nationwide shipper survey of commodities shipped in the United States. It follows the 1993 and 1997 CFS and its predecessor, the 1977 Commodity Transportation Survey. The Bureau of Transportation Statistics and the Census Bureau cosponsor the CFS as part of the quinquennial (every 5 years) Economic Census (BTS and Census 2003). The Census Bureau collects CFS data from a sample of manufacturing, mining, and wholesale trade industries in the United States. The survey excludes shipments by establishments classified in the North American Industry Classification System (NAICS) as farms, forestry, fishing, government agencies, construction, transportation, and most retail and service industries. The 2002 survey did not capture most shipments from logging establishments because under NAICS, the classification of this industry moved from manufacturing (in-scope for the CFS) to agriculture (out-of-scope for the CFS). Further, because the CFS is a survey of domestic establishments and measures shipments leaving an establishments facility, it includes exports but not imports (unless the imported goods are received by an in-scope domestic business at the port of entry and reshipped by that business). Although the initial 1993 CFS design included establishments from the oil and gas extraction industry, all three surveys exclude shipments of crude petroleum by this industry because of the way these companies record and report shipment information.
Reliability of the CFS Data Used in this Report
The CFS data presented in this report are derived from a sample survey and may differ from the actual, unknown values for the entire population of businesses they represent. Statisticians define this difference as the total error of the estimate. When describing the accuracy of survey results, it is convenient to discuss total error as the sum of sampling error and nonsampling error. Sampling error is the average difference between the estimate and the result that would be obtained from a complete enumeration of the sampling frame conducted under the same survey conditions. Nonsampling error encompasses all other factors that contribute to the total error of a sample survey estimate.
The sampling error of the estimates reported in the CFS can be estimated from the selected sample because the sample was selected using probability sampling. Common measures related to sampling error are the sampling variance, the standard error, and the coefficient of variation (CV). The sampling variance is the squared difference, averaged over all possible samples of the same size and design, between the estimator and its average value. The standard error is the square root of the sampling variance. The CV expresses the standard error as a percentage of the estimate to which it refers.
Nonsampling errors are difficult to measure and can be introduced through inadequacies in the questionnaire, nonresponse, inaccurate reporting by respondents, errors in the application of survey procedures, incorrect recording of answers, and errors in data entry and processing. Data users should take into account both the measures of sampling error and the potential effects of nonsampling error when using the CFS estimates. See the CFS source cited below for detailed discussion of reliability of the CFS data and estimates of standard errors. Additional information on (1) comparability of 2002 CFS with the 1993 and 1997 CFS, (2) reliability of the CFS estimates, and (3) sample design, data collection, and estimation is available at http://www.bts.gov/cfs/prod.html.
SOURCE: U.S. Department of Transportation, Research and Innovative Technology Administration, Bureau of Transportation Statistics and U.S. Department of Commerce, U.S. Census Bureau, 2002 Economic Census: Transportation Commodity Flow Survey, Final Report, December 2003, EC02TCF-US.