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Box E - Source and Reliability of the CFS Data and Other Sources Used in this Report
The CFS data presented in
this report are estimates derived from a sample survey. Because the CFS is a
sample survey and uses estimation methods and procedures to estimate total
freight activity for the entire United States, the data are subject to
sampling and nonsampling errors. Sampling errors
occur because one set of businesses sampled in a survey may have different
characteristics from another sample set of companies selected from the entire
universe of businesses. Nonsampling errors occur
because of all other factors that may contribute to the total errors in a
sample survey, such as nonresponse, response errors,
and differences in interpretation of questions by respondents. See the CFS
source cited below for detailed discussion of reliability of the CFS data and
estimates of standard errors.
The supplementary data on
crude petroleum, logging, imports, and farm shipments used in this report are
not based on surveys and therefore are not subject to sampling errors. However,
the data are subject to nonsampling errors that may
also occur in censuses and other forms of data collection. The impact of these
nonsampling errors on the supplementary data is not
fully quantifiable. Clearly, because the supplementary data did not cover all
out-of-scope industries and government shipments, the estimates of overall
national freight activity are incomplete.
In this report, the BTS
estimates of the value, tons, and ton-miles of crude petroleum and petroleum
products shipments by pipelines are based on pipeline data from the Federal
Energy Regulatory Commission (FERC). BTS converted FERC information on barrels
of petroleum and petroleum products transported into tons and ton-miles. Estimates
of waterborne shipments not captured in the CFS are based on information from
the U.S. Army Corps of Engineers (USACE) and the Census Bureau’s Foreign Trade
Division (FTD). The USACE reports data on the tons and ton-miles of domestic
and foreign waterborne commerce. The FTD reports information on the value and
tons of waterborne U.S. international trade. BTS used data from both the USACE and Census Bureau to
develop a set of values, tons, and ton-miles for all domestic and foreign
waterborne freight.
This report also compares the
CFS data with data from the Surface Transportation Board’s Carload Rail Waybill
Sample. The preliminary analysis of rail freight activities reported by the CFS
and the Waybill shows some differences in the overall change in volume of
freight moved between 1993, 1997, and 2002. While the CFS shows a slight
increase in rail tonnage, the Waybill shows a larger increase in rail tonnage
over the same period. These differences for rail single mode and intermodal truck and rail combination
are being analyzed, and BTS will present any findings after additional processing of the CFS during
2004.
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, Bureau of Transportation Statistics and U.S.
Department of Commerce, U.S. Census Bureau, “2002 Economic Census: Transportation Commodity Flow Survey,
Preliminary Report,” December 2003, EC02TCF-US(P).
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