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Acknowledgments US Department of Transportation - BTS

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longer derived exclusively from U.S. government administrative records, but from Revenue Canada. Maritime weightdata are initially processed and edited by the Foreign Trade Division, U.S. Census Bureau (Census) as part <strong>of</strong> theoverall edits and quality checks performed on all U.S. international merchandise trade data. After Census processing,the U.S. Army Corps <strong>of</strong> Engineers (<strong>US</strong>ACE) and the Maritime Administration (MARAD) perform additional maritimespecificprocessing and quality edits on maritime-related data elements, including the weight <strong>of</strong> maritime imports andexports. The <strong>US</strong>ACE and MARAD began performing this function in October 1998 after the Foreign WaterborneTrade data program was transferred from the Census Bureau. Prior to October 1998, the <strong>US</strong>ACE historicallyperformed additional specialized edits at the port level, including reassignment <strong>of</strong> some tonnage data to the actualwaterborne port rather than the reported U.S. Customs port.TABLE 1-58. Freight Activity in the United States: 1993, 1997, 2002, and 2007TABLE 1-59. Value, Tons, and Ton- Miles <strong>of</strong> Freight Shipments within the United States by DomesticEstablishment, 2007TABLE 1-62. U.S. Hazardous Materials Shipments by <strong>Transportation</strong> Mode, 2007TABLE 1-63. U.S. Hazardous Materials Shipments by Hazard Class, 2007These data are collected via the 1997 Commodity Flow Survey (CFS) undertaken through a partnership between theU.S. <strong>Department</strong> <strong>of</strong> Commerce, Census Bureau (Census), and the U.S. <strong>Department</strong> <strong>of</strong> <strong>Transportation</strong>, Bureau <strong>of</strong><strong>Transportation</strong> Statistics. For the 1997 CFS, Census conducted a sample <strong>of</strong> 100,000 domestic establishmentsrandomly selected from a universe <strong>of</strong> about 800,000 multiestablishment companies in the mining, manufacturing,wholesale trade, and selected retail industries. It excluded establishments classified as farms, forestry, fisheries,governments, construction, transportation, foreign, services, and most retail.Reliability <strong>of</strong> the EstimatesAn estimate based on a sample survey potentially contains two types <strong>of</strong> errors-sampling and nonsampling. Samplingerrors occur because the estimate is based on a sample, not on the entire universe. Nonsampling errors can beattributed to many sources in the collection and processing <strong>of</strong> the data and occur in all data, not just those from asample survey. The accuracy <strong>of</strong> a survey result is affected jointly by sampling and nonsampling errors.Sampling VariabilityBecause the estimates are derived from a sample <strong>of</strong> the survey population, results are not expected to agree withthose that might be obtained from a 100-percent census using the same enumeration procedure. However, becauseeach establishment in the Standard Statistical Establishment List had a known probability <strong>of</strong> being selected forsampling, estimating the sampling variability <strong>of</strong> the estimates is possible. The standard error <strong>of</strong> the estimate is ameasure <strong>of</strong> the variability among the values <strong>of</strong> the estimate computed from all possible samples <strong>of</strong> the same size anddesign. Thus, it is a measure <strong>of</strong> the precision with which an estimate from a particular sample approximates theresults <strong>of</strong> a complete enumeration. The coefficient <strong>of</strong> variation is the standard error <strong>of</strong> the estimate divided by thevalue being estimated. It is expressed as a percent. Note that measures <strong>of</strong> sampling variability, such as the standarderror or coefficient <strong>of</strong> variation, are estimated from the sample and are also subject to sampling variability. Standarderrors and coefficients <strong>of</strong> variation for CFS data presented in this report are given in Appendix B <strong>of</strong> the 1997Economic Census report, and are available online www.census.gov/econ/wwwse0700.html.Nonsampling ErrorsIn the CFS, as in other surveys, nonsampling errors can be attributed to many sources, including 1) nonresponse; 2)response errors; 3) differences in the interpretation <strong>of</strong> questions; 4) mistakes in coding or recoding the data; and 5)other errors <strong>of</strong> collection, response, coverage, and estimation.A potentially large source <strong>of</strong> nonsampling error is due to nonresponse, which is defined as the inability to obtain allintended measurements or responses from selected establishments. Nonresponse is corrected by imputation.TABLE 1-60. Value <strong>of</strong> U.S. Land Exports to and Imports from Canada and Mexico by Mode

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