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Database Analysis 63 Table 4-17. Hazardous materials in FARS and TIFA, 19992004. Number Hazmat in cargo of cases Coded as Hazmat in 551 FARS, not in TIFA Coded as Hazmat in 706 both FARS and TIFA Coded as Hazmat in 343 TIFA, not in FARS Only a small part of the discrepancy is explained by differences in the ability to determine if the vehicle was transporting hazardous materials. In eight of the cases marked in FARS as carry- ing hazardous material, the TIFA survey was unable to determine if the vehicle held hazmat, while there were 33 cases in FARS where the analyst left the hazmat variable unknown, but the TIFA survey showed that the vehicle was carrying hazardous material. In most of the cases (880), the coding of hazardous material was directly contradictory (i.e., coded as hazmat in one and as not hazmat in the other). Generally, there are a number of reasons to believe that the identification of hazardous material in the cargo is more accurate in TIFA than FARS. First, it is difficult in FARS to perform direct consistency checks on the hazmat variables, since FARS does not capture any other information about the cargo. Moreover, the FARS data collection protocol does not include direct contact with the carrier, driver, reporting officer, or other potential source, but relies primarily on the police report and other investigative documents. Moreover, the TIFA data collection protocol is based on a telephone survey of the motor carrier, driver, dispatcher, or safety director of the truck involved in the crash, as well as the reporting officer, so those sources are questioned directly. In addition, the TIFA data include other information about the cargo, so it is possible to perform basic checks on the accuracy of the hazmat coding, such as whether the truck was carrying cargo at all. The comparison of TIFA and FARS records showed that 264 of the cases coded in FARS were loaded with hazardous materials, while the TIFA survey showed that those trucks were empty at the time of the crash. Finally, it should be noted that the TIFA survey specializes in trucks, while the FARS file cov- ers all vehicle types. This focus on trucks allows the TIFA survey to go into greater depth and to develop more expertise in the details and varieties of truck operations. FARS analysts cover all vehicle types, and while the FARS file is a quality crash data source, it is not reasonable to expect FARS to have a higher degree of detail and accuracy than a file that has the advantage of narrowly focusing on only one vehicle type. In sum, while no doubt there are errors in the TIFA file, it is likely to be more reliable for analyzing truck crashes than FARS. 4.3.5 Usefulness of the Data for Determining Root Causes Because the TIFA file incorporates the relevant data, a discussion of usefulness is deferred to Section 4.4 on the TIFA file. 4.3.6 Data Quality FARS includes multiple layers of quality control. Cases are entered using computer software that includes validity and consistency checks. The validity checks ensure that the values entered are possible for the field. For example, if a field has valid values for one through seven, but an