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Estimating seat belt effectiveness using matched-pair cohort methods

Harborview Injury Prevention and Research Center, 325 Ninth Avenue, Box 359960, Seattle, WA 98104-2499, USA.
Accident Analysis & Prevention (Impact Factor: 1.87). 02/2003; 35(1):143-9. DOI: 10.1016/S0001-4575(01)00087-2
Source: PubMed

ABSTRACT Using US data for 1986-1998 fatal crashes, we employed matched-pair analysis methods to estimate that the relative risk of death among belted compared with unbelted occupants was 0.39 (95% confidence interval (CI) 0.37-0.41). This differs from relative risk estimates of about 0.55 in studies that used crash data collected prior to 1986. Using 1975-1998 data, we examined and rejected three theories that might explain the difference between our estimate and older estimates: (1) differences in the analysis methods; (2) changes related to car model year; (3) changes in crash characteristics over time. A fourth theory, that the introduction of seat belt laws would induce some survivors to claim belt use when they were not restrained, could explain part of the difference in our estimate and older estimates; but even in states without seat belt laws, from 1986 through 1998, the relative risk estimate was 0.45 (95% CI 0.39-0.52). All of the difference between our estimate and older estimates could be explained by some misclassification of seat belt use. Relative risk estimates would move away from 1, toward their true value, if misclassification of both the belted and unbelted decreased over time, or if the degree of misclassification remained constant, as the prevalence of belt use increased. We conclude that estimates of seat belt effects based upon data prior to 1986 may be biased toward 1 by misclassification.

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    • "In some cases, pairs can share characteristics that are unmeasurable (e.g. Cummings et al., 2003b; Martin and Lenguerrand, 2008; Viano et al., 2008), and a more classic example of this would be twin studies. Similarly to the latter, this study in fact constructs a dataset with observations of inherently matched pairs, because each pair of drivers originates from the same accident and experienced the same weather, the same crash location, the same road geometry, etc. "
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    • "Many studies have assessed the effect of individual variables, such as age, sex, safety belt use and seating position in the vehicle on the risk of death or severe injury to the driver or passengers of vehicles involved in road crashes (Bédard et al., 2002; Braver et al., 1998; Cummings et al., 2003b; Evans, 2001a,b; Huelke and Compton, 1995; Li et al., 2003; O'Donnell and Connor, 1996; Smith and Cummings, 2004; Yau, 2004; Zhang et al., 2000). To control for the confounding effect of crash severity (for example, the protective effect of the driver's safety belt may be overestimated if unbelted drivers tend to be involved in more severe crashes), some researchers have applied matched-byvehicle analyses, either with the double pair comparison method proposed by Evans (1986) or with the more efficient regression matched-pair analysis methods (Cummings et al., 2003a). "
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