Comparison of reporting of seat belt use by police and crash investigators: variation in agreement by injury severity

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Abstract

Purpose: To evaluate agreement between police and trained investigators regarding seat belt use by crash victims, according to injury severity. Methods: We used data from the National Accident Sampling System Crashworthiness Data System (CDS) for front seat occupants, 16 years and older, in crashes during 1993–2000. Crashworthiness Data System investigators determined belt use from vehicle inspection, interviews, and medical record information; their assessment was considered the gold standard for this analysis. Occupant severity of injury was categorized in five levels from no injuries to death. We estimated the sensitivity, specificity, and area under receiver operating characteristic curves for police reports of belt use. Results: Among 48,858 occupants, sensitivity of a police report that a belt was used was 95.8% overall and varied only modestly by injury severity. Specificity of a police report that a belt was not used was 69.1% overall; it was the lowest among the uninjured (53.2%) and greatest among the dead (90.4%). The area under the curve was 0.82 (95% confidence interval 0.82–0.83) overall; this was lowest among those not injured (0.75, 95% confidence interval 0.74–0.76) and increased with injury severity to 0.91 (95% confidence interval 0.90–0.93) among those who died. Conclusion: Police usually classify belted crash victims as belted, regardless of injury severity. But they often classify unbelted survivors as belted when they were not. This misclassification may result in exaggerated estimates of seat belt effectiveness in some studies.

Introduction

In studies of seat belt use and traffic crash outcomes, errors in assessment of seat belt use may bias the estimates of any benefit afforded by wearing a belt in a crash. Seat belt misclassification is differential when the proportion misclassified is related to an outcome, such as death (Kelsey et al., 1996). Some authors have suggested that differential misclassification has produced exaggerated estimates of benefit in some studies, biasing the risk ratios toward 0; this might have occurred either because some dead crash victims were incorrectly classified as unbelted, or some survivors were incorrectly classified as belted, or both (Robertson, 1976, Partyka, 1988, Kahane, 2000, Robertson, 2002). Misclassification is non-differential when errors in assessing belt use are unrelated to the study outcome. Non-differential misclassification tends to bias risk ratios toward 1 (Kelsey et al., 1996), resulting in an underestimate of any benefit associated with seat belt use.

One of us has reported that both differential and non-differential misclassification of belt use occurs to some degree, but the amount of error in recent data suitable for a matched-cohort analysis was trivial, bias toward 1 and 0 was balanced, and the misclassification did not appreciably influence the risk ratio estimate (Cummings, 2002). Although that study was limited to front-seat occupant pairs in which one or both died, not all studies of seat belts are limited to those pairs.

In this paper, we compared the assignment of seat belt status as recorded by police with the assignment made by trained investigators who relied chiefly on vehicle inspection to determine belt use after a crash. We assessed how agreement on seat belt use varied by the injury severity of the crash victims. Several studies have estimated the association between seat belt use and death using a matched-pair cohort study design that was limited to front seat occupants in the same vehicle (Evans, 1986a, Evans, 1987, Kahane, 1986, Kahane, 2000, Cummings et al., 2003a); this design can estimate seat belt effectiveness for all pairs, using only information from pairs with at least one death (Evans, 1986b; Cummings et al., 2003b, Cummings et al., 2003c). We therefore limited our analysis to front seat occupant pairs in the same vehicle.

Section snippets

Methods

The National Highway Traffic Safety Administration collects information on a sample of all motor vehicle crashes reported to the police in the United States. Approximately 5000 crashes are investigated annually by trained investigators and the crash data are entered in the National Accident Sampling System Crashworthiness Data System (CDS) (NASS, 2001). The CDS contains data on seat belt use from two sources, police crash reports and CDS investigators. We evaluated the CDS data for all front

Results

The CDS data set contained 49,115 front seat occupants, aged 16 years or older involved in motor vehicle crashes with complete information on age, injury severity, and seat belt use as reported by police and CDS investigators. The mean age of occupants was 36.4 years and 55.4% were male. Most occupants were drivers (78.2%) and 70.1% of occupants were belted according to the CDS investigators. About one-fourth (28.7%) had no injuries, 54.6% had minor injuries (ISS 1–8), 7.5% had moderate

Discussion

We found that agreement between CDS investigators and police on the reporting of seat belt use and non-use in a motor vehicle crash increased with increasing injury severity among occupants of fatal and non-fatal crashes combined as measured by κ and the area under receiver operator characteristic curves. Agreement between CDS investigators and the police did not vary in a systematic way with injury severity among occupants in vehicles with a fatality.

Some investigators (Robertson, 1976, Evans,

Acknowledgements

This work was supported by grants R49/CCR002570 and R49/CCR019477-01 from the Centers for Disease Control and Prevention, Atlanta, GA. We would like to thank Christopher Mack for programming assistance with the CDS data files and Robert Kaufman, a CDS crash investigator for the past 12 years, for his advice on CDS procedures and variables, and comments on this manuscript.

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