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—A motorcyclist not wearing a helmet in 1991 has a single motorcycle crash and sustains a skull fracture and a small laceration of the leg. Police and emergency medical services respond to the scene and transport the motorcyclist to a hospital. A motorcyclist in 1992, now wearing a helmet because of the helmet law, experiences the identical crash and sustains only a small laceration of the leg. The motorcyclist gets up, curses his luck, and departs. This incident is never reported. As this counterfactual example shows, counting of crashes is an extremely fragile denominator for calculation of rates.Counts of crashes are, in reality, counts of reported crash events. Police count three types of motorcycle crashes: fatal, nonfatal injury, and property damage only. Crashes that result in death are reliably noted by both police and coroner. Crashes that result in severe injury are generally noted by police and
Kraus JF, Peek C, McArthur D, Williams A. Motorcycle Helmet Use and Injuries-Reply. JAMA. 1995;274(12):941. doi:10.1001/jama.1995.03530120033026