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Environmental Epidemiology
Published in Lorris G. Cockerham, Barbara S. Shane, Basic Environmental Toxicology, 2019
Another measure of disease occurrence is the standardized mortality ratio (SMR), a measure of risk relative to that of another population called the standard or reference population. In many studies the standard population is the control or unexposed group, while the study population is one that has experienced exposure to some agent. For example, using death rates of U.S. white males as a standard, Rinsky et al. (1981) found a SMR of 560 (p < 0.001) for leukemia in workers exposed to benzene in the rubber industry. A value of 100 would have indicated there was no excess risk of cancer compared to the standard population. The SMR considers differing age structures between the study and standard populations and the use of this method is known as the indirect adjustment method. The SMR method of analysis is frequently used in occupational cohort studies which are described in more detail in the following sections. The SMR is a measure of relative risk for a particular cause of death. In a hypothetical example, if 50 bladder cancer deaths were observed in a group of workers involved in a particular industrial process and only 10 deaths were expected, then the SMR for this cause of death would be 500 (50/10 × 100). An interpretation of this example is that if the workers were dying at the same rate as the standard or unexposed population, one would have expected only 10 deaths, but in this example there were 50 deaths or 5 times more than expected.
Motorcycle antilock braking systems and fatal crash rates: updated results
Published in Traffic Injury Prevention, 2022
Data were analyzed for fatal crashes and registrations occurring during 2003–19. The rate of driver fatal crash involvements per 10,000 registered vehicle years for each motorcycle model, both ABS and non-ABS versions, were computed. If ABS does not affect the risk of fatal motorcycle crashes, then the fatal crash rate for each study motorcycle should not vary by whether or not it has ABS. Under this assumption, an expected count of drivers involved in fatal crashes was computed for each ABS motorcycle model as the product of the fatal crash rate per registered vehicle year for the non-ABS version and the number of registered vehicle years of the ABS version. A rate ratio estimating the effect of ABS was calculated as the sum of the observed number of drivers in fatal crashes for ABS motorcycles (O) divided by the sum of their expected number of drivers in fatal crashes (E). This is also known as the standardized mortality ratio (SMR). It standardizes the exposure distributions of the two study groups to limit possible bias due to, in this study, some motorcycles being more likely to have the ABS option than others. Using formulas derived by Silcocks (1994), a 95% confidence interval for the SMR was computed as (L, U), where: where βp(a,b) is the 100×pth percentile from the beta distribution with parameters a and b. Furthermore, the SMR was computed for motorcycle types: cruiser/standard, touring, sport touring, sport/unclad sport, supersport, and other. These motorcycle types are known to have widely varying driver death rates and rates of speeding in fatal crashes (Teoh and Campbell, 2010), so it is possible that ABS may be more effective for some motorcycle types than for others, on average.