American journal of epidemiology
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Comparative Study
Using head-on collisions to compare risk of driver death by frontal air bag generation: a matched-pair cohort study.
US air bag regulations were changed in 1997 to allow tests of unbelted male dummies in vehicles mounted and accelerated on sleds, resulting in longer crash pulses than rigid-barrier crashes. This change facilitated depowering of frontal air bags and was intended to reduce air bag-induced deaths. Controversy ensued as to whether sled-certified air bags could increase adult fatality risk. ⋯ In head-on collisions involving only passenger cars, the adjusted risk ratio was 1.04 (95% confidence interval: 0.85, 1.29). Increased fatality risk for drivers with sled-certified air bags was not observed. A borderline significant interaction between vehicle type and air bag generation suggested that sled-certified air bags may have reduced the risk of dying in head-on collisions among drivers of pickup trucks.
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Immortal time is a span of cohort follow-up during which, because of exposure definition, the outcome under study could not occur. Bias from immortal time was first identified in the 1970s in epidemiology in the context of cohort studies of the survival benefit of heart transplantation. It recently resurfaced in pharmaco-epidemiology, with several observational studies reporting that various medications can be extremely effective at reducing morbidity and mortality. ⋯ The author shows that for time-based, event-based, and exposure-based cohort definitions, the bias in the rate ratio resulting from misclassified or excluded immortal time increases proportionately to the duration of immortal time. The bias is more pronounced with a decreasing hazard function for the outcome event, as illustrated with the Weibull distribution compared with a constant hazard from the exponential distribution. In conclusion, observational studies of drug benefit in which computerized databases are used must be designed and analyzed properly to avoid immortal time bias.
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The joint effects of tobacco use and body mass on mortality have not been well characterized, although evidence regarding the effect of smoking on the association between body mass and mortality is accumulating. To study the joint effects of these important risk factors, the authors conducted a prospective cohort study of 148,173 men and women aged > or =35 years in Mumbai, India. Subjects were recruited during 1991-1997 and then followed for approximately 5-6 years (1997-2003). ⋯ Body mass and all forms of tobacco use had independent as well as multiplicative joint effects on mortality risk. Tobacco use and undernutrition are serious problems in India. The current study indicates that obesity may emerge as a serious public health problem with which tobacco use may interact.
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Between 1987 and 2005, the authors conducted a case-control study nested within the entire Swedish population to investigate whether loss of a child due to death is associated with the risk of amyotrophic lateral sclerosis (ALS). The study comprised 2,694 incident ALS cases and five controls per case individually matched by year of birth, gender, and parity. Odds ratios and their corresponding 95% confidence intervals for ALS were estimated by using conditional logistic regression models. ⋯ Loss of a child due to malignancy appeared to confer a lower risk of ALS (odds ratio = 0.5, 95% CI: 0.3, 0.8) than loss due to other causes. These data indicate that the risk of developing ALS decreases following the severe stress of parental bereavement. Further studies are needed to explore potential underlying mechanisms.
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A prospective cohort of 74,332 men and women was used to investigate the association of body mass index and height with suicide, anxiety, and depression. Participants in the Nord-Trøndelag Health Study (Norway, 1984-1986) (HUNT 1) were aged 20 years or more and followed up until December 31, 2002. Anxiety and depression were measured with the Hospital Anxiety and Depression Rating Scale (HADS) in 1995-1997. ⋯ In fully adjusted models, there was no association of height with the incidence of suicide or depression. Raised body mass index is associated with an increased risk of depression but reduced risk of suicide in men and women. The mechanisms underlying these different associations require clarification.