American journal of epidemiology
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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.