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The challenge that researchers interested in biomarkers face is to identify measures that can reflect aging better than simply knowing an individual's chronologic age. Ideally, one might follow a cohort of people and use various parameters to estimate the risk of dying, but such a study at best takes a long time. Instead, investigators have tried to model factors that seem to be associated with estimated age. Even using mortality can present problems. Risk factors for mortality are often risk factors for disease. We know that aging is associated with an increased prevalence of chronic diseases. There is an ongoing debate about the appropriateness of including disease states in studies of aging. Some hold out for a model of pure aging, independent of disease, and prefer to study only those persons or organs that are free of apparent disease. Others argue that disease is a concomitant of aging and to
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