The community aspects of disease, arising from environmental hazards of many kinds, gave rise to collective "therapy" that combined preventive medicine and engineering in a new field of public health. The expectation of life, this is a well-known parameter, perhaps the most often used measure of health to be found in the demographic literature. The categories of cause of death that are used in vital statistics are elaborations of those developed for purposes of medical diagnosis. Since the purpose of diagnosis is treatment, the causal system that physicians use has evolved to correspond to the methods successful in treating individual patients. In a homogeneous population, where everyone is subject to the same risk, the reduction in the rate and the increase of the expectation of life are the same thing, and no one can object to using either one as the criterion. But in a heterogeneous population, a value judgment in the selection of the criterion is inescapable.