Relations among standard epidemiologic measures in a population.

Recent developments in population mathematics apply to measurement issues in epidemiology. In particular, they demonstrate explicitly the relations that prevail among incidence, prevalence, case-fatality, mortality, and duration of illness in a population at a moment in time, rather than in a cohort of persons followed through time (or in a population artificially assumed to be stationary). They indicate explicitly how certain common indicators such as the ratio of deaths to new cases should be interpreted. They also suggest possible new strategies for estimating certain measures, but these would require some reorientation of current approaches to measurement.