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Epidemiology is the study of the distribution and determinants of health-related conditions in the population. As such, it is concerned with issues such as the total number of cases of a specific disorder in the population (prevalence) and the number of new cases that arise within a given time frame (incidence). These data are most often derived from cross-sectional surveys, which have become increasingly sophisticated over four "generations" of epidemiological research, and now consist of nation-wide studies involving interviews with thousands of randomly-selected individuals. In addition to counting individual cases, epidemiology is concerned with quantifying the degree to which certain factors may predispose people toward certain disorders, while others may be protective. These effects are captured with statistics such as the relative risk and the odds ratio. There are specific research designs that are used to gather these data, including case-control and