Epidemiology studies the variations in health in populations, according to different factors and parameters. In this field, probability and statistics are used in order to provide a quantitative description and analysis of the variations in exposure and disease, as well as of the effects of possible preventatives. Thus, one goal of epidemiology is to establish statistical generalizations about health and disease in populations. Consequently, it is important to understand how statistical generalizations are established and what use one can make of them to establish medical knowledge or to design public health policies.