Up to now we have almost exclusively dealt with “person-based” studies. By this we mean that both the variable “risk factor”and the variable “health outcome”are defined for individual persons, who are human beings. Typical factors are age, nicotine consumption, housing standard, a diagnosis and a medical treatment received, defined for each person in the target population. Likewise, the outcome, which may for example be the appearance of lung cancer or the cure from malaria, concerns individuals. Studying the influence of a factor means comparing the frequency of the outcome in subgroups determined by different levels of the factor; see Sect. 15.1.