Epidemiologists traditionally have used two study designs—case control studies and prospective cohort studies—that typically use samples of unrelated people (i.e., that do not include multiple family members in the same study) (e.g., Kleinbaum et al. 1982). In case control studies, investigators compare genetic or environmental risk factors for cases (e.g., alcoholics) and for unaffected control subjects (e.g., nonalcoholics). In prospective cohort studies, researchers follow a large cohort of initially unaffected people over an extended period of time to identify risk factors that predict subsequent disease onset. Geneticists, in contrast, typically use family studies and extended pedigree methods, often focusing on families in which multiple members are affected by a given disorder. In these studies, investigators analyze the co-occurrence among family members of a disorder and various genetic markers, attempting to identify markers that are inherited only by members who also develop the disorder. Case control studies using multiple genetic markers, however, which typically can have larger sample sizes and therefore greater statistical power, may soon replace traditional family studies (e.g., Risch 2000).
