a research project that examines longitudinal data obtained in the past to track the incidence of a particular disorder over time and its association with various risk factors. For example, a researcher could examine the health records of smokers and nonsmokers to follow the path of an illness from before its manifestation through to its diagnosis and treatment. It is distinct from prospective research generally, which begins with individuals who are apparently healthy in the present and moves forward to investigate whether a specific disorder will occur over time.