Ans: Based on the exposure, subjects are enrolled in the cohort studies. Subjects are enrolled based on their interest in the disease. If the disease is present or not in the case of a case-control study. Disease status and exposure are accessed in both the subjects. The cohort studies are done for many years. The other study is upon any new outbreak of a disease. A wide range of health problems is studied using both cohort and case-control studies. They also deal with two branches of diseases. Infectious branch and non-infectious branch. A snapshot of the populations’ health is provided in the cross-control study. So, it deals with prevalence and not incidence.