ABSTRACT
Whereas prevalence studies aim to describe how much disease is present at a particular point in time, cohort and case-control studies aim to explore what may have caused the disease in the first place. A cohort study is usually conducted prospectively and over a long period. Subjects in the study cohort are followed up over a period of time. The information that is collected on exposure to the risk factor can then be analysed in order to ascertain how many subjects, both exposed and not exposed, develop the disease. Retrospective cohort studies are also possible, and are common in occupational epidemiology and disease outbreak investigations. Relative risk should be used in a cohort study to assess the likelihood of developing the disease in subjects who have been exposed to the risk factor, relative to those who have not been exposed to it. Attributable and population attributable risks can also be calculated, and the Chi-squared test can be employed.
