A cohort study is an observational study design in which subjects are usually selected based on their risk factor exposure and followed over time to evaluate whether they develop the outcome of interest (usually disease).
Cohort studies are commonly used in behavioral medicine research to investigate associations in which experimental designs are unethical or too costly. In a cohort design, participants who have not experienced the outcome of interest are selected, usually based on whether or not they have been exposed to the risk factor of interest. Therefore, a cohort study design is efficient when the exposure is relatively rare but the outcome of interest is common. For example, a cohort design was used in a study of the association of prenatal polychlorinated biphenyl (PCB) exposure with behavior issues and cognitive disability (Lai et al. 2002). A cohort design is...