A cohort study is an observational study in which a group of subjects are followed over time. Information is collected about the subjects’ characteristics and exposures and then they are followed to see which subjects experience the health outcome of interest. For example, the outcome of interest could be a heart attack, cancer or liver disease. The number of people who experienced the outcome is compared between those who were exposed to a factor and those who were not exposed. For example, the exposure of interest may be alcohol, a medication or high cholesterol. It is very important that the investigator is not involved in choosing whether or not a subject is exposed, and instead only observes what happens.