A cohort study, commonly used in medical and nutritional epidemiology, is a longitudinal study that samples a cohort (a group of people that is defined by having experienced the same event, e.g. birth or graduation, in a selected time period) and follows it over time to investigate people’s exposure to certain factors (e.g. the consumption of certain foods) and the occurrence of particular health outcomes. For example, a cohort study may measure the dietary patterns over time of a group of people who were born between 1960 and 1970 to find out which dietary patterns are associated with the development of diabetes or particular cancers. There are two types of cohort studies. Prospective studies follow a cohort of people who differ by certain factors (e.g. smokers and non-smokers) to study which medical conditions appear in what part of the cohort (e.g. after 20 years more smokers than non-smokers have developed lung cancer). Retrospective studies study a cohort after a medical condition (e.g. lung cancer) has occurred in a part of the group to trace back which factors (e.g. smoking) may have contributed to this.