A form of longitudinal study that follows a group of people, or ‘cohort’, who share a common characteristic or experience within a defined period. For example, a group of people who were born on the same day or in a particular period form a birth cohort. The typical aim of a cohort study is to determine the effect on the group of an experience, or ‘treatment’. This is achieved by forming a comparison group who may be the general population from which the cohort is drawn, or another cohort of people that have not had the treatment but are otherwise similar. See also panel data.