A type of observational study that takes a group (cohort) of people and follows their progress over time in order to measure outcomes such as disease or mortality rates and make comparisons according to the type of exposure (treatments or interventions) that they received. For example, comparing mortality between one group of patients that received a specific treatment and one group which did not (or between two groups that received different levels of treatment). Cohorts can be assembled in the present and followed into the future (a ‘concurrent’ or ‘prospective’ cohort study) or identified from past records and followed forward from that time up to the present (a ‘historical’ or ‘retrospective’ cohort study).