A cohort is a group of subjects who have shared a particular event together during a particular time span.

Cohorts may be tracked over extended periods in a cohort study. The cohort can be modified by censoring, i.e. excluding certain individuals from statistical calculations relating to time periods (e.g. after death) when their data would contaminate the conclusions.

A cohort study is a particular form of longitudinal study.

Matched cohort study

Prospective cohort study

Retrospective cohort study