In statistics and demography, a cohort is a group of subjects who have shared a particular event together during a particular time span (e.g., people born in Europe between 1918 and 1939; survivors of an aircrash; truck drivers who smoked between age 30 and 40). 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.