Cohort (statistics)

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.

The term cohort can also be used where membership of a group is defined by some factor other than a time-based one: for example, where a study covers workers in many buildings, a cohort might consist of the people who work in a given building.

Demography often contrasts cohort perspectives and period perspectives. For instance, the total cohort fertility rate is an index of the average completed family size for cohorts of women, but since it can only be known for women who have finished child-bearing, it cannot be measured for currently fertile women. It can be calculated as the sum of the cohort's age-specific fertility rates that obtain as it ages through time. In contrast, the total period fertility rate uses current age-specific fertility rates to calculate the completed family size for a notional woman were she to experience these fertility rates through her life.