Cohort

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In statistics and demography, a cohort is a group of subjects — most often humans from a given population — defined by experiencing an event (typically birth) in a particular time span. For example, Irish women born in the year 1950 would form a cohort, when studied from a point of view such as their health or mortality, or education, or marriages. A cohort study often tracks a cohort over extended periods of time and returns to the same sample groups decades later. The cohort can be modified by censoring. Censoring means excluding individuals whose follow-up is lost from statistical calculations from the time the individuals are lost onwards.

Demography often contrasts cohort 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 product 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. In life table, a cohort refers to a specific rate of fecundity.