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method used in studies to describe an aggregate of individuals having in common a significant event in their life histories, such as year of birth (birth cohort) or year of marriage (marriage cohort). The concept of cohort is useful because occurrence rates of various forms of behaviour are often influenced by the length of time elapsed since the event defining the cohort-e.g., the rate of fertility is influenced by the event of marriage. The idea of cohorts is similar to that of generations, although some writers avoid the latter term because it also has other connotations.