cohorts

A cohort is a group of individuals who share a defining characteristic or experience within a specific period. The term originates from the Latin cohors, historically a division of Roman soldiers; in modern usage it covers both abstract groups and concrete units such as student classes or patient registries.

In research, a cohort refers to a group followed over time to observe outcomes. A cohort study collects data prospectively, following participants from exposure to outcome, or retrospectively, using existing records. Cohorts enable estimation of incidence and risk factors and are useful for examining the natural history of diseases. Limitations include loss to follow-up, confounding, and the time and cost required.

In demography and epidemiology, birth cohorts are groups born in the same period, often used to study how social or environmental changes affect outcomes across life stages. The term cohort effect describes differences among age groups arising from shared experiences rather than aging alone.

In other contexts, cohorts describe groups entering a program together, such as a student cohort or a new-hire cohort in a company. In military history, a cohort was a unit within a legion, typically comprising several hundred soldiers; the size and organization varied by era. The concept remains widely used to denote any defined group moving forward together.