cohort, cohort analysis The term cohort originally referred to a Roman military unit, but it is now used to identify any group of people with a time-specific common experience, such as graduating from school in the same year, or cohorts defined by time of marriage or widowhood. Cohort analysis refers to any study in which there are measures of some characteristics of one or more cohorts at regular intervals after the defining event.

Cohort analysis as a method of research was developed by demographers and applied primarily to the study of fertility. The most common type of cohort analysis uses age-groups (birth cohorts), for example five- or ten-year age-bands, to study mortality rates. The individuals within the bands move through the ageing process together as a cohort, and often become identified as a distinct group, such as the ‘Baby-Boomers’ or ‘Ageing-Hippies’. This approach is especially common in secondary analysis, as age is a commonly recorded item of information in population census data-sets, and in data from registers and administrative records. Cohort analysis can also be applied to repeated cross-sectional survey data-sets, where samples are large enough to distinguish a number of cohorts defined by (for example) age, or year of first childbirth. Proxy cohort data or analysis is achieved by tracking the characteristics of ten-year age-groups through successive decennial population censuses or equivalent large data-sets.

The critical problem of cohort analysis is to differentiate age, cohort, and period effects. Age effects are associated with growing older; cohort effects are common to people born at the same time; period effects are due to the shared experience of particular historical events—for example the Second World War. Unfortunately there is no simple way of disentangling these. This is important substantively: because the effects are confounded, different interpretations of change are usually possible (for example, observed cultural, political, or social changes may be due to cohort effects or ageing), and conclusions have to be correspondingly tentative. When informed by sound theory, however, cohort analysis is a powerful analytic technique (see H. Blossfeld , ‘Career Opportunities in the Federal Republic of Germany: A Dynamic Approach to Study Life Course, Cohort and Period Effects’, European Sociological Review, 1986
).

co·hort / ˈkōˌhôrt/ • n. 1. [treated as sing. or pl.] an ancient Roman military unit, comprising six centuries, equal to one tenth of a legion.2. [treated as sing. or pl.] a group of people banded together or treated as a group: a cohort of civil servants. ∎  a group of people with a common statistical characteristic: the 1940–44 birth cohort of women.3. a supporter or companion. ∎  an accomplice or conspirator.

COHORT

A cohort refers to a group of people that were born at the same period of time. They are likely to share some common experiences such as social, cultural, and historical influences that are unique to them. Examples of well-known cohorts include "baby boomers" and "Generation Xers." Cohort effects arise when changes in performance are due circumstances specific to a particular time and place, rather than age. Thus, if looking at physical development, children born during times of conflict or war may have retarded physical growth due to stress and food deprivation. Therefore, growth pattern norms might be inaccurate if based on this cohort. Another example would be gender differences in vocational aspirations for adolescents. Career choices by males and females could depend on whether fifteen-year-olds born in 1985 participated in the study versus fifteen-year-olds born in 1935. Both cross-sectional and longitudinal research designs are susceptible to cohort effects. A cross-sequential design may be used when cohort effects are suspected.

See also:DEVELOPMENTAL NORMS; STAGES OF DEVELOPMENT

Bibliography

Miller, Scott. Developmental Research Methods, 2nd edition. Englewood Cliffs, NJ: Prentice-Hall, 1998.

Katherine M.Robinson

cohort body of infantry in the ancient Roman army; also transf. XV. — (O)F. cohorte or L. cohors, cohort- enclosure, company, crowd, f. CO- + *hort-, as in hortus garden.

Cohort

a division in the Roman army; a band of warriors. See also band, company.

Examples: cohort of acquaintances, 1719; of bright cherubim, 1667; of Christian fathers, 1858; of infantry, 1489; of priests, 1874; of social regenerators, 1871; of warriors, 1500.