Where does cohort collide?

In epidemiology a cohort is a defined group of people who share a starting characteristic, usually a common exposure or a common time of entry, and who are then followed forward in time so that the rate of a later outcome can be compared between them. Grimes and Schulz's 2002 Cohort studies: marching towards outcomes set out how such a study tracks a group from suspected cause toward effect, and how it differs from a case-control study that starts from the outcome and works backward. The referent is a group assembled to be watched for what happens to it, defined by exposure and outcome, not a whole generation born in the same years, a fighting unit of the Roman army, or a class of students moving through a course.[1]

Unit

One followed group, sized by its number of people and its person-years of observation

Recovery means

Not applicable

Measured by

Followed prospectively so that incidence rates and risk ratios can be compared between exposed and unexposed members

In demography a cohort, most often a birth cohort, is the set of all people born in the same period who are then tracked together as they age, so that the experience of one generation can be told apart from the effects of simply getting older and from the effects of a particular year. Ryder's 1965 The Cohort as a Concept in the Study of Social Change argued that a birth cohort carries its own history through the age structure and is the natural unit for studying social change, the basis for separating age, period, and cohort effects. The referent is a whole generation defined only by when its members were born, not a group assembled around an exposure, a Roman army unit, or an entering class of students.[2]

Unit

One birth cohort, all the people born within a defined span of years

Recovery means

Not applicable

Measured by

Followed across the life course so that its rates can be separated into age, period, and cohort components

In education a cohort is a group of students who enter a program at the same time and move through it together as a fixed unit, taking the same courses in the same order and forming a stable peer group along the way. Maher's 2005 The Evolving Meaning and Influence of Cohort Membership examined how such a group shapes learning and belonging, the idea behind cohort-based programs and cohort learning. The referent is an intake of learners bound together by a shared schedule, not a group watched for a health outcome, a generation defined by birth year, or a fighting unit of the Roman army.[3]

Unit

One entering class of students who progress through a program together

Recovery means

Not applicable

Measured by

Tracked by shared enrolment and by progression and completion rates through the program