Longitudinal studies in other disciplines
Longitudinal studies in demography are conceptually similar to those in related disciplines such as epidemiology or genetics. However, the nature of demographic enquiries means that longitudinal studies are frequently population based studies. In epidemiology, participants for cohort studies are recruited in one of two ways:
- by diagnosis with a particular disease. For example, prostate cancer.
- by exposure status. For example, two groups of people one given a standard treatment and one given a new treatment.
The terminology used for longitudinal studies is varied and differs by discipline. Perhaps most worrying for demographers is the way in which the word “cohort” is used to describe a particular type of study. In demography the cohort 
, and its membership, is a clearly defined entity which does not vary over time. However, in cohort studies, the word cohort is used merely to describe the group of people chosen for the study and therefore the constitution of the cohort can vary during the study.