Cohort studies: history of the method - I. prospective cohort studies

The term "cohort study" was introduced by Frost in 1935 to describe a study that compared the disease experience of people born at different periods, in particular the sex and age specific incidence of tuberculosis and the me thod was extended to the study of non-communicable disease by Korteweg who used it 20 years later to analyse the epidemic of lung cancer in the Nether lands. Such studies are now best described as generation studies or generat ion cohort studies to distinguish them from the common type of study that i s now carried out that consists in defining groups of individuals distingui shed by some variable (such as place of residence, occupation, behaviour, o r environmental exposure)and following them up to see if the incidence or m ortality rates vary with the selected variable. This type of study is now o ne of the most important tools for epidemiological investigation. initially called prospective studies, because the information characterising the ind ividuals in the cohorts was recorded before the onset of disease, they are now preferably called cohort studies and distinguished as prospective cohor t studies, if the information obtained relates to the subjects at the time the study is started and they are then followed, or retrospective cohort st udies, if the information characterising the individuals was recorded somet ime in the past (for example, the receipt of radiotherapy, or entry to a sp ecific occupation). Studies of either type have the great advantage that they avoid all the mos t important sources of bias that may affect case-control studies, but the d isadvantage that because incidence rates and more specifically mortality ra tes are commonly low large numbers of subjects have to be followed for seve ral (if not many) years to obtain statistically significant results. Several early prospective studies are described: Namely, those of 34000 mal e British doctors, 190000 male and female American citizens with different smoking habits, some 5000 middle aged residents of Framingham with differen t blood pressures, blood cholesterol levels, etc, and 13000 children born i n the UK in one week in 1946 with different family backgrounds.