What are Cohort studies?
A cohort study is an epidemiological study design called an observational study. Observational studies differ from experimental studies in that the researcher does not control the assignment of people to groups. Instead the groups are 'observed'. Unlike experimental studies, observational studies do not look at the effectiveness of an intervention. A cohort study is an analytical observation study, i.e. it has a comparsion (control) group. Cohort studies follow up two or more groups from exposure to outcome. They compare the experience of one group exposed to a factor (exposed group) with that of the other which was not exposed to the factor (control group). If the exposed group has a higher or lower frequency of an outcome than the unexposed, then an association between exposure and outcome is evident. Cohorts look at causes and natural history of diseases and are also useful for examining prognosis of people who already have the disease. In a cohort study, precise choice of cohort depends on the nature of disease under investigation. It is worth noting that the term 'cohort' is also used in relation to a group of people who share a similar experience at a point in time, e.g. birth cohort. A cohort can sometimes be used to mean a group of individuals followed over a period of time to examine the cause or progress of disease (or physiological variable) or to chart its long term effects.
Cohort studies can be designed to be prospective or retrospective. Prospective design allows exposure to risk factors to be assessed directly and confounding variables to be considered. A retrospective design is effective for diseases with a long development time. Sometimes referred to as historical cohort studies, they offer the advantage of speed and low cost compared to a prospective cohort. Historical cohort studies should not be confused with case-control studies (also retrospective). Cohorts track people forward in time from exposure to outcome. Case control studies trace backwards from outcome to exposure.
