In a cohort study, a group(cohort) of people (subjects) who are similar is studied over a period of years (a longitudinal study) to determine if there is a relationship between exposure to a risk factor and development of a disease. At the beginning of a prospective study, none of the subjects has the disease. They are followed up for a number of years, and at the end of the period, those who have developed the disease are compared with those who have not. In a retrospective study, the researchers look back, by studying hospital records for example, at what has happened in the past, comparing subjects who have developed the disease with those who have not. In a cohort study and in a case-control study the subjects are only observed and there is no intervention such as drug treatment or surgery. A cohort study is a more reliable method of identifying the cause of disease than a case-control study. But for proof of cause, a trial is needed.

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