In a cohort study, a defined population with known information on exposure is followed up. Information on diseases or causes of death is collected. Finally, the correlation of these outcomes with the exposure is analyzed. Subjects who presently have a certain condition and/or receive a particular treatment are followed over time and compared with another group who are not affected by the condition under investigation. For research purposes, a cohort is any group of individuals who are linked in some way or who have experienced the same significant life event within a given period. There are many kinds of cohorts, including birth (e.g., all those who born between 1970 and 1975), disease, education, employment, family formation, etc. Any study in which there are measures of some characteristic of one or more cohorts at two or more points in time is cohort analysis.
In some cases, cohort studies are preferred to randomized experimental design. For instance, since a randomized...