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A Cohort Study is a study in which 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 (for example, 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 controlled study to test the effect of smoking on health would be unethical, a reasonable alternative would be a study that identifies two groups, a group of people who smoke and a group of people who do not, and follows them forward through time to see what health problems they develop.

In general, Cohort analysis attempts to identify cohorts effects: Are changes in the dependent variable (health problems in this example) due to aging, or are they present because the sample members belongs to the same cohort (smoking vs. non smoking)? In other words, cohort studies are about the life histories of sections of populations and the individuals who comprise them. They can tell us what circumstances in early life are associated with the population's characteristics in later life - what encourages the development in particular directions and what appears to impede it. We can study such developmental changes across any stage of life in any life domain: education, employment, housing, family formation, citizenship and health

Graphical illustration of cohort study

Definition ||Advantages ||Disadvantages