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A Cohort Study is a study in which patients 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 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.
Cohort studies are not as reliable as randomized controlled studies, since the two groups may differ in ways other than in the variable under study. For example, if the subjects who smoke tend to have less money than the non-smokers, and thus have less access to health care, that would exaggerate the difference between the two groups.
The main problem with cohort studies, however, is that they can
end up taking a very long time, since the researchers have to wait for
the conditions of interest to develop. Physicians are, of course, anxious
to have meaningful results as soon as possible, but another disadvantage
with long studies is that things tend to change over the course of the
study. People die, move away, or develop other conditions, new and promising
treatments arise, and so on. Even so, cohort studies are generally preferred
to case control studies , since they involve
far fewer statistical problems and generally produce more reliable answers.