Question

what is the difference between case control and cohort trial with examples for each trial?

Answer 1

Case-control and cohort studies are observational studies that lie near the middle of the hierarchy of evidence. These types of studies, along with randomised controlled trials, constitute analytical studies

In case-control, one starts from the outcomes i.e. cases and bases/controls/referents (matched or not), and tries to study what the exposure was. Using the 'rare disease assumption', both odds ratio and relative risk can be used to study association between exposure and outcome. However, odds ratio is preferred because the prevalence of the disease outcome is already identified.

Cohort is when one already has determined the exposure in the study cohort and tries to study the association of exposure to disease outcome within that cohort in a retrospective manner.

Examples :

Case-control study

Demonstration of the link between tobacco smoking and lung cancer, by Richard Doll and Bradford Hill. They showed a statistically significant association in a large case–control study. Opponents argued for many years that this type of study cannot prove causation, but the eventual results of cohort studies confirmed the causal link which the case–control studies suggested, and it is now accepted that tobacco smoking is the cause of about 87% of all lung cancer mortality in the US.

Cohort study

Framingham Heart Study, which recruited over 5,209 male and female participants in 1948 from around the area of Framingham, MA. It has continued to serve as a source of data for cardiovascular risk factors.

A second cohort was recruited in 1971 and a third in 2002. The study has made important contributions to the understanding of heart health. The researchers are now looking into how genetic factors may affect cardiovascular risk.

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