<unk>  versus cohort on a timeline. "OR" stands for "odds ratio" and "RR" stands for "relative risk". A<unk>  (also known as<unk> –referent study) is a type of observational study in which two existing groups differing in outcome are identified and compared on the basis of some supposed causal attribute.<unk> es are often used to identify factors that may contribute to a medical condition by comparing subjects who have that condition/disease (the "cases") with patients who do not have the condition/disease but are otherwise similar (the<unk> s"). They require fewer resources but provide less evidence for causal inference than a randomized<unk> led trial. A<unk>  produces only an odds ratio, which is an inferior measure of strength of association compared to relative risk.
Case–control study
Natural history study
Nested case–control study
Cohort study
Retrospective cohort study
Prospective cohort study
Randomized controlled trial
Population study