Observational studies include cross-sectional, case-control and cohort studies. Of these 3 study designs, the cohort method is the preferred design for confirming causal (antecedent) associations of health outcomes.

An extension of the cohort design would be an “experiment” to see whether the expected outcome is found in a group receiving the exposure (antecedent) with a control group not receiving the exposure.

A “natural experiment” would follow a cohort exposed to a natural phenomenon of health consequences and then compare the health effects in the exposed population with those not exposed to the phenomenon.

“Experimental” studies are referred to as “experimental epidemiology.” They are applied as “clinical trials” to assess the efficacy of some intervention (or procedure), as surgical technique, type of facility, or type of health service, to name a few. The more common term used in Epidemiology is “randomized trial.”