Some quasi-experimental designs would be included in this category. Clinical trials and community trials can be in this category. In a cohort study, members of a defined population receive different exposures and their outcomes are compared. Assignment of exposure is not in the hands of the investigator. In a cohort study, we have at least some information about people at the level of the individual.
Cohort studies can be prospective or retrospective.
Case-control study Cases are persons with the outcome of interest, and controls are drawn from the same population that produced the cases. The exposure of both groups is then compared. This is also a type of observational study. Case-control studies can be prospective or retrospective. The intervention of interest is applied across an entire population or group.