Observational studies refer to the broad class of epidemiological study designs characterized by the fact that exposure is not assigned by the investigator. Rather, the investigator passively observes as nature takes its course (Aschengrau & Seage, pp. 140-141).

Two subsets of observational study are descriptive and analytical studies

• Descriptive observational studies are not designed to test a causal hypothesis, but collect information on the distribution of disease patterns in terms of the characteristics of person, place, and time (Aschengrau & Seage, p. 99).
• Analytical observational studies assemble a population for study for the purposes of testing a causal hypothesis concerning the relationship between exposure and disease. These include the cohort and case-control studies.