An observational study is a study in which inferences are drawn or hypotheses tested through observational methods. Two common varieties are the descriptive study, where events are simply observed and described as they take their natural course, and the analytic epidemiologic study, which does not include any intervention or experimentation; examples include the case-control study and the cohort study. As human experimentation is fraught with ethical problems and often not feasible, many important epidemiologic discoveries have had to rely entirely on observational studies.
John M. Last
"Observational Studies." Encyclopedia of Public Health. . Encyclopedia.com. (October 20, 2018). http://www.encyclopedia.com/education/encyclopedias-almanacs-transcripts-and-maps/observational-studies
"Observational Studies." Encyclopedia of Public Health. . Retrieved October 20, 2018 from Encyclopedia.com: http://www.encyclopedia.com/education/encyclopedias-almanacs-transcripts-and-maps/observational-studies