• 1) analytical Study design
• 1.1) Observational study design
• 1.1.1)case-control
• 1.1.2) cohort (prospective and retrospective)
• 1.2) interventional

• 2) Descriptive study design
• 2.1) individual
• 2.1.1) case reports
• 2.1.2) case series
• 2.1.3) cross sectional survey
• 2.2) population (ecological study )

• 3) experimental study design

In fields such as epidemiology, social sciences, psychology and statistics, an observational study draws inferences from a sample to a population where the independent variable is not under the control of the researcher because of ethical concerns or logistical constraints. One common observational study is about the possible effect of a treatment on subjects, where the assignment of subjects into a treated group versus a control group is outside the control of the investigator.This is in contrast with experiments, such as randomized controlled trials, where each subject is randomly assigned to a treated group or a control group.

A case–control study (also known as case–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. Case–control studies 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 controls).[1] They require fewer resources but provide less evidence for causal inference than a randomized controlled trial. A case–control study produces only an odds ratio, which is an inferior measure of strength of association compared to relative risk.