Overview of clinical study designs.

The goal of a clinical study is to determine the factors associated with a disease and to assess the efficacy and safety of an investigational drug, procedure, or device. Since clinical study designs vary due to unique requirements of individual studies, the aims of this report are to educate researchers on the different types of studies and to assist researchers in choosing the optimal study type to fulfill their individual requirements. Clinical studies are classified into the two main types, observational studies and clinical trials, depending on the presence or absence of an intervention. Observational studies include case-control studies, cohort studies, and cross-sectional studies. Case-control and cohort studies may be prospective or retrospective, and case-control studies may be nested or not. Clinical trials may be pragmatic and may be controlled or noncontrolled; randomized or nonrandomized; open label or blinded; and parallel, crossover, or factorial. These observational and clinical trial designs are reviewed. Each type of clinical study has advantages and disadvantages. Therefore, researchers must consider these in choosing the design best suited for achieving their study objectives.