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.

Cho S, et al. J Clin Neurol. 2020 Apr;16(2):237-244. https://doi.org/10.3988/jcn.2020.16.2.237

Moon JY, et al. Korean J Radiol. 2021 Jul;22:e98. https://doi.org/10.3348/kjr.2020.1428

Kim HY, et al. J Korean Med Sci. 2017 May;32(5):803-809. https://doi.org/10.3346/jkms.2017.32.5.803

Shin S, et al. Allergy Asthma Immunol Res. 2024 May;16(3):300-307. https://doi.org/10.4168/aair.2024.16.3.300

Prader–Willi syndrome (PWS) is a rare genetic disorder characterized by an insatiable appetite that leads to morbid obesity. Previous studies reported health problems in adults with PWS. However, studies on younger adults are lacking, and there are no specific studies of endocrine and metabolic illness in this age group. We performed a retrospective cohort study of 68 individuals with PWS aged 19 to 34 years at Samsung Medical Center. The prevalence of endocrine and metabolic illnesses were compared with those in an age-, sex-, and BMI-matched healthy control group. Young adults with PWS had a higher prevalence of metabolic syndrome (35.3% vs. 4.4%), type 2 diabetes mellitus (50.0% vs. 5.4%), hypertension (30.8% vs. 16.1%), dyslipidemia (38.2% vs. 14.7%), decreased bone density (26.4% vs. 0.9%), and sleep apnea (32.3% vs. 4.4%) than controls (all p < 0.05). The PWS group that maintained recombinant human growth (rhGH) treatment in adulthood had a lower probability of having a BMI ≥ 30 at the last follow-up (odds ratio = 0.106 (0.012–0.948), p = 0.045). Endocrine and metabolic illnesses in individuals with PWS may have already started in the early teens; therefore, appropriate screening and early intervention are important. Better understanding of the natural history of PWS and age-related complications will lead to better-quality medical care for individuals with PWS.