Overview Observational studies are studies in which subjects are observed for the effect of exposures on outcomes without intervention by researchers Often done when randomization is not possible for logistical or ethical reasons Descriptive Studies Case report login to view 3 more bullets Case series login to view 3 more bullets Prospective and Retrospective Studies Prospective studies login to view 3 more bullets Retrospective studies login to view 2 more bullets Case-Control Study Retrospective (almost always) Compares patients with and without disease to asses their odds of a certain exposure Odds ratio (OR) is measure of disease association login to view 3 more bullets "How much more likely is it that patients with cirrhosis have been exposed to heavy alcohol use compared to controls?" Cohort Study Can be either prospective or retrospective login to view 2 more bullets Compares groups with and without an exposure to assess associations with subsequent disease Relative risk (RR) is measure of disease association login to view 3 more bullets "How much more likely are patients to develop cirrhosis if they are exposed to heavy alcohol use?" Cross-Sectional Study Exposure and outcome are assessed simultaneously Ascertains association not causality Prevalence is measure of disease association "How much higher is the proportion of cirrhosis patients with heavy alcohol use compared to those without cirrhosis?" Ecological Study Exposure and outcome assessed at the population level login to view 1 more bullet Prevalence and incidence are measures of disease association Often used for rapid hypothesis generation "How much higher is the proportion of cirrhosis patients in a country where average alcohol use is high compared to a country in which alcohol is illegal and of low use?" Disadvantage is the potential for "ecological fallacy" login to view 2 more bullets Sibling Studies Measures relative influence of environment vs genetics on trait variation Twin concordance study login to view 1 more bullet Adoption study login to view 3 more bullets