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 detailed description of a patient's symptoms, signs, treatment, and disease course conducted for hypothesis generation or to raise awareness of emerging diseases, rare pathologies or unusual presentations not useful for hypothesis testing Case series a group of case reports of patients with a similar exposure, treatment, or disease no comparison group not useful for hypothesis testing Prospective and Retrospective Studies Prospective studies patients enrolled before data collection participants followed over time data collected on exposures and outcomes as they arise Retrospective studies involve collecting historical information from a sample of patients enrollment occurs after exposure and outcome have already occurred 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 OR = odds of exposure among cases/odds of exposure in non-cases = ad/bc odds of exposure among cases = # cases with exposure/# cases without exposure = a/c odds of exposure among non-cases = # non-cases with exposure/# non-cases without exposure = b/d "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 In prospective studies, the outcome of interest has not yet occurred and patients are tracked prospectively for development of the outcome In retrospective studies, the outcome of interest has already occurred Compares groups with and without an exposure to assess associations with subsequent disease Relative risk (RR) is measure of disease association RR= incidence rate in exposed group/incidence rate in unexposed group = [a/(a+b)]/[c/(c+d)] incidence rate in exposed group = # of exposed cases/(# total exposed cases and non-cases) = a/(a+b) incidence rate in unexposed group = # of unexposed cases/(# total unexposed cases and non-cases) = c/(c+d) "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 population defined geographically or temporally 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" making spurious risk associations between populations which may not represent true risk association at the individual level e.g., assuming that people who live in a country with high alcohol use rates must be more likely to get cirrhosis by virtue of living in that country Sibling Studies Measures relative influence of environment vs genetics on trait variation Twin concordance study compares how often monozygotic and dizygotic twins both have the same trait or disease Adoption study 2 designs examines differences and similarities between child and adoptive vs birth parents examines differences and similarities between siblings raised by adoptive vs birth parents