Noel Weiss identifies when it is appropriate to use a case-control study design, a cohort study design or a clinical trial to answer a research question. This lecture was taped at the 2004 Epidemiology, Biostatistics and Clinical Research Methods Summer Session co-sponsored by the Seattle VA Epidemiologic Research and Information Center (ERIC) and the University of Washington. From the Series:Epidemiology: Introduction to Epidemiologic Methods