Many different study designs are used to address different types of clinical questions. The two broad categories are the experimental study, which is the randomized controlled trial, and observational studies, which includes all of the other basic study designs (cohort [prospective and retrospective] studies, case-control [etiologic] studies, cross-sectional [prevalence] studies, case series, and case reports). These study designs address basic categories of clinical questions: treatment, diagnosis, prognosis, and harm. This article focuses on some of these study designs, highlighting the impact of patient selection, inclusion, and exclusion criteria on results and outcomes through the use of specific examples.