These four stages of information development take us from recognition of a health problem through its solution. Evoking the philosophy and work of the Centers for Disease Control and Prevention (CDC), they constitute the Public Health Approach to problem solving. The first two stages fall under the realm of descriptive epidemiology. Stages three and four involve analytic epidemiology, with its capacity to evaluate both putative cause-and-effect relationships and interventions based on this etiologic understanding. Etiology is the science of causation.