Proper scientific sampling is an important element in the study of populations. The population of interest may be a general population, such as all people in the United States 18 years of age or older, or a targeted subpopulation, such as people in the United States 18 years of age or older living in poverty or living in urban areas. Many epidemiologic studies involve gathering information from such populations. The population of interest in such studies may be specific—such as physicians, people in homeless shelters, people with a specific chronic disease—or may involve any population that can be accurately defined. Drawing an appropriate sample of the target population is the foundation on which such studies must be built. An improper sample can negate everything that the study wishes to discover. This entry describes different techniques that can be used for drawing a proper sample and when they are most appropriate. ...