stratified sampling
the process of selecting a sample from a population comprised of various subgroups (strata) in such a way that each subgroup is represented. For example, in a study of college students, a researcher might wish to examine people from different majors (e.g., social sciences, physical sciences, humanities). The selection procedure within each of these strata may be random or systematic. In stratified random sampling, a chance process (e.g., a random number generator) is used to select individuals, whereas in stratified systematic sampling an objective, orderly procedure is applied to choose individuals (e.g., listing all of the students within each major alphabetically and choosing every 10th case).