cluster sampling
a tiered method of obtaining units for a study. A population is first subdivided into smaller groups or clusters (often administrative or geographical), and a random sample of these clusters is drawn. The process is then repeated for each sampled cluster until the required level is reached. An example would be sampling voters in a large jurisdiction (e.g., a state) by randomly choosing subgroups (e.g., counties) and then further subgroups (e.g., towns and cities) until individual participants are obtained for a study.