Multistage Sampling
Multistage Sampling is a sampling strategy (e.g., gathering participants for a study) used when conducting studies involving a very large population. The entire population is divided into naturally-occurring clusters and sub-clusters, from which the researcher randomly selects the sample.
For example, you want to conduct a survey of salespeople for a nationwide retail chain with stores all over the country. You could randomly select states, randomly select counties in each state, randomly select stores in each county, and randomly select salespeople in those stores.