An economical method for sampling a scattered population. When, as is usually the case, a geographical population is scattered, it would be uneconomic to visit the scattered individuals chosen by simple random sampling. Instead, the population is subdivided into a large number of geographically compact regions (the clusters) and a random sample of clusters is selected. These are referred to as the primary sampling units (PSUs). In single-stage cluster sampling all members of the selected cluster are interviewed. In multi-stage cluster sampling, further subdivisions take place.