Sampling from large populations
Two-stage sampling is a sampling scheme that is related to cluster sampling, but is of most use for large populations when the individuals are very widely separated in some sense. For example, many polls are conducted to obtain national information about voting intentions or consumer purchases, and there is a high cost associated with travelling between different regions.
In two-stage sampling, the population is separated into groups of adjacent individuals called primary sampling units. These primary sampling units are typically large — for example a country might be split into 20 or more regions. A small number of these are selected according to some sampling scheme, then individuals are sub-sampled within each selected primary unit.
Costs are reduced by limiting sampling to a small number of primary units. For example, if individuals are only sampled from within say 5 regions, travelling and accommodation costs will be considerably reduced.