Two-stage sampling
This simple diagram illustrates what is meant by two-stage sampling. Each box of 8 crosses represents a primary unit. The controller at the top adjusts the number of primary units sampled — the total sample size is n = 16.
The diagram also gives an idea of the relative cost of the sample schemes — sampling primary units is usually relatively expensive since they are usually different geographical areas. The fewer primary units, the cheaper the survey but also the less accurate the estimates (see next page).
Two-stage sampling is equivalent to...
In practice however there are usually too many individuals in each primary unit to sample all of them, and it would be too expensive to use all primary units.