Two-stage sampling
This simulation demonstrates how accuracy decreases when fewer primary units are sampled. Each vertical band of 8 crosses is a primary unit.
Firstly move the slider two-thirds to the right to make the primary units more dissimilar. Then use the control at the top right to change the number of primary units and take several samples to see how the standard deviation of the sample mean changes. The diagram also shows the possible cost of the sampling scheme.
When the primary units are dissimilar,
The number of primary units that are sampled is a trade-off between accuracy and cost.