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.

Cost and accuracy (advanced)

In the example above, there was a considerable cost involved with travel between the primary units, so the total cost is reduced when fewer primary units are sampled. Unfortunately, the accuracy of the resulting estimate is usually lower in this situation.

The number of primary units to sample is therefore a trade-off between accuracy and cost. The details are beyond the scope of CAST.

As with cluster and stratified sampling, formulae for the standard deviation of the resulting estimates are beyond the scope of an introductory course.