Cluster Sample

For a cluster sample, a population is first divided into smaller groups with similar characteristics to the whole population called clusters. One or more clusters are randomly selected. All members in the selected clusters form the sample. A member of the population cannot be included in more than one cluster. Consider an example. A group of researchers conduct a study to examine the basic mathematical skills of all the eight-graders in a city. They consider each school in the city as a separate cluster. The researchers then select one cluster to collect data from. Cluster sampling can be conducted as follows.

Cluster sampling is particularly useful for populations that have a wide geographic spread where simple random sampling is difficult to apply. Note that already-existing groups such as cities and schools can be used as clusters.