Cluster sampling involves selecting clusters, which are groups of sampling units. For example, clusters might be schools, hospitals, or geographical areas, and sampling units might be students, patients, or citizens. Cluster sampling can provide efficiency in frame construction and other survey operations. However, it can also result in a loss in precision of your estimates, compared to a nonclustered sample of the same size. To minimize this effect, units within clusters should be as heterogeneous as possible for the characteristics of interest.