4. Cluster sampling: A cluster random sample is a two-step process in which the entire population is divided into clusters or groups, usually geographic areas or districts like villages, schools, wards, blocks, etc. It is more commonly used in epidemiologic research than in clinical research. It is most practical to be used in large national surveys. The clusters are chosen randomly. All individuals in the cluster are taken in the sample. Usually it requires a larger sample size. Cluster sampling is very useful when the population is widely scattered and it is impractical to sample and select a representative sample of all the elements.