Cluster samples may be chosen where individuals fall naturally into groups or clusters. In the above scenario, a sample of the clusters is chosen at random, and then a random sample of units is chosen from within this selection of clusters. The villages are naturally formed groups or clusters in the region. Ref: Encyclopaedic Companion to Medical Statistics, Editors Brian S. Everitt and Christopher R. Palmer, Second Edition; Oxford Handbook of Medical Statistics, By Janet L. Peacock, Philip J. Peacock, Oxford University Press 2011, Page 55.