Cluster Random Sampling : Applicable when units of population are natural groups or clusters. Use of CRS in India: Evaluation of immunization coverage. WHO technique used in CRS: 30*7 technique (Total= 210 children) 30 clusters, each containing 7 children who are 12-23 months age and are completely immunized for primary immunization (till measles vaccine) Clusters are heterogenous within themselves but homogenous with respect to each other. Sampling interval is also calculated in CRS. Accuracy: Low error rate only +/- 5% Limitation: Clusters cannot be compared with each other. Ref: Simple Biostatistics by Indrayan and Indrayan 1st edition Pgno: 35