For National Immunization Programme, children surveyed in cluster sampling are

Cluster Random Sampling Applicable when units of population are natural groups or clusters Use in India: Evaluation of immunization coverage WHO technique used: 30 x 7 technique (total = 210 children) WHO technique used in CRS: 30 x 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 heterogeneous within themselves but homogenous with respect to each other Sampling interval is also calculated in CR5 Accuracy: Low error rate of only ± 5% Limitation: Clusters cannot be compared with each other.