This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Biostatistics, Subject name: Social & Preventive Medicine
Sampling method used in assessing immunization status of children under immunization programme is - 
Select and explain. 
A. Systemic sampling, B. Stratified sampling, C. Group sampling, D. Cluster sampling
D: Cluster sampling
Answer: D: Cluster sampling 
Explanation: Ans. is 'd' i.e., Cluster sampling Important facts of cluster random sampling (CRS)o Cluster sampling is used in India to evaluate immunization coverage.o WHO used 39 x 7 technique (total = 210 children) for cluster sampling in which there are 30 clusters, each containing 7 children who are 12 - 23 months old and are completely immunized for primary immunization (till measles vaccine at 9 month)o Clusters are heterogenous within themselves but homogeneous with respect to each othero Main objective of cluster sampling is to reduce costs by increasing sampling efficiency; in contrast to stratified sampling where; the main objective is to increase precision.o Cluster sampling is quiet accurate with low error rate of only +- 5%.o Cluster sampling can also calculate sampling interval.o The main limitation of cluster sampling is that cluster cannot be compared with each other.