This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Biostats, Subject name: Social & Preventive Medicine
WHO recommended Sampling method for Immunisation coverage evaluation 
Select and explain. 
A. Simple random sampling, B. Cluster random sampling, C. Stratified random sampling, D. Convenience sampling
B: Cluster random sampling
Answer: B: Cluster random sampling 
Explanation: Cluster random sampling (CRS) 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 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). Accuracy: Low error rate of only +- 5%. Limitation: Clusters cannot be compared with each other.