This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Social & Preventive Medicine
Estimation of percentage of children immunized in community as per WHO is to be done by: 
Select and explain. 
A. Multistage random sampling, B. Cluster random, C. Systematic random sampling, D. All of the above
B: Cluster random
Answer: B: Cluster random 
Explanation: WHO recommends cluster random sampling (CRS) to estimate the percentage of children immunized in the community, paicularly in developing nations. Their recommended strategy is 30x7 sampling. Ref: Medical biostatistics, 1st edition pg: 201