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
In the WHO recommended EPI Cluster sampling for assessing primary immunization coverage, the age group of children to be surveyed is:- 
Select and explain. 
A. 0-12 months, B. 6-12 months, C. 9-12 months, D. 12-23 months
D: 12-23 months
Answer: D: 12-23 months 
Explanation: Cluster random sampling: Used for immunization coverage evaluation Error rate for immunization coverage evaluation - +- 5% WHO recommended technique - 30 x 7 Total sample size - 210 Intercluster disparity - even the cluster are not comparable to each other; To remove intercluster disparity, we use DESIGN effect 30 clusters selected by systematic random sampling