## **Core Concept**
The World Health Organization (WHO) recommends the Expanded Programme on Immunization (EPI) Cluster sampling method for evaluating immunization coverage. This method involves selecting clusters and then assessing a sample of children within those clusters to estimate vaccination coverage. The age group of children surveyed is crucial for accurately reflecting recent immunization practices.

## **Why the Correct Answer is Right**
The correct answer, **12-23 months**, is chosen because this age range allows sufficient time for children to have completed their primary immunization series, which typically occurs within the first year of life, and also accounts for potential delays in vaccination. Children in this age group would have had ample opportunity to receive all vaccines included in the primary series, making it an ideal age range for assessing primary immunization coverage.

## **Why Each Wrong Option is Incorrect**
- **Option A (6-12 months):** This age range might be too early for all children to have completed their primary immunization series, especially if there were delays in vaccination.
- **Option B (9-15 months):** While closer to the correct range, this still might not capture all children who have completed their primary series, potentially leading to underestimation of immunization coverage.
- **Option D (18-36 months):** This range, although likely to capture all primary immunizations, may be too broad and could reflect older cohorts' vaccination practices, which might not accurately represent current immunization coverage.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that the EPI cluster survey is a method used to estimate vaccination coverage in a population. The **12-23 months** age group is specifically chosen for assessing primary immunization coverage because it balances the need for children to have completed their primary series with the desire to reflect current vaccination practices.

## **Correct Answer:** .