In cluster sampling used for vaccine coverage surveys, the EPI coverage survey methodology uses 30 clusters of 7 children each. A limitation unique to cluster sampling compared to simple random sampling is:

• A Reduced precision due to intracluster correlation (design effect) ✓
• B Cannot provide population estimates without a sampling frame
• C Each unit has an unequal probability of selection
• D Cannot be used for rare disease studies

Explanation

In cluster sampling, individuals within the same cluster tend to be more similar to each other than to individuals in other clusters (intracluster correlation — ICC). This reduces the effective sample size compared to a simple random sample of the same total n, quantified by the design effect (DEFF = 1 + ICC × (cluster size − 1)). A design effect >1 means precision is worse than SRS; standard errors must be inflated accordingly. This is the key statistical limitation. Systematic or stratified sampling can provide population estimates without a complete frame; cluster sampling actually selects all units in chosen clusters (equal probability within selected clusters).

Reference: Park's Textbook of Preventive and Social Medicine, 27th ed.

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