In the WHO 30-cluster sampling method for assessing immunization coverage, villages are selected with probability proportional to their population size, and then a fixed number of children is sampled per village. The selection of villages by population size is an example of:

• A Simple random sampling
• B Stratified random sampling
• C Probability proportional to size sampling ✓
• D Non-probability convenience sampling

Explanation

In probability proportional to size sampling, larger units stand a higher chance of selection, which compensates for unequal cluster sizes so the final estimates remain unbiased. This differs from stratified sampling because no attempt is made to sample every subgroup separately. Although individual children within clusters are conveniently sampled rather than randomly, the primary-stage selection of villages remains a formal probability mechanism based on size, so the overall design is not classified as pure convenience sampling.

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

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