In a district immunisation survey using 30-cluster EPI methodology, clusters are selected by probability proportional to size (PPS). The primary advantage of PPS over simple random cluster selection is:
- A Larger populations have a proportionally higher probability of being selected, giving all children an equal chance of selection ✓
- B It reduces within-cluster correlation and lowers design effect
- C It eliminates the need for a complete sampling frame of children
- D It allows stratification by age-group within each cluster
Explanation
In PPS sampling, each cluster's probability of selection is proportional to its population size. This ensures that each individual child in the district has an approximately equal probability of being selected overall—a key requirement for unbiased estimation. Without PPS, small villages with few children and large wards with many children would have equal cluster selection probability, over-representing children from small villages. PPS does not reduce within-cluster correlation or design effect; those are addressed through sample size calculations using DEFF.
Reference: Park's Textbook of Preventive and Social Medicine, 27th ed.
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