A study compares mortality rates between two cities. City A has a crude death rate (CDR) of 18/1000, while City B has a CDR of 12/1000. After age-standardization using the direct method with a standard population, both cities show an age-standardized rate of 14/1000. The most likely explanation is:

• A City A has better healthcare facilities than City B
• B City B has higher disease-specific mortality in younger adults
• C The standard population used was from City B
• D City A has a higher proportion of elderly individuals compared to City B ✓

Explanation

When crude rates differ but age-standardized rates are similar, the difference in crude rates is explained by different age structures rather than true differences in mortality risk. City A's higher CDR is due to a larger proportion of elderly (who have higher age-specific mortality), not a higher disease burden. Age-standardization removes this compositional difference, revealing comparable underlying mortality.

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

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