An ecological study finds a strong positive correlation (r = 0.85) between per capita fat intake and breast cancer rates across 20 countries. A subsequent individual-level cohort study shows no association. This discrepancy is best explained by:
- A Ecological fallacy — group-level correlations may not hold at the individual level ✓
- B Lead-time bias in the ecological study
- C Berkson's bias in the cohort study
- D Length-time bias in case identification
Correct answer: A. Ecological fallacy — group-level correlations may not hold at the individual level
Explanation
The ecological fallacy (also called the aggregation bias) occurs when inferences about individuals are drawn from group-level data. Country-level fat intake correlates with many other factors that differ between countries, and the association may not hold when individual-level data are examined. This is the fundamental limitation of ecological studies.
Reference: Park's Textbook of Preventive and Social Medicine, 27th ed.
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