The unit of observation and analysis in an ecological study is:
- A The population or group ✓
- B The individual subject
- C The discordant twin pair
- D The matched case-control set
Correct answer: A. The population or group
Explanation
An ecological study examines correlations between exposure levels and outcome rates using aggregates such as countries, states or districts, never individuals. Because exposure and outcome are measured at group level, an observed association may not hold for any individual member of the group, which is the ecological fallacy. Individual-level designs such as cohort, case-control and cross-sectional studies analyse data at the person level.
Reference: Gordis Epidemiology, 6th ed.
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