What is a key limitation of ecological studies?

• A. Cannot estimate group-level prevalence
• B. Requires individual measurements
• C. Cannot infer individual-level associations
• D. Not applicable in public health

Answer: (C)

Ecological studies use data that are aggregated at the group level, so they can reveal patterns and associations across populations but cannot tell whether individuals with higher exposure actually have a higher risk. This limitation comes from the ecological fallacy: relationships observed for groups don’t necessarily hold for individuals within those groups. That’s why the strongest drawback is the inability to infer individual-level associations from ecological data. For completeness: group-level prevalence can be estimated from aggregated data, these studies don’t require individual measurements, and they are indeed used in public health, so those points aren’t limitations.