This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Dental
All of the following are Experimental / Interventional studies except: 
Select and explain. 
A. Randomised control trials ., B. Field trials., C. Community trials., D. Ecological studies.
D: Ecological studies.
Answer: D: Ecological studies. 
Explanation: Ecological study (Correlational study):

Type of analytical (observational) epidemiological study which provide the ‘least satisfactory type of evidence on causality’
Units of study: Population
Advantage: Data can be used from populations with different characteristics
Potential problem: Socio-economic confounding
Ecological fallacy: Is an error of interpretation of statistical data in an ecological study, whereby characteristics are ascribed to a group of individuals which they may not possess as individuals