All of the following are Experimental / Interventional studies except:

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