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: Social & Preventive Medicine
To test the association between risk factor and disease, which of the following is the wakest study design - 
Select and explain. 
A. Case-control study, B. Ecological study, C. Coho study, D. Cross-sectional study
B: Ecological study
Answer: B: Ecological study 
Explanation: Ans. is 'b' i.e., Ecological study To test the association between risk factor and diseaseo Most reliable, overall --> systematic review and meta-analysis of RCTso Most reliable individual study --> Randomized controlled trial.o Most reliable observational analytic study Coho studyo Least reliable study and least reliable observational analytic study --> Ecological study.