This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Epidemiology, Subject name: Social & Preventive Medicine
To test the association between risk factors and disease, which of the following is the weakest 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: Ecological studies, as they donot take into consideration individual data of exposure & diseases, have poorest evidence of determining association. Hence they are considered as weak studies. These are done at point of time & commonly used in nutritional surveys. Order from strong to weak study determining association:Randomized Control Trial > Retrospective Coho Study > Prospective Coho Study > Case control study > Cross sectional study > Ecological study.