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
[{'content': 'To test the association between risk factor and disease, which of the following is the weakest study design \nSelect and explain. \nA. Case - control study, B. Ecological study, C. Coho study, D. Cross sectional study', 'role': 'user'}, {'content': 'Answer: B \nExplanation: Coho study proceeds forward from cause to effect Case-control study proceeds backward from effect to cause An ecological study is an observational study defined by the level at which data are analyzed, namely at the population or group level, rather than individual level.', 'role': 'assistant'}]