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
[{'content': 'Incidence rate can be calculated from: \nSelect and explain. \nA. Coho study, B. Case control study, C. Cross sectional study, D. Descriptive study', 'role': 'user'}, {'content': 'Answer: A \nExplanation: Only Coho study follows patient for a definite time period. It proceeds from cause to effect. So incidence can be calculated. Besides relative risk as well as attributable risk are also derived from coho study. While case control study study yields only estimate of relative risk. Cross sectional study yields prevalence rate. Ref: Park 21st edition, page 71.', 'role': 'assistant'}]