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
Selection bias occurs mainly in 
Select and explain. 
A. Coho study, B. Case control study, C. RCT, D. All the above
B: Case control study
Answer: B: Case control study 
Explanation: It's usually unwise to choose a control group of patients with one disease. This is because hospital controls are often a source of 'selection bias'. Many hospitals may have diseases that are also influenced by factors under study. In selection bias, cases and controls may not be representative of actual cases and controls in the general population. Ref: 25th edition, Park's Textbook of Preventive and Social Medicine, Page no. 79, 81