Selection bias occurs mainly in

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