Bias Arising in Case-Control Studies from Selection of Controls from Overlapping Groups

Austin H (Department of Epidemiology, School of Public Health and the Comprehensive Cancer Center, University of Alabama at Birmingham, USA), Flanders W D and Rothman K J. Bias arising in case-control studies from selection of controls from overlapping groups. International Journal of Epidemiology 1989, 18: 713–716. Three examples are used to demonstrate that the selection of controls from categories that overlap can lead to bias. Case-control studies that use friend controls and the practice of age-matching controls to cases within a specified number of years (caliper matching) are examples of the selection bias described in the paper. The bias resulting from the use of friend controls can be large and, since the use of friend controls is common, this source of bias is of considerable practical consequence. The use of friend controls or the selection of controls according to ther variables with categories that overlap should be avoided.