Investigation of design and bias issues in case-control studies of cancer screening using microsimulation

Using a microsimulation approach, the authors examined design and bias issu es in case-control studies of cancer screening. Specifically, they looked a t the impact on the odds ratio of the way in which exposure to screening is defined, the type of age matching, the time scale used, and the criteria u sed to determine control eligibility. The results showed that defining expo sure as "ever/never" screened produced, as expected, a serious bias in favo r of screening. Defining exposure as being screened no later than the time the case's cancer is diagnosed has a serious bias against screening. An alt ernative exposure definition-screening can occur no later than the time the case would have been clinically diagnosed-eliminates the bias against scre ening. Further, the results showed that the type of age matching and the ti me scale used can produce a bias against screening and that this bias can b e quite strong when case-control studies are performed in populations with a periodic screening program that is the only source of screening. Finally, control eligibility criteria had little effect.