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 issues in case-control studies of cancer screening. Specifically, they looked at 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 used to determine control eligibility. The results showed that defining exposure as "ever/never" screened produced, as expected, a serious bias in favor 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 alternative exposure definition--screening can occur no later than the time the case would have been clinically diagnosed--eliminates the bias against screening. Further, the results showed that the type of age matching and the time scale used can produce a bias against screening and that this bias can be 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.