In a case-control study of the relationship of radiation exposure and thyroid cancer, 50 cases admitted for thyroid cancer and 100 "controls" admitted during the same period for treatment of hernias were studied. Only the cases were interviewed, and 20 of the cases were found to have been exposed to x-ray therapy in the past, based on the interviews and
medical records. The controls were not interviewed, but a review of their hospital records when they were admitted for hernia surgery revealed that only 2 controls had been exposed to x-ray therapy in the past.
Based on the descriptive given above, what source of bias is least likely to be present in this study?
a. Recall bias
b. Bias due to controls being nonrepresentative of the nondiseased population
c. Bias due to use of different methods of ascertainment of exposure in the cases and
controls
d. Bias due to loss of subject from the control group over time
e. Selection bias for exposure to x-ray therapy in the past