Only about 80 percent of both cases and controls were included in the data from the previous table. In other words, about 20 percent of both cases and controls did not report information on abortion history. Suppose that the cases who did not report an abortion history were less likely to have an induced abortion than the controls. Why would they be less likely to have an abortion but be unwilling to report it, and what would be the effect on the risk of breast cancer from abortion