Diagnostic tests such as CAT scans are not perfect. A test can make two errors. It can call a diseased person healthy – a false negative. This is like acquitting a person guilty of a crime. Or a test can falsely call a healthy person diseased – a false positive. This is like convicting an innocent person of a crime that she did not commit. There is a trade-off between false negatives and false positives. To achieve fewer false negatives we incur more false positives.
Physicians do not want to be wrong. Since error is possible we must choose which side to err towards. That is we must choose between two wrongness. We have chosen to reduce false negatives at the expense of false positives. Why this is so is illustrated by screening mammography for breast cancer.
A woman who has cancer which the mammogram picks up is thankful to her physician for picking up the cancer and, plausibly, saving her life.
A woman who does not have cancer and whose mammogram is normal is also thankful to her physician. The doctor does not deserve to be thanked as she played no hand in the absence of the patient’s cancer. But instead of thanking genes or the cosmic lottery, the patient thanks the doctor.
