The first hurdle is a test's sensitivity. Sensitivity answers this question: What percentage of people with a given type of cancer will have their cancer detected when a given screening test is used? If a test is not very sensitive, there will be many "false negative" results, and persons with cancer will go undetected. A test that yields many false negatives will obviously not be very good at reducing cancer deaths, giving a false sense of security to people who actually have the disease.