Why is mortality the measure of the effectiveness of a screening test?
Why not case survival? Mortality refers to the number of deaths from the disease within the whole population screened. Case survival refers only to the number of people with the disease remaining alive at a certain point in time after diagnosis. Changes in lung cancer mortality rates (rates of death from lung cancer) are the accepted measure of screening effectiveness. The major reason that case survival cannot be used when determining the effectiveness of screening is that it does not take into account specific biases that affect its measurement. These biases are lead time, length, and overdiagnosis bias. Screening tests are performed in ostensibly healthy people who do not have symptoms of cancer. If the screening detects a cancer, the time of diagnosis is advanced (made earlier). The time between a screening diagnosis and death will be longer just because of early diagnosis, even if the screen does not ultimately change the time of death. Secondly, studies of other types of cancer s
