Lead time bias
Lead time is the length of time between the detection of a disease (usually based on new, experimental criteria) and its usual clinical presentation and diagnosis (based on traditional criteria).
Lead time bias is the bias that occurs when two tests for a disease are compared, and one test (the new, experimental one) diagnoses the disease earlier, but there is no effect on the outcome of the disease — it may appear that the test prolonged survival, when in fact it only resulted in earlier diagnosis when compared to traditional methods. It is an important factor when evaluating the effectiveness of a specific test.
Relationship between screening and survival
By screening, the intention is to diagnose a disease earlier than it would be without screening. Without screening, the disease may be discovered later once symptoms appear.
Even if in both cases a person will die at the same time, because the disease was diagnosed early with screening, the survival time since diagnosis is longer with screening. No additional life has been gained (and indeed, there may be added anxiety as the patient must live with knowledge of the disease for longer). For example, most people with the genetic disorder Huntington's disease are diagnosed when symptoms appear around age 50, and they die around age 65. The typical patient therefore lives about 15 years after diagnosis. With a genetic test, it is possible to diagnose this disorder at birth. If this newborn baby dies around age 65, he or she will have "survived" 65 years after diagnosis, without having actually lived any longer than the people who were diagnosed late in life.
Looking at raw statistics, screening will appear to increase survival time (this gain is called lead time). If we do not think about what survival time actually means in this context, we might attribute success to a screening test that does nothing but advance diagnosis.