Lead time bias

Lead time bias is the length of time between when a disease becomes present in a person's body and its usual clinical presentation. It is an important factor when evaluating the effectiveness of a specific treatment.

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 we diagnosed the disease 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).

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.