Lead Time Bias

We assume that early detection saves lives.

Sometimes it does. Sometimes it just creates Lead Time Bias.

The Scenario

Imagine two men, Bob and Dave. Both develop cancer at age 50. Both will die at age 60.

Bob does not get screened. He feels sick at age 59. He is diagnosed. He dies at 60. Survival time: 1 year.

Dave gets screened at age 50. The doctor finds the cancer. Dave lives with the diagnosis for 10 years. He dies at 60. Survival time: 10 years.

The Illusion

The statistics say screening increased survival by 9 years.

It did not. Dave died at the same age as Bob.

The screening did not add years to his life. It added years to his illness. He spent 10 years worrying, getting treated, and suffering side effects. Bob spent those 9 years living his life.

Overdiagnosis

This is compounded by overdiagnosis. Screening finds slow-growing cancers that would never have killed the patient.

We treat them anyway. We claim we "cured" them. In reality, we just treated a harmless anomaly.

The Diagnosis

Survival rates are misleading. Mortality rates are the truth.

If a screening program increases "5-year survival" but does not change the total number of deaths, it is just Lead Time Bias.