After a city-wide mammography screening program begins, the five-year survival rate for breast cancer rises from 78% to 91%, yet the annual breast cancer mortality rate remains unchanged. The most likely explanation for the improved survival figure is:

• A Length bias
• B Lead time bias ✓
• C Selection bias against enrollment of severe cases
• D Confounding by stage at diagnosis

Explanation

Screening detects tumors earlier in their natural history, so the clock starts sooner even though the date of death is unchanged. Survival measured from diagnosis therefore lengthens without anyone living longer, which is lead time bias; mortality rates, unlike survival, are unaffected. Length bias refers instead to overrepresentation of slow-growing tumors among screen-detected cases, which is a different phenomenon.

Reference: Gordis Epidemiology, 6th ed.

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Written and medically reviewed by the StethoPrep medical team.