In a city-wide breast cancer screening programme, women whose tumours are detected by mammography show 5-year survival of 92%, while women diagnosed clinically show 5-year survival of 70%. Death rates from breast cancer in both groups are unchanged. The inflated survival among screen-detected cases is best explained by:

• A Selection bias
• B Recall bias
• C Confounding by age
• D Lead-time bias ✓

Explanation

Screening detects cancer earlier in its natural history, so the interval between detection and death lengthens even though the time of death does not change. Survival from diagnosis therefore rises spuriously. This is the classic lead-time bias. Selection bias concerns differential enrolment, recall bias concerns misremembered past exposure, and age confounding would not specifically inflate only screen-detected survival while mortality stays flat.

Reference: Park's Textbook of Preventive and Social Medicine, 27th ed.

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