A mammography screening programme reports better 5-year survival among screen-detected breast cancers than among clinically detected cancers. Critics note that screening disproportionately picks up slow-growing tumours that remain in the detectable pre-clinical phase for a long time, inflating apparent survival without reducing deaths. This criticism describes:

• A Length bias ✓
• B Lead time bias
• C Neyman's bias
• D Berkson's bias

Explanation

Length bias arises because indolent, slowly progressing tumours spend longer in the detectable pre-clinical phase and are therefore more likely to be caught by periodic screening, whereas aggressive fast-growing cancers surface clinically between screening rounds. Screen-detected cases are thus enriched for better-prognosis tumours, improving survival statistics without lowering mortality. Lead time bias is different: it merely shifts the diagnosis date earlier, lengthening measured survival without changing the course of disease.

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

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Written and medically reviewed by the StethoPrep medical team.