A new screening programme detects prostate cancer at age 62 years, whereas clinically the same tumours would have been diagnosed at age 67 years. Death occurs at age 72 years in both screened and unscreened men. Survival after diagnosis appears 5 years longer in screened men although nothing about the course of disease changed. This artefact is called:
- A Length bias
- B Lead time bias ✓
- C Neyman's bias
- D Hawthorne effect
Correct answer: B. Lead time bias
Explanation
Lead time is the interval between detection by screening and usual clinical diagnosis. When lead time exists but early treatment does not delay death, survival measured from diagnosis looks inflated purely because diagnosis moved earlier. Length bias refers to over-representation of slowly progressing disease in screened populations, Neyman's bias is survival-related selection in case-control studies, and the Hawthorne effect is behaviour change due to being observed.
Reference: Gordis Epidemiology, 6th ed.
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