City X introduces a widespread mammography screening program. Five years later, the average survival time after breast cancer diagnosis in City X is longer than in City Y, which has no screening program. However, the age-specific breast cancer mortality rates are identical in both cities. The apparent survival advantage in City X is best explained by:
- A Selection bias
- B Recall bias
- C Lead time bias ✓
- D Confounding by age
Correct answer: C. Lead time bias
Explanation
Screening detects disease earlier in its natural history without delaying the time of death. Survival measured from diagnosis therefore appears prolonged even though the actual course and mortality are unchanged, which is lead time bias. The identical mortality rates confirm no real benefit has occurred. Length bias is the tempting distractor here, but length bias refers to screening preferentially detecting slow-growing, better-prognosis tumors, not earlier detection inflating survival time from diagnosis.
Reference: Park's Textbook of Preventive and Social Medicine, 27th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.