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Lead-time bias — USMLE Step 3 MCQ

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ModeratePreventive Medicine, Epidemiology, BiostatisticsLead-time biasUSMLE Step 3

A cancer screening program detects tumors 2 years earlier than usual care. Five-year survival after diagnosis improves, but disease-specific mortality in the population does not change. Investigators question why survival appears better without fewer deaths. Which of the following is the most likely explanation for this patient's findings?

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Correct answer: D — Lead-time bias

Earlier diagnosis can lengthen measured survival from diagnosis without changing the time of death; this is lead-time bias.

Reference: USPSTF Methods; Epidemiology Texts