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Length-time bias — USMLE Step 3 MCQ

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HardPreventive Medicine, Epidemiology, BiostatisticsLength-time biasUSMLE Step 3

A screening program preferentially detects slow-growing prostate cancers with long asymptomatic phases. Screen-detected patients appear to have better prognosis than clinically detected patients, but mortality benefit is small. Which of the following is the most likely explanation for this patient's findings?

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Correct answer: C — Length-time bias

Screening is more likely to detect indolent disease with longer preclinical duration, making outcomes look better among screen-detected cases.

Reference: USPSTF Methods; Epidemiology Texts