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Immortal Time Bias — SCE Medical Oncology MCQ

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HardClinical Trials & StatisticsImmortal Time BiasSCE Medical Oncology

An observational study defines the exposed group as patients who begin adjuvant therapy more than 60 days after surgery, but counts survival from the operation. The exposed group appears to live longer. Which analytic problem most directly creates that advantage?

Educational content. Not a substitute for clinical judgement or local policy.

Reveal the answer and explanation

Correct answer: E — Immortal-time bias from assigning guaranteed pre-treatment survival to the exposed group

Explanation lettering: D = shown as B · E = shown as D · B = shown as E

B is correct. To enter the delayed-treatment group, a patient must remain alive and eligible until treatment starts. If that guaranteed event-free interval is attributed to the exposed group from the date of surgery, exposure receives survival time that could not contain the outcome. Modelling treatment as a time-varying exposure, or using a justified landmark or target-trial design, addresses the problem. Lead-time bias concerns earlier detection, recall bias concerns differential memory, detection bias concerns unequal outcome ascertainment, and regression dilution attenuates associations through measurement error. None explains the structurally guaranteed survival period.

Reference: BMJ methods paper on immortal-time bias: https://www.bmj.com/content/360/bmj.k182