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Pragmatic vs Explanatory Trials — SCE Medical Oncology MCQ

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HardClinical Trials & StatisticsPragmatic vs Explanatory TrialsSCE Medical Oncology

A retrospective study labels patients as “immunotherapy exposed” if they completed at least 12 weeks of treatment, but starts survival follow-up for everyone on the date metastatic disease was diagnosed. Patients dying before week 12 are classified as unexposed. Which bias most directly exaggerates benefit in the exposed group?

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Reveal the answer and explanation

Correct answer: D — Immortal-time bias from using future survival to define baseline exposure

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

E is correct: patients must survive 12 weeks to enter the exposed group, and the guaranteed-survival interval is misclassified. A concerns advancing diagnosis without changing death; B concerns preferential detection of slow disease; C concerns unequal outcome ascertainment; D concerns extreme measurements moving toward average.

Reference: Core concepts: immortal-time bias in pharmacoepidemiology (Clinical Pharmacology and Therapeutics 2023): https://pmc.ncbi.nlm.nih.gov/articles/PMC10204604/