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Immortal Time Bias — SCE Medical Oncology MCQ

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ModerateClinical Trials & StatisticsImmortal Time BiasSCE Medical Oncology

A researcher explains 'immortal time bias' in observational studies. A study shows patients starting adjuvant therapy >60 days post-surgery had better survival. What is the bias?

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Correct answer: C — Immortal time bias — patients starting >60 days must have survived ≥60 days (immortal during waiting), giving artificial survival advantage; proper methods (landmark analysis) eliminate this

Immortal time bias: period during which outcome cannot occur is misclassified. Patients who start late must have survived to that point. Landmark analysis or time-dependent covariates correct this.

Reference: Lévesque et al BMJ 2010; Suissa et al Am J Epidemiol 2008