A baseline-group log-rank test is inappropriate. Receiving surgery is determined at month 3, so classifying patients by that future decision gives the surgery group guaranteed survival without treatment failure to the decision time, creating <immortal time bias>. Fitness also strongly confounds surgery and prognosis. One can perform a month-3 <landmark analysis> among patients still at risk, or model surgery as a <time-dependent covariate>, while adjusting for the clinical variables driving the decision.

Solution

Past exam of the mathematics course of the University of Cambridge / 2025 / iii / Paper 207 / 4 / b / Solution

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Immortal time bias occurs when exposure classification requires an individual to remain event-free for a period that is then incorrectly credited to the exposed group.


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 Incoming links: Immortal time bias