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A Structural Description of Biases That Generate Immortal Time
Miguel A Hernán1,2, Jonathan A C Sterne3,4,5, Julian P T Higgins3
1From the CAUSALab, Department of Epidemiology, Harvard T.H. Chan School of Public Health, Boston, MA.
Epidemiology (Cambridge, Mass.)
|November 4, 2024
Summary
Immortal time bias in survival analysis stems from selection or misclassification after treatment assignment, not the immortal time itself. Target trial emulation using longitudinal data synchronizes eligibility and assignment at follow-up start to prevent this bias.
Area of Science:
- Epidemiology
- Biostatistics
- Clinical Trials
Background:
- Immortal time bias can occur in survival analyses due to post-eligibility information or post-assignment criteria.
- This bias arises when follow-up begins at treatment assignment, but eligibility or treatment is determined later.
Purpose of the Study:
- To explain the origins of immortal time bias in survival analysis.
- To present methods for preventing immortal time bias, particularly through target trial emulation.
Main Methods:
- Reviewing analytic approaches that prevent immortal time bias.
- Utilizing longitudinal data to emulate target trials from the time of treatment assignment.
- Explicitly defining eligibility and treatment assignment synchronization at the start of follow-up.
Main Results:
- Selection or misclassification, not immortal time itself, generates bias.
- Target trial emulation effectively prevents immortal time by aligning eligibility and assignment.
Conclusions:
- Understanding the sources of immortal time bias (selection/misclassification) is crucial.
- Target trial emulation is a robust strategy to mitigate immortal time bias in survival analyses.
- Synchronizing eligibility and assignment at follow-up initiation is key to accurate emulations.
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