Abstract
Purpose of Review
Immortal time occurs when study subjects’ person-time is misclassified. For example, if exposure is assigned over time, but treated as a binary “ever-exposed” variable, subjects in the exposed group are “immortal” prior to their exposure. We describe immortal time and the context in which it introduces bias and describe several approaches to avoid immortal time bias via design or mitigate it through analysis.
Recent Findings
Several authors have described examples of immortal time bias in clinical epidemiology, pharmacoepidemiology, and perinatal epidemiology. Solutions to immortal time bias include analyses that appropriately account for time-varying exposure, and design solutions that align exposure with the start of follow-up.
Summary
Immortal time bias is pervasive in epidemiology. It can cause substantial bias. It is, however, easily avoided and can be controlled using appropriate analytic and design strategies.
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Platt, R.W., Hutcheon, J.A. & Suissa, S. Immortal Time Bias in Epidemiology. Curr Epidemiol Rep 6, 23–27 (2019). https://doi.org/10.1007/s40471-019-0180-5
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DOI: https://doi.org/10.1007/s40471-019-0180-5