A Structural Description of Biases That Generate Immortal Time
Open accessAbstract
Immortal time may arise in survival analyses when individuals are assigned to treatment strategies based on post-eligibility information or selected based on post-assignment eligibility criteria. Selection based on eligibility criteria applied after treatment assignment results in immortal time when the analysis starts the follow-up at assignment. Misclassification of assignment to treatment strategies based on treatment received after eligibility results in immortal time when the treatment strategies are not distinguishable at the start of follow-up. Target trial emulation prevents the introduction of immortal time by explicitly specifying eligibility and assignment to the treatment strategies, and by synchronizing them at the start of follow-up. We summarize analytic approaches that prevent immortal time when longitudinal data are available to emulate the target trial from the time of treatment assignment. The term “immortal time bias” suggests that the source of the bias is the immortal time, but it is selection or misclassification that generates the immortal time, leading to bias.
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Cites
A structural approach to selection bias
Hernán, M. A., Hernández-Díaz, S., & Robins, J. M. (2004) · Epidemiology · doi:10.1097/01.ede.0000135174.63482.43 · Cited by 2,844 · cited 1× · see citing works
“18 The analysis in Figure 1B also includes immortal time.”
Self-citationBNT162b2 mRNA Covid-19 Vaccine in a Nationwide Mass Vaccination Setting
Dagan, N., Barda, N., Kepten, E., Miron, O., Perchik, S., Katz, M. A., Hernán, M. A., Lipsitch, M., Reis, B., & Balicer, R. D. (2021) · New England Journal of Medicine · doi:10.1056/nejmoa2101765 · Cited by 2,527 · cited 1× · see citing works
“In randomized trials, immortal time due to selection is not expected — accidental data deletions are unlikely and investigators do not typically exclude the early follow-up from their analysis (vaccine trials are often a problematic exception, as discussed in the Appendix of Dagan et al., 2021 19 ).”
Self-citationRobins, J. (1986) · Mathematical Modelling · doi:10.1016/0270-0255(86)90088-6 · Cited by 2,451 · see citing works
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Cardiovascular events in individuals treated with sulfonylureas or dipeptidyl peptidase 4 inhibitors
Turchin, A., Petito, L. C., Hegermiller, E., Carnahan, R., DeVries, A., Goel, S., Lansang, M. C., McDonnell, M. E., Nair, V., Priest, E., et al. (2025) · JAMA Network Open · doi:10.1001/jamanetworkopen.2025.23067 · Cited by 14 · see citing works
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- Authors
- Hernán, M. A., Sterne, J. A. C., Higgins, J. P. T., Shrier, I., & Hernández-Díaz, S.
- Affiliations
- CAUSALab, Department of Epidemiology, Harvard T.H. Chan School of Public Health, Boston, MA
- University of Bristol (GB)
- Jewish General Hospital (CA)
- Venue
- Epidemiology
- Published
- 2024-11-04
- Volume
- 36
- Issue
- 1
- Pages
- 107-114
- ISSN
- 1044-3983 (ISSN-L)
- 1531-5487
- PMC
- PMC11598638
- OpenAlex
- W4404021068
- Cited by
- 99
- References
- 3
- Citations by year
- 2026: 52 · 2025: 46 · 2024: 1
- FWCI
- 34.11
- Type
- journal article
- Language
- en
- Open access
- green
- pmc.ncbi.nlm.nih.gov
- Locations
- Epidemiology (publishedVersion)
- PubMed Central (publishedVersion)
- PubMed (publishedVersion)
- Explore Bristol Research (publishedVersion, cc-by)
- Europe PMC (PubMed Central) (acceptedVersion)
- Bristol Research (University of Bristol) (acceptedVersion, cc-by)
- Funders
- National Institute for Health and Care Research (NIHR203807)
- National Institutes of Health (AI102634)
- Medical Research Council (HDR-23013)
- National Institute of Allergy and Infectious Diseases (R37AI102634)
- Eunice Kennedy Shriver National Institute of Child Health and Human Development (R01HD097778)
- Topics
- Physical Sciences › Mathematics › Statistics and Probability › Advanced Causal Inference Techniques
- Physical Sciences › Mathematics › Statistics and Probability › Statistical Methods in Clinical Trials
- Physical Sciences › Mathematics › Statistics and Probability › Statistical Methods in Epidemiology
- Keywords
- immortal time bias, target trial emulation, treatment eligibility criteria, treatment allocation, selection bias, survival analysis, longitudinal data
- MeSH terms
- Humans, Time Factors, Bias, Survival Analysis
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