Immortal Time Bias in the Analysis of Time-to-Event Data in Orthopedics

Dirk R. Larson, Cynthia S. Crowson, Katrina L. Devick, David G. Lewallen, Daniel J. Berry, Hilal Maradit Kremers

Research output: Contribution to journalArticlepeer-review


Many outcomes in arthroplasty research are analyzed as time-to-event outcomes using survival analysis methods. When comparison groups are defined after a time-delayed exposure or intervention, a period of immortal time arises and can lead to biased results. In orthopedics research, immortal time bias often arises when a minimum amount of follow-up is required for study inclusion or when comparing outcomes in staged bilateral vs unilateral arthroplasty patients. We present an explanation of immortal time and the associated bias, describe how to correctly account for it using proper data preparation and statistical techniques, and provide an illustrative example using real-world arthroplasty data. We offer practical guidelines for identifying and properly handling immortal time to avoid bias. Please visit the following for a video that explains the highlights of the paper in practical terms.

Original languageEnglish (US)
Pages (from-to)3372-3377
Number of pages6
JournalJournal of Arthroplasty
Issue number10
StatePublished - Oct 2021


  • censoring
  • immortal time bias
  • survival analysis
  • time-to-event
  • total joint arthroplasty

ASJC Scopus subject areas

  • Orthopedics and Sports Medicine


Dive into the research topics of 'Immortal Time Bias in the Analysis of Time-to-Event Data in Orthopedics'. Together they form a unique fingerprint.

Cite this