title: Implications of immortal person-time when outcomes are nonfatal.
abstract: OBJECTIVE
  The amount of immortal time bias in studies with nonfatal outcomes is unclear. To quantify the magnitude of bias from mishandling of immortal person-time in studies of nonfatal outcomes.
  
  
  METHODS
  We derived formulas for quantifying bias from misclassified or excluded immortal person-time in settings with nonfatal outcomes, assuming a constant rate of outcome. In the situation of misclassified or excluded immortal person-time, the quantification includes the immortal time and corresponding events mistakenly attributed to the exposed group (misclassified) or excluded from study (excluded) that must be attributed to the comparison group.
  
  
  RESULTS
  With misclassified immortal person-time, the magnitude of bias varies according to the incidence rate ratio of immortal time and comparison group as well as the rate ratio of immortal time and exposed group: toward null for both ratios less than 1, no bias for both ratios equal to 1, away from null for both ratios greater than 1. For one ratio less than 1 and the other greater than 1, the direction and magnitude of bias can be obtained from the formula provided. With excluded immortal person-time, the magnitude of bias is associated with the incidence rate ratio of immortal time and comparison group: toward null for the ratio less than 1, no bias for the ratio equal to 1, and away from null for the ratio greater than 1.
  
  
  CONCLUSIONS
  Bias due to immortal person-time in studies with nonfatal outcomes can vary widely and can be quantified under assumptions that apply to many studies.
authors: Optum Epidemiology, Waltham, MA. Electronic address: caihua.liang@optum.com.
authors: Caihua
authors: C
authors: Liang
authors: Optum Epidemiology, Waltham, MA; Division of Pharmacoepidemiology and Pharmacoeconomics, Department of Medicine, Harvard Medical School/Brigham and Women's Hospital, Boston, MA.
authors: John D
authors: JD
authors: Seeger
authors: Optum Epidemiology, Waltham, MA; Department of Health Services, Policy, and Practice, School of Public Health, Brown University, Providence, RI.
authors: David D
authors: DD
authors: Dore
mesh_terms: D015982:Bias; D015340:Epidemiologic Research Design; D000077270:Exenatide; D005500:Follow-Up Studies; D006801:Humans; D007004:Hypoglycemic Agents / Q000627:therapeutic use; D009026:Mortality; D010195:Pancreatitis / Q000188:drug therapy / Q000401:mortality; D010455:Peptides / Q000627:therapeutic use; D012189:Retrospective Studies; D013997:Time Factors; D016896:Treatment Outcome; D014688:Venoms / Q000627:therapeutic use
chemical_list: D007004:Hypoglycemic Agents; D010455:Peptides; D014688:Venoms; D000077270:Exenatide
keywords: Immortal person-time; Immortal time bias; Pharmacoepidemiology