Evidence-appraisal glossary
Immortal time bias
Immortal time bias is a distortion in cohort studies where a stretch of follow-up is guaranteed event-free because of how exposure was defined. A person must survive long enough to receive a treatment, so that waiting time gets wrongly credited to the treated group, faking a survival benefit.
Also called: Immortal time, Survivor treatment selection bias, Time-dependent bias.
What it is
"Immortal time" is a span of follow-up during which, by the study's own design, the outcome (often death) could not happen. It typically arises when a person is classified as "treated" using information that only becomes available after follow-up starts, such as filling a prescription weeks later. To reach that treatment, they had to survive the gap, so that gap is "immortal." Misclassifying or excluding it hands the treated group a spurious head start, usually exaggerating benefit.
How to use it when reading a study
Ask: when did follow-up start (time-zero), and when was exposure assigned? If exposure is defined by something that happens after time-zero, suspect this bias. Watch for treatments that look surprisingly protective in observational data. Credible studies neutralize it with time-varying exposure analysis, landmark analysis, or by aligning exposure with the start of follow-up.
This is a plain-language methodology definition for reading research. It is general education, not medical advice.