Immortal time bias
Immortal time bias is a type of bias that can occur in observational research when the study design allows for a period of time during which the outcome of interest cannot occur, often referred to as “immortal time”.
Simply put, immortal time bias occurs when information from a future event is incorporated into the analysis already at baseline (that is, start of follow-up), making all individuals who experience that future event immortal up until that point in time.
For example, consider a study examining the effect of a new drug on mortality in patients with a certain disease. If the new drug is administered at some time point after diagnosis, for example 6 months later, patients who receive the drug will be immortal during those first 6 months. This leads to an overestimation of the effect of the new drug on mortality, as those who did not receive the drug will include those who died before having a chance to get treated.
Special features
Immortal time bias will not occur in randomised controlled trials, as they are performed “in real time” – making it impossible to use future information. However, it can occur in many different types of observational studies, including cohort studies, case-control studies, and cross-sectional studies. In these studies, it is avoided by carefully defining the study population and the time period over which outcomes are assessed.
Want to learn more?
Red Door Analytics AB is a registered company in Sweden
CEO: Michael Crowther
Org. number: 559351-8359
Terms & Conditions | Privacy Policy