What the hell is an Immortal Time Bias?
The appellation is very poetic indeed. What is hidden behind?
I searched on the internet and found three papers which unveiled the mystery.
An immortal time bias occurs when individuals of one of the two groups that are compared (e.g.treated patients and control patients) are guaranteed for one period (called immortal time) to be alive if the outcome of interest is all cause mortality (or not to incur the condition of interest if the outcome is a disease). The period of immortal time must be situated after the cohort entry and before the end point (i.e. during the follow up time).
For example if date of birth is the date of cohort entry and death is the outcome of interest, Popes or Oscar Winners live longer than others. The explanation of this life time discrepancy is only the immortal time bias. You have to be alive long enough (and thus not to die) to become Pope or to win Oscars!
Below are the full text of the three papers I found on the internet treating of the immortal time bias scope: