Strange question from me, but try to follow me. I do not remember or name correctly a type of bias in cohort study which is pretty clear in my mind. I try to explain: Let's assume that I want to test the rate of adverse events for a drug name "ABC" in a cohort of patients, half of which is naive for treatment with "ABC", and other half is being treated with ABC since 10 years. Obviously, the rate of Adverse Events is way higher in the naive cohort, and that's why patients that were being treated for a decade were already selected to be "resistant" to the adverse events of the ABC drug. Obviously, this result is biased.
The question is: how to name this type of bias? My guess is to name it as a type of "immortality bias" (because patients being treated for several years are sort of "immortal" in relation to the adverse events of the drug), but i firmly believe that there is another way to refer to it.
Thanks to anyone that'll try to help me.