Explanation of immortal time bias in randomized clinical trials

In a randomized controlled trial, the start time for both groups is the same, which eliminates unexplained or missing intervals. The treatment is initiated at the same time, creating a common starting point for comparing time to events. In observational studies, immortal time bias arises when there is a gap of time between hospital discharge and the initiation of treatment. During this time, multiple events could occur, leading to misclassification of treatment time. Observational studies should account for this immortal time as unexposed to the treatment.