Figure 1.
Illustration of immortal time bias in the Sin and Tu observational cohort study of inhaled corticosteroids in patients discharged with COPD.27
The top patient received a prescription of ICS within 90 days after discharge and is classified as an ICS user for the entire follow-up time. The time between cohort entry and the first ICS prescription is thus immortal (thick line), since the subject must survive to receive this first ICS prescription, and is also misclassified as exposed to ICS when in fact unexposed, leading to immortal time bias.