16 February 2021 - Observational studies are commonly used to evaluate the association between a risk factor or “exposure” and the time that elapses until an outcome of interest occurs (eg, smoking decreasing the time to cardiovascular death). 

This relationship may be assessed by analysing natural variation in the exposure and patient outcomes. 

Such studies may be subject to immortal time bias, meaning that, during the period of observation, there is some interval during which the outcome event cannot occur. The research participants are “immortal” in that they must survive long enough to receive the intervention being studied.

Read JAMA Guide to Statistics and Methods