Employed workers generally have lower death rates than the general population, because the severely ill and chronically disabled are less likely to be employed. This can result in an underestimate of effect for occupational exposures. For example, if an occupational cohort had an exposure that actually increased their risk of an outcome two-fold, if they were compared to the general population, the risk ratio would likely be less than 2.0, because the general population is less healthy than the employed to start.