Competing risks bias arising from an omitted risk factor.

The authors describe a form of selection bias that may arise when a second disease selectively removes from the population persons susceptible to the primary disease of interest. Two examples of this bias are given: 1) a lack of association between an exposure and the primary disease may appear as an inverse association, and 2) a direct association between exposure and primary disease may be greatly attenuated. These examples of bias require the presence of an unknown risk factor in addition to the exposure of interest.