A quantitative evaluation of competing risks in occupational studies.

Mortality risk is competing because each individual is subject to several competing causes of death but can only die from one. Adjustment for competing risks will allow more meaningful comparisons of cause-specific mortality of two populations, especially if dying from all other causes is significantly different between the two populations. In this paper, a method has been developed for adjustment of competing causes of death in the calculation of relative risk. It identifies three factors as determining the significance of competing risks: (1) magnitude of the overall mortality risk of the study population; (2) differential risk or adjustment factor for all other causes between two populations; and (3) age interval used in mortality calculation. Thus, the impact of competing risks is increased if the mortality risk of the study population is high, if the differential risk for all other causes is large or if wide age intervals are used in the mortality calculation. An example from refinery cohort data shows that in certain age groups unadjusted for competing risks the relative risk is overestimated by 9%. The impact of competing risks in this particular example is relatively small.