Why can a proportionate mortality ratio mislead when a workforce is compared with the general population?

1. Proportionate mortality expresses each cause as a share of all deaths, and those shares must sum to 100%. 2. If one cause is unusually rare in a workforce, its small share leaves a larger share to be divided among every other cause. 3. So a raised proportionate mortality for injuries can appear without any real excess of injury deaths. 4. Workers are typically healthier than the population as a whole, since chronic and disabling conditions keep people out of work, which is the healthy worker effect. 5. That makes the third option the opposite of the usual pattern. 6. Live births never appear in proportionate mortality, and a ratio above 1.0 is precisely what flags an excess, so it can and does exceed 1. _Source: CDC Principles of Epidemiology in Public Health Practice (3rd ed., US federal, public domain), Lesson 3 "Measures of Risk", section Definition of proportionate mortality_