How to Game Mortality Data

There is a great illustration in this BMJ article of what I discuss in Chapter 2 of my book: the type of mortality that matters. In the figure below from the paper, note that, as the new diagnoses of each of the cancers rise (green lines), the attendant “Deaths” (red lines) stay unchanged. If you look at the Y-axis of each graph, it tells us that the unit of measurement is “Rate per 100,000 people.” So the red lines represent population mortality.

“Population mortality” means that the denominator for this value is all people in the population who are at risk for the disease in question. This means that for prostate cancer, for example, we include only those men who have a prostate and exclude all women and men who have had a prostatectomy. Population mortality stands in contradistinction to case fatality. The latter is defined as deaths among all the people diagnosed with the disease. So, for prostate cancer, case fatality would be deaths among all men who have been diagnosed with prostate cancer.

It is not difficult to see how case fatality is a somewhat circular, even self-referential, measure of our diagnostic prowess, but says very little about how well we are doing with disease treatment. If we have tests that are capable of picking up the most minute of diseases, those that are not likely to cause death in the first place, then the denominator becomes inflated with this noise, while the numerator, the actual fatalities, does not change. This leads of course to an apparent reduction in deaths from the disease, but a reduction that is an artifact of overdiagnosis. Population mortality, on the other hand, cannot be gamed this way, as you can see in the figure above. This is the only mortality that gives us honest feedback without a bias about how we are doing with our early detection and other interventions. In the case of the cancers in the figure, the answer is “not so well.”