All-cause mortality

All-cause mortality counts deaths from any cause at all, which is why it cannot be improved by moving deaths from one category into another. It is the outcome in medical research that is hardest to argue with, and the hardest to shift.

Its value lies in what it forbids. A treatment can reduce deaths from one disease while causing deaths from something else, and a study looking only at the first will report a success. All-cause mortality catches that trade, because every death counts against the total regardless of what is written on the certificate.

The cost of this strictness is size and time. Deaths from any single cause are rare enough in a healthy population that a study needs very many people, followed for very many years, before a difference in the total becomes visible. This is why most research on aging reports something else instead — a blood marker, a functional score, a diagnosis rate — and why claims about living longer usually rest on a measure that is not death.

When such a result is reported as a percentage, the percentage is almost always relative. A treatment that cuts the chance of dying in a given year from two in a thousand to one in a thousand halves the risk, which sounds substantial, and moves one person in a thousand, which may not.