Every death certificate answers two questions: what happened, and what started it. The answer to the second is the underlying cause of death, and it is the one that becomes a national statistic. How it is chosen, and how the choosing has changed, explains a surprising share of the trends in cause-specific mortality.
The certificate. A doctor records a chain: the immediate cause on the first line, then the conditions that led to it, down to the one that began the sequence. Pneumonia, due to immobility, due to a stroke: the stroke is the underlying cause. Other conditions that contributed without being in the chain go in a separate section. The chain is a clinical judgement made under time pressure, sometimes by a doctor who did not know the patient.
The rules. The International Classification of Diseases, now in its tenth revision in most countries with the eleventh arriving slowly, assigns a code to every condition and a set of rules for choosing the underlying cause when the certificate is ambiguous or implausible. The rules are detailed and occasionally counter-intuitive; they exist so that the same certificate produces the same code in Copenhagen and Chicago. They are revised, and each revision moves deaths between causes.
Automation. Most countries now assign the underlying cause by software — IRIS in much of Europe, the American system before it — that applies the rules to the text of the certificate. Automation improved consistency, and changes to the software and to the rules it applies both move the results: when England and Wales updated their coding rules in 2011, deaths recorded as dementia rose sharply in a single year, not because more people died of dementia but because many deaths previously coded to stroke were now coded to vascular dementia; a switch to new software in 2014 produced a second step. Any series that crosses a change in rules or software has a step in it.
Garbage codes. A share of deaths are coded to causes that cannot be underlying — "senility", "heart failure" without a reason, "unspecified" — and that share varies from a few per cent in the Nordic countries to a third or more elsewhere. The Global Burden of Disease project reassigns these by algorithm before comparing countries, which is one reason its figures differ from national ones.
The pandemic as a case study. COVID-19 got its own codes within weeks, and countries applied them differently: some counted any death within 28 days of a positive test, some required the disease to be on the certificate, some required it to be the underlying cause. The reported death tolls diverged accordingly, which is why excess mortality became the comparison of choice.
Multiple causes. The underlying cause is one line of the certificate; the whole certificate is more informative. Multiple-cause data — every condition mentioned — show, for instance, how often diabetes or dementia is present at deaths attributed to something else. A growing number of countries publish them, and they change the picture of what people die with as against of.
Reading a cause-specific trend. Before believing that deaths from a cause rose or fell, three questions: did the classification revision, the coding rules or the software change across the period; did certification practice change, as it did for dementia when doctors were encouraged to record it; and is the trend visible in the underlying-cause series only, or in multiple-cause data too? A trend that survives those questions is probably real.