Framing & context

The redefined measure

The step in the line is the year the definition changed.

a.k.a. series break · definitional change · recoded series · discontinuity · the ruler changed

Long series outlive the rules that made them. A classification is revised, a diagnostic threshold moves, a question is reworded, an instrument gets more sensitive — and the number jumps on the day the change took effect, with nothing in the world having moved. The chart plots both régimes as one line, so the step reads as an event. It is usually disclosed and almost never on the chart.

How to spot it

A step that lands exactly on a boundary — a new year, a new form, a new system.
Find out what changed that year. The methodology note is where the change lives; the chart rarely repeats it.
Check the total. A reclassification moves cases between categories, so when every category jumps and the total does not, the categories are what changed.
The jump beats the trend. A series that drifts a few percent a year does not gain half its value in twelve months on its own.
Ask whether every category moved in the same direction. A real shock pushes one way; a recode has winners and losers, because the cases that left one heading arrived at another.

The fix

Break the line where the definition broke, and label the break. Where the publisher has measured the discontinuity — statistical agencies often do, by coding the same records both ways and publishing a conversion factor — apply it and show both versions, so the reader can see how much of the move was the world and how much was the rulebook. Treat that factor as a measurement rather than a constant: it was estimated on a particular year’s records and carries its own error. If no conversion exists, the honest chart is two series that stop and start, plus a sentence about why they cannot be joined.

In the gallery

Dark navy deck slide, masthead “Mortality Desk · United States · 1998 → 1999”, headlined “American death changed shape in a single year”. A subhead reads “Every death certificate filed in the country, both years, age-adjusted to one standard population. Total mortality barely moved — 875.8 per 100,000 to 881.9, up 0.7% — but underneath it the causes rearranged themselves. Alzheimer’s disease nearly doubled and climbed four places up the leading-cause table. Influenza and pneumonia lost a third of its toll in twelve months.” Under the label “Change in age-adjusted death rate, 1998 to 1999”, eight bars run from a zero line on an axis marked −25%, 0%, +25%, +50%, +75%, captioned “deaths per 100,000, year-2000 standard population”. Each row carries the two rates it is drawn from: Alzheimer’s disease 8.6 → 16.5, a long gold bar labelled +91.9%; kidney disease, noted as nephritis, nephrotic syndrome and nephrosis, 9.8 → 13.1, +33.7%; septicemia 8.9 → 11.3, +27.0%; hypertension 5.4 → 6.3, +16.7%; chronic lower respiratory diseases 42.0 → 45.8, +9.0%; stroke 59.5 → 61.8, +3.9%; all causes 875.8 → 881.9, a sliver of a bar at +0.7%; and influenza and pneumonia 34.6 → 23.6, a teal bar running the other way at −31.8%. Three ticked boxes along the bottom read “Every death, both years — 2,337,256 certificates in 1998 and 2,391,399 in 1999, nothing sampled, nothing modelled”, “Age-adjusted, one standard — the same year-2000 standard population on both sides of the comparison”, and “One office, one system — the same national vital statistics system published both years”. Down the right, a card reads “+91.9%, Alzheimer’s disease, age-adjusted death rate, 1998 to 1999. No other leading cause of death moved half as far.” A second card headed “The leading-cause table, 1998 → 1999” lists Alzheimer’s disease 12th → 8th in gold, diabetes 7th → 6th, septicemia 11th → 10th, hypertension 15th → 13th, influenza and pneumonia 6th → 7th and chronic liver disease 10th → 12th, noting that atherosclerosis left the fifteen leading causes after at least four decades and aortic aneurysm and dissection entered at fifteenth. A gold note headed “Nothing modelled here” says these are completed death certificates, not estimates — every death registered in the United States in 1998 and in 1999, age-adjusted by the same office to the same standard population; total mortality moved 0.7%, and what changed is what Americans died of. Beneath it, in small grey type: “Cause of death classified by the International Classification of Diseases; 1999 tabulations use the Tenth Revision.” A footnote credits the National Center for Health Statistics, “Deaths: Final Data for 1999”, National Vital Statistics Reports 49(8), 2001, table 8, and calls the slide a Mortality Desk demonstration.

The epidemic that arrived with a new codebook

The redefined measure