New Zealand has 5.3 million people, so almost every number anyone argues about here sits on a denominator small enough that noise runs the show. A region of 32,000. A school of 200. A district with fifteen thousand ratepayers.
This page answers the two questions that follow: did it actually change, and is the worst area actually worst. The second answer is usually no — it is usually the smallest.
Nothing is uploaded. The arithmetic runs on the server for one request and nothing is written down.
First, the preconditions
16 areas, 331 events, an overall rate of 6.5 per 100,000. Not one area escapes the 99.8% limits drawn for an area of its own size — so the ranking below is a ranking, not a finding.
The funnel
Rate against population, with the limits an area of each size would produce at the overall rate. The limits flare out on the left because small populations produce wild rates — that is arithmetic, not a story about those places. An area is only remarkable when it escapes the limits drawn for its own size.
The limits are stepped because counts are whole numbers. A smooth curve would imply a precision the arithmetic does not have.
| # | Area | Events | Population | Rate /100,000 | 95% range | Expected count | |
|---|---|---|---|---|---|---|---|
| 1 | Tasman / Te Tai-o-Aorere | 6 | 58,000 | 10.3 | 3.8 – 22.5 | 0 – 8 | |
| 2 | West Coast / Te Tai Poutini | 3 | 32,000 | 9.4 | 1.9 – 27.4 | 0 – 5 | |
| 3 | Nelson / Whakatū | 5 | 55,000 | 9.1 | 3.0 – 21.2 | 0 – 8 | |
| 4 | Wellington / Te Whanganui-a-Tara | 48 | 550,000 | 8.7 | 6.4 – 11.6 | 23 – 48 | |
| 5 | Gisborne / Te Tairāwhiti | 4 | 52,000 | 7.7 | 2.1 – 19.7 | 0 – 7 | |
| 6 | Waikato | 38 | 500,000 | 7.6 | 5.4 – 10.4 | 21 – 44 | |
| 7 | Otago / Ōtākou | 16 | 250,000 | 6.4 | 3.7 – 10.4 | 8 – 24 | |
| 8 | Taranaki | 8 | 125,000 | 6.4 | 2.8 – 12.6 | 2 – 14 | |
| 9 | Bay of Plenty / Te Moana-a-Toi | 22 | 350,000 | 6.3 | 3.9 – 9.5 | 13 – 32 | |
| 10 | Canterbury / Waitaha | 40 | 660,000 | 6.1 | 4.3 – 8.3 | 29 – 56 | |
| 11 | Auckland / Tāmaki Makaurau | 102 | 1.7m | 6.0 | 4.9 – 7.3 | 89 – 131 | |
| 12 | Marlborough / Te Tauihu-o-te-waka | 3 | 52,000 | 5.8 | 1.2 – 16.9 | 0 – 7 | |
| 13 | Northland / Te Tai Tokerau | 11 | 200,000 | 5.5 | 2.7 – 9.8 | 5 – 20 | |
| 14 | Manawatū-Whanganui | 13 | 260,000 | 5.0 | 2.7 – 8.6 | 8 – 25 | |
| 15 | Southland / Murihiku | 5 | 103,000 | 4.9 | 1.6 – 11.3 | 1 – 12 | |
| 16 | Hawke's Bay / Te Matau-a-Māui | 7 | 180,000 | 3.9 | 1.6 – 8.0 | 4 – 19 |
About 0.8 of them would be expected outside the 95% band by chance alone, and 0 are. Nothing here needs explaining.
Tasman / Te Tai-o-Aorere tops the table at 10.3 per 100,000 — but on 6 events over 58,000 people its own interval runs from 3.8 to 22.5, which contains the overall rate of 6.5. It is at the top because it is small, not because it is worse.
3 events over 32,000 people is a rate of 9.4 per 100,000 — and anything from 1.9 to 27.4. That is not a precise number with error bars on it. It is a range, and the middle of it is not more true than the ends.
What it does not know
It does not know why. An area outside the limits has something to explain — a different population, a different way of recording, or a real difference. This page says the number is unusual, and nothing at all about which.
Recording is not the same everywhere. A district that records more carefully will look worse. On small counts that difference alone can move an area from the middle to the top.
The populations matter as much as the counts. A rate is a fraction, and half of it comes from a denominator somebody else estimated. If the population is a projection rather than a count, that uncertainty is not in any interval on this page.
One year is one year. The area at the top this year is usually not the area at the top next year, and that is not improvement — it is the same arithmetic running again.