December 8, 2017

Attributing risk

Some time in the next week or so, we should be getting the ACC Christmas Sermon, where we get told about how many accidents happen on Christmas Day. From last year’s version in the Herald

Every year, more than 3400 claims are lodged with ACC for Christmas Day incidents, costing the country almost $3million.

As I always point out, that’s a lot less than the number lodged on a typical day that isn’t Christmas.  On the other hand, many of those 3400 are genuinely Christmas-caused injuries; accidents that would not have happened on some random day in summer.

You can look at Christmas-attributable risk by considering individual cases and counting the number that involve new toys, Christmas trees, batteries inserted in appropriately, misuse of wrapping paper, etc, etc. Or, you can compare Christmas to an otherwise similar day.

Rafa at Simply Statistics writes about a more serious example.

The official death toll from Hurricane María in Puerto Rico is 55. That’s 55 people whose death can be specifically and clearly attributed to the hurricane. However, the number of recorded deaths from all causes in September was 2838, which is 455 above the average for September in recent years. The next largest exceedance in the past seven years was just over 200 in November 2014.

Attributing deaths on a case-by-case basis to a disaster like María is hard; it would be hard to make those sorts of decisions even without the continuing post-hurricane disruption. Another example is deaths due to the 2003 power outage in New York, where there were 6 officially-attributed deaths but a spike of 90 in the total death statistics.

Sometimes we want to look at specifically attributable cases: when snow shuts down the roads, we probably want to count the number of snow-caused crashes without subtracting the number of snow-prevented ones. But for natural disasters it’s probably the total excess deaths we want.

December 7, 2017

If this goes on…


If you click through, things are less local and immediate: ATMs could be extinct in Australia within 30 years


A projection of data from the Reserve Bank of Australia by has found ATMs could be a distant memory in Australia by 2036.

2036 is in 19 years, and 19 is less than thirty, so I suppose that counts as within 30 years. So how did they do the projection? There’s not much detail in the story and I couldn’t find any on

The story says

According to, the number of ATM withdrawals per month has fallen from a high of 73 million in 2010 to just 47 million this year. If the trend continues at the same rate, ATM use will reach zero in three decades.

Now, I can fit a straight line to data. They teach you this in statistics. They often also teach you not to do it with just two points, but whatever

Ok, maybe had more data or more detailed data or something, but the information in the story is all we have, and it doesn’t really support either “2036” or “30 years”

I don’t know how long ATMs will last. And I don’t think does either. But they do know how to get a free mention in the Herald.

December 4, 2017

Compared to what?

From Stuff

Your summer pavlova costs more than 40 per cent more to make this year than it did 10 years ago – and commentators think that trend will continue.

That’s true, but prices now and prices ten years ago are in different currencies, and so shouldn’t just be compared like numbers.

Using the RBNZ inflation calculator, about half the apparent price increase is just currency conversion; a 2007 dollar is worth about 1.2 2017 dollars.

On top of that, incomes have changed over the past ten year. The median annual household income is up about 36%, so if pavlova is less affordable than in 2007 it’s mostly because of something like housing costs, not the price of cream and kiwifruit.

December 1, 2017


  • Testing drug sniffer dogs: “The dogs are mainly used to confirm what we already suspect,” says Fulmer. “When the dogs come out, about 99 percent of the time we get an alert. And it’s because we already know what’s in the car; we just need that confirmation to help us out with that.”  At least with the biosecurity beagles at Auckland Airport, there’s no incentive on the handler’s part for false positives.
  • Security researcher Matt Blaze talks to US Congress about voting security: “The most reliable and well-understood method to achieve this is through an approach called risk-limiting audits. In a risk limiting audit, a statistically significant randomized sample of precincts have their paper  ballots manually counted by hand and the results compared with the electronic tally. …The effect of risk-limiting audits is not to eliminate software vulnerabilities, but to ensure that the integrity of the election outcome does not depend on the herculean task of securing every software component in the system.” 
  • The Grattan Institute (in Australian) has a report (PDF) on adverse events in hospitals: Strengthening safety statistics: how to make hospital safety data more useful.  Peter Davis has an opinion piece in the Dominion Post on what NZ could do
  • Statistical population genetics of New York rats: they stick to their neighbourhoods, just like the humans. Sarah Zhang in the Atlantic.
  • ProPublica found that Facebook won’t let you target ads based on race, or even on ethnicity — but it will let you target “African American” under “Behaviors”, sub-category “Multicultural Affinity”.   Facebook said “The rental housing ads purchased by ProPublica should have but did not trigger the extra review and certifications we put in place due to a technical failure.” The last five words of that sentence are interesting — they don’t actually add anything, but they kind of sound like they do.
November 29, 2017

Expensive road is expensive

Under the headline Auckland residents prepared to pay to fix congested road, Bernard Orsman writes about an AA survey of residents of the Devonport peninsula.  There were three options for upgrading Lake Road, costing $10 million, $40 million, or $70 million.  Of the roughly half of respondents who preferred the $70 million proposal, about half (ie, 25% of people) were willing to pay targeted rates, and two-thirds of these (17%) were willing to pay $50 -$200/year.

“People are willing to pay something extra, but they want to see it happening faster as a result. AA members want to see benefits within the next five years – not 10,”

One calculation that’s left for the reader is how much would actually be needed to make up the difference between a $40m and $70m budget.  According to the local free paper, there are 8,328 households in the Devonport peninsula.  At $200/year each, that’s $1.6m/year.  Over ten years, that’s about half the gap — which might actually be reasonable.  On the other hand, $50/year each for 5 years is less than 10% of the gap — and most weren’t willing to support even this, in a survey where it didn’t actually cost anything.

November 23, 2017

State caricatures

This map of most disproportionately consumed Thanksgiving side dishes, from 538, is circulating again


As I’ve pointed out before, these aren’t the most commonly eaten in each state, they’re the ones that are most different from the rest of the country — a sort of caricature of the nation’s food geography. It’s actually worse than that, since this is from a relatively small poll and didn’t even record what state people were in, just what region.

Since 538 makes their data available, we can do other maps. Here’s the most commonly consumed side-dish


It’s much less interesting, but even this overstates the geographic variation.

Here, on a red-to-yellow heat scale, is the proportion of respondents who have mashed potatoes

and green beans/green bean casserole

There’s nothing necessarily wrong with the ‘most disproportionate’ map, as long as you recognise what it’s doing. But saying, as 538 did, “When you get past the poultry and check out the side dishes, though, the regional distinctions really come out” tends to hide that point.

More complicated than that

Science Daily

Computerized brain-training is now the first intervention of any kind to reduce the risk of dementia among older adults.

Daily Telegraph

Pensioners can reduce their risk of dementia by nearly a third by playing a computer brain training game similar to a driving hazard perception test, a new study suggests.

Ars Technica

Speed of processing training turned out to be the big winner. After ten years, participants in this group—and only this group—had reduced rates of dementia compared to the controls

The research paper is here, and the abstract does indeed say “Speed training resulted in reduced risk of dementia compared to control, but memory and reasoning training did not”

They’re overselling it a bit. First, these are intervals showing the ratios of number of cases with and without the three types of treatment, including the uncertainty


Summarising this as “speed training works but the other two don’t” is misleading.  There’s pretty marginal evidence that speed training is beneficial and even less evidence that it’s better than the other two.

On top of that, the results are for less than half the originally-enrolled participants, the ‘dementia’ they’re measuring isn’t a standard clinical definition, and this is a study whose 10-year follow-up ended in 2010 and that had a lot of ‘primary outcomes’ it was looking for — which didn’t include the one in this paper.

The study originally expected to see positive results after two years. It didn’t. Again, after five years, the study reported “Cognitive training did not affect rates of incident dementia after 5 years of follow-up.”  Ten-year results reported in 2014, showed relatively modest differences in people’s ability to take care of themselves, as Hilda Bastian commented.

So. This specific type of brain training might actually help. Or one of the other sorts of brain training they tried might help. Or, quite possibly, none of them might help.  On the other hand, these are relatively unlikely to be harmful, and maybe someone will produce an inexpensive app or something.

November 20, 2017


  • “The Wired Brain: how not to talk about an AI-powered future” from  Ines Montani
  • “How neural networks build up their understanding of images” from Distill
  • “Many pub and restaurant chains do this kind of thing a bit, but none does it with the ruthlessness of Wetherspoon, whose grand, cavernous spaces fill up each day thanks in large part to a menu that reads like several hundred carefully targeted microaggressions against the immediate community.” Graphics and data analysis from the Financial Times
  • Maps of cycling to work in Melbourne.  The inner north is flat and has nice wide streets suitable for adding bike lanes.
  • “At 10am 14 November 2017 NZST, millions of people around the world suddenly had high blood pressure.” from John Pickering
  • Keith Ng is joining the Herald as a data journalist!
  • The US Supreme Court doesn’t like numbers ““When I read all that social science stuff and the computer stuff, I said, ‘Is there a way of reducing it to something that’s manageable?’” said Justice Breyer, who is nevertheless expected to vote with the court’s liberal bloc. It’s easy to imagine a situation where the answer for this and many other cases is, simply, “No.” The world is a complicated place.
  • Map of changes in tourist numbers in the South Island, from StatsNZ
  • “Next time you see some poll breathlessly claiming that 21 percent of Americans support executing anyone whose name starts with “G”, or that 18 percent of Millennials believe themselves to be the reincarnation of Kublai Khan, take it with a grain of salt. It’s a lot easier to give a stupid answer on a survey than to actually truly hold a nuts belief” from Noah Smith
  • Porcupine graph: annual projections by the International Energy Agency for rates of installation of new solar panels, from Auke Hoekstra
November 19, 2017

Hyperbole or innumeracy?

From the Herald (and also from NewstalkZB, apparently originally at South Africa’s The Citizen)

He is also said to own a custom-built Mercedes Benz s600L that is able to withstand AK-47 bullets, landmines and grenades. It also features a CD and DVD player, internet access and anti-bugging devices. The Citizen reported that Mugabe – who is a trained teacher – also owns a Rolls-Royce Phantom IV: a colonial-era British luxury car so exclusive, only 18 were ever manufactured. The vintage black car is estimated to be worth more than Zimbabwe’s entire GDP. (emphasis added)

Several people on Twitter, starting with Richard Easther, had the same reaction: that this doesn’t look remotely plausible.  It’s like the claims that Labour’s water levies would make cabbages cost $18 and a bottle of wine $75 — extraordinary claims demand, if not extraordinary evidence, at least some evidence.

So, how is it that you’d decide this number was implausible? Well, in one direction, you might try to guess the GDP of Zimbwawe.  If Zimbabwe had a smaller population than NZ you’d probably know it was a small country, so we can say there’s at least 5 million people.  So, if the per-capita GDP was only $1, it would still add up to $5 million, and that’s a very expensive car.  Since you’d expect the population to be more than 5 million and the per-capita GDP to be a lot more than $1, the figure is looking implausible.

In the other direction, you might look up the current GDP of Zimbabwe — $16 billion — or the lowest it’s been in recent years — $4.4 billion in 2008 — and note that you could by several wide-body jets for that much.

That’s enough to know something is strange. If you wanted more detail you could search for prices of Rolls-Royce Phantom IVs or of the most expensive cars ever sold, and find that, yes, there’s three or four orders of magnitude missing.

Or, you could look at the first line of the story

Zimbabwe embattled president Robert Mugabe is reportedly worth more than $1 billion despite his country being one of the poorest in the world.

Or the last line

Rolls Royce Phantoms cost a minimum of just under $698,000, but custom-built versions are sold for as much as $1.74 million. Media in South Africa reported the combined cost of the cars was about $6.98 million.

and again, there’s no way the claim about the car vs the GDP could be true — a used one couldn’t be worth thousands of times more than a new one.

So, where could it have come from?  My guess is that the claim was originally hyperbole: that someone did say “his car’s worth more than the Zimbabwe GDP” but they didn’t mean it literally. Over repetitions, the rhetorical figure turned into an “estimate”, and was quoted without any real thought.

What’s harder to understand is someone thinking a CD and DVD player is the height of motoring luxury.

November 17, 2017


Q: Can I improve my chances of winning Lotto by…

A: No.

Q: But….

A: No.

Q: …

A: Just no.

Q: … by buying a ticket?

A: Ok, yes. But not by very much.

Q: You sound like you’ve been asked about Lotto odds a lot.

A: There’s a larger-than-usual jackpot in the NZ Powerball

Q: Enough to make it worth buying a ticket?

A: If you like playing lotto, sure.

Q: No, as an investment.

A: I refer the honourable gentleman to the answer given some moments ago

Q: Huh?

A: No.

Q: But $35 million. And a 1 in 38 million chance of winning. And 80c tickets.  Buying all the tickets would cost less than $30 million. So, positive expected return.

A: If you were the only person playing

Q: And if I’m not?

A: Then you might have to share the prize

Q: How many other people will be playing?

A: Lotto NZ says they expect to sell more than a million tickets

Q: Compared to 38 million possibilities that doesn’t sound much

A: That’s tickets. Not lines.

Q: Ah. How many lines?

A: They don’t say.

Q: Couldn’t the media report that instead of bogus claims about a chemist in Hawkes Bay selling better tickets?

A: Probably not. I don’t think Lotto NZ tells them.

Q: That story says it would take 900 years to earn the money at minimum wage. How long to get it by playing Powerball?

A: At, say, ten lines twice per week?

Q: Sure.

A: 36900 years.