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.

November 15, 2017

Summarising house prices

From the Herald (linking to this story)


To begin with, “worst” is distinctly unfortunate now we’ve finally got a degree of political consensus that Auckland house prices are too high. “Best” might be too much to hope for, but at least we could have a neutral term.

More importantly, as the story later concedes, it’s more complicated than that.

It’s not easy to decide what summary of housing prices is ideal.  This isn’t just about mean vs median and the influence of the priciest 1%, though that comes into it.  A bigger problem is that houses are all individuals.  Although the houses sold this October are, by and large, not the same houses that were sold last October, the standard median house price summary compares the median price of one set of houses to the median price of the other set.

When the market is stable, there’s no real problem. The houses sold this year will be pretty much the same as those sold last year. But when the market is stable, there aren’t interesting stories about real-estate prices.  When the market is changing, the mix of houses being compared  can change. In this case, that change is the whole story.

In Auckland as a whole, the median price fell 3.2%. In the old Auckland City — the isthmus — the median price fell 17%. But

Home owners shouldn’t panic though. That doesn’t mean the average house price has fallen by anything like that much.

The fall in median has been driven largely by an increasing number of apartments coming onto the market in the past year.

That is, the comparison of this October’s homes to last October’s homes is inappropriate — they aren’t similar sets of properties.  This year’s mix has many more apartments; apartments are less expensive; so this year’s mix of homes has a lower median price.

The story does admit to the problem with the headline, but it doesn’t really do anything to fix it.  A useful step would be to separate prices for apartments and houses (and maybe also for townhouses if they can be defined usefully) and say something about the price trends for each.   A graph would be a great way to do this.

Separating out changes in the mix of homes on sale from general house price inflation or deflation is also helpful in policy debates. Changing the mix of housing allows us to lower the price of housing by more than we lower the value of existing houses, and would be valuable for the Auckland public to get a good feeling for the difference.

Bogus poll headlines justified

The Australian postal survey on marriage equality was a terrible idea.

It was a terrible idea because that sort of thing shouldn’t be a simple majority decision.

It was a terrible idea because it wasn’t even a vote, just a survey.

It was a terrible idea because it wasn’t even a good survey, just a bogus poll.

As I repeatedly say, bogus polls don’t tell you anything much about people who didn’t vote, and so they aren’t useful unless the number voting one particular way is a notable proportion of the whole eligible population. In the end, it was.

A hair under 50% of eligible voters said ‘Yes’, just over 30% said ‘No’, and about 20% didn’t respond.

And, in what was not at all a pre-specified hypothesis, Tony Abbott’s electoral division of Warringah had an 84% participation rate and 75% ‘Yes’, giving 63% of all eligible voters indicating ‘yes’.


PS: Yay!

November 13, 2017

Stat of the Week Competition: November 11 – 17 2017

Each week, we would like to invite readers of Stats Chat to submit nominations for our Stat of the Week competition and be in with the chance to win an iTunes voucher.

Here’s how it works:

  • Anyone may add a comment on this post to nominate their Stat of the Week candidate before midday Friday November 17 2017.
  • Statistics can be bad, exemplary or fascinating.
  • The statistic must be in the NZ media during the period of November 11 – 17 2017 inclusive.
  • Quote the statistic, when and where it was published and tell us why it should be our Stat of the Week.

Next Monday at midday we’ll announce the winner of this week’s Stat of the Week competition, and start a new one.


November 6, 2017

Stat of the Week Competition: November 4 – 10 2017

Each week, we would like to invite readers of Stats Chat to submit nominations for our Stat of the Week competition and be in with the chance to win an iTunes voucher.

Here’s how it works:

  • Anyone may add a comment on this post to nominate their Stat of the Week candidate before midday Friday November 10 2017.
  • Statistics can be bad, exemplary or fascinating.
  • The statistic must be in the NZ media during the period of November 4 – 10 2017 inclusive.
  • Quote the statistic, when and where it was published and tell us why it should be our Stat of the Week.

Next Monday at midday we’ll announce the winner of this week’s Stat of the Week competition, and start a new one.


November 4, 2017

A few details

Seeing a headline like


might cause an unwary person to think that eating purple kumara would reduce their risk of colon cancer by seventy-five per cent.

You, of course, would be suspicious and would want to read the story.

He found that when fed to three generations of mice bred with colon cancer, using the same gene which caused the disease in humans, purple kumara reduced the number of polyps by two-thirds or more.

So, the study is in mice. Mutant mice.  And it didn’t reduce the risk of colon cancer in these mice — which was basically 100% — it reduced the number of developing tumours.

It’s true that the mutation is one that occurs in people, too. About one in ten thousand people is born with the mutation that the mice had — these people have the mutation in every cell in their bodies, and they all get colon cancer if they don’t have major surgery.  And in the majority of ordinary people who get colon cancer, part of the  process is a mutation in this same gene in one cell.  So, the mutant mice are relevant.  There isn’t any problem with the research being in mice, just with the headline. Especially as further down in the story we hear about the equivalent dose of kumara in humans

“To eat 1kg of sweet potato every day is too hard.”

and that the kumara seems to have most potential as a way to produce a concentrated extract.

So far, there’s not much evidence either way on whether anthocyanins (basically, purple food other than beets or dragonfruit) really prevent cancer in humans.  Animal studies such as this one give good reasons to be hopeful; the history of other micronutrient-based prevention trials give good reasons to be skeptical.