Posts written by Thomas Lumley (2645)

The Herald asks “Is John Carter the biggest flop of all time?”

While there isn’t an official definition of `flop’, it’s worth noting that John Carter has grossed $184 million in two weeks.  That’s quite a lot of money. For example, it’s more than the first Harry Potter movie managed.

In one of our Stat of the Week nominations, the commenter takes issue with ” There is an incident [of intimate partner violence] reported to the police every 7 minutes and only 18% of cases are reported”

According to this there are 417,000 instances of physical intimate partner violence every year. With a population of around 4 million, and half women, this means that up to 40% (bad stat alert!) could be affected every year.

Leaving for the moment whether the 18% reporting rate is well-founded,  the ‘up to’ is critical here.  If the occurrences were independent, 417,000 instances would be 40%of NZ women, but of course they aren’t independent.  People don’t just abuse their partners once, they keep doing it until they are stopped.  With 417,000 instances, the actual number of women attacked per year could as easily be 4% as 40%.

A similar issue came up with the elimination of TV7, when Jonathan Coleman divided the number of monthly viewers of TV7 by four and called it an estimate of weekly viewers.  That would be appropriate if watching TV7 one week was independent of watching it the next week, but of course it’s not.  The real number of weekly viewers will be very close to 100% of the number of monthly viewers, not 25%.

The issue is very important in legal settings.  British paediatrician Roy Meadow famously wrote (and testified in court) “one sudden infant death is a tragedy, two is suspicious and three is murder, until proved otherwise”.  If sudden infant deaths were independent, then three in one family would be very rare, but we don’t have good evidence that they are independent.  On top of this,  multiple murder of infants, especially without leaving any signs, is also rare: knowing that a rare thing happened doesn’t tell you which one it was.

What provoked this post, though, was a reader’s email reminding us of the joke about the man who always carried a bomb when he flew, on the grounds that two bombs on the same airline would be incredibly unlikely.  The point of the joke is that his bomb obviously seems independent of a hypothetical other bomb, so the safety is unaffected.  In fact, if you manage to carry a bomb onto a plane it’s either a small regional NZ flight that doesn’t have screening, or there’s been a failure of the security system.  While it’s not terribly worrying that the security system failed, it does make you wonder what else isn’t being done according to plan. Perhaps you should reconsider your flight plans.

From a talk I’m giving tomorrow to the Canterbury Mathematical Association:

“Bogus polls look a bit like real surveys on the outside, but don’t have any of the inner machinery that you need to make them work.”

A Campbell Live trailer just asked “Why is the disease that’s at epidemic levels about to be targeted by government cuts?”

In what is unfortunately an urban legend, Willie Sutton was supposedly asked “Why do you rob banks?”, and replied “Because that’s where the money is”.

There is no point in the NZ medical system spending too much time and effort on assessing cost-effectiveness in idiopathic pulmonary fibrosis, Fabry syndrome, or Hashimoto’s encephalitis.  Reducing the cost of diabetes treatment, on the other hand,  can free up a lot of money to spend on other illnesses.  There’s nothing surprising or shocking about targeting diabetes for cost savings, especially in a low-cost public health system like ours.

The real question is whether the savings in money come at too high a price in patient care, not why diabetes is one of the targets.

This week’s installment in the stream of stories that have found The Answer To Obesity says the problem isn’t just diet or exercise, it’s plastic bottles.   The Herald says

Man-made chemicals present in homes, schools, offices, cars and food are probably contributing to the sharp rise in obesity and diabetes in Western societies, according to a review of scientific literature published yesterday.

Until now lifestyle factors such as lack of exercise and poor diet were believed to be the primary causes of the increased incidence of both conditions, whose proliferation has strained global health budgets.

If you separate out diabetes, it’s not true that obesity has strained global health budgets.  The incidence of heart attacks, for example, continues to go down all over the world.  The rate of decline has slowed a bit, but increases are still theoretical and aren’t straining anything.  However, if we stipulate that obesity is bad, are bisphenol A, PCBS, and phthalates responsible?

All 240 studies they reviewed – whether in test-tubes, on animals or on humans – had been peer-reviewed and published in scientific journals.

It occurred to me that using the bus prediction system to estimate traffic congestion would make a nice student research project.  Bus flow along Dominion Rd or through Newmarket, say, gives a lot of information about traffic congestions.  This information might be useful for improving bus predictions, but also might be useful for drivers as a way of getting real-time congestion data for a much larger set of roads than is currently available.

Unfortunately, the terms of use for the prediction system say “No part of the MAXX Website may be distributed, resold, published, copied, reproduced, transmitted or stored and you are not permitted to incorporate the material or any part of it in any other work or publication (whether in hard copy, electronic or any other form) without ATs express written consent.”  I’m not sure if this is actually true, but it’s a fight I don’t need to have.

Stuff is reporting a survey on attitudes to overseas ownership of NZ farms.   New Zealanders aren’t just opposed to Chinese buyers of the Crafar farms because they’re racist — they say so themselves.

According to the story, the survey asked

“The Chinese company Shanghai Pengxin wants to buy the Crafar farms; do you support or oppose selling the farms to this Chinese company?”

and then

“Do you agree, or disagree, with this statement: ‘I don’t care what the nationality of the company is, I don’t want the farms to be sold to a foreign buyer’.

Not surprisingly, people who had said they didn’t want Shanghai Pengxin as the buyer went on to say that it wasn’t just Chinese buyers they were opposed to.

If you really want to find out whether people feel differently about a Chinese buyer, this isn’t the best way.  You want to ask different samples of people about Chinese buyers and buyers from, say, the UK, or Monaco, or some other country with more NZ farm ownership than China.   That way, people’s natural reluctance to admit to anti-Chinese bias won’t distort the results.

A survey like that would actually be interesting, since it’s not at all clear whether anti-Chinese bias is a big factor or a small one.

We don’t get a lot of opportunities to say nice things about statistics in the press, but the Herald has a good example today in a report on young Maori health.  They report both the numbers of hospital admissions for Maori and non-Maori, and the relative risk.   It wouldn’t hurt if they had linked to a definition of bronchiectasis, though.

On 3 News last night, we got a tour of South Auckland, just missing various drunken disturbances.   The video starts “3News has uncovered some shocking figures about the amount of alcohol some offenders are drinking” , and the webpage story says  “new research has revealed the alarming impact alcohol is having on crime“.

That’s approximately truish.  The `new research’ is a report dated December 2010 by the New Zealand Drug Use Monitoring (NZ‐ADUM) research programme at Massey University.  They surveyed 800 arrestees in Whangarei, Auckland Central, Wellington Central, and Christchurch Central, asking them about their drug and alchol use.   And there was a lot of drug and alcohol use — evidently, police arrests are selecting for people who are insufficiently risk-averse in all sorts of ways.  The figures possibly qualify as ‘shocking’, with arrestees reporting an average of 12 standard drinks both on typical days when they drink, and on the day they were arrested.  Some people reported much higher consumption, and some reported none.

3News compares this to “The average New Zealander drinks around two standard drinks a day”, which looks like this StatsNZ figure on total alcohol sales.  In addition to the problems of comparing self-reported drinking with sales data, the national average is over all days and the arrestee average is over days when they drank.  Since the arrestees reported drinking an average of about 1 day in 3, the correct comparison to 2 drinks/day is 12/3=4 drinks per day, about twice the NZ average.    The arrestees also consumed more tobacco, more cannabis, more P, more opioids, more ecstasy, and more of everything else than the average Kiwi; in fact, as far as I can tell alcohol showed a smaller difference than any of the other drugs — mostly because the average for all New Zealanders is fairly high.

The other notable statistic about drinking is that the average drinks per day that they drank was the same as the average drinks on the day they were arrested.  That is, although many of the arrestees were drunk when they were arrested, they were no drunker than they usually are on about one day in three.   The relative risk of arrest given their heavy drinking for these arrestees is thus about 3, which is a lot smaller than I would expect even without the headlines.  If you read the report, while alcohol is unquestionably a problem, it is far from the only problem that the arrestees have.

This isn’t in the NZ media yet, but it will probably turn up soon.  A second CERN experiment, ICARUS,  has repeated the measurement of neutrino speed made by the famous OPERA experiment: the same neutrino source, the same distance, but different measurement equipment.   And the neutrinos arrived on-time, not 60ns early. Since the OPERA results violate relativity  and have other practical and theoretical problems , it’s not that hard to decide which set of numbers to believe.

As Prof. Matt Strassler says

This is the way it works in science all the time. A first experiment makes a claim that they see a striking and surprising effect. A second experiment tries to verify the effect and instead shows no sign of it. It’s commonplace. Research at the forefront of knowledge is much more difficult than people often realize, and mistakes and flukes happen on a regular basis. When something like this happens, physicists shrug and move on, unruffled and unsurprised.