July 2, 2016

Dark and full of terrors

Two items from the Herald this week:

70-year-old licked by dog nearly dies from blood poisoning

One paracetamol in pregnancy could raise risk of autism

There has long been an argument over whether the daily news reports should show more good news (that isn’t sports). Supporters of the status quo argue that the bad news is the important news. That might be true for wars and rumors of war, but it isn’t true in stories fishing for a scary health problem.

The dog story reported a cause of sepsis that has been diagnosed about once every two years in the UK over the past quarter-century. Since there are about 30,000 cases of sepsis per year in the UK, that’s 0.00% of cases, to two decimal places.  It wouldn’t be surprising if the health benefits of having a dog were larger than this, and it’s pretty clear the benefits in happiness are.

The paracetamol story is the other sort of unnecessary health scare: something that probably isn’t true and certainly isn’t supported by enough evidence for a public-health warning.  Since paracetamol has nothing to recommend it as a recreational drug, women who take it during pregnancy are probably doing so for a good reason, and it’s going to be hard to distinguish the effects of that reason from those of the drug.

In fact, the study  (Table 2)found that mothers who took paracetamol during pregnancy had slightly lower risk of autism-spectrum symptoms in their children than those who didn’t. However, the mothers who took paracetamol looked as if they should have already have been at lower risk than those who didn’t. When the researchers attempted to control for this, there was basically the same risk of autism-spectrum symptoms in the children of those who took paracetamol as those who didn’t.

However, the basically-no-difference could be separated out into a higher level of symptoms  in boys and a lower level in girls, by a bit more than half a point on a scale where 15 points is the threshold for likely autism.  This got reported in the story as

There was also a link with paracetamol and signs of autism – but only in boys.

And the “One paracetamol” headline? There was absolutely no analysis in the paper comparing ‘one paracetamol’ to no paracetamol.

July 1, 2016

Too good to check

Twitter is a good way to get news and rumours of news from across the world, but it also exposes you to a lot of items that are either deliberate fraud or just ‘too good to check’.  Here’s one: it claims to compare maps of the ‘Leave’ vote with BSE prevalence.


It’s clear what idea the author was going for, and it’s also clear that it has to be unfounded as well as malicious. The BSE prions weren’t preferentially consumed in farming areas — people in cities eat hamburgers, too — and nvCJD is not only very rare, but primarily affects movement rather than political beliefs.

However, it’s not inconceivable that farming areas which experienced losses from BSE and then later from foot-and-mouth would be anti-government and possibly anti-European. Some correlation, even a strong correlation, would be possible for that reason.

If you cared about the truth, there’s a simple two-word Google search you could do before passing on the maps: BSE Scotland. Yes, there was mad cow disease north of the border. You could also note the implausibility of having exactly the same map layout, and a color scheme that was just a grayscale version of the modern one.

Thinking about the numbers

More students cheat in exams, and most are in Auckland, says the Herald.

This story combines two frequent StatsChat themes: denominators, and being careful about what was actually measured.

Auckland, as we have noted before, has a higher population than other regions.  As you will recall, it’s about a third of the NZ population, so it looks like making up about 50% of those caught cheating is excessive. That’s the sort of work that the paper might do for you — as well as checking if 1/3 is still about right as the proportion of students sitting NCEA exams (it seems to be).

On similar lines, if you look just at the totals without denominators, you’ll miss some notable values.  Northland had 25 students caught cheating, which is more than the much-larger Waikato and Canterbury regions. You’d expect about 10 at the national average rate and about 15 at the Auckland rate.

Much more important is the question of what proportion of those cheating were caught — to say things like

Again Central Plateau and the Cook Islands had no cheaters, and Wairarapa and Southland students were also honest

or to draw conclusions about trends over time assumes that you’re not missing many.

The story says

NZQA received 1,314,207 entries in NCEA and New Zealand Scholarship examinations from 145,464 students last year.

The 290 attempts at cheating that were caught come to just under 0.2% of students and just over 0.02% of exams.  Maybe I’m just cynical, but I’d be surprised if the real rate was as low as one exam in a thousand, let alone five times lower.

June 28, 2016

Updated Super Rugby Predictions

I posted predictions for Round 15 before the international games. I have just updated the predictions because I had not taken into account that the Chiefs versus Crusaders game is to be on a neutral ground.

See the updated post atSuper 18 Predictions for Round 15

NRL Predictions for Round 17

Team Ratings for Round 17

The basic method is described on my Department home page.

Here are the team ratings prior to this week’s games, along with the ratings at the start of the season.

Current Rating Rating at Season Start Difference
Cowboys 9.60 10.29 -0.70
Storm 8.57 4.41 4.20
Broncos 6.39 9.81 -3.40
Sharks 5.71 -1.06 6.80
Bulldogs 4.96 1.50 3.50
Raiders 2.90 -0.55 3.50
Eels 1.23 -4.62 5.90
Panthers -0.59 -3.06 2.50
Roosters -1.35 11.20 -12.60
Dragons -2.11 -0.10 -2.00
Sea Eagles -2.14 0.36 -2.50
Warriors -2.56 -7.47 4.90
Rabbitohs -3.23 -1.20 -2.00
Titans -3.37 -8.39 5.00
Wests Tigers -5.23 -4.06 -1.20
Knights -17.13 -5.41 -11.70


Performance So Far

So far there have been 118 matches played, 71 of which were correctly predicted, a success rate of 60.2%.
Here are the predictions for last week’s games.

Game Date Score Prediction Correct
1 Panthers vs. Rabbitohs Jun 24 28 – 26 6.30 TRUE
2 Knights vs. Dragons Jun 25 18 – 30 -12.00 TRUE
3 Sharks vs. Warriors Jun 25 19 – 18 14.10 TRUE
4 Bulldogs vs. Broncos Jun 25 40 – 14 -2.30 FALSE
5 Titans vs. Raiders Jun 26 22 – 30 -2.40 TRUE
6 Storm vs. Wests Tigers Jun 26 29 – 20 18.10 TRUE
7 Cowboys vs. Sea Eagles Jun 27 30 – 26 16.50 TRUE


Predictions for Round 17

Here are the predictions for Round 17. The prediction is my estimated expected points difference with a positive margin being a win to the home team, and a negative margin a win to the away team.

Game Date Winner Prediction
1 Roosters vs. Bulldogs Jun 30 Bulldogs -3.30
2 Broncos vs. Storm Jul 01 Broncos 0.80
3 Warriors vs. Titans Jul 02 Warriors 4.80
4 Wests Tigers vs. Panthers Jul 02 Panthers -1.60
5 Sharks vs. Eels Jul 02 Sharks 7.50
6 Raiders vs. Knights Jul 03 Raiders 23.00
7 Rabbitohs vs. Cowboys Jul 03 Cowboys -9.80
8 Sea Eagles vs. Dragons Jul 04 Sea Eagles 3.00


June 27, 2016

Graph of the week

From the BBC coverage of the Brexit referendum. With great power comes great responsibility.


  • In Brexit, the YouGov estimate that I mentioned last week was pretty accurate, but the result genuinely was too close to call.  The real-time forecasts from Chris Hanretty at the University of East Anglia seemed to work well.
  • If you combine turnout estimates with voting estimates by age group, the proportion of 18-24 year olds voting Remain (75% of the 36% turnout) was less than the proportion of 65+ year olds (39% of the 83% turnout).  Turnout matters.
  • You’re much more likely to survive a cardiac arrest on TV than in real life.
  • What theoretical physicists (statisticians, etc,) look like when working (Dr Katie Mack, astrophysics/cosmology). No lab coats; no brighly coloured Erlenmeyer flasks.

Stat of the Week Competition: June 25 – July 1 2016

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 July 1 2016.
  • Statistics can be bad, exemplary or fascinating.
  • The statistic must be in the NZ media during the period of June 25 – July 1 2016 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.


June 23, 2016

Or the other way around

It’s a useful habit, when you see a causal claim based on observational data, to turn the direction around: the story says A causes B, but could B cause A instead? People get annoyed when you do this, because they think it’s silly. Sometimes, though, that is what is happening.

As a pedestrian and public transport user, I’m in favour of walkable neighbourhoods, so I like seeing research that says they are good for health. Today, Stuff has a story that casts a bit of doubt on those analyses.

The researchers used Utah driver’s-licence data, which again included height and weight, to divide all the neighbourhoods in Salt Lake County into four groups by average body mass index. They used Utah birth certificates, which report mother’s height and weight, and looked at 40,000 women who had at least two children while living in Salt Lake County during the 20-year study period.  Then they looked at women who moved from one neighbourhood to another between the two births. Women with higher BMI were more likely to  move to a higher-BMI neighbourhood.

If this is true in other cities and for people other than mothers with new babies, it’s going to exaggerate the health benefits of walkable neighbourhoods: there will be a feedback loop where these neighbourhoods provide more exercise opportunity, leading to lower BMI, leading to other people with lower BMI moving there.   It’s like with schools: suppose a school starts getting consistently good results because of good teaching. Wealthy families who value education will send their kids there, and the school will get even better results, but only partly because of good teaching.

June 22, 2016

Eat up your doormats

Q: Did you see food allergies are caused by diet?

A: That makes sense, I suppose.

Q: Does it make sense that low-fibre diets are why people get peanut allergy more now?

A: Ah. No.

Q: Why not?

A: Because fibre in the typical diet hasn’t changed much in recent years and peanut allergies have become much more common.

Q: It could still be true that adding more fibre would stop people getting peanut allergies, though?

A: Could be.

Q: And that’s what the research found?

A: Up to a point.

Q:  Mice?

A: Mice.

Q: But peanut allergy and dietary fibre?

A: Yes, pretty much. And a plausible biological reason for how it might work.

Q: So it’s worth trying in humans?

A: Probably, though getting little kids to eat that much fibre would be hard.

Q: But the story just says “a simple bowl of bran and some dried apricots in the morning”

A: Sadly, yes.

Q: So how much fibre did they give the rats?

A: They compared a zero-fibre diet to 35% fibre

Q: Is 35% a lot?

A: Well, it’s more than All-Bran, and that was their whole diet.

Q: A more reasonable dose might still work, though?

A: Sure. But you wouldn’t want to assume it did before the trials happened.