Stats Chat

From the BBC, Hans Rosling on the Ebola epidemic

(That’s a diagram of the data collection system behind him)

(via Harkanwal Singh)

Stories were coming out recently about new cancer research led by Bryony Telford in Parry Guilford’s lab at Otago, and I’d thought I’d use it for an example of translation from Scientist to English. It’s a good example for news because it really is pretty impressive, because it involved a New Zealand family with familial cancer, and because the abstract of the research paper is well written — it’s just not written in ordinary English. Combining the abstract with the press release and a bit of Google makes a translation possible.

This will be long. (more…)

Q: Did you see that eating a bowl of quinoa every day helps you live longer?

A: No.

Q: There’s story on Stuff (well, from the West Island branches). Is it true?

A: Hard to say.

Q: Well, does the research claim it’s true?

A: Hard to say.

Q: Why? Didn’t they link?

A: No, they linked, and the paper is even open-access. It just doesn’t say anything about the effects of quinoa.

Q: But the story said “A new study by Harvard Public School of Health has found that eating a daily bowl of the protein-packed, gluten-free grain significantly reduces the risk of premature death from cancer, heart disease, respiratory disease and diabetes.”

A: Sadly, yes.

Q: This is your correlation and causation thing again, isn’t it?

A: No, the paper just doesn’t mention quinoa. It talks about grains and cereals.

Q: Ok. So they just didn’t break out the data for quinoa separately. It’s still a grain and a cereal, isn’t it?

A: Yes, as long as you aren’t even more pedantic than me. But it’s not just data analysis. They didn’t even ask their study participants about eating quinoa.

Q: So? Some of the grain they ate must have been quinoa, and there’s no reason to expect it’s different from other grains, is there? Won’t it all get averaged in somehow?

A: I suppose so. But there can’t have been that much of it getting “averaged in”

Q: Why not? You old folks may not have caught on, but quinoa’s getting popular now.

A: The study was in people over 50. That’s older than both of us. Even assuming we weren’t the same person.

Q: Even so. Things are changing. People have more adventurous diets. It’s not the twentieth century any more.

A: It is in the study.

Q: Huh?

A: The dietary data were collected in 1995 and 1997, from people with average age 61 years.

Q: Oh.

From the Stuff front page

Now, no-one (maybe even literally no-one) is denying that foreign drivers are at higher risk on average. It’s just that some of us feel exaggerating the problem is unhelpful. The quoted sentence is true only if “the tourist season” is defined, a bit unconventionally, to mean “February”, and probably not even then.

When you click through to the story (from the ChCh Press), the first thing you see is this:

Notice how the graph appears to contradicts itself: the proportion of serious crashes contributed to by a foreign driver ranges from just over 3% in some months to just under 7% at the peak.  Obviously, 7% is an overstatement of the actual problem, and if you read sufficiently carefully, the graphs says so.  The average is actually 4.3%

The other number headlined here is 1%: cars rented by tourists as a fraction of all vehicles.  This is probably an underestimate, as the story itself admits (well, it doesn’t admit the direction of the bias). But the overall bias isn’t what’s most relevant here, if you look at how the calculation is done.

Visitor surveys show that about 1 million people visited Canterbury in 2013.

About 12.6 per cent of all tourists in 2013 drove rental cars, according to government visitor surveys. That means about 126,000 of those 1 million Canterbury visitors drove rental cars. About 10 per cent of international visitors come to New Zealand in January, which means there were about 12,600 tourists in rental cars on Canterbury roads in January.

This was then compared to the 500,000 vehicles on the Canterbury roads in 2013 – figures provided by the Ministry of Transport.

The rental cars aren’t actually counted, they are treated as a constant fraction of visitors. If visitors in summer are more likely to drive long distances, which seems plausible, the denominator will be relatively underestimated in summer and overestimated in winter, giving an exaggerated seasonal variation in risk.

That is, the explanation for more crashes involving foreign drivers in summer could be because summer tourists stay longer or drive more, rather than because summer tourists are intrinsically worse drivers than winter tourists.

All in all, “nine times higher” is a clear overstatement, even if you think crashes in February are somehow more worth preventing than crashes in other months.

Banning all foreign drivers from the roads every February would have prevented 106 fatal or serious injury crashes over the period 2006-2013, just over half a percent of the total.  Reducing foreign driver risk by 14%  over the whole year would have prevented 109 crashes. Reducing everyone’s risk by 0.6%  would have prevented about 107 crashes. Restricting attention to February, like restricting attention to foreign drivers, only makes sense to the extent that it’s easier or less expensive to reduce some people’s risk enormously than to reduce everyone’s risk a tiny amount.

Actually doing something about the problem requires numbers that say what the problem actually is, and strategies, with costs and benefits attached. How many tens of millions of dollars worth of tourists would go elsewhere if they weren’t allowed to drive in New Zealand? Is there a simple, quick test would separate safe from dangerous foreign drivers, that rental companies could administer? How could we show it works? Does the fact that rental companies are willing to discriminate against young drivers but not foreign drivers mean there’s something wrong with anti-discrimination law, or do they just have a better grip on the risks? Could things like rumble strips and median barriers help more for the same cost? How about more police presence?

From 2006 to 2013 NZ averaged about 6 crashes per day causing serious or fatal injury. On average, about one every four days involved a foreign driver. Both these numbers are too high.

Team Ratings for Round 4

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.

Performance So Far

So far there have been 24 matches played, 16 of which were correctly predicted, a success rate of 66.7%.

Here are the predictions for last week’s games

Predictions for Round 4

Here are the predictions for Round 4. 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.

Team Ratings for Round 7

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.

Performance So Far

So far there have been 40 matches played, 26 of which were correctly predicted, a success rate of 65%.

Here are the predictions for last week’s games.

Predictions for Round 7

Here are the predictions for Round 7. 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.

• From Dylan Cleaver and Harkanwal Singh at the Herald, an interactive graphic for exploring comparisons between players over the history of one-day internationals.
• From the Herald Data blog, or with interactive versions at his website, a visualisation by Michael Lascarides of how each innings progressed. Mostly this is for whole teams,but here’s Martin Guptill’s double century against the West Indies

Most stories about population genetic ancestry tend to be based on pure male-line or pure female-line ancestry, which can be unrepresentative.  That’s especially true when you’re looking at invasions — invaders probably leave more Y-chromosomes behind than the rest of the genome.  There’s a new UK study that used data on the whole genome from a few thousand British people, chosen because all four of their grandparents lived close together.  The idea is that this will measure population structure at the start of the twentieth century, before people started moving around so much.

Here’s the map of ancestry clusters. As the story in the Guardian explains, one thing it shows that the Romans and Normans weren’t big contributors to population ancestry, despite their impact on culture.

The “It’s not paranoia if..” issue

• A new initiative, Data Justice, concerned with widespread commercial data collection and analysis as a threat to privacy and equality.

• Some of the reasons that data-based decision making goes bad

• Trying to get “open” data in New Jersey: “initially refused to answer The Jersey Journal’s OPRA request because it didn’t make it on the agency’s standardized OPRA form, which wasn’t available on the NBHA website. Even after a reporter noted that in 2009 the state Supreme Court ruled standardized forms aren’t necessary, Earl wouldn’t accept a request on anything but the agency’s form.”

• Reidentification of ‘anonymised’ data: ‘When data is released after applying current ad-hoc de-identification methods, the privacy risks of re-identification are not just unknown but unknowable.’

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 March 27 2015.
• Statistics can be bad, exemplary or fascinating.
• The statistic must be in the NZ media during the period of March 21 – 27 2015 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.

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