Posts filed under General (3156)

Every year, the Department of Statistics at the University of Auckland offers summer scholarships to a number of students so they can work with our staff on real-world projects. We’ll be profiling the 2013-2014 summer scholars on Stats Chat.

Ashley (right) is working with Dr Paul Murrell on a project entitled  Grid-based graphs in R.

Ashley explains:

“There’s a wonderful piece of software called graphviz that does a great job of making node and edge graphs that look really good. My research is about expanding an R package called gridGraphviz, which has graphviz lay out a graph, then uses the R ‘grid’ graphics package to draw the graph.

“This research is useful as current node and edge packages in R draw graphs using R’s default ‘graphics’ package. Building a package that uses the ‘grid’ package means we can take advantage of all the flexibility that ‘grid’ allows, including making interactive graphs and exporting our plots in a variety of useful formats. We also hope it will make very good-looking graphs.”

More about Ashley:

“I have a BA in philosophy, and have just finished my second year of a BSc in Statistics. I would like to train to be a high school teacher after I graduate.

“When I finished my compulsory high-school maths classes, I swore I would never use maths again, and set about becoming a philosopher. Much to my surprise, I found that philosophy and logic led me to become very, very interested in mathematics, enough that I decided I wanted to return to university and learn all about it.

“Statistics is, for me, a meeting place of my interest in mathematics and computer programming. It can allow us to produce something that can communicate useful ideas about the world to other people. It’s a form of literacy I honestly had no idea I was missing out on until I came back to learn about it.

“This summer, I’m going to spend a few weeks travelling around the South Island treating myself to some wonderful summer weather.”

• Why you might want to avoid a dementia test, from the Washington Post. The usual issues about testing, but well presented.
• False positives from drug-sniffing dogs, and consequences. (local relevance)
• In the spirit of our fly/drive posts, from Chad Orzel: how fast should you drive?

But, of course, time behind the wheel is time not doing anything else, and if we’re going to properly calculate an optimum speed, we really ought to put some monetary value on that, as well.

• I was out of town when Stuff ran the story about men and women having different brain wiring. Language Log has a collection of posts by their contributors and others. Executive summary: good research, interesting findings, pity about the interpretation and coverage.

• Less research needed:

“In any case, the current uncertainty about any benefit from helmet wearing or promotion is unlikely to be substantially reduced by further research” – Ben Goldacre and David Spiegelhalter

• The Cahill-Keyes map projection, at Wired. All flat maps of the round world’s imagined corners have to distort, but the Cahill-Keyes map largely confines the distortion to the oceans. (and a Javascript version)

• NZ cartographer Chris McDowall (@fogonwater) “When presented with an unlabelled map depicting a random part of the country, I can identify most places purely on the basis of their shape. But, when I close my eyes, their forms fade away …. The maps on this page are an attempt to translate my head landscapes into cartographic artefacts. I am trying to recreate of what I see when I close my eyes“

I’ve been looking at our links in and out over the past year.  Leaving out Twitter, Facebook, various RSS readers, and Reddit, we get the most traffic from Kiwiblog and Simply Statistics, with consistent appearances from Observational Epidemiology, Lindsay Mitchell, Learn and Teach Statistics, Andrew Gelman, Public Address, Anti-Dismal, and my other blog.

Unsurprisingly, our largest outward traffic is to Stuff and NZ Herald, in about a 2:3 ratio.  Wikipedia is next, including informative technical references such as the Optional Stopping Theorem,  Berkson’s Paradox, and the Asch conformity experiments, and cultural phenomena you might not have encountered, such as the Bechdel test, Satoshi Kanazawa, and ‘Hitler Has Only Got One Ball’.

The other main media sites you clicked through to were the BBC, the Guardian, the Washington Post, and the New York Times.

The referendum preliminary results are in, and as you’d expect, there is a substantial majority for “No”, but it’s also substantially smaller than the number who voted National at the last election. Everyone will be able to say the results support their own views, so I won’t bother.

What is interesting is the relationship between turnout and vote. Before going further, think about what you’d expect. (more…)

Q: Have you read the latest issue of PLoS One?

A: What do you mean? PLoS One doesn’t have “issues”, it’s online-only and publishes papers as soon as they are ready.

Q: Well, have you read the study that led to the headline ‘exam results are influenced by genes, not schools‘, which the Herald says is in the latest issue of PLoS One?

A: Perhaps they mean this paper about heritability of exam results.

Q: Yes, that one. Couldn’t they have just linked to it?

A: My sources tell me linking is harder than it looks.

Q:  Whatever. How did they find out that schools don’t matter?

A: That’s not quite what they found out.

Q: Well, they found 60% of education was due to genes, not schools, didn’t they?

A: What would that even mean?

Q: I thought I got to ask the questions.

A: <sigh>

Q: Ok, what did they find?

A: They found that identical twins had a correlation of about 0.8-0.9 between their exam results, and non-identical twins had a weaker correlation, about 0.5-0.6.  If you assume that the only difference between identical and non-identical twin pairs is that the identical twins share more genes, and that the genetic and non-genetic contributions just add, you can estimate how much of the variation between twins was due to genes and how much is due to environmental factors. And they end up with estimates from 40% to 60% for the genetic part.

Q: How is that different from the headline?

A: The effects aren’t going to be additive in reality — genetics isn’t going to let you pass a British history test if you haven’t ever studied any British history — so the heritability is just a summary of variation in the current population under the current conditions. What the study really finds is that British schools currently differ from each other less than British kids do. If you made a lot of kids from Beijing or Buenos Aires take the British GCSE tests (or vice versa) they’d probably do really badly, and that would definitely be due to their schooling, not their genes.

Q: And what do the researchers say?

A: Pretty much what I said. The Herald quotes them further down in the story

“Since we are studying whole populations, this does not mean that genetics explains 60 per cent of an individual’s performance, but rather that genetics explains 60 per cent of the differences between individuals, in the population as it exists at the moment.

“This means that heritability is not fixed – if environmental influences change, then the influence of genetics on educational achievement may change too.”

Q: If schools were improved, would the heritability of exam results increase or decrease?

A: That’s a very interesting question. We don’t know. It could be that better schools would have more benefits for people who currently do poorly for genetic reasons, and would reduce heritability; it could be that they would have more benefits for people who currently do poorly for environmental reasons, and would increase heritability; it could be that they would have more benefits for people who currently do well for genetic reasons, and would increase heritability; it could be that they would have more benefits for people who currently do well for environmental reasons, and would decrease heritability. Or it could be that they wouldn’t use exams.

Q: What does this research tell us about the UK’s falling position on the PISA international education comparison? Or is that the fault of Facebook and email, like the UK Schools Minister says?

A:  Well, it’s not genetic.  The genes of the UK population don’t change fast enough. It’s probably not due to Facebook, either, but at least that’s conceivable.

Q: Are the results surprising?

A: Not especially. Similar correlations are seen in IQ test results, where we do know from changes over time that environmental differences can have a large impact. It’s a bit surprising that test scores are slightly more heritable than IQ test results.

Q: Could you give a less controversial example, perhaps something like height?

A: Height is an excellent example.  The fact that siblings have similar heights shows the large genetic component when environments are similar; the fact that most people are taller than their parents or grandparents shows the large environmental component when genetics are similar. If you look in a wealthy Western country, height is about 80% heritable. In medieval Europe it would have been much more sensitive to environment, since the nobility were much healthier and better-nourished than the peasants. And if you mixed together people from medieval Europe and modern Europe, about half the variability would be due to which era they came from.

It’s time for our annual holiday drive-vs-fly post.

Last year I looked at safety: for trips where either flying or driving is feasible, flying is much safer.

This year, let’s look at fuel economy and carbon emissions.  Holiday flights tend to be  full, which makes them more efficient. According to random people on the internet, a Boeing  737 with winglets uses about 3 litres/seat/100km, and according to more reliable sources, Air New Zealand’s smaller turboprop Q300’s will use about 5 litres/seat/100km. (Car and jet fuels are different, but they both have an energy density of about 36MJ/litre and roughly two hydrogens per carbon, so are similar for this purpose.)

There are two further complications: firstly, take-off and landing use more fuel (based on Figure 3 in this report, they add about 250km of effective distance). And secondly, the global warming impact of emissions at high altitude is greater than the same amount of CO2 at ground level, by a factor that is uncertain, but in the range 1.3-2.9.

According the Ministry of Transport national fleet data, the average car in NZ seems to use about 10 litres/100km in real life. In terms of fuel use for long-distance flight, a car with 3-4 people would be comparable to flying on a full 737, and a car with 2 people would be comparable to flying on a full Q300 turboprop. For short NZ distances, the take-off/landing cost adds a good 50%, and the extra impact of high-altitude emissions means that cars win out, though by a relatively small margin if there’s only one person in the car.

A direct comparison like this misses the real emissions problem with planes, though. Hardly anyone drives to the Cook Islands or Australia over Christmas — planes let you travel further.

The New Zealand media is self-regulating – that is, it governs its own, and the Press Council is the port of call for public complaints about print media. Two complaints that have been upheld recently focused on the use/abuse of statistical information. I’m posting about these  not to take a dig at my esteemed colleagues, but to point out how we can avoid falling into a hole and/or creating a damaging, discriminatory or dangerous urban myth.

The first complaint concerns the The Press (Weekend) newspaper, which published an article on Saturday October 12, 2013, about health data from the Canterbury District Health Board concerning the increase in the sexually-transmitted infection chlamydia in the region since 2011. The headline of the article was Luck of the Irish has downside in sex-disease stats. The intro read “Irish workers helping with the rebuild are sharing the love but it seems they may also be helping to spread sexual disease.”

The Press Council noted there was no statistical information given to support the statements linking the Irish to the chlamydia. The link between the Irish nationals and the chlamydia statistics was of the newspaper’s making and not supported by any reported information, making the report inaccurate and discriminatory.  Read the full decision here.

The second complaint concerns the Dominion Post. On August 12, 2013, under the headline Boys slip further in school’s co-ed class, the paper published a story and table about achievement rates in 2012 for NCEA level 3 students in its circulation area, with the table reporting on highest and lowest achieving schools. The table gave pass rates for the highest achieving schools, but failure rates for the lowest achieving schools. Under the heading “Lowest Achieving Hawke’s Bay Schools” the table listed Wairoa High School 43.8% not achieved; Dannevirke High School: 40%; Taradale High School: 36.2%. The school complained that this conveyed a misleading impression that only 36.2% of its students had passed. In fact, 63.8% had.

It turns out that the original NZQA figures showed the number of year 13 students who had NOT passed NCEA level 3. The newspaper said that it had decided to turn the figures around to assist readers and also show how well most schools and students had performed.  In upholding the complaint, the Press Council said “A table provides readers with a quick and ready means of assessing data. But when a comparison is being made it is important that the data is presented in such a way as to make the comparison valid. The use of two differing measures of data in the same table was therefore confusing and misleading.” The editor said, ” … I do accept that we would have been better advised to have used only one measure throughout. I am happy to give an undertaking that we will not be using that format again.” Read the full decision here.

From Stuff, where GJ Gardner are asking for a GST exemption for their industry

“For example, if the average Kiwi purchases a house worth $600,000, they end up paying $90,000 in GST.”

That’s not a house+land worth $600k, that’s just the value of the new building. In Auckland, the ratio of land to building value averages about 60:40, so you’d expect that to be $1.5 million of real estate. The average Kiwi isn’t quite buying at that level.

At that 60:40 ratio, even if there were no increase in construction costs as a result, the impact of removing GST would be to reduce the price of new homes by about 5%. Auckland prices have increased by twice that much this year already, so it’s not going to solve the problem. In fact, the price reduction would almost certainly be smaller than that — if prices are constrained by available money, and GST is decreased, people will spend some of the savings on more expensive construction.

GJ Gardner are right that the only solution to housing prices is more homes, but in Auckland it’s either going to be necessary to decrease the price of land or reduce the amount of land a home occupies to have a big impact on prices.

This Herald story improves a lot after the first few paragraphs. But it would have to.

Almost three-quarters of the depressed New Zealanders who have gone to Sir John Kirwan’s website depression.org.nz are no longer depressed after finishing the six lessons the site offers.

The story goes on to say that an evaluation of the website found that only 3% of the first 13000 people who registered ended up finishing the six lessons.

Public health officials say the result makes the website, and the $5 million-a-year advertising campaign around it, one of the New Zealand’s most successful public health campaigns.

Based on the 3% figure applied to the current 40000 registrants that doesn’t sound at all plausible: the relevant figure would have to be the 26% who completed at least two lessons, 48% of whom were no longer depressed. That comes to about 5000 people out of 40000. They estimate that 10-15% would have recovered without any intervention, or about 5000 out of 40000.  So, any evidence of public-health benefit needs some information about how representative the 26% were: if they included everyone who was going to recover anyway, there’s no benefit; if they were completely representative, there’s a big benefit. Both extremes are pretty unlikely, but we can’t tell any more from the information in the story.

The evaluation report may have more detail that really does show this website has been effective, but the report doesn’t seem to be public (yet).

[Update: I hadn’t noticed that this had also been nominated for Stat of the Week, with much the same arguments.]