Posts from October 2017 (33)

The StatsNZ twitter account has been tweeting creatively decorated graphs.  The first one I noticed was

which prompted a discussion on Twitter about why the graph actually still worked well even though you’d normally want to avoid this sort of thing.

Then, a few days ago, I saw this one via Harkanwal Singh

Unlike the cars, which worked as labels, the motorbikes don’t do anything except provide a distraction. They’re a bit better at a smaller size, where they reinforce the local trends in the graph, but I still don’t think they’re a net positive

Then today (again via Harkanwal)

Yes, ok, I get that it’s a joke. But pumpkins prices rise  at this time of year in New Zealand because it’s Spring. Halloween isn’t a big driver. And most of our pumpkins aren’t even orange (which is why Stuff has a story on alternative things to carve). And winter isn’t a single point in time. And the decoration distracts from the potentially-important observation that prices didn’t really drop last winter.  And the vertical axis doesn’t say what the units are (average retail price per kilogram, it turns out).

And… just no.

Figure.NZ has the version you want if you’re after information.

A rugby team that has won a lot of games this year is likely to do fairly well next year: they’re probably a good team.  Someone who has won a lot of money betting on rugby this year is much less likely to keep doing well: there was probably luck involved. Someone who won a lot of money on Lotto this year is almost certain to do worse next year: we can be pretty sure the wins were just luck. How about mutual funds and the stock market?

Morningstar publishes ratings of mutual funds, with one to five stars based on past performance. The Wall Street Journal published an article saying (a) investors believe these are predictive of future performance and (b) they’re wrong.  Morningstar then fought back, saying (a) we tell them it’s based on past performance, not a prediction and (b) it is, too, predictive. And, surprisingly, it is.

Matt Levine (of Bloomberg; annoying free registration) and his readers had an interesting explanation (scroll way down)

Several readers, though, proposed an explanation. Morningstar rates funds based on net-of-fee performance, and takes into account sales loads. And fees are predictive. Funds that were good at picking stocks in the past will, on average, be average at picking stocks in the future; funds that were bad at picking stocks in the past will, on average, be average at picking stocks in the future; that is in the nature of stock picking. But funds with low fees in the past will probably have low fees in the future, and funds with high fees in the past will probably have high fees in the future. And since net performance is made up of (1) stock picking minus (2) fees, you’d expect funds with low fees to have, on average, persistent slightly-better-than-average performance.

That’s supported by one of Morningstar’s own reports.

The expense ratio and the star rating helped investors make better decisions. The star rating and expense ratios were pretty even on the success ratio–the closest thing to a bottom line. By and large, the star ratings from 2005 and 2008 beat expense ratios while expense ratios produced the best success ratios in 2006 and 2007. Overall, expense ratios outdid stars in 23 out of 40 (58%) observations.

A better data analysis for our purposes would look at star ratings for different funds matched on fees, rather than looking at the two separately.  It’s still a neat example of how you need to focus on the right outcome measurement. Mutual fund trading performance may not be usefully predictable, but even if it isn’t, mutual fund returns to the customer are, at least a little bit.

• From Politico:  “Is Washington Bungling the Census?”

• From Wired: China’s planned ‘Social Credit’ score

• Florida man arrested for meth that was actually donut glaze.  Georgia police ‘drug recognition expert’ can recognise drug use that even blood tests can’t detect. In NZ, though, 95% of those who fail a roadside impairment test are positive on follow-up blood tests.

• A McDonalds promotion in Canada advertises some high-level prizes and a 1 in 5 chance of winning. Some guy bought 100 orders of large fries, thinking he’d get forty prizes (2 tickets per order).  It’s actually two half-tickets per order, so he won 23 prizes. Mostly cheeseburgers. The moral: (a) read the instructions, and (b) most of the prizes are always just cheeseburgers (or the moral equivalent in other lotteries).

• rawgraphs.io is a new tool for producing fairly attractive graphs quickly from spreadsheet data

• Mark Hanna wrote a guide to the New Zealand Official Information Act

• Andrew Gelman: Advice for Science Writers

• [update] I nearly forgot Chris McDowall’s graphs of electorate vs party vote in the NZ election, from the Spinoff

• Computer maps: then

Computer visualization of population density in Los Angeles County, 1971 https://t.co/IIAD7xjiGp pic.twitter.com/BkOXgbaXrI

— Eric Fischer (@enf) October 14, 2017

and now (from Wikipedia)

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 3 2017.
• Statistics can be bad, exemplary or fascinating.
• The statistic must be in the NZ media during the period of October 28 – November 3 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.

(more…)

If you’d like to comment on or debate any of this week’s Stat of the Week nominations, please do so below!

Team Ratings for the Mitre 10 Cup Finals

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 74 matches played, 51 of which were correctly predicted, a success rate of 68.9%.
Here are the predictions for last week’s games.

Predictions for the Mitre 10 Cup Finals

Here are the predictions for the Mitre 10 Cup Finals. 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 the Currie Cup Final

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 44 matches played, 30 of which were correctly predicted, a success rate of 68.2%.
Here are the predictions for last week’s games.

Predictions for the Currie Cup Final

Here are the predictions for the Currie Cup Final. 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.

Three weeks ago there was a story in the Herald headlined Research reveals that divorce does run in the family (via Antonio Rinaldi on Twitter). The headline, as you’d expect, got the news wrong: the first sentence of the story is

Numerous studies have shown that children of divorced parents are more likely to get divorced when compared to those who grew up with parents who remained married.

The new claim was that divorce ‘runs in the family’ for genetic reasons. The researchers say

“Nearly all the prior literature emphasised that divorce was transmitted across generations psychologically.

“Our results contradict that, suggesting that genetic factors are more important.”

Now, when someone comes up with a finding that contradicts previous research and that they claim even they were surprised by, I’d want pretty good evidence. I’d want to look at what they actually found, and to see some discussion of how much the evidence is specific to adoptive families in a fairly homogeneous society such as Sweden.  In a perfect world, I’d want the story to have some independent input from someone who knows what ‘heritability’ means.  And I’d still worry about publication bias — maybe the academic journal would have published a paper saying ‘no, it’s still just environment’, but I bet the Herald wouldn’t have a story.

How good is the evidence in the story? Well, it has a link to the Daily Mail.

It’s pretty common for British science linkbait that turns up in the NZ papers to just link to the UK media. But here the research paper doesn’t even exist yet. The story says “will be published in an upcoming issue Journal Psychological Science.” Three weeks later, it’s still upcoming — this isn’t the usual problem of the embargo ending the day before a paper actually appears.

I can’t find a preprint or any other source of details, and as far as I can tell, the primary source for this story is a press release from Virginia Commonwealth University.

This isn’t science news.  It’s academic marketing.

There’s a story in a lot of the British media (via Robin Evans on Twitter) about a plan to raise speed limits near highway roadworks. The speed limit is currently 50mph and the proposal is to raise it to 55mph or 60mph.

Obviously this is an significant issue, with potential safety and travel time consequences.  And Highways England did some research. This is the key part of the description in the stories (presumably from a press release that isn’t yet on the Highways England website)

More than 36 participants took part in each trial and were provided with dashcams and watches incorporating heart-rate monitors and GPS trackers to measure their reactions.

The tests took place at 60mph on the M5 between junction 4a (Bromsgrove) to 6 (Worcester) and at 55mph on the M3 in Surrey between junction 3 and 4a.

According to Highways England 60% of participants recorded a decrease in average heart rate in the 60mph trial zone and 56% presented a decrease on the 55mph trial.

That’s a bit light on detail — how many more than 36; does 60% decrease mean 40% increase; are they saying that the 4 percentage point difference between 55 and 60mph is enough to matter or not enough to matter?

More importantly, though, why is a heart rate decrease in drivers even the question?  I’m not saying it can’t be. Maybe there’s some good reason why it’s reliable information about safety, but if there is the journalists didn’t think to ask about it.

A few stories, such as the one in the Mirror, had a little bit more

“Increasing the speed limit to 60mph where appropriate also enables motorists who feel threatened by the close proximity of HGVs in roadworks to free themselves.”

Even so, is this a finding of the research (why motorists felt safer, or even that they felt safer)? Is it a conclusion from the heart rate monitors? Is it from asking the drivers? Is it just a hypothetical explanation pulled out of the air?

If you’re going to make a scientific-sounding measurement the foundation of this story, you need to explain why it answers some real question. And linking to more information would, as usual, be nice.

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 October 27 2017.
• Statistics can be bad, exemplary or fascinating.
• The statistic must be in the NZ media during the period of October 21 – 27 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.

(more…)