Posts from August 2013 (54)

From the Twitters

Some people are suggesting the 99.9% bus punctuality stats might be made up. I am shocked. Truly shocked.

— AkTransportBlog (@AkTransportBlog) August 21, 2013

There followed a lively discussion, with comparisons to election results in various unpleasant places.

As I’ve suggested before, the really damning aspect of the 99.9% bus punctuality stats is that they might actually be honest. That is, `punctuality’ has been defined in a way that it does not reflect passenger experience and is of almost no interest to passengers. A bus is ‘punctual’ if it starts its route (which I assume means the driver starts up the ticket system) no more than 5 minutes late. What happens later doesn’t matter, as long as the bus does eventually reach its destination.

The real-time prediction system (although it may not be great at predicting the future) knows where the buses have been, so it would be feasible to set up a punctuality summary that actually measured punctuality — perhaps the proportion of major timepoints where the bus was no more than 5 minutes late or 1 minute early in departing.

I mention this point again because I’m teaching Design of Clinical Trials at the moment, and one of the big issues is ‘surrogate outcomes’. That is, people running studies are often tempted to measure something convenient rather than what patients care about: diabetes trials may measure blood sugar rather than heart attacks and kidney failure; heart disease trials may measure blood pressure rather than heart attacks or strokes; cancer trials may measure tumour size rather than survival or symptoms.  This matters, because there are important examples where a treatment improves a surrogate outcome, but makes the real outcomes worse.

There is no way that the Auckland bus punctuality statistics are accurate measurements of something that matters to passengers. It’s not a matter of life and death as it is in clinical trials, but it’s still a waste of time and money.  And since the Auckland bus system is actually pretty good, it makes Auckland Transport and the Council look unnecessarily stupid.

Oh dear, mea culpa. I was alerted by Pierre du Plessis (pduplessis1@gmail.com) that my predictions were a bit bizarre and on checking I found I had entered an incorrect parameter value. Here are the correct predictions for this week. Note that the success probability below is what would have been obtained if I had entered the correct parameter value previously.
 

Team Ratings for Round 3

Here are the team ratings prior to Round 3, along with the ratings at the start of the season. I have created a brief description of the method I use for predicting rugby games. Go to my Department home page to see this.

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 6 matches played, 3 of which were correctly predicted, a success rate of 50%.

Here are the predictions for last week’s games.

Predictions for Round 3

Here are the predictions for Round 3. 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 24

Here are the team ratings prior to Round 24, along with the ratings at the start of the season. I have created a brief description of the method I use for predicting rugby games. Go to my Department home page to see this.

Performance So Far

So far there have been 168 matches played, 103 of which were correctly predicted, a success rate of 61.31%.

Here are the predictions for last week’s games.

Predictions for Round 24

Here are the predictions for Round 24. 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.

The US Census Bureau has put together an interactive map of languages other than English spoken in the US — you can look at all speakers of the language, or just those who are not fluent in English, for a range of languages.

Here’s the map for (preferentially) French speakers, showing concentrations in Maine (near French Canada) and Louisiana (Cajun) that I’d expect, groups in the major cities, and a cluster I don’t understand in northwestern Georgia

Diversity edition

• Statistical classification (data mining, learning) algorithms will learn to match your classifications. Sometimes that’s not quite what you want
• People are starting to comment more on entirely-male conference speaker lineups. Here’s a simple calculator for the probability distribution of number of people from some group (eg women), for a given proportion in the target population. For example, if you select 10 people from a population that’s 15% female, you should have only a 1/9 chance of not getting any women.
• Map of scientific collaborations, based on co-authorship of books and papers. (click for a really big version)

Once again, the Herald is relying on sympathetic magic in a nutrition story (previous examples)

1. Walnuts: These nuts look just like a brain, so it makes sense that they’re packed with good stuff for your grey matter.The British Journal of Nutrition reported that eating half a cup of walnuts a day for eight weeks increased reasoning skills by nearly 12 per cent in students. 

There’s no way that the appearance of a natural food could possibly be a guide to its nutritional value — how would the walnut know that it’s good for human brains, and why would it care? Pecans, which look a bit like brains, don’t contain the levels of n-3 fatty acids that are supposed to be the beneficial component of walnuts, and fish and flax seeds, which do contain n-3 fatty acids, don’t look like brains.

The story gets two cheers for almost providing a reference: searching on “British Journal of Nutrition walnuts reasoning skills” leads to the paper. It’s a reasonable placebo-controlled randomised experiment, with participants eating banana bread with or without walnuts.  The main problem is that the researchers tested 34 measurements of cognitive function or mood, and found a difference in just one of them.  As they admit

The authors are unable to explain why inference alone was affected by consumption of walnuts and not the other ‘critical thinking’ subtests – recognition of assumption, deduction, interpretation, and evaluation of arguments.

The prior research summarised in the paper shows the same problem, eg,  one dose of walnuts improved one coordination test in rats, but a higher dose improved a different test, and the highest dose didn’t improve anything.

This week there were 5 nominations.

We’re awarding Nathaniel Wilson’s nomination of the NZ Herald‘s “complete ignorance of how averages work”:

Can I please nominate the New Zealand Herald, for displaying a complete ignorance of how averages work?

“Aucklanders’ rates bills have arrived in letterboxes and the figures
have come as a shock to some homeowers who have seen rises of 10 per
cent – despite the council promising an average increase of 2.9 per
cent.”

Because averages and maximums are not the same thing.

Thomas posted more about that story:

Obviously there’s nothing inconsistent about the average being 2.9% and the maximum being 10%. NZ’s average income is about $48000, but I take home somewhat more than that, and the CEO of Fonterra makes a whole lot more, and he may well not be the maximum. The average and the maximum are different. That’s not a shock.

The other point, that our nominator doesn’t make, is that rate increases are capped at 10%, and that all the people who hit the cap last year already knew that they would be seeing an increase this year, and roughly how much it would be. I know this because I live in Onehunga, where property values have gone up quite a lot, and I’m one of the people with a large rate increase. Since I read the rates notice I received last year I’m not at all shocked. I don’t have to say whether I’m happy or not, but it certainly wasn’t a surprise.

Congratulations Nathaniel, and thank you for the other nominations. Please keep them coming in!

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 August 23 2013.
• Statistics can be bad, exemplary or fascinating.
• The statistic must be in the NZ media during the period of August 17 – 23 2013 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!

There’s an interesting piece on cannabis risks at Project Syndicate. One of the things they look at is the correlation between frequent cannabis use and psychosis.  Many people are, quite rightly, unimpressed with the sort of correlation, since it isn’t hard to come up with explanations for psychosis causing cannabis use or for other factors causing both.

However, there is also some genetic data.  The added risk of psychosis seems to be confined to people with two copies of a particular genetic variant in a gene called AKT1. This is harder to explain as confounding (assuming the genetics has been done right), and is one of the things genetics is useful for. This isn’t just a one-off finding; it was found in one study and replicated in another.

On the other hand, the gene AKT1 doesn’t seem to be very active in brain cells, making it more likely that the finding is just a coincidence.  This is one of the things bioinformatics is good for.

In times like these it’s good to remember Ben Goldacre’s slogan “I think you’ll find it’s a bit more complicated than that.”