Posts from January 2014 (43)

The Herald has a story that  has something for everyone.  On the front page of the website it’s labelled “Support for lower speed limit“, but when you click through it’s actually about the tighter tolerance (4km/h, rather than 10km/h) for infringement notices being used on the existing speed limits.

The story is about a real poll, which found about 2/3 support for the summer trial of tighter speed limits. Unfortunately, the poll seems to have had really badly designed questions. Either that, or the reporting is jumping to unsupportable conclusions:

The poll showed that two-thirds of respondents felt that the policy was fair because it was about safety. Just 29 per cent said that it was unfair and was about raising revenue.

That is, apparently the alternatives given for respondents combined both whether they approved of the policy and what they thought the reason was.  That’s a bad idea for two reasons. Firstly, it confuses the respondents, when it’s hard enough getting good information to begin with. Secondly, it pushes them towards an answer.   The story is decorated with a bogus clicky poll, which has a better set of questions, but, of course, largely meaningless results.

The story also quotes the Police Minister attributing a 25% lower death toll during  the Queen’s Birthday weekends to the tighter tolerance

“That means there is an average of 30 people alive today who can celebrate Christmas who might not otherwise have been,” Mrs Tolley said.

We’ve looked at this claim before. It doesn’t hold up. Firstly, there has been a consistently lower road toll, not just at holiday weekends.  And secondly, the Ministry of Transport says that driving too fast for the conditions is a only even one of the contributing factors in 29% of fatal crashes, so getting a 25% reduction in deaths just from tightening the tolerance seems beyond belief.  To be fair, the Minister only said the policy “contributed” to the reduction, so even one death prevented would technically count, but that’s not the impression being given.

What’s a bit depressing is that none of the media discussion I’ve seen of the summer campaign has asked what tolerance is actually needed, based on accuracy of speedometers and police speed measurements. And while stories mention that the summer campaign is a trial run to be continued if it is successful, no-one seems to have asked what the evaluation criteria will be and whether they make sense.

(suggested by Nick Iversen)

1. Xiaoji Chen (陈晓霁) has graphs of air pollution in some cities in China, spiralling to show the seasons

2. The Cooper-Hewitt collection at the Smithsonian lets you search by colour, with up to five representative colours for each piece

I found this from Chris McDowall’s page that summarises a set of photos by their dominant colours.

There’s a lot of information loss in reducing a photo to four or five pixels, but finding good ways to reduce information is exactly what statistics is about

Usually it’s a problem for opinion polls that respondents tend to answer based on political or group affiliation rather than their actual opinion about the real issue.  Today’s Herald has a poll where that’s basically the point. This is a real poll, not one of those bogus clicky things, but the question was “Who would you trust most to feed you cat over the holidays?”

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Now, to start with, just over half of NZ households do not haz cat, so the question is pretty meaningless for them. Even for the 48% with feline overlords,  the answers supplied didn’t include anyone you might actually get to feed your cat over the holidays. (And isn’t the kitten in the photo a bit young to be left alone like that?)

The choices were one MP (out of 121), one big-city mayor (out of, say, four to six), one internet celebrity (out of an indeterminate set), and one former MP and climate-change denier. No women. No-one on the paper’s New Zealanders of the Year list. Unsurprisingly, one in three of the people they managed to get to answer the question looked at the options and said something along the lines of “Do not want”. 

It’s unusual for a statistician to say this, but sometimes getting a properly representative sample doesn’t really help all that much. The one person on the list who is actually known for his commitment to animal welfare came last.

(picture via @ChrisKeall)