Posts from September 2015 (45)

Update: the data Graeme Edgeler was given didn’t mean what he (reasonably) thought they meant and this analysis is no longer operative. There isn’t good evidence that the law has any substantial beneficial effect.  See Nikki Macdonald’s story at Stuff and Graeme’s own post at Public Address.

The usual objection to a “three-strikes” law imposing life sentences without parole, in addition to the objections against severe mandatory minimums, is

• It doesn’t work; or
• It doesn’t work well enough given the injustice involved; or
• There isn’t good enough evidence that it works well enough given the potential for injustice involved.

New Zealand’s version of the law is much less bad than the US versions, but there are still both real problems, and theoretical problems (robbery and aggravated burglary both include crimes of a wide range of severity).

Graeme Edgeler (who is not an enthusiast for the law) has a post at Public Address arguing that there is, at least, evidence of a reduction in subsequent offending by people who receive a first-strike warning, based a mixture of published data and OIA requests.

Here’s his data in tabular form, showing second convictions for offences that would qualify under the three-strikes law. The red cell is ‘first strike’ convictions, the other rows did not count as strikes because the law isn’t retrospective.

The first and last rows are directly comparable five-year periods. Offences that now qualify as ‘strikes’ are down 20% in the last five-year period; second convictions are down a further 62%. Data in the middle row isn’t as comparable, but there is at least no apparent support for a general reduction in reoffending in the last five-year period.

The overall 20% decrease could easily be explained as part of the long-term trends in crime, but the extra decrease in second-strike offences can’t be.  It’s also much larger than could be expected from random variation. The law isn’t keeping violent criminals off the streets, but it does seem to be deterring second offences.

Reasonable people could still oppose the three-strikes law (and Graeme does) but unless we have testable alternative explanations for the large, selective decrease, we should probably be looking at arguments that the law is wrong in principle, not that it’s ineffective.

Team Ratings for the Grand 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 200 matches played, 118 of which were correctly predicted, a success rate of 59%.
Here are the predictions for last week’s games.

Predictions for the Grand Final

Here are the predictions for the Grand 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.

Team Ratings for Round 8

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 54 matches played, 38 of which were correctly predicted, a success rate of 70.4%.
Here are the predictions for last week’s games.

Predictions for Round 8

Here are the predictions for Round 8. 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 9

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 32 matches played, 23 of which were correctly predicted, a success rate of 71.9%.
Here are the predictions for last week’s games.

Predictions for Round 9

Here are the predictions for Round 9. 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.

A familiar trope of alternative cancer therapy is the patients who were given just months to live and are still alive years later.  The implication is that their survival is surprising and the cancer therapy was responsible.  Falling foul of the post hoc ergo propter hoc fallacy isn’t the big problem here. The big problem is that it’s not surprising that some people live a lot longer than the median.

Our intuition for variation is developed on measurements that aren’t like cancer survival. Most adults are pretty close to the average height — very few are more than a foot taller or shorter. Most people in Western countries die at close to the average age: for example, by the NZ life tables, the median life expectancy for NZ men born now is 81 years, and the tables predict half will die between 73 and 88 years.

For many types of cancer, survival isn’t like that. Here’s a graph from a big Canadian study of breast cancer

The median survival for women with stage IV cancer in this group is about a year; half of them are still alive after a year. About half of those are still alive after two years; about half of those are still alive after three years, and some live much longer.

Variation in cancer survival — the long tail — should be welcome, but not surprising. Some people will be alive three, four, five or more years after ‘being given a year to live’.  We should be just as cautious about crediting them with finding a cure as we should be about blaming those who died sooner.

It is a little difficult to choose a period for the predictions. I have predicted up to October 3 because Tonga plays a further game on October 4, and I want to have updated their rating based on the result of the Tonga-Namibia game before predicting their game against Argentina.

Team Ratings at 29 September

The basic method is described on my Department home page.

Here are the team ratings prior to 29 September along with the ratings at the start of the Rugby World Cup.

Performance So Far

So far there have been 19 matches played, 16 of which were correctly predicted, a success rate of 84.2%.
Here are the predictions for previous games.

Predictions for 29 September to 03 October

The prediction is my estimated expected points difference with a positive margin being a win to the first-named team, and a negative margin a win to the second-named team.

• Good story in the Herald: A survey from Southern Cross Health Insurance estimates that about 750,000 people in NZ regularly take vitamins and dietary supplements. Contrary to what some supporters say, there have been quite a lot of randomised trials of vitamin supplements — it’s just that they tend to give disappointing results.  There are a few exceptions (such as folate in pregnancy) and a few arguably open questions (such as higher-dose vitamin D) but not many.

• Entertaining legal report in NBR: NZ Honey is fighting the Ministry for Primary Industries over whether it makes unsupported health claims for mānuka honey. The fun part is that the controversy isn’t whether there’s evidence for health benefits, it’s over whether brand names such as “Manuka Doctor” and “Manuka Pharm” are so clearly just puffery that no-one would mistake them for health claims

• People often claim spinach is very high in iron. Other people often claim this is due to a misplaced decimal point in early German research, and that Popeye didn’t know what he was talking about and spinach is a terrible source of iron. Still other people look into this more carefully and find that the error was not German, or a misplaced decimal point, and that Popeye’s health claims weren’t iron-related. In the end, spinach actually is a moderately good source of iron. (Karl Kruszelnicki, ABC Science)

A detail from Andrew Chen’s visualisation of all the election polls in NZ:

His full graph is somewhat interactive: you can zoom in on times, select parties, etc. What I like about this format is how clear it makes the poll-to-poll variability.  The poll result for, say, National isn’t a line, it’s a cloud of uncertainty.

The cloud of uncertainty gets narrower for minor parties (as detailed in my cheatsheet), but for the major parties you can see it span an entire 10-percentage-point grid cell or more.

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 2 2015.
• Statistics can be bad, exemplary or fascinating.
• The statistic must be in the NZ media during the period of September 26 – October 2 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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If you’d like to comment on or debate any of this week’s Stat of the Week nominations, please do so below!