September 12, 2017

Currie Cup Predictions for Round 10

Team Ratings for Round 10

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.

Current Rating Rating at Season Start Difference
Sharks 4.36 2.15 2.20
Western Province 4.04 3.30 0.70
Lions 3.41 7.41 -4.00
Cheetahs 3.23 4.33 -1.10
Blue Bulls 0.09 2.32 -2.20
Pumas -7.60 -10.63 3.00
Griquas -10.29 -11.62 1.30

 

Performance So Far

So far there have been 27 matches played, 18 of which were correctly predicted, a success rate of 66.7%.
Here are the predictions for last week’s games.

Game Date Score Prediction Correct
1 Griquas vs. Lions Sep 08 17 – 34 -8.50 TRUE
2 Western Province vs. Cheetahs Sep 09 57 – 14 3.20 TRUE
3 Pumas vs. Sharks Sep 09 25 – 27 -8.50 TRUE

 

Predictions for Round 10

Here are the predictions for Round 10. 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.

Game Date Winner Prediction
1 Pumas vs. Western Province Sep 15 Western Province -7.10
2 Griquas vs. Sharks Sep 16 Sharks -10.10
3 Lions vs. Blue Bulls Sep 16 Lions 7.80

 

September 11, 2017

Stat of the Week Competition: September 9 – 15 2017

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 September 15 2017.
  • Statistics can be bad, exemplary or fascinating.
  • The statistic must be in the NZ media during the period of September 9 – 15 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…)

September 10, 2017

Should there be an app for that?

As you may have heard, researchers at Stanford have tried to train a neural network to predict sexual orientation from photos. Here’s the Guardian‘s story.

Artificial intelligence can accurately guess whether people are gay or straight based on photos of their faces, according to new research that suggests machines can have significantly better “gaydar” than humans.

There are a few questions this should raise.  Is it really better? Compared to whose gaydar? And WTF would think this was a good idea?

As one comment on the study says

Finally, the predictability of sexual orientation could have serious and even life-threatening implications to gay men and women and the society asa whole. In some cultures, gay men and women still suffer physical and psychological abuse at the hands of governments, neighbors, and even their own families.

No, I lied. That’s actually a quote from the research paper (here). The researchers say this sort of research is ethical and important because people don’t worry enough about their privacy. Which is a point of view.

So, you might wonder about the details.

The data came from a dating website, using self-identified gender for the photo combined with the gender they were interested in dating to work out sexual orientation. That’s going to be pretty accurate (at least if you don’t care how bisexual people are classified, which they don’t seem to). It’s also pretty obvious that the pictures weren’t put up for the purpose of AI research.

The Guardian story says

 a computer algorithm could correctly distinguish between gay and straight men 81% of the time, and 74% for women

which is true, but is a fairly misleading summary of accuracy.  Presented with a pair of faces, one of which was gay and one wasn’t, that’s how accurate the computer was.  In terms of overall error rate, you can do better that 81% or 74% just by assuming everyone is straight, and the increase in prediction accuracy in random people over the human judgment is pretty small.

More importantly, these are photos from dating profiles. You’d expect dating profile photos to give more hints about sexual orientation than, say, passport photos, or CCTV stills.  That’s what they’re for.  The researchers tried to get around this, but they were limited by the mysterious absence of large databases of non-dating photos classified by sexual orientation.

The other question you might have is about the less-accurate human ratings.  These were done using Amazon’s Mechanical Turk.  So, a typical Mechanical Turk worker, presented only with a single pair of still photos, does do a bit worse than a neural network.  That’s basically what you’d expect with the current levels of still image classification: algorithms can do better than people who aren’t particularly good and who don’t get any particular training.  But anyone who thinks that’s evidence of significantly better gaydar than humans in a meaningful sense must have pretty limited experience of social interaction cues. Or have some reason to want the accuracy of their predictions overstated.

The research paper concludes

The postprivacy world will be a much safer and hospitable place if inhabited by well-educated, tolerant people who are dedicated to equal rights.

That’s hard to argue with. It’s less clear that normalising the automated invasion of privacy and use of personal information without consent is the best way to achieve this goal.

Why you can’t predict Epsom from polls

The Herald’s poll aggregator had a bit of a breakdown over the Epsom electorate yesterday, suggesting that Labour had a chance of winning.

Polling data (and this isn’t something a statistician likes saying) is essentially useless when it comes to Epsom, because neither side benefits from getting their own supporters’ votes. National supporters are a clear majority in the electorate. If they do their tactical voting thing properly and vote for ACT’s David Seymour, he will win.  If they do the tactical voting thing badly enough, and the Labour and Green voters do it much better, National’s Paul Goldsmith will win.

Opinion polls over the whole country don’t tell you about tactical voting strategies in Epsom. Even opinion polls in Epsom would have to be carefully worded, and you’d have to be less confident in the results.

There isn’t anywhere else quite like Epsom. There are other electorates that matter and are hard to predict — such as Te Tai Tokerau, where polling information on Hone Harawira’s popularity is sparse — but in those electorates the polls are at least asking the right question.

Peter Ellis’s poll aggregator just punts on this question: the probability of ACT winning Epsom is set at an arbitrary 80%, and he gives you an app that lets you play with the settings. I think that’s the right approach.

September 6, 2017

Threshold and discards

There have been a few discussions on Twitter about what happens to votes for parties who don’t make the threshold of 5% or one electorate. I’m going to try to make it clearer than is feasible in 140 characters, but without mentioning quotients.

If you voted for a party that gets less than 5% of the party vote and does not win any electorate, your party vote is not used in determining the list seats.  It doesn’t get reassigned, reweighted, or re-anything. It just isn’t used — exactly as if those people hadn’t cast a Party vote. (Electoral Act, section 191 (4)). Last time, about 150,000 votes were set aside at this point.

The votes for the parties that are left (2.3 million, last time) are now used to allocate 120 seats.  Complicated procedures are used to work out a number of votes per seat, call it N.  The total for each party is divided by N, and rounded to the nearest whole  number, so you need at least ½N to get one seat, 1½N to get two seats and so on. (That’s not how the Electoral Act describes it; this is the equivalent ‘Webster’ method rather than the ‘Sainte-Laguë’ method).

So what are the implications?

I don’t like the term ‘wasted’ vote — if you’re voting for, say, Ban 1080 or Aotearoa Legalise Cannabis, it’s presumably not in the expectation of getting representation in Parliament, but more as a way of making your views known.  However, if your intent is to increase representation in Parliament of people whose views you support, this is the basic guideline

  • If the opinion polls show a party is nowhere near the 5%/one electorate threshold, the expected impact of increased votes for that party  on the composition of Parliament is very small (compared to a major party)
  • If the opinion polls show a party is close to the 5% threshold (in either direction) and isn’t certain to get an electorate, the expected impact of increased votes for that party  on the composition of Parliament is relatively large (compared to a major party)
  • If a party is reasonably certain to get an electorate or to get over the 5% threshold, he expected impact of increased votes for that party  on the composition of Parliament is about the same as for a major party.
September 5, 2017

NRL Predictions for Finals Week 1

Team Ratings for Finals Week 1

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.

Current Rating Rating at Season Start Difference
Storm 15.68 8.49 7.20
Broncos 6.96 4.36 2.60
Raiders 4.07 9.94 -5.90
Sharks 2.98 5.84 -2.90
Cowboys 2.25 6.90 -4.70
Panthers 2.00 6.08 -4.10
Eels 1.26 -0.81 2.10
Roosters 0.93 -1.17 2.10
Sea Eagles -0.33 -2.98 2.60
Dragons -0.94 -7.74 6.80
Bulldogs -3.55 -1.34 -2.20
Wests Tigers -3.72 -3.89 0.20
Rabbitohs -3.84 -1.82 -2.00
Warriors -7.23 -6.02 -1.20
Titans -9.03 -0.98 -8.10
Knights -9.54 -16.94 7.40

 

Performance So Far

So far there have been 192 matches played, 116 of which were correctly predicted, a success rate of 60.4%.
Here are the predictions for last week’s games.

Game Date Score Prediction Correct
1 Cowboys vs. Broncos Aug 31 10 – 20 0.50 FALSE
2 Eels vs. Rabbitohs Sep 01 22 – 16 9.20 TRUE
3 Roosters vs. Titans Sep 02 20 – 16 15.20 TRUE
4 Sea Eagles vs. Panthers Sep 02 28 – 12 -1.60 FALSE
5 Storm vs. Raiders Sep 02 32 – 6 13.10 TRUE
6 Knights vs. Sharks Sep 03 0 – 18 -7.30 TRUE
7 Dragons vs. Bulldogs Sep 03 20 – 26 8.40 FALSE
8 Wests Tigers vs. Warriors Sep 03 28 – 16 6.60 TRUE

 

Predictions for Finals Week 1

Here are the predictions for Finals Week 1. 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.

Game Date Winner Prediction
1 Roosters vs. Broncos Sep 08 Broncos -2.50
2 Storm vs. Eels Sep 09 Storm 17.90
3 Sea Eagles vs. Panthers Sep 09 Panthers -2.30
4 Sharks vs. Cowboys Sep 10 Sharks 4.20

 

Currie Cup Predictions for Round 9

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.

Current Rating Rating at Season Start Difference
Sharks 4.88 2.15 2.70
Cheetahs 4.29 4.33 -0.00
Lions 3.05 7.41 -4.40
Western Province 2.99 3.30 -0.30
Blue Bulls 0.09 2.32 -2.20
Pumas -8.12 -10.63 2.50
Griquas -9.93 -11.62 1.70

 

Performance So Far

So far there have been 24 matches played, 15 of which were correctly predicted, a success rate of 62.5%.
Here are the predictions for last week’s games.

Game Date Score Prediction Correct
1 Blue Bulls vs. Griquas Sep 01 44 – 42 15.50 TRUE
2 Sharks vs. Cheetahs Sep 02 45 – 15 3.50 TRUE
3 Lions vs. Pumas Sep 02 29 – 28 16.80 TRUE

 

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.

Game Date Winner Prediction
1 Griquas vs. Lions Sep 08 Lions -8.50
2 Western Province vs. Cheetahs Sep 09 Western Province 3.20
3 Pumas vs. Sharks Sep 09 Sharks -8.50

 

Mitre 10 Cup Predictions for Round 4

Team Ratings for Round 4

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.

Current Rating Rating at Season Start Difference
Canterbury 18.66 14.78 3.90
Taranaki 4.65 7.04 -2.40
Tasman 4.59 9.54 -5.00
Counties Manukau 4.49 5.70 -1.20
North Harbour 4.09 -1.27 5.40
Wellington 3.23 -1.62 4.80
Waikato 2.42 -0.26 2.70
Otago 1.40 -0.34 1.70
Auckland -1.03 6.11 -7.10
Manawatu -3.04 -3.59 0.50
Bay of Plenty -4.60 -3.98 -0.60
Northland -7.27 -12.37 5.10
Hawke’s Bay -10.51 -5.85 -4.70
Southland -19.68 -16.50 -3.20

 

Performance So Far

So far there have been 22 matches played, 14 of which were correctly predicted, a success rate of 63.6%.
Here are the predictions for last week’s games.

Game Date Score Prediction Correct
1 Auckland vs. Waikato Aug 30 27 – 35 7.00 FALSE
2 Bay of Plenty vs. Wellington Aug 31 10 – 31 -0.10 TRUE
3 Hawke’s Bay vs. Canterbury Sep 01 10 – 53 -21.20 TRUE
4 Otago vs. Manawatu Sep 02 40 – 30 8.10 TRUE
5 Southland vs. Northland Sep 02 13 – 44 -3.50 TRUE
6 Taranaki vs. Counties Manukau Sep 02 30 – 27 4.40 TRUE
7 Waikato vs. Tasman Sep 03 29 – 31 1.30 FALSE
8 North Harbour vs. Auckland Sep 03 57 – 10 -0.50 FALSE

 

Predictions for Round 4

Here are the predictions for Round 4. 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.

Game Date Winner Prediction
1 Wellington vs. Hawke’s Bay Sep 06 Wellington 17.70
2 Counties Manukau vs. North Harbour Sep 07 Counties Manukau 4.40
3 Canterbury vs. Southland Sep 08 Canterbury 42.30
4 Manawatu vs. Bay of Plenty Sep 08 Manawatu 5.60
5 Auckland vs. Taranaki Sep 09 Taranaki -1.70
6 Northland vs. Waikato Sep 09 Waikato -5.70
7 Tasman vs. Wellington Sep 10 Tasman 5.40
8 Hawke’s Bay vs. Otago Sep 10 Otago -7.90

 

September 4, 2017

Before and after

We’re in the interesting situation this election where it looks like political preferences are actually changing quite rapidly (though some of this could be changes in non-response that don’t show up in actual voting).

On Thursday, One News released a poll by Colmar Brunton that found Labour ahead of National by 43% to 41% for the first time in years.  Yesterday, NewsHub released a Reid Research poll with Labour back behind National 39% to 43%.

“Released” is important here. The Colmar Brunton poll was taken over August 26-30. The Reid Research poll was taken over August 22-30. That is, despite being released  later, the Reid Research poll was (on average) taken earlier. Comments (and even analysis) of polls often ignore the interview time and focus on the release date, but here we can see why the code of conduct for pollers requires the interview period to be described.

A difference of 4 percentage points in Labour’s support is quite large for two polls of this size (though not out of the question just from sampling error). If the polls were really discrete events four days apart, it would be plausible to argue they showed Labour’s support had stopped increasing — that the Ardern effect had reached its limit. If the two polls were taken over exactly the same period, the most plausible conclusion would be that the true support was in between and that we knew nothing more about Labour’s trajectory. With the Sunday poll actually taken slightly earlier, the difference is still likely to mostly be noise, but to the (very limited) extent that it says anything about trajectory, the story is positive for Labour.

Stat of the Week Competition: September 2 – 8 2017

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 September 8 2017.
  • Statistics can be bad, exemplary or fascinating.
  • The statistic must be in the NZ media during the period of September 2 – 8 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…)