October 18, 2014


1. There’s a conference coming up in Canada on “Fairness, Accountability, and Transparency in Machine Learning”, a topic I wrote a little about for the Listener

Questions to the machine learning community include:

  • How can we achieve high classification accuracy while eliminating discriminatory biases? What are meaningful formal fairness properties?
  • How can we design expressive yet easily interpretable classifiers?
  • Can we ensure that a classifier remains accurate even if the statistical signal it relies on is exposed to public scrutiny?
  • Are there practical methods to test existing classifiers for compliance with a policy?

(via mathbabe.org)

2. From Nate Silver at fivethirtyeight.com

Democrats may not be wrong. The polls could very well be biased against their candidates. The problem is that the polls are just about as likely to be biased against Republicans, in which case the GOP could win more seats than expected.

This sort of slowly varying bias is probably one of the reasons the NZ election polls weren’t very good: not only did they have more variability than you’d expect given the sample sizes, but averaging didn’t cancel out much of the error.

3. Yesterday was Spreadsheet Day. Flee in terror! (via @kara_woo)

4. An informative  visualisation of what the world eats, over time. (via Harkanwal Singh)


When barcharts shouldn’t start at zero

Barcharts should almost always start at zero. Almost always.

Randal Olson has a very popular post on predictors of divorce, based on research by two economists at Emory University. The post has a lot of barcharts like this one


The estimates in the research report are hazard ratios for dissolution of marriage. A hazard ratio of zero means a factor appears completely protective — it’s not a natural reference point. The natural reference point for hazard ratios is 1: no difference between two groups, so that would be a more natural place to put the axis than at zero.

A bar chart is also not good for showing uncertainty. The green bar has no uncertainty, because the others are defined as comparisons to it, but the other bars do. The more usual way to show estimates like these from regression models is with a forest plot:


The area of each coloured box is proportional to the number of people in that group in the sample, and the line is a 95% confidence interval.  The horizontal scale is logarithmic, so that 0.5 and 2 are the same distance from 1 — otherwise the shape of the graph would depend on which box was taken as the comparison group.

Two more minor notes: first, the hazard ratio measures the relative rate of divorces over time, not the relative probability of divorce, so a hazard ratio of 1.46 doesn’t actually mean 1.46 times more likely to get divorced. Second, the category of people with total wedding expenses over $20,000 was only 11% of the sample — the sample is differently non-representative than the samples that lead to bogus estimates of $30,000 as the average cost of a wedding.

October 16, 2014

Do you feel lucky?

I’m glad to say it’s been quite a while since we’ve had this sort of rubbish from the NZ papers, but it’s still  going across the Tasman (the  Sydney Morning Herald)

If you’re considering buying a lottery ticket, you’d better make sure it’s from either Gladesville or Cabramatta, which are now officially Sydney’s luckiest suburbs when it comes to winning big. 

NSW Lotteries has released statistics that show the luckiest suburbs across all lotto games in NSW and the ACT, as well as other tips for amateurs hoping to ring their bosses tomorrow morning to say they wouldn’t be coming in to work. 

Of course, the ‘luckiest’ suburbs are nothing of the sort: just the ones where the most money is lost on the lotteries. Cabramatta has improved a lot in recent years, but it’s still not the sort of place you’d expect to see called ‘lucky’.

October 15, 2014

Currie Cup Predictions for the Semi-Finals

Team Ratings for the SemiFinals

The basic method is described on my Department home page. I have made some changes to the methodology this year, including shrinking the ratings between seasons.

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
Lions 6.03 0.07 6.00
Western Province 6.02 3.43 2.60
Sharks 4.36 5.09 -0.70
Blue Bulls 1.02 -0.74 1.80
Cheetahs -3.53 0.33 -3.90
Pumas -8.20 -10.00 1.80
Griquas -10.10 -7.49 -2.60
Kings -14.91 -10.00 -4.90


Performance So Far

So far there have been 40 matches played, 29 of which were correctly predicted, a success rate of 72.5%.

Here are the predictions for last week’s games.

Game Date Score Prediction Correct
1 Kings vs. Pumas Oct 10 26 – 25 -2.20 FALSE
2 Lions vs. Cheetahs Oct 11 47 – 7 11.30 TRUE
3 Western Province vs. Sharks Oct 11 20 – 28 8.70 FALSE
4 Blue Bulls vs. Griquas Oct 11 46 – 12 13.70 TRUE


Predictions for the SemiFinals

Here are the predictions for the SemiFinals. 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 Lions vs. Sharks Oct 18 Lions 6.70
2 Western Province vs. Blue Bulls Oct 18 Western Province 10.00


ITM Cup Predictions for the ITM Cup Finals

Team Ratings for the ITM Cup Finals

Here are the team ratings prior to the ITM Cup Finals, 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.

Current Rating Rating at Season Start Difference
Canterbury 13.65 18.09 -4.40
Tasman 10.97 5.78 5.20
Counties Manukau 6.58 2.40 4.20
Auckland 6.05 4.92 1.10
Taranaki 5.06 -3.89 9.00
Hawke’s Bay 0.84 2.75 -1.90
Manawatu -2.66 -10.32 7.70
Wellington -2.74 10.16 -12.90
Otago -3.98 -1.45 -2.50
Northland -4.42 -8.22 3.80
Waikato -5.74 -1.20 -4.50
Southland -6.31 -5.85 -0.50
Bay of Plenty -9.23 -5.47 -3.80
North Harbour -10.13 -9.77 -0.40


Performance So Far

So far there have been 70 matches played, 44 of which were correctly predicted, a success rate of 62.9%.

Here are the predictions for last week’s games.

Game Date Score Prediction Correct
1 Counties Manukau vs. Auckland Oct 08 41 – 18 1.40 TRUE
2 Waikato vs. Bay of Plenty Oct 09 29 – 12 5.70 TRUE
3 Otago vs. Manawatu Oct 10 25 – 38 5.40 FALSE
4 Wellington vs. North Harbour Oct 11 58 – 34 9.10 TRUE
5 Hawke’s Bay vs. Southland Oct 11 20 – 20 13.20 FALSE
6 Auckland vs. Northland Oct 11 38 – 10 13.60 TRUE
7 Taranaki vs. Canterbury Oct 12 23 – 26 -5.00 TRUE
8 Tasman vs. Counties Manukau Oct 12 16 – 21 12.40 FALSE


Predictions for the ITM Cup Finals

Here are the predictions for the ITM 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.

Game Date Winner Prediction
1 Hawke’s Bay vs. Northland Oct 17 Hawke’s Bay 9.30
2 Manawatu vs. Southland Oct 18 Manawatu 7.70
3 Taranaki vs. Auckland Oct 18 Taranaki 3.00
4 Tasman vs. Canterbury Oct 18 Tasman 1.30


October 14, 2014

Does it make any more sense this time?

From the Herald today

“The average annual weekly wage increase of $28.06 was not enough to offset a $30,000 increase in the national median house price and an increase in the average mortgage interest rate from 5.52% to 5.86%,” the survey found.

We did this one last time, in June. Today’s story is better in that it links to the Massey report. It could still do with a bit of interpretation.

Quick, without a calculator, roughly what would be a large enough weekly wage increase to offset a $30,000 increase in the national median house price?  Would we need to up the $28.06 by ten percent, or  ten dollars, or a factor of ten?

[Update: I should also note that the word "weekly" wasn't in the description of wage increases last time, so this is a definite improvement]

Ada Lovelace Day

October 14 is Ada Lovelace Day, an international celebration of the achievements of women in science, technology, engineering and maths.

New Zealand has (only) three female Professors of Statistics, the top position in our UK-style academic ranking. They work in very different areas of statistics, but with related applications to ecological and environmental monitoring, an area of particular interest in New Zealand.

Going north to south:

  • Marti Anderson is at Massey University in Albany (and was previously at the University of Auckland). Her research is in multivariate analysis — techniques for analysing ecological data on multiple species together, rather than one at a time — mostly applied to marine species
  • Shirley Pledger retired this year from Victoria University. Her research is on capture-recapture methods for counting animals. It’s often impossible to get a complete census of a species even in a limited area, but you can mark the individuals you catch, release them, and observe how often you catch them again. The simplest approaches to estimation are easy but unrealistic; she has worked on more sophisticated and sensible models.
  • Jennifer Brown is head of the Maths & Stats department at the University of Canterbury. Her main statistical research is on sampling techniques for monitoring sparse or patchy populations: either rare animals and plants, or invasive weeds. Sampling systematically or purely at random are both very wasteful; ‘adaptive’ sampling designs allow you to take advantage of finding a clump of your target species without biasing the overall results.


October 13, 2014

Context from everyday units

From @JohnDonoghue64 on Twitter


From the Guardian, a few years ago

Perhaps, as with metric and imperial measurements, such comparisons should be given convenient abbreviations: SoWs (size of Wales), SoBs (size of Belgium), OSPs (Olympic swimming pools), DDBs (buses) and so on. Thus the Kruger national park in South Africa measures 1 SoW (Daily Telegraph), as do Lesotho (London Evening Standard) and Israel (Times), whereas Lake Nzerakera in Tanzania is 2 SoBs (Observer).

At times the most carefully calibrated calculations can go awry. So we learn that Helmand province in Afghanistan is “four times the size of Wales” (Daily Telegraph, 2 December 2009) only to find a few weeks later that it has apparently shrunk to “the size of Wales” (Daily Telegraph, 29 January 2010).

For the benefit of NZ readers, a badger appears to weigh about the same as three female North Island brown kiwi, two typical merino fleeces, or half a case of Malborough sav blanc. That should help you get a grasp on the size of the Lindisfarne Gospels.

Herald data blog starts

The Herald’s Data Editor, Harkanwal Singh,  announces the online site’s new ‘Data Blog’, with the first new post being a map of NZ internet affordability created by Jonathan Brewer.

This has got to be a Good Thing for data literacy in the local media.

Stat of the Week Winner: October 4 – 10 2014

Thanks for your nominations last week for our Stat of the Week competition!

This week, we’ve chosen the following to be our winner:

“‘Rates of participants brushing their teeth rose from 53 per cent to 73 per cent during the trial.’

This story got quite a bit of coverage, the stat actually relates to an increase in the number of people texting and saying they had brushed their teeth (far from the same thing).

The Herald got the context correct in their coverage:

This still seems to just be evidence that people will say anything when nagged enough.”

Congratulations Ben Moore!