Posts from August 2020 (28)

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.

Performance So Far

So far there have been 18 matches played, 13 of which were correctly predicted, a success rate of 72.2%.
Here are the predictions for last week’s games.

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.

Team Ratings for Round 14

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 104 matches played, 70 of which were correctly predicted, a success rate of 67.3%.
Here are the predictions for last week’s games.

Predictions for Round 14

Here are the predictions for Round 14. 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.

• Newshub has a story about so-called ‘lucky’ Lotto stores.  I’ll recycle a previous response.
• The Productivity Commission are arguing that the extra week in lockdown was unnecessary and very expensive. Their analysis is wrong; it does not seem to consider whether and how much the extra week reduced the risk of needing a second lockdown, which was part of the reason for doing in.  I’m not saying the extra week was the right decision — you can’t tell, without modelling the extra risk, which they didn’t do.  It’s like saying insurance is not cost-effective because your house didn’t burn done. Insurance may or may not be cost-effective, but that isn’t how you tell.
• Ed Yong at the Atlantic, on why there’s so much we don’t know about COVID immune response: Immunology Is Where Intuition Goes to Die
• The Human Gene Nomenclature Committee has changed the names of a bunch of genes. Not because they’re named after unpleasant historical figures, but because Excel keeps trying to turn them into dates:  SEPT1, OCT4, MARCH1.  Spreadsheets are useful (and Excel is the world’s most popular statistical software), but you do need to keep a sharp eye on them
• Newshub reports on an attempt to get Pharmac to pay for a drug that costs half a million dollars per patient per year.  I’ll outsource the basic statistical comparison to Matt Nippert on Twitter — the total cost would be about a quarter of Pharmac’s budget (and I’ll just note that this is slightly more than it spends on cancer.)
• If you thought our Census had problems, look at the US.  The American Statistical Association and the American Association for Public Opinion Research are among the groups who want the data collection extended rather than shortened.

Team Ratings for Round 6

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 10 matches played, 9 of which were correctly predicted, a success rate of 90%.
Here are the predictions for last week’s games.

Predictions for Round 6

Here are the predictions for Round 6. 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 16 matches played, 11 of which were correctly predicted, a success rate of 68.8%.
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.

Team Ratings for Round 13

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 96 matches played, 64 of which were correctly predicted, a success rate of 66.7%.
Here are the predictions for last week’s games.

Predictions for Round 13

Here are the predictions for Round 13. 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.

At the moment, NZ policy is to test everyone with suitable symptoms consistent with COVID-19, and potential contacts of cases, but not to go around randomly bothering healthy people for community surveillance.  Looking at some numbers explains why that’s a good strategy, and also gives a way to think about what it takes for other surveillance strategies to be useful.

The first question is how well you do just by testing symptomatic people, when we know some people never develop symptoms, and other people only develop symptoms after passing on the virus.  Professor Nick Wilson and various co-workers^* studied this problem back in May.  They did a lot of computer simulations of what would happen if you introduced one COVID case to ‘an island nation’ where the coronavirus had been eliminated but there was still widespread testing.  Under the assumption that about 40% of cases ended up getting tested, they found that an outbreak had a fifty-fifty chance of being detected when there were only six active cases, but that a reasonable worst case was 50-100 active cases at the time of detection.

You can disagree with the particular assumptions being made (they did this way back in May, the coronavirus equivalent of the Sony Walkman era) but it’s a reasonable ballpark guide.   The idea is that you maybe don’t routinely test absolutely everyone with a cold, but you do test everyone with some (new or worsening) respiratory symptom plus shortness of breath, or fever, or loss of sense of smell, or various other combinations.  We’ve gotten lucky: flu-like illnesses are much less common so far this year than in a usual year, so right now we don’t need to test as many people as they modelled.

So, taking the reasonable worst case, suppose at some point there are 50-100 people out there in NZ with coronavirus and we’ve been unlucky enough that none of them got tested (or a few got tested and the tests were false negatives).  How many random people would we need to test to pick up this outbreak?

Fifty people in NZ is one person in 100,000, so we’d need to test about 100,000 people to have a chance of finding a case.  A simple statistical rule of thumb says that testing about 300,000 people would make us pretty sure to find a case.  Outbreaks grow fast;  if there’s an outbreak with 50 people this week, it had maybe  15 people last week. To get a worthwhile improvement in detecting even the worst outbreaks we’d need to test 100k-300k healthy people each week. That isn’t happening.  Random community testing could be useful if we knew where to look. If we had one suspected case in a town of 10,000 people it might be worth just testing as many people as we could, to try to  get ahead of the contact-tracing process. But if you don’t know where to look there isn’t much point in looking.

Sewage testing is another promising possibility for picking up outbreaks, but these numbers show that it’s not going to be easy.  The testing has to be reliable enough that we’d be prepared to take some fairly major and expensive actions based on finding the virus, but sensitive enough to pick up just 50 or so cases.  It currently isn’t clear whether or not that’s possible, but ESR have the expertise (and some funding) needed to work on the question.  Reliable wastewater testing would be very helpful in the situation we’re now in, where there’s a  suggestion of transmission in NZ but not good evidence — but unreliable wastewater testing would just make things worse.

The take-home message is that we’re probably going to find the next outbreak by testing someone with symptoms. That person might very well have no known contact with international travellers.  If you might be that person, you should call Healthline to ask about getting tested.

^* Statisticians will recognise Matt Parry; any Kiwi who hasn’t been hiding in a cave on Mars with their fingers in their ears should recognise Ayesha Verrall and Michael Baker.

We aren’t getting international tourism at the moment, which is obviously a problem for those working in the international tourism industry¹, and to some extent a problem for everyone because of the hit to the economy.

I saw some speculation on Twitter today about how big international tourism actually is, and about the extent to which NZ tourism expenditure staying in NZ would offset the losses.   Now, there will obviously be gaps, where foreign and domestic tourists don’t do the same things (eg, domestic tourists don’t buy long-distance plane tickets from Air New Zealand), but what about the totals?

Overall, tourism (as defined in the tourism satellite account) brought in $17 billion in the year ending June 2019.  Nearly $4 billion of it was actually international education lasting less than a year, leaving a bit over $13 billion in ‘real’ tourism.  That’s just behind dairy, but roughly equal to meat and wood products together.  The short-term international-education component of ‘tourism’ was a bit a head of fruit exports.

Domestic expenditures on international tourism aren’t completely captured, but the Household Expenditure Survey estimates that all NZ households together spent $2 billion on “overseas accomodation prepaid in NZ” and $4.5 billion on “international air transport” in the year to June 2019.  That’s going to miss food bought overseas and admission tickets to cultural experiences, and some overseas accomodation, but it still looks as though redirecting NZ tourism locally would leave a big hole.  The Household Expenditure Survey does miss out on business travel that isn’t a household expenditure, but it seems more of a stretch that business travel spending will just be redirected to NZ.

¹ I was surprised to find this, in one sense at least, includes me, since international students here for less than 12 months are counted in the ‘tourism satellite account’.