Stats Chat

From Sophie Jones on Twitter:

If you zoom in on the fine print, that’s 50.6% of 15096 people preferring Pepsi Max over full-sugar Coca-Cola.  You could quibble about the comparison — should this be restricted to cola drinkers (or non-cola drinkers); what happened to the diet versions; how about L&P? — but it’s a comparison.

More obviously, 50.6% is very close to 50%.  You  might ask what the margin of error was for a sample of 15000. It’s more than 0.6%: these results are consistent with just a coin toss.  It might taste like victory, but only if victory doesn’t taste very distinctive.

On the other hand, Pepsi lost the cola wars in New Zealand, so the starting point might reasonably not be 50:50.  This survey doesn’t convincingly show that Pepsi Max is preferred over Coca-Cola by a majority even in blind two-way comparisons, but it does show it’s not far behind. And, in context, that’s probably worth advertising.

The Covid vaccine is safe and effective and it’s good that most eligible people are getting it. But how much protection does it give? If you look at the NZ statistics on who gets Covid, it seems to be extraordinarily effective: the chance of ending up with (diagnosed) Covid for an unvaccinated person is about 20 times higher than for a vaccinated person.

That’s probably an overestimate. People who are vaccinated are at lower risk for immunological reasons: the vaccine really works.  We’re also at lower risk for social reasons: if you’re vaccinated, your friends and family and people you interact with are also more likely to be vaccinated, so they are less likely to give you the virus. That’s partly due to equity problems in the vaccine rollout and partly just to what social-network people call homophily:  you tend to hang out with people similar to you. The immunological reason will hold true over summer; the social reason perhaps less so if people travel. 

Also, because elimination came so close to working in Auckland, the virus has been fairly effectively suppressed in most of the New Zealand population.  On top of the clustering of unvaccinated people, there’s very strong clustering of the current outbreak — it’s mostly in Auckland, but it’s not at all evenly spread within Auckland.  Even if you’re in Auckland you probably know either no-one or lots of people who have been infected.  If you’re in know no-one, you’re at lower risk– and you’re probably vaccinated. As we go from a more or less localised outbreak to many little outbreaks, this additional clustering will go away and the apparent benefit of vaccination will fall.

How much will it fall (and why am I sure)? In the USA, you’re currently about 6 times as likely to get a Covid diagnosis if you’re unvaccinated (according to the CDC). In the UK, the ratio comparing unvaccinated people to those with a Pfizer vaccination within four three months is 4-5.  That fits with the estimates of how effective the vaccine is, biologically, against Delta, plus a bit of social clustering.  The ratio in NZ will be heading that way over time.

So: vaccines, yes, but also masks and distancing and meeting people outside when you can and getting tested if you have symptoms and not going to isolated places that don’t even have enough of their own health care.  Don’t give the virus an inch.

Team Ratings for the Finals

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 52 matches played, 31 of which were correctly predicted, a success rate of 59.6%.
Here are the predictions for last week’s games.

Predictions for the Finals

Here are the predictions for the 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.

Team Ratings for Round 11

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 70 matches played, 54 of which were correctly predicted, a success rate of 77.1%.
Here are the predictions for last week’s games.

Predictions for Round 11

Here are the predictions for Round 11. 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 48 matches played, 22 of which were correctly predicted, a success rate of 45.8%.
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 the Semi-Finals

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 48 matches played, 28 of which were correctly predicted, a success rate of 58.3%.
Here are the predictions for last week’s games.

Predictions for the Semi-Finals

Here are the predictions for the Semi-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.

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 63 matches played, 49 of which were correctly predicted, a success rate of 77.8%.
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 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 42 matches played, 19 of which were correctly predicted, a success rate of 45.2%.
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 Week 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 43 matches played, 23 of which were correctly predicted, a success rate of 53.5%.
Here are the predictions for last week’s games.

Predictions for Week 14

Here are the predictions for Week 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.

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 38 matches played, 29 of which were correctly predicted, a success rate of 76.3%.
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.