Stats Chat

Via Russell Brown on Bluesky, the Herald has a report on the increases in people being charged with cannabis possession. Charges fell by about 1/3 from 2017 to 2021, in parallel with increasing evidence that arrests for possession didn’t really have social license, but then started rising and now are back at nearly 2017 levels.

So what do the police say? Well, the Herald reports

Director of the National Organised Crime Group Detective Superintendent Greg Williams says wastewater testing in the Auckland and Northland region shows cannabis consumption spiking in July 2024.

“If you look at that charging data, it actually perfectly almost reflects what looks like a significant increase in cannabis consumption.”

We can look at the charging data, and the Herald does. We can’t look at the wastewater cannabis data, though.  On the same day in the Herald there was a story on the newest results from wastewater drug analyses. The story reported estimates of meth, MDMA, and cocaine use. As expected, there’s a lot more meth than anything else, but there’s a potentially worrying increase in cocaine (it’s not so much that cocaine is worse than meth, but it’s a new supply chain).  There was no comment in the story on cannabis use.  There were related stories at One News and RNZ and Newstalk ZB.

If you go to the NZ Police webpage on wastewater drug testing you see

The drugs tested for include methamphetamine, MDMA, cocaine, fentanyl, and heroin. These reports focus on methamphetamine, MDMA and cocaine as these drugs are routinely detected by the programme.

At PHF Science (former ESR) you can find plenty of pages talking about their efforts in testing for meth, MDMA and cocaine, such as this one on the 2024 spike in meth, or this research paper with the mind-numbing details of how they do the testing, or this drug harm page where they say

To date, wastewater testing has been used to measure consumption of illicit drugs including methamphetamine, MDMA, cocaine, heroin and fentanyl. 

Neither the police nor PHF Science publish cannabis-use estimates from wastewater.  The reason they don’t publish the estimates is they aren’t very good.  According to a research report from PHF Science,

However, certain characteristics of cannabis – such as it being lipophilic, not dissolving well in water and its tendency to stick to surfaces such as wastewater pipes – have made analysis in wastewater more difficult. Additionally, due to the considerable chemical differences between cannabis and the other illicit substances being monitored it cannot be added to the same analysis workflow. At this stage there is still too much uncertainty for cannabis measurements to be reliably quantifiable. However, the monitoring data can still be used in trend analyses

They do measure cannabis at five sites around the country, and as the research report says, the data could still be used in trend analyses. But popping up with a claim about two regions from undisclosed data about one time period isn’t a credible trend analysis.

What other data are there?

I don’t find the NZ Drug Trends Survey all that convincing on a detailed level, but its questions asking people who admit to using illegal drugs about which drugs they use should also be ok for trend analyses, and their cocaine reports show a similar trend to the wastewater data. They see a decrease and then increase in daily or weekly cannabis use over the time period we’re talking about, but to a much smaller extent: 68% of respondents at the peak, then down to 57%, then up to 70% for the most recent data. That’s about a 15% decrease and corresponding in regular cannabis use among regular drug users.  Also, a big spike in population cannabis use would increase the number of regular drug users, and show up as a decrease in the proportion regularly using other drugs, which we don’t see.

The NZ Health Survey asks about drug use. The Drug Foundation has collected their data (along with other data sources) and it doesn’t show a pattern anything like the police charging data (click to embiggen, as always)

So, I’m not convinced by the bare assertion that wastewater data show the police are just picking up the same fraction of a varying drug-user population. If the police want to use trends in the cannabis wastewater data to influence public policy they should publish the complete data series, with all the attached caveats from the scientists behind the testing (who I do trust).

Joao Barbosa posted these two maps of Paris on Bluesky: votes in the mayoral election, and cat ownership. If you’re one of the dozens of people on the internet who aren’t American, the political colour scheme is the way you expect.

You can probably come up with explanations for the left-wing lean of the cats if you’re a cat person. And even more so if you’re a dog person.

Another useful map is this one from a report on the risk of gentrification in Paris caused by the 2024 Olympics. It’s a map of income: light colours are high income, dark colours are low income.

There seems to be a general rule that all choropleth maps of a given place reduce to one of a very small number of basic patterns.  There’s an XKCD comic about this for the USA, and Kieran Healy has also written about the two basic US maps

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 104 matches played, 69 of which were correctly predicted, a success rate of 66.3%.
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 20

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 133 matches played, 102 of which were correctly predicted, a success rate of 76.7%.
Here are the predictions for last week’s games.

Predictions for Round 20

Here are the predictions for Round 20. 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 7

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 30 matches played, 19 of which were correctly predicted, a success rate of 63.3%.
Here are the predictions for last week’s games.

Predictions for Week 7

Here are the predictions for Week 7. 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 12

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 55 matches played, 41 of which were correctly predicted, a success rate of 74.5%.
Here are the predictions for last week’s games.

Predictions for Round 12

Here are the predictions for Round 12. 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 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.

Performance So Far

So far there have been 24 matches played, 14 of which were correctly predicted, a success rate of 58.3%.
Here are the predictions for last week’s games.

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.

Team Ratings for Week 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.

Performance So Far

So far there have been 21 matches played, 14 of which were correctly predicted, a success rate of 66.7%.
Here are the predictions for last week’s games.

Predictions for Week 4

Here are the predictions for Week 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.

Yesterday I noted RNZ had just quoted an Auckland Transport claim about the cost of driving that was implausible on the face of it, and didn’t seem to have done any checking or provided any explanation.  Today, the same story is in the Herald, with the same  lack of explanation.

Here’s the RNZ quote; the Herald one is almost identical

Auckland Transport said before the Iran conflict began late last month, the cost of public transport was roughly the same as the cost of driving a vehicle with single occupancy in Auckland.

It’s now costing people nearly double to drive their own cars.

“The cost of petrol has risen at least 50 cents per litre since then, with a 15-kilometre single person commute now costing roughly 80 cents per kilometre, which is equal to about $12 for the total trip.”

So where does this come from? In comments to yesterday’s post, David Welch pointed me to the IRD page on the cost of driving.  The “Tier 1” cost in 2024-5 was $1.17/km for petrol cars. That’s higher than 80c/km, and it’s also not the right comparison — it’s an average cost per km. That is, it includes a per-km share of the fixed costs of having a car. Auckland Transport are (or should be) talking about just the extra per-km cost of using the car to commute. Taking a bus won’t make your car loan go away.

The IRD view on the cost of running a car is their “Tier 2” number, which is only 37c/km.   That, interestingly, is close to half the 80c/km that Auckland Transport is claiming. Since they say that this is double what last year’s cost was, their estimate of last year’s cost is interestingly close to the IRD Tier 2 value and might come from the same methods?

I found an estimate of national average petrol prices as $2.66/L  from December last year.  That would be an increase of 44c/L compared to the one Auckland station I checked yesterday.  If, instead, we take Auckland Transport’s “at least 50c/L”, the increase in running costs would be 5c/km for a vehicle that gets 10 km/L, and less than that for a more typical single-commuter vehicle, so again we can’t get the AT figure.

Even without trying to work out and replicate their calculations, however, we can say one simple thing.  Petrol prices have not yet nearly doubled, so they can’t have caused driving costs, however defined, to have nearly doubled.

On the other hand, the conclusion that people should consider switching to public transport is true: we want to save the potentially scarce supply of oil for people and industries who don’t have any alternatives.

Update: Greater Auckland have also reprinted the claim from AT, again without comment.

• RNZ passes on a claim by Auckland Transport  that Cost of driving 15km in Auckland nearly double that of public transport. Specifically, they say we have “a 15-kilometre single person commute now costing roughly 80 cents per kilometre, which is equal to about $12 for the total trip.” That seems a lot?  I won’t claim to be an expert on driving costs, but the nearest petrol station for which I can see data (Mobil on K Rd)  is charging $3.099/litre.  To get 80c/km you would need to use a bit over a 1/4 litre per km (more precisely, 0.258l/km). This is 25.8 L/100km (for our US readers, about 9 mpg).
• The US celebrated Pi Day this week, and there was a niche popular video describing how to estimate π by coin tossing.  Toss a coin until there are more heads than tails. The expected value of the proportion at this point (which is obviously more than 1/2 and at most 1) is π/4, so by repeating this procedure lots and lots of times and averaging the results you can estimate π/4, and thus π.  This is not a good way to compute π, but it’s surprising that it works at all (sadly, the explanation for why it works is not very illuminating)
• From newsroom, “Mazda NZ’s Driving Good promotion promises the company will donate five trees to environmental charity Trees That Count and that “over each vehicle’s five-year warranty term, these five trees will not only mitigate any environmental impact from CO2 emissions, but they will significantly contribute to the ecosystems in which they are planted”.” Whether this is true turns out to be sensitive to your interpretation of “mitigate”.  There is a sense in which planting any trees provides some mitigation of the environmental impact from CO2 emissions, but you might think Mazda was saying something more like “will absorb as much CO2 as the car emits”. If that’s what you think, you agree with Lawyers for Climate Action, who say it would take 41,000 trees to absorb a comparable amount of CO2.
• StatsNZ said “food prices were up 2.5 percent in January 2026 compared with December 2025. The correct increase was 2.1 percent.”  They’ve fixed it, and it didn’t impact the CPI.