“Causal” is only the start
Jamie Morton has an interesting story in the Herald, reporting on research by Wellington firm Dot Loves Data.
They then investigated how well they all predicted the occurrence of assaults at “peak” times – between 10pm and 3am on weekends – and otherwise in “off-peak” times.
Unsurprisingly, a disproportionate number of assaults happened during peak times – but also within a very short distance of taverns.
The figures showed a much higher proportion of assault occurred in more deprived areas – and that, in off-peak times, socio-economic status proved a better predictor of assault than the nearness or number of bars.
Unsuprisingly, the police were unsurprised.
This isn’t just correlation: with good-quality location data and the difference between peak and other times, it’s not just a coincidence that the assaults happened near bars, nor is it just due to population density. The closeness of the bars and the assaults also argues against the simple reverse-causation explanation: that bars are just sited near their customers, and it’s the customers who are the problem.
So, it looks as if you can predict violent crimes from the location of bars (which would be more useful if you couldn’t just cut out the middleman and predict violent crimes from the locations of violent crimes). And if we moved the bars, the assaults would probably move with them: if we switched a florist’s shop and a bar, the assaults wouldn’t keep happening outside the florist’s.
What this doesn’t tell us directly is what would happen if we dramatically reduced the number of bars. It might be that we’d reduce violent crime. Or it might be that it would concentrate around the smaller number of bars. Or it might be that the relationship between bars and fights would weaken: people might get drunk and have fights in a wider range of convenient locations.
It’s hard to predict the impact of changes in regulation that are intended to have large effects on human behaviour — which is why it’s important to evaluate the impact of new rules, and ideally to have some automatic way of removing them if they didn’t do what they were supposed to. Like the ban on pseudoephedrine in cold medicine.
Thomas Lumley (@tslumley) is Professor of Biostatistics at the University of Auckland. His research interests include semiparametric models, survey sampling, statistical computing, foundations of statistics, and whatever methodological problems his medical collaborators come up with. He also blogs at Biased and Inefficient See all posts by Thomas Lumley »
One of the things we found after the change in the purchase age from 20 to 18 was that disorder went down among 18-20 year olds as they moved from drinking in unsupervised settings to supervised settings. One of the few good things about the change.
In Auckland the police have been doing their “last drink” survey since the 1990s where they ask people arrested where they had their last drink. They do have some idea where the trouble is coming from.
6 years ago
Yes, they’ve probably got a very good idea where the problem *is* coming from. What they don’t know is where it *would be* coming from under a different legal regime.
6 years ago
There are some natural experiments that could be used. Sometimes bars that have breached enough regulations have their alcohol license suspended for a number of days. The changes before, during and after would be useful.
http://www.stuff.co.nz/auckland/local-news/5469483/Auckland-bars-breach-liquor-laws
If the authorities did their sweeps over a short time frame (which they usually do) then the bar suspensions could all happen at the same time. N is probably still going to be a problem but it should give some useful info.
6 years ago
Wont the police be able to say from domestic violence call outs that the ‘location’ has shifted again. It wont matter to a victim if an assault occurred after coming home from drinking at a bar or the drinking occurred entirely at home.
Its a bit like US and its attachment to guns, most gun victims are in and around the home, but doesnt get the publicity. They too like to talk about ‘access’ while avoiding the real problem.
6 years ago