Crimes and data limitations
There’s another neat interactive map at Herald Insights, this time of meshblock-level data on assaults in public places reported to the police. It’s a good job, but I still want to talk about some of the limitations forced on the graphics by the available data.
The most obviously strange thing about the map is the hospitals: in Auckland, two of the big hotspots are Auckland City Hospital and Middlemore Hospital. When we talk about hospitals being dangerous places, we usually mean surgical errors or drug-resistant bacteria, not assault. There are two relatively boring reasons that the hospitals stand out, which they share with the town and city centres: the hospitals have a lot of people in them, and they are public places.
In the Herald map, the colour coding reflects the number of reported victimisations, not a rate per capita. That’s because the right population of capitas is hard to define and not readily available. We know the number of people who live in each meshblock, and the number who work in each meshblock, but the assaults are those in public places. When you’re at home — and for many people, when you’re working — you aren’t in a public place. There isn’t easily-available data on the number of people in public places, at meshblock resolution, for the whole country.
The other question the hospital data should raise when talking about the most dangerous areas is ‘dangerous to whom’? Who gets assaulted in hospitals? I don’t know, but I’d expect staff are at reasonably high risk, and so are people who have already been fighting before they arrived. It might well be that hospitals are pretty safe for people just going there because their chest hurts or they decided to mow the lawn barefoot.
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 »
The meshblocks also have another quirk worth knowing and that is of double counting. When you have two areas that border each other the offences that occur on the common border are counted in both (there isn’t a delineation down the middle of the street for example). If two bordering areas have 50 crimes each the total of them does not necessarily equal 100 – it might only be 80 or even 70. In some cases the double counting is very significant because that street might be a particularly busy foot-traffic area and, generally, the off-street area is not public space (i.e. fewer people walking around).
This isn’t to say these charts and ways of packaging stats are not useful, but they have limits of their usefulness at the macro level – they’re more for generalist use.
8 years ago
There are (potential) data mis-recording reasons this can happen as well. Pretty much all of the PD’s data I have worked with here in the US will get calls from the hospital if they know someone was seriously assaulted (in particular if someone was shot). If the victim refuses to provide details, the hospital often goes down in the report as the location, although it should be missing for many analyses. It is a non-trivial proportion in two of the places I have dealt with – in that serious assaults are more often than not these mis-assigned cases.
This is a general problem with crime data, it is not straightforward what address to put down, and many places are not standardized. Sometimes the police station is a crime hot spot due to this data error.
I do know of examples of hospitals being legitimate crime hot spots though. One is an in-patient psychiatric unit that has a high level of assaults. Another is a hospital that persistently had thefts from vehicles.
8 years ago
Hi Andy. We checked with the police specifically regarding hospitals – they only list hospitals as location if the assault had happened there. In case they didn’t know, the location was listed as unknown.
8 years ago