You won't find meth labs in places where you're not looking for them

I just read this 2018 article by Ashley Howell, David Newcombe, and Daniel Exeter (all University of Auckland), published in the journal Policing (gated, but there is a summary of some of it here). The authors report on the locations of clandestine methamphetamine labs in New Zealand, based on data from police seizures between 2004 and 2009.

It's an interesting dataset and paper, and they report that:

In the unadjusted spatial scan, there were five locations in the study area with significantly high clandestine methamphetamine laboratory rates (Fig. 2). The ‘most likely’ cluster, centred in Helensville (north-west of Auckland), had a RR of 4.14 with 59 observed CLRT incidents compared to 15.1 expected incidents. A similarly high cluster (RR = 4.09, P = 0.000) was found in the Far North TA.

In other words, there were four times as many lab seizures in Helensville and the Far North than would be expected, if lab seizures were randomly distributed everywhere. The other locations were Hamilton, West Auckland, Central Auckland, and there was a sixth cluster centred on Papakura in some of their analyses. This bit also caught my eye (emphasis mine):

In addition, 26 laboratories (2%) were found at storage units, 21 (2%) discovered in motel or hotel rooms, and another 27 were abandoned in public areas, including cemeteries, parks, roadsides to school yard dumpsters and even the parking lot of a police station.

I wonder how much effort it took for police to find that last one? The paper gives some insights into where the most meth labs have been seized by police. However, we should be cautious about over-interpreting the results, because by definition, you can only seize labs in locations where you are looking for them. So, if police are more diligent or exert more effort in searching for meth labs in Hamilton or the Far North, we would expect to see more lab seizures there, even if there were actually fewer labs than in other locations.

To be fair, the authors are aware of this, and in the Discussion section they note that:

Reports of clandestine laboratory seizures may also be prone to subjectivity. There is no way to be certain that CLRT incident density is not a symptom of a greater police presence or different policing priorities.

However, that doesn't stop them from noting in the abstract that:

Identifying territorial authorities with more clandestine laboratories than expected may facilitate community policing and public health interventions.

It is true that identifying areas with more meth labs than expected would give information about resource allocation. The problem is that this paper doesn't tell us where meth labs are, it only tells us where police have found them.

[HT: The inimitable Bill Cochrane]