Shocking comparisons

Stuff (The Sunday Star-Times) has a story about taser use statistics that illustrates the importance of the question ‘compared to what?’.

The paper tells us that nearly 1/3 of taser discharges have been at people considered by police to have mental health issues. Is that a lot? What proportion would you expect? If they know, they should be telling us, and if they don’t know, the statistic is pretty meaningless.   We aren’t even told how the proportion is changing over time. We do know that these uses were explicitly contemplated when NZ Police did their 2006 trial of the taser “The taser can be used by Police when dealing with: unarmed (or lightly armed) but highly aggressive people, individuals displaying irrational or bizarre behaviour, and people under the influence of mind altering substances, solvents or alcohol.”

Later on we get more useful (though not new) information from analysis of the pilot period of Taser use, where the weapons were more likely to be discharged when police attended a mental health emergency (27% chance) than when they made a criminal arrest (10% chance).  This is at least answering a meaningful and relevant question, and shows a large difference, though it’s still not clear how big a difference you would expect. Mental health emergencies get police involvement because they are emergencies; many criminal arrests are much more boring and routine.

The story quotes Judi Clements of the Mental Health Foundation as saying “Once you start giving that sort of weapon to police it’s highly likely it’s going to be used”. That’s a reasonable concern, but the numbers in the story, to the extent that you can interpret them, don’t really support it.   There have been 212 taser discharges over two years, from between 600 and 900 tasers, or less than one discharge per five years per taser.  We aren’t told anything about what rate of use would be appropriate, but 0.2 uses per taser-year doesn’t seem all that ‘highly likely’.

Finally,

Further statistics released to the Sunday Star-Times under the Official Information Act show there are serious reliability issues around the weapons.

That’s based on lots of them needing repairs, not on failing to work, since

Despite the problems, only one weapon had failed to fire and administer a shock since they had been rolled out.

that is, 99.5% reliability in use.

It’s important for society to keep tabs on the use of tasers by law enforcement, especially because of the potential for misuse, but reporting of free-floating numbers doesn’t qualify.