Saturday, June 03, 2006

Michael Gove on Question Time

In recent weeks I've been reading a books on logic - Jamie Whyte's brilliant Bad Thoughts - A guide to clear thinking and Madsen Pirie's How to win every argument.

I don't know if all this has sharpened the old wits up a bit, but when I heard Michael Gove claim on Question Time that people who carried knives were more likely to be victims of knife crime, and implied that it they shouldn't carry knives because of this, the potential flaw in the reasoning was obvious.

Gove appears to be repeating claims made by the police:
A Met spokesman said: "If you carry a knife you are more likely to become a victim of crime and more likely to get a criminal record. Bringing out a knife escalates the scale of violence.
We are invited to believe, then, that carrying a knife makes one more likely to become a victim of crime. This is, when you think about it, a bit unlikely since you are only less likely to be attacked if armed, and more likely to be able to defend yourself. Likewise the claim that carrying a knife makes you more likely to be a perpetrator of crime is on the face of it unlikely. A moment's thought should at least make us question this claim. Someone unarmed who gets into a violent altercation with another unarmed person is presumably just as likely to commit a crime and get a criminal record - but they would probably be guilty of a lesser offence than the person who is armed with a knife.

If the statisticians have compared probability of becoming a victim (or perpetrator) among the general public to the probability amongst those who carry knives, they may well find that the risk is higher among the knife carriers. But this doesn't prove that the increased risk is due to their knife carrying. It might equally be due to some other factor. In order to prove the causality they have to control for these other factors. From the way the police claims are made, I don't think this has been done.

The increased tendency to be a victim is more likely to be due to the fact that people who are involved in violent parts of society are more likely to be victims of crime, and that they arm themselves against that probability. In other words the heightened risk is due to their position in society rather than their carrying knives, which is a symptom of the danger rather than a cause of it.

The conclusion the police and Mr Gove reach - that you should not carry a knife because it "escalates the scale of violence" rests on very shaky foundations. All this is not to say that the apparent increase in knife crime (or its reporting) is not a worry, but a flawed argument is not going to help.