All entries for Monday 12 December 2005

December 12, 2005

Probability neglect

Nice term in one of the Sunday papers: probability neglect, the tendency to consider the consequences of something happening, but not the probability. So fear of child abduction, for example, is much more pronounced than it was thirty years ago, even though the risk hasn’t changed in that time. People are much more worried about things which are very unlikely to happen, but which would be dreadful if they did, than things which are relatively common but less serious.

But why does probability neglect happen? It’s partly a psychological thing; imagining something happening and how we would feel about it is easy, natural, intuitive, but thinking about probability is difficult, mathematical, unfamiliar. And it’s also partly a media problem; we have much more news reporting now than we did, say, thirty years ago. There were no 24 hour news channels then, only a small number of TV channels, and no internet. The news machine is voracious, and so when there is an accident or a disaster or any sort of human tragedy, it is reported and analysed endlessly. This makes us think that events which are actually very rare happen frequently, and perversely, events which are relatively common are under-reported precisely because they’re not news.

As an exercise in trying to make myself think about probability, I went and found some statistics relating to the riskiness of different modes of travel. Before you read on, what’s the safest way to travel? How much safer do you think it is to travel by car than to walk? A bit? A lot? Is a train safer than a plane?

The best numbers I could find were fatalities per billion passenger kilometres for 1999, and they look like this:-

Mode of transport Deaths per billion passenger km
Air 0.02
Boat 0.3
Rail 0.9
Car 2.8
Pedal cycle 41
Pedestrian 49
Motor cycle 112

(It was a bad year for rail fatalities because of the Ladbroke Grove crash which killed more than thirty people; rail fatalities are normally less than half this figure.)

The trouble is, if you worry irrationally about things, then rational argument is by definition unlikely to change your mind. If you’re a parent, it’s extremely difficult to think probabilistically about your children; if you sleep with a baseball bat by your bed for reassurance, you’re not going to be swayed by figures showing that burglary is in decline. The best we can hope for, I think, is to understand that we’re often irrational and try to remember that now and again.

*Update 13th Dec*: I’ve since discovered this fascinating list of the odds of dying of just about anything (PDF) which reassures me that there are at least some things which are both unlikely and largely ignored. My odds of dying by being confined to or trapped in a low-oxygen environment, for example, are vanishingly small, and as if that weren’t good news enough, I hardly ever worry about it!

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