Micromorts, horses and ecstasy

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understandinguncertainty.org was produced by the Winton programme for the public understanding of risk based in the Statistical Laboratory in the University of Cambridge. The aim was to help improve the way that uncertainty and risk are discussed in society, and show how probability and statistics can be both useful and entertaining.

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DJS, Times, 10th February 2009

After the chairman of the Advisory Council on the Misuse of Drugs suggested that taking ecstasy is no more dangerous than horse-riding, can we expect horse-mad Fionas around the country to sadly hang up their jodhpurs and start to pop disco-biscuits on health and safety grounds? It’s an interesting image but unlikely to occur: even if Professor David Nutt’s sums do add up, the calls for his resignation suggest that comparing risks involves a lot more than just counting bodies.

Let’s start by taking a cold-hearted look at the statistics. There are claimed to be around 500,000 weekly users of ecstasy, and so with 30 ecstasy-related deaths per year that’s around one death per million uses. Is that a lot or not? To make comparisons with other, legal, activities we need a friendly unit of deadly risk – fortunately, risk analysts in the US have given the wonderful name of micromort to a 1-in-a-million chance of dying. So someone taking ecstasy is exposing themselves to around 1 extra micromort, but how does this compare with daily living? 50 people are killed each day by accidents and violence in England and Wales, and so on average we all have to face around 1 micromort a day just between getting up and going to bed. But this is just an average and we can choose our own micromorts in different ways: according to the Rail Safety and Standards Board, an average person experiences a micromort by driving 230 miles in a car, riding 6 miles on a motorbike, travelling 1500 miles in a train or by taking 3 flights. If we really want to splash out we could go hang-gliding (8 micromorts every time we go up) or scuba-diving (5 for each trip down). Horse-riding is more difficult to judge. Professor Nutt reports 10 rider deaths and 100 extra road traffic accidents each year, say 25 deaths in total: if a million people rode horses each week this would mean around half a micromort each ride. So maybe a bit less than taking ecstasy but not hugely different.

We could try making these very rough calculations more precise, but it would quickly start to miss the point. Regardless how carefully the figures are calculated, people see risks differently if they are judged to be the fault of distrusted organisations and affect the vulnerable. Radon gas leads to over 1000 deaths from lung cancer a year, but it just seeps out of the ground and mainly affects middle-aged smokers so nobody gets angry and waves placards calling for improved protection. Compare that to mobile-phone masts, so far unproved to cause any childhood cancers but subject to vociferous campaigns.

But the comparison between horse-riding and taking ecstasy has been carefully chosen to avoid these issues, since both are voluntary activities and both affect young people, so there must be some other reason for the indignation in this case. Certainly those who actually take the risks don’t seem to mind – in both cases there’s a simple trade-off between risk and enjoyment, and fun wins every time. But other (generally older) people clearly feel the need to take a moral stance - I suspect this may be as much about young people being irresponsible as about the activity being illegal. In a similar way a sense of moral outrage can lead society to spend vast sums in trying to prevent train crashes, although saving very few micromorts in the process.

As a statistician, it would be nice if we could just do the sums and tell people what is safer than what. But comparing risks is a complicated business that involves people as well as arithmetic, and so Professor Nutt should not be surprised at the fuss he has caused.

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