Tuesday, 8 November 2011

Nonsensical probabilities about asteroid risk

There is an article in today's Evening Standard in which, rather depressingly, someone who should know better (Roger Highfield, Editor of New Scientist) reels off a typically misleading probability about asteroid strike risk (this is in response to the news today that an asteroid was within just 200,000 miles of Earth).

Quoting a recent book by Ted Nield he says (presumably to comfort readers) that

Our chances of dying as a result of being hit by a space rock are something like one in 600,000. 
There are all kinds of  ambiguities about the statement that I won't go into (involving assumptions about  random people of  'average' age and 'average' life expectancy) but even ignoring all that, if the statement is supposed to be a great comfort to us then it fails miserably. That is because it can reasonably be interpreted as providing the 'average' probability that a random person living today will eventually die from being hit by a space rock. Assuming a world population of 7 billion that's about 12,000 of us. And 12,000 actual living people is a pretty large number to die in this way. But it is about as meaningful as putting Arnold Schwarzenegger in a line up with a thousand ants and asserting that the average height of those in the line-up is 3 inches tall. The key issue here is that large asteroid strikes are, like Schwarzenegger in the line-up, low probability high impact events. Space rocks will not kill a few hundred people every year as implied by the original statement, just as there are no 3-inch tall beings in the line-up. Tens of thousands of years pass between them killing any more than a handful of people. But eventually one will wipe out most of the world's population. 

What Ted Nield should have stated (and what we are most interested in knowing) was the probability that a large space rock (one big enough to cause massive loss of life) will strike Earth in the next 50 years.

Indeed, I suspect that (using Nield's own analysis) this probability would be close to the 1 in 600,000 quoted (given that incidents of small space rocks killing a small number of people are very rare). You might argue I am splitting hairs here but there is an important point of principle. Nield and Highfield avoid stating an explicit probability of a very rare event (such as in this case a massive asteroid strike) because there is a natural resistance (especially from non-Bayesians) to do so. For some reason it is more palatable in their eyes to consider the probability of a random person dying (albeit due to a rare event), presumably because it can more easily be imagined. But, as I have hopefully shown, that only creates more confusion.