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Monthly Archives: December 2015

Governments, the media, and other pundits appear to conspiring to cause confusion about probability, although I suspect that the truth is more likely to be that are just don’t understand what they are talking about.

I live in Cumbria in the UK, and last weekend we suffered from severe flooding, and the news seems to have been dominated by people talking about  a “once in a hundred year event” that seems to have occurred several times in the last decade. These comments always seem to lead on to people saying that it can’t be a “once in a hundred year event”, or that it must be caused by something else if it has happened more than once. This displays a total ignorance of probability, and educated people who talk such rubbish should be ashamed of themselves.

A “once in a hundred year event” is actually an event that has a probability of 1% of happening in any one year, just like a coin has a 50% chance of  tails when tossed, or a dice has a 1 in 6 chance of turning up as a 6 when rolled. The “once in a hundred year event” can happen twice is succeeding years, or twice in a decade, and still remain an event that has a probability of 1% of happening in any one year. Similarly, a coin that is tossed 10 times can land as a tail more than 5 times, and a dice that is rolled 12 times can turn up as a six more than twice. Using the phrase”once in a hundred year event” is not only misleading, but wrong, as it implies that something can only happen once in any hundred years.

So, this is a plea to all of you out there who comment of the chance of an event happening. Please state your views as “there is a chance of x% that the event will happen in any one year” and not as “x times in a hundred years”. The first is mathematically correct and is not misleading, the second is not correct and is immensely misleading.