Posts Tagged fat tail

The benefits of pessimism

Normal Distribution

Normal distribution - thin tail

Last week I learnt a new piece of jargon. A ‘fat-tail event’ is something that you thought was virtually impossible, but it happened anyway. In theory, it could be very good or very bad, but it usually refers to something extremely unpleasant, such as a financial crisis.

The phrase comes from statistics. Many randomly occurring events (such as the height of the person you sit next to on the bus) are assumed to follow what is called a Normal Distribution (the classic ‘bell-shaped curve’). So you are more likely to sit next to someone around average height and less likely to sit next to someone really short or really tall. With the Normal Distribution the probability of something really unusual happening tails off really rapidly the further away you get from the average — it has a thin tail.

Fat Tail

See the fat tail!

However, some things in the real world don’t follow the Normal Distribution curve. Instead of a thin tail, they have a fat tail. This means that certain extreme possibilities are more likely than you might think.

I was quite pleased to be able to use my newly discovered jargon in a session on negotiation skills I was running last week. I was talking about the usefulness of assessing any negotiated deal by imagining how it would look if subsequent events turned out a lot better or a lot worse than you were expecting (e.g. your fixed-rate mortgage doesn’t look so good if the Bank of England cuts rates to zero).

A black swan

Does this black swan have a fat tail?

A related term for unexpected events is a Black Swan, coined by author Nassim Nicholas Taleb. This is the unexpected event which you could not have predicted based on your previous experience and derives from the fact that, until they were discovered in the 17th century, most Europeans thought that black swans could not exist.

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