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Hello and welcome to more or less on the BBC World Service.

With me, Tim Hartford, opening the curtains and letting the light in on the lazy use of numbers.

Now, loyal listener Les Halpin recently wrote to us with a question that brought us up short.

Les has motor neurone disease, a condition which is both fatal and rare.

But rare is a strange word.

Here is Les's email, read by an actor.

It struck me that the way we use words is clearly at odds with their precision as numbers.

In particular, on TV the other day, a lightning strike on an individual was described as rare.

Probably 12 strikes per year, three fatal.

At the same time, something like motor neurone disease is described as a rare disease

with 5,000 sufferers and 1,500 deaths per year.

What does rarity really mean?

There's no precise, universally accepted definition of rare.

But strangely enough, there is a widely accepted definition of rare disease.

It's a disease that affects fewer than 0.0.5% of the population, one in 2,000 people.

Both the UK authorities and the European Union use that definition.

Still, that doesn't quite answer Les Halpin's question.

What do we think we're saying when we use a word such as rare?

And this isn't a question for a mathematician or a statistician.

...