Hello, this is the shorter World Service Edition of more or less.

First broadcast on Friday the 12th of August.

I'm Tim Harford, and in this week's program we will be asking why swimming records are broken so often and what the perfect distance is if you want to run for health and fitness.

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

We are the program which looks at the numbers in the news and in life, and I'm Tim Harford. Later in the program we will be exploring the perfect running distance. But first, as we record this program, six world records

have been broken in the first five days of the Olympic swimming. That's quite a

rate. It's partly thanks to British gold medalist Adam Petey. He managed to break the world

100 meter breaststroke record twice in two days, which is almost showing off. But that's the thing

about swimming. Swimmers always seem to be breaking world records. In the

London Olympics of 2012 only one individual track record was broken. David Radiche's

rather brilliant 800 meters, while a Jamaicans shattered the men's 4 by 100 relay record.

Two excellent records on the track, but in the pool nine records fell.

So what's going on? Are swimming records really more likely to be beaten? And if so, why?

Rick Madge is a Canadian swim coach. He's also a blogger with a taste for numbers. He's been investigating these questions.

Oh, it's absolutely true. In track and field, for instance, since 1972,

roughly 10% of the track and field events result in world records.

And if you look at swimming, we're up close to 40%.

Now, if you look since 2000, roughly 6% of the track and field events get world records, and yet swimming

is still up around 40%.

So that shows definitely that swimming continues to have world records and it's not tapering off at all.

And what explains the difference? The strokes have full body movements that are very

complex. They involve virtually all parts of the body and they're moving through water

...