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

We're your statistical guide to the numbers in the news and in life and I'm Tim Harford.

And if you're eating as you're listening to this, well, sorry.

Round worm, whip worm and hookworm.

We know that perhaps more than a quarter of the world's population is infected with one or more of these worms.

This is David Taylor Robinson from the University of Liverpool.

They live in the gut and they're transmitted through

contaminated food or directly from soil and they can burrow through the skin of the

feet and they particularly affect kids living in poverty in areas where there's poor sanitation.

We'll hear more from David later. Now worms are not

usually fatal but they can make children ill and possibly contribute to

malnutrition, stunted growth and absences from school. This has led to the idea that in areas where

parasitic worms are common, children should simply be routinely treated. That's a very

attractive idea and many development experts including the World Health Organization

recommend masteworming programs as a highly cost effective way to help poor children.

Many discussions of the subject refer to a very famous, very influential study that was conducted in Kenya.

The study's authors were two economists, Michael Kramer of Harvard and Edward Miguel of the University of California Berkeley.

We worked starting in 1998 on a large school-based deworming project.

This is Edward Miguel. He and Michael Kramer set up something

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