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Digital advertising fraud cost companies an estimated $16bn last year. Often the clicks or downloads generated by the ads they paid for came not from people, but robots.

Alex Hewson, from mobile advertising firm M &C Saatchi, describes the scale of the problem and the tricks some fraudsters use. And Gary Danks, managing director of Machine Advertising explains how his company is tracking fraudulent app downloads.

The gaming of the online advertising system raises an age-old issue in economics - the principal agent problem. Jerry Z Muller, author of The Tyranny of Metrics, explains and also warns of the dangers inherent in setting targets in business and economics.

(Picture: A hand touching a screen and icons. Credit: Getty Images)