In testing is a new content ad system that works on a CPA (cost per action) model, as opposed to a CPC (cost per click) or CPM (cost per thousand) model.

The search engine has invited a number of web site owners to take part in the pilot programme that will see site owners getting extra payments if their visitors take some kind of beneficial action on the advertisers’ sites – such as buying a product or requesting further information. Source

The driving force behind this new model is that CPA ads would tend to “be less susceptible to click fraud” and yield greater flexibility.