Following on the heels of Google (no surprise there then)... Yahoo is to introduce a new ranking model for its Pay Per Click..

Previously they had a nice simple straightforward ranking mechanism whereby the ads were ranked entirely by bid amount. But all that is set to change on July 23rd when they introduce the concept of "Ad Quality" - like Google's "Quality Score" - whereby ads are ranked by both bid amount and quality score.

This Quality Score is calculated using factors like Click Through Rate.

Oh well - can't complain I suppose - good idea really.