Google AdWords ranking

I was discussing the way rankings in this program work yesterday with a reader. The fact is that the ranking of your ad is based on CPC (cost-per-click) that you bid/pay and CTR (click-thru-rate). I mistakenly thought it was also influenced by PageRank.
Google AdWords: FAQ And this is why my ads sometimes show up on top: We occasionally show Google AdWords Select ads in the top spots if they qualify for Premium placement. This is determined by a combination of cost-per-click and clickthrough rate.