When Google launched AdWords Select and started to shift from CPM-based pricing to CPC-based pricing, they couldn’t afford to lose all the revenue from their CPM program (which was called AdWords at the time). They were still a pretty small company, and Yahoo/Overture was a formidable competitor. So, to ensure revenue was maximized, they ranked ads in the CPC program according to a very simple rule: Ad Rank = Max CPC * CTR If you take a minute to look at this more closely, you’ll quickly see that ad rank is, in fact, equal to CPM or dollars-per-impression. This was the simple but brilliant insight that made AdWords so powerful — advertisers only had to pay when they got clicks, users would see more relevant ads because ads with high CTRs were more likely to appear high in the results, and Google was making as much money as possible from these ads. The equation for ad rank is a whole lot more complicated these days and now includes thresholds for appearing at the top of the page, landing page factors, and more. But at the heart of it, the original principle still applies: if Google can show more relevant ads, they will get more clicks, have happier users and make more money. And, the key component for achieving this is CTR. The importance of CTR to Quality Score is a bit like the importance of TF-IDF to SEO. While there are hundreds of factors that go into ranking in paid or organic search, these long-established principles are still some of the most important ones. In the 80/20 rule, these are your 80 percent factors and the first ones you should pay attention to. Get the full story at Search Engine Land