They have several. To answer your question, if I was to pick up only one, the first that came to my mind was "pageviews". So, everything that increases pagevie

They have several. To answer your question, if I was to pick up only one, the first that came to my mind was "pageviews". So, everything that increases pageviews is good. Well, not quite, because so is CTR, and in fact doubling CTR allows you to halve pageviews. Anyway, whatever they think of, the ultimate step is money earned. No single path leads to this, but several, like pageviews, CTR, click value, searches per day, other services integration, etc... Edited: You're thinking about A/B testing but I don't know if you're aware there's also multivariate testing, which Google does use (its optimizer easily creates A/B and multivariate).