I've always found that if my QS is 7 or higher, I'm hitting the defined CPA I have for my company's AdWords campaigns. Any lower, and it doesn't work, regardless of how good any of the other metrics look.
To me, this is obvious, but maybe I'm just benefiting from clearly defined goals and stronger than usual correlation. However, Larry's analysis suggests my experience isn't that uncommon, and maybe complacency is the bigger problem.
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