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As I understand it, your rank is a function of your CTR multiplied by your max bid. But exactly which CTR is used in this calculation? Is it the CTR for a particular term since the day you added it to your account, the CTR for that term over the past month, the past week, CTR from yesterday or?
Can anyone confirm that it's the CTR over the last 1,000 impressions?
limitup, it is actually the all-time CTR for the keyword that matters, with the most recent 1000 impressions being weighted a bit more heavily.
One way to look at it: every single time a keyword has an impression, it's CTR has changed. The same is true if it has both an impression and a click. The AdWords system works on this basis, and weights to most recent 1000 impression more heavily. However, if you have a truly horrible 'most recent 1000', your past history, assuming it is at least reasonably strong, will save you.