Google has a requirement of a minimum click through rate that you must meet to keep your keywords from being slowed or disabled. (More info about ads being disabled in Keyword Status).
This CTR referrers only to the keyword, and not to the various ads you may have.
This CTR is also determined by your CTR on Google’s site only. You will see overall CTR rates and this will include data from partner searches if you have that feature enabled.
The minimum CTR rate for the first position is 0.5%. As the positions decrease (from 2, 3, etc), the minimum CTR rate slow decreases as well.
Google has not released exact statistics for each position. However, its believed that a 0.3% will be enough for a position 6-8 ad (bottom of the first page) not to be disabled.
To learn more about managing keywords and optimizing ads, please see our Creating Your Own Pay Per Click Account page.