I've read that ctr as well as contributing to the Quality Score of an ad is also used to rate adgroups > campaigns > accounts. Is there any solid evidence that the scope really extends beyond the ad level?
Does say a low ctr on one adgroup affect to some degree the quality score of ads in other adgroups. It sounds unreasonable but people claim that it does.
You may find this interesting: See: http://www.ewhisper.net/blog/what-determines-the-account-quality-score’-for-adwords/ Personally, I think the account level QS is so minor that unless you're account is in horrible, horrible shape you'll never even notice the impact. I haven't.