IanCP - 10:28 pm on Mar 8, 2012 (gmt 0)
My view of CTR comes down to the "relevance" of the Ads offered.
Rubbish Ads = Rubbish CTR and vice versa.
On a day to day basis, I can instantly tell a rubbish ad day. I can only assume quality, relevant ads have been exhausted for my genre that day.
Your mileage might vary.