I've spent much time lately in creating systematic tests for my ads and have gotten a huge boost in CTR as a result. Now as the low hanging fruit is disappearing, I'm seeing less of a boost in each round, but one thing that is still seeing gains is in my CPC for tested ads. I was wondering if anyone could point me in the right direction for tools or information about determing when a boost in CPC is statistically significant? I currently use http://www.splittester.com/ for testing CTR significance, any tool similar to that would be great.