true_INFP - 8:02 am on May 8, 2010 (gmt 0)
CTR alone is not important. What matters is eCPM. Even if your CTR decreases, you can actually earn more money (because the CPC of the new ads may be higher).
So compare your eCPMs, not CTRs. (That's also what the Google auction system is officially supposed to do.)