It's unbelievable how inconsistent ad format CTR are and subsequently RPM. For example, when I run text ads only (yes even the latest version which for me has a much lower CTR) I have far and away the highest CTR. And the CPC/RPM aren't bad either. However, every now and then I test opting into all ad formats and for a while it seems I get very little impressions from image/rich media and it APPEARS to help the CPC prices on text ads clicks. Then I get today's scenario; huge spike in image/rich media which drops my CTR dramatically which results in a crappy earnings day. I'm pretty sure this 3rd scenario happens toward the end the month. But it's hard to say if it happens EVERY month. For the last year or so I ran text ads only because after all my testing it resulted in the highest earnings. However, about 3 months ago I opted into all ad formats and it appears to help dramatically for like a week - and then "hurts" earnings/CTR for say the next 2 weeks. It's such a roller coaster. I feel to really maximize earnings you have to manage ad formats like a stock portfolio - it's ridiculous.