I was wondering what's the most effective way to set up the channels? should I set up the channels by websites? like? channel 1 : website1.com channel 2 : website2.com channel 3 : website3.com OR by each unique ad unit channel 1 : website1_sidebar_rect channel 2 : website1_top_leaderboard channel 3 : website2_sidebar_large_rect etc The only thing about the 2nd one is that it's then difficult to see the overall stats (impression, CTR, eCPM, etc) of the ENTIRE site.
I use one channel for one site. And there is no need to make different channels for diff ads. You yourself will get to know which ad position helps you earn more even if you have only one channel for a site.
How would you know which ad formats are performing best when you only use one channel for all of them ?
Keep changing the ad position and observe at what positions you earning max. Its not that tough. Or else you can go for diff channels too.