I have always used only one ad (250x250), above the fold, on all my pages for two reasons -
1. I don't want my pages to appear too ad heavy
2. The 250 always serves a maximum of 3 ads per block
3. I was told that the first served ads always pay the most, and subsequent ads pay much less.
I thought that I would test this on one of my popular pages, so I added a second 250x250 near the bottom of the page. I created a unique channel for each ad. What I found was surprising. Is it OK to say the actual EPC?
Anyway, the EPC was (is) exactly 3 TIMES more than my old standard first ad block above the fold.
The CTR is less than 1/10 of the first ad block, somewhat less than expected.
First block (EPC=x) (served in small font) -
Ad #2 (bogus, different subject URL - I will be blocking this)
Ad #3 (also served on Google search results)
Second block (EPC=3x) (served in medium font) -
This raises some interesting questions, such as -
1. Should a webmaster block the URLs of the first ads shown, thereby ensuring that the subsequent higher-paying ads are displaying? This can only be determined by experimentation.
2. Why does Google insist on telling us that the most expensive ads are always served first, when in fact, they are not? In one experiment, I was unsatisfied with the EPC I was getting on one page, so I wiped the ads (I blocked all 3 URLs that were being displayed). The new ads, which appeared within hours, paid TWICE the EPC I had been getting. This proved to me that there are MORE CRITERIA than just ad price, that determine the order of ad presentation. Perhaps the biggest factor is that all purchased ads must be served, which is going to require adjustments to the ad serving order, when the budgets for the low-paying ads are relatively huge compared to the budgets for the high-paying ads. Or something like that. Make any sense?
Some advertisers will pay for position, but not volume? (high bid, small budget) (start-ups seeking visibility)
Some advertisers will pay for volume, but not position? (low bid, high budget) (Walmart, Target, etc.)
3. What is YOUR explanation for why the highest-paying ads are not always shown first? Do you try to do anything about it?