Does single ad unit have a highest payout?

Anybody has tested this?

[fischermx]

I recently changed some part of my site to show a couple of single ad units, instead of one big square showing 4-5 ads.
I did this because I wanted to test if my earnings would go better, expecting having the highest paying ads on this unites.
However, I feel like this is not correct, I mean, the ads being shown there, are not necesarly the more expensive.
What do you think?

[jchampliaud]

I mean, the ads being shown there, are not necesarly the more expensive

How do you know that?

[fischermx]

Some guessing, here, it goes: In other ad units that show 4 ads, the higher positioned ads are usually the most expensive.
And I see that the middle or lower ads are many times showing in my 1-ad unit.