Hello
I know I wrote an article about “Did you take the display order of your
AdSense ADs into account?” in which I said that it is a known fact that
the first AdSense Ad Unit in the HTML receives the highest paying Ads.
However reviewing my AdSense statistics today after replacing my
Large Rectangles in my content with medium rectangles I am not
that sure anymore. Or at least the theses needs an additional comment.
I still think that the highest paying Ads are served in the first Ad Unit that
appears in your HTML. However this does not necessarily mean that the
highest paying Ads are shown in the first Ad Unit. I think this effect depends
on your Ad Unit sizes and if you allow image Ads or not.
First some statistics:
My first AdSense Ad Unit (visible and in HTML) on my forum is a
Large Rectangle serving image and text Ads.
My second AdSense Ad Unit (visible and in HTML) on my forum is a
Medium Rectangle serving image and text Ads (switched from a
Large Rectangle a few days ago).
Now comparing the combined stats of those two Ad Units
(to list them on the detailed level I track them would be to much work 
)
the CTR of my medium rectangle (which is the second Ad Unit) is 118% higher
then the CTR for my large rectangle (which is the first Ad Unit).
The eCPM of my medium rectangle is 165% higher then the eCPM for my large rectangle.
The average CPC is 138% higher for my medium rectangle then the CPC for my
large rectangle.
But how can that be if my medium rectangle is the second Ad Unit on
my page and the highest paying Ads are served in the first Ad Unit?
Let me explain my theory:
Each Advertiser creates different advertising creatives (Banners) in different sizes
for his campaigns. Most of them will use Ad sizes that are industry standard which are:
- 300 x 250 - (Medium Rectangle)
- 180 x 150 - (Rectangle)
- 160 x 600 - (Wide Skyscraper)
- 728 x 90 - (Leaderboard)
This only applies to image Ads and RichMedia Ads. Ads which all have a defined size.
Text Ads are independent from the size of the Ad Unit they are displayed in.
So what does this mean?
It means that depending on the Ad Unit sizes you use on your website different
contingents of Ads to display are available.
So let’s do a simple example:
Let’s take one webpage of a website using my Ad Units setup I described above.
Large Rectangle format:
20 Advertisers created an image Ad for that Ad Size and
they compete with 100 text ad Advertisers for this Ad spot.
120 Advertisers are now competing for this Ad Spot
Medium Rectangle:
because this is industry standard Ad size 150 Advertisers created an Image Ad
for this Ad Size and they compete with 100 text ad Advertisers for this Ad spot.
250 Advertisers are now competing for this Ad Spot
And because Google AdSense is auctioning the display of Ads for each Ad Unit
in real time more competition means higher CPC which translates to higher eCPM
for you as publisher.
This means the more ads that are available for an Ad Unit the higher your eCPM.
By enabling image Ads as well as text Ads for your Ad Units you increase the
competition for your Ad Unit and with this your eCPM.
By choosing industry standard Ad Unit sizes you increase competition because
more Creatives (Banners) are available for these sizes then for non standard
Ad Unit sizes and with this you increase your eCPM.
This is the only way I can explain the discrepancy I see in my AdSense Stats without
discarding the well known fact that the first Ad Unit gets the highest paying Ads.
In conclusion:
Enabling Image Ads (at least for industry standard Ad Unit sizes) is important to
increase your revenue. And only if you compare Ad Units of the same size or text
only Ads then the fact is true that the first Ad Unit gets the highest paying Ads.
What are your thoughts on this?
Do you notice the same when checking your AdSense statistics?