Msg#: 4289523 posted 3:01 pm on Mar 30, 2011 (gmt 0)
Google officially said in the AdSense Blog:
As you might already know, a great way to increase your AdSense earnings is to place multiple ad units on webpages that are rich with content. Here's a way to maximize your revenue with multiple ad units: Make sure the ad unit with the highest CTR is the first ad unit in the HTML code of your page. We've heard that there may be some confusion on this, so here's some clarification on how we serve ads to a page once we know which ads win the auction.
Currently, the first ad unit on a page always shows the top ads that win the ad auction. Also, if there aren't enough ads in our ad inventory to fill all of the ad units on a page, the first ad unit on the page will display ads first. This is why I recommend using custom channels to determine which of your ad units has the highest CTR, and then placing that ad unit first in the HTML code.
For the purposes of this post, the first ad unit on the page is defined as the first instance of the ad code within the HTML of the page. Keep in mind that the first ad unit in the source code is not always the first ad unit that your users will see when the page finishes loading in their browser. For example, if you use DIV tags, our system may recognize an ad unit which users see at the bottom of the page as the first ad unit.
Now that you know how to make the most out of multiple ad units, try this tip and watch your earnings grow.
On my medical forum I have a large rectangle that is loaded first and a medium rectangle that is loaded second. The medium rectangle, even though it is loaded second, has a much higher eCPM (near double) then the large rectangle that is loaded first. It also has a higher CTR. Both Ad Units are tracked by different channels.
That is why I made the experiment and put the old "put the best performing Ad first in your source code" to the test.
And the result is: No change in revenue whatsoever
But why is that? I am not that sure.
One reason could be that AdSense is able to track down performance to an Ad Unit level and serve the highest paying Ads to the best performing Ad Unit independent of its loading order.
Another reason could be that the 2 Ads I made the test with are different size and because of different ad auction competition (the medium rectangle eCPM is much higher even though it is loaded 2nd because there is more competition for this ad unit) the loading order only has influence on Ads of the same size.
Msg#: 4289523 posted 3:11 pm on Mar 30, 2011 (gmt 0)
My opinion is that four full days is not near enough time to do a reasonable test.
You have to take into account other things too - how many impressions? How many clicks? Are your ad units set to both text and image? Are both of them capable of displaying text and image? (AdSense doesn't carry image ads for all the available ad unit sizes, for example) I don't mean you have to tell US the answers to all these questions, but you need to take them all into account (and more) before you can come to any conclusions.
And yes, over the years, across the board, the first loading ad unit on most if not all of my sites is the highest earning.
Msg#: 4289523 posted 3:38 pm on Mar 30, 2011 (gmt 0)
Image and Text Ads enabled on both ad units and both rectangles medium and large are capable of serving image ads. I only ran the test for 4 days because I had to disable my wallpaper campaigns for this test which meant I was losing money during the test. I hoped that the effect would compensate this but it was marginal at best.
The test was run with around 40k AIs per ad unit and around 1500 clicks. It is not much but the numbers are very consistent when compared to the same time frames the weeks before.