I just wandered why the CTR one of my sites declined at the AdLink.
The page is like this
Content Navigation code for 120x90 AdLink code for 300x250
<!-- google_ad_section_start(weight=ignore) -->
Logo with links to other thematic sub domains
The 300x250 shows ads matching the content of the page, but the 120x90 AdLink shows 3 themes complete unrelated to the site. The themes can only be from the logo area, where links are to the other sub domains. The links are by pictures and ALT text. 2 ALT texts contain words responsible for the 3 AdLink themes.
But all this is in the <!-- google_ad_section_start(weight=ignore) -->
We've had a major decline in overall CTR over the past six weeks which is probably down to the performance of Ad Links. Certainly there are a lot more Ad Links "search boxes" appearing than usual. And as we introduced <ad_section> tags a couple of months back, there's a possibility they're related. We're testing as we speak, but this not one where we'll be able to get instant A-B test results.