Google AdSense targets ads by content of web pages from which the ads are served.
Sometimes pages get out of control, exposing irrelevant content for Google bots to analyze for ads selection. It may be an inappropriate comment by a site’s visitor or, a navigation menu item Google happened to adore. Suddenly you start noticing CTR going down (or watch it low from the beginning).
Fortunately there’s a way to point the robots at the right text chunks of your site (or isolate “wrong” parts from undue attention).
For that just enclose your articles including titles (or any other info you deem relevant) in these HTML tags:
<!-- google_ad_section_start -->
[Your relevant content here]
<!-- google_ad_section_end -->
This is called “Section targeting“.
Certain content management systems may feature this out of the box so that you start getting all the targeting benefits right away.
For example, Typo-XP theme of WordPress blogging platform was reported to have this in place in some versions (the version used inSTEREO-blog had to be hacked for that thou). So it seems to be a good habit to get software (like probably most things actually) from reliable sources.
Anyway, stock solutions can’t (and don’t) include everything. So here’s a little trick you could use even in a good CMS.
The Google Section targeting tag also has a reverse form used for exclusion. Besides isolating the whole parts of the site it can be helpful eliminating “poison” words.
An example of such a word would be a “Blog” in a blog. In fact, many bloggers complain about overly emphasized blogging theme in the ads served on their sites while they don’t blog about blogging at all.
To fix that do this on the appropriate files in your site system:
<!-- google_ad_section_start(weight=ignore) -->Blog<!-- google_ad_section_end -->