Google AdSense works by displaying ads relevant to a page. To get relevant ads, Google AdSense crawler crawls your site and extract keywords from the pages it visit. When a visitor visits a page on your site, Google will serve ads based on the extracted keywords that Google AdWords advertisers bid on.
With that in mind, it's important for Google AdSense crawler to get the right keyword from your page, as if it doesn't, it will either display irrelevant ads or worst, PSAs or Public Service Avertisements. Irrelevant ads gets less click from your site's visitors, and PSAs don't pay at all.
Keyword targeting for Google AdSense in a sense is similar to keyword targeting in SEO or Search Engine optimization, with some unique targeting features of its own.
Your page's content is where most of the keywords would come from. Try to repeat keywords that you want to target, but don't overdo it as it'll be unnatural to your site's visitors and would just drive them away. Even worse, you could be tagged for keyword spamming by Google.
Site title should be definitive. Using Page 1, Untitled, or anything similar doesn't has any keywords that could help you serve relevant AdSense ads. If your page is about AdSense, remember to have AdSense somewhere in the title.
AdSense crawler doesn't crawl images, but they sure can read your <img> tags. Use meaningful name for your image, and be sure to include meaningful description, or alt text to your image as well.
Section targeting is an AdSense feature that allows you to emphasize certain part of your page in keywords targeting. You can also use section targeting to ignore certain section of your page such as the sidebars or the footer.
Meta tags are some information inserted in the <head> area of your site. It's not visible in page when rendered by web browsers, but is useful for robots crawling your site. While the meta tags are no longer used for SEO, it's still applicable in targeting your site for more relevant AdSense ads.
The 2 meta tags you can use for targeting is the keywords and description tags. You can enter keywords relevant to the page in both meta tags, though the description should be in more human-readable form.
<meta name=”keywords” content=”keyword1 keyword2“> <meta name=”description” content=”Description of your site“>
Google by default serve ads based on your visitor's browsing history in what they call as interest based advertising. This aims to serve more targeted ads for AdSense, but it can give adverse effect, depending the nature of your site.
If you have implemented all the above tips but there are still some ads that you don't want to appear in you page, you can just use the Competitive Ad Filter feature.
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