Here's a little observation from my site statistics I'd like to share with those of you interested in SEO. I've been looking at the Adsense ads displayed on my pages and thinking a lot about how Google determines ad relevance based on the content of those pages. I've noticed that the ads Google decides are relevant are not always as relevant as they could be from my perspective. I assumed, and still believe I'm partially correct in assuming, that the ads served are determined not only by my page content, but by the bid amount of the advertiser. Then I started looking at my referral logs to see what phrases people are using to find my site. Aside from being impressed and surprised by the fact that I average about 20 distinct visitors each day, I noticed some very familiar key phrases in my referrals - phrases that seem very similar to the ads being displayed in those cases where I don't think the ad necessarily matches the content of my pages. I'm going to go out on a limb and say that there is a connection between the search term used to find one's site, and the resulting Adsense ad(s) that are displayed on that site. This becomes very interesting when you consider the fact that these key phrases that appear in my server logs are being extracted from clickthrough data from the natural search engine results that got the visitor to my site in the first place (as opposed to other sponsored results). In essence, Google’s natural results are generating the traffic to their own paid results, and it appears they incorporate some of the same referral data that shows up in my logs.

I've believed for a long time that a lot can be learned about Google's natural search algorithm by examining the more transparent algorithm of Adsense's contextual ad serving. I say more transparent because if you're both an Adwords advertiser and Adsense publisher, you get to see a direct cause and effect relationship. Although there are differences between the algorithms, I think they're based on the same basic engine. That's what makes Google Google. The above finding shows further evidence of that. My theory at this point is that Google's natural ranking algorithm and their contextual ad serving algorithm are very much intertwined and, quite possibly, use an iterative process between one another to produce both the best natural search results and the most relevant ads that pay the most money on any given site. It's a given that some folks have known this for a long time and other folks will simply disagree, but this is what I'm seeing. Maybe it'll help you make more money and/or attract more visitors to your site.