The ProductWiki blog has published the results of an A-B test in which they find that Shopping.com Merchant Listings outperforms Google AdSense by a factor of 4.6, in terms of revenue returned to publishers.

The primary reason given by ProductWiki for this difference is relevance. Google attempts to determine the most relevant ads with no help from the publisher. Shopping.com allows publishers to provide hints about the products being discussed in the content so that Shopping.com can serve up ads and images for those products.

	Shopping.com	AdSense
Clickthrough rate (%)	29%	6.5%
Revenue per click ($/click)	$0.21	$0.19
eCPM ($/1000 impressions)	$59	$13

You would expect to see very relevant results [on Adsense] since the theme of our site is consumer products, but it’s often not the case. In the above example [for the Creative Zen Vision MP3 player], we see an ad for “creative products” and for a Zen Vision case, not exactly what we were going for…

Google does have something that Shopping.com does not – breadth. Their database of ads is much more extensive than anything Shopping.com has to offer beyond products; they have ads for blogs, publishers, services, etc. So how do you get the best of both worlds: the relevancy and style of Shopping.com ads coupled with the breadth of Google AdSense? Simple. Google needs to allow publishers to control what ads are displayed and how they are styled. Another possible improvement is to allow advertisers to classify their ads across broad categories (product, service, blog, etc.) and then publishers could exclusively select ads from those categories.

Google’s bias has always been to assume that they are smarter than the publisher at selecting ads, and to rely upon textual and link analysis rather than trusting in metadata. This approach will only carry you so far, as seen in the relevance challenges described here.

Eric Giguere has followed up with some good suggestions on how to improve the effectiveness of the ProductWiki campaign, such as using section targeting to help Adsense identify the relevant sections of the page.

With Google AdSense is taking hints about most relevant sections of a page, it seems a small additional step for it to take hints about the most relevant kinds of ads to display.

While not all content publishers will be able to provide accurate hints, the Shopping.com results show the considerable benefits that result when ad hints are applied effectively, as suggested by the ProductWiki blog.

Either Google AdWords and AdSense will grow as a platform that allows publishers and third parties to contribute domain specific intelligence to matching the right ads to the right content, or alternative contextual networks will establish strong beachheads in different verticals.