What to make of the numbers? First off, it's quite interesting to see that Comscore measures, on average, a significantly higher number of uniques across all types of sites. Comscore's numbers are three to three and a half times higher, according to Comscore.
Secondly, for sites that are using Google's Adsense network, the undercounting is not as dramatic (that's the second chart.). As Comscore's charts note, there seems to be a "significant bias in Google Ad Planner data" toward "sites that carry more ad impressions from Google."
In short: If you were a media planner using Google Ad Planner, and you were looking for larger sites, you would be led to sites that are running Google AdSense, on average, over sites that do not. Net net: This data indicates that Google Ad Planner pushes ad dollars to Google sites over non-Google sites. This makes sense - Google has data on Google users, after all. So that data might naturally bias toward Google-related sites.
But as like John Battelle said in his coverage: "Such a tool must be neutral and not bias advertisers toward buying on Google properties or those that have Google ads, which of course is going to be a perceived bias in any case. Such is the price of being Very Big."
This data once again raises the question, long asked, of how Google is measuring in the first place. Most believe Google must be leaning heavily on its Toolbar data, and this data does nothing to counter that argument. The strong bias toward Google network sites is suspicious - one can imagine that folks who might install the Google toolbar are clearly already biased toward visiting Google-related sites, for example.
But Google will not acknowledge any use of the Toolbar. Instead it said in its announcement: "Google Ad Planner combines information from a variety of sources, such as aggregated Google search data, opt-in anonymous Google Analytics data, opt-in external consumer panel data, and other third-party market research."
to be continued...
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