Complaining about how comScore (NSDQ: SCOR) undercounts unique visitors compared to their internal numbers is a fairly constant refrain from web publishers. So when Google (NSDQ: GOOG) Ad Planner was released with the promise of better figures (i.e. higher unique visitor counts) than comScore, Federated Media head John Battelle was initially enthused. But after comparing the first set of numbers between the two, Battelle writes on his Searchblog, that Google Ad Planner is hardly the hoped for comScore-killer. While the comparison data Battelle received is from comScore, he says given that comScore’s reputation depends on not juicing the stats, he is inclined to trust that the research is bias-free. Battelle’s review follows others, such as ad agency and web publishers, who also found that Google Ad Planner was, in the words of Forbes.com CEO Jim Spanfeller, “are as bad or worse as anybody else’s out there.”

SEE ALSO: First Reviews Are In: Google AdPlanner Not Likely To Rival ComScore, Nielsen

Taking a sample of 20,163 sites, comScore compared reporting on monthly uniques with Google Ad Planner. Then, comScore looked at data for 5,398 sites that are part of the Google AdSense system. The first chart shows that on average, Google Ad Planner’s unique visitor counts were roughly one-third of comScore’s.  Battelle says the second chart shows that for those sites participating in AdSense, the undercounts are less dramatic. Battelle accepts the view of comScore CEO Magid Abraham, who has said that Google may be using its Ad Planner service to push more sites into AdSense. For example, Google hasn’t provided details of how the use of its Toolbar data might affect the numbers. When it launched Ad Planner last month, Google said the data was based on “aggregated Google search data, opt-in anonymous Google Analytics data, opt-in external consumer panel data, and other third-party market research.” And while Google previously used comScore data for its analytics, comScore says it doesn’t anymore.

Both charts after the jump.