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From - comscore.com/Insights/Press_Releases/2009/5/comScore_Announced_Media_Metrix_360

"The new approach combines person-level measurement from comScore's proprietary 2 million person global panel with Web site server metrics in order to account for 100 percent of a Web site's audience."

It's possible that Yahoo and Google are providing server metrics via javascript tagging to comScore. That would give them direct access to the traffic data. I believe that Quantcast and maybe Nielsen both offer something similar as well.

[DannyBee]

"It's possible that Yahoo and Google are providing server metrics via javascript tagging to comScore."

Your suggestion is essentially that Yahoo and Google are providing non-public data from which revenue growth/etc can almost be calculated, to outside companies?

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The incentive is ad dollars. Ad companies want third party verification.

[nostrademons]

Google runs all their own advertising. They have no need for third party verification.

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That explains why debaserab2 was able to find b.scorecardresearch for Yahoo, but not Google.