At first, Digg Chief Executive Jay Adelson thought he just couldn't count. By Digg's estimate, 15 million individuals were visiting the news bookmarking site each month. But advertisers said the site wasn't nearly that popular. Other yardsticks, like the one used by comScore, put the numbers between 1 million and 2 million a month. Adelson figured he was misreading his company's server logs. "We thought, 'O.K., maybe it's us,'" says Adelson.

So Digg hired Web analytics firm WebSideStory to audit the traffic. The firm installed unseen tracking tags on Digg's pages that yielded results matching Digg's own. Still, some advertisers were - and remain - unconvinced and point to lower figures as justification for paying lower ad rates.

Such travails of tracking Web traffic are neither new nor unique to Digg. But lately, they've become so pronounced that the Interactive Advertising Bureau, an association that counts hundreds of Web publishers among its members, has asked two of the most prominent Web measurement firms to submit to an audit by the Media Rating Council, a media ratings watchdog.

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