There's only one media-ratings system no one takes seriously: Web site rankings. You never see Harvey Weinstein on Entertainment Tonight telling viewers not to believe the latest box-office tallies. But confront any Internet mogul with the numbers published by the two big Web ratings houses, Nielsen/NetRatings and comScore Media Metrix, and you're in for an earful about how they're wrong, wrong, wrong.

Web publishers are so confident about this because they have their own numbers. A site's internal log files track every single page served, and to where. When publishers compare these internal counts against third-party ratings, they usually conclude that Nielsen and comScore's official rankings are too low. If nothing else, the two companies are often way out of sync. My fellow Slate scribe Adam L. Penenberg pondered this "fuzzy math" last year. Nielsen measured Wired News' audience at 1.096 million, while comScore reported an equally precise-sounding 1.87 million. Which was right, Penenberg wondered—1 million, 2 million, or neither?

It's obvious why nobody trusts site rankings—the "official" numbers from Nielsen and comScore are often so far apart that you couldn't trust them even if you wanted to. But site owners have a real stake in these numbers. Advertisers use them to decide where to spend their budgets, and the inconsistent counts make buyers wary. So, if Web moguls think the published statistics are inaccurate, why don't they just release their own numbers? They will—if you're a prospective advertiser on the phone. Otherwise, they prefer to keep house stats under wraps because they see them as competitive info.

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