While Slate's stats come straight from the server logs, Nielsen and comScore employ panelists who get modest compensation to install tracking software on their computers. The companies use different formulas to extrapolate total traffic from their panel measurements.
There are problems with both methodologies. Notice that Slate counts unique browsers instead of unique visitors. It's easy to count browsers by tagging each one with a cookie. But if the same person uses one browser at home and another at work, that will cause an overcount. You get an undercount if the same browser is used by more than one person. You can only guess roughly how many surfers have deleted or disabled their cookies, which can scramble the count both ways. And some unknown number of browsers aren't people, but robots crawling sites to index them for search engines or price-comparison sites. (Google identifies itself, but some content-scrapers deliberately camouflage themselves to avoid being blocked.)
Altogether these factors usually add up to massive overcounting—in this case, 8 million browsers instead of 4 million unique visitors. Web publishers swap notes with each other and apply their own math to adjust for multibrowser users and cookie haters, but the result is an approximation rather than a straight count. Slate subscribes to NetRatings' advanced client reports for a second opinion.
The ratings companies, for their part, can't accurately measure people surfing from universities, government agencies, and large corporations. That's because many such places prohibit workers from installing the tracking software. Academics, government workers, and corporate professionals—that sounds like a good chunk of Slate's core readership. This undercounting makes for a good argument that Nielsen and comScore are undercounting sites like Slate. (Nielsen tries to even out the work-surfing numbers by conducting random-dial phone surveys, while comScore solicits a larger panel—120,000 to Nielsen's 28,000.)
The two agencies agree on the number of Slate's December visitors, but the results weren't so cozy for the top news sites. (To compare the Nielsen and comScore ratings for the Net's top five news sites, click here.)
As for page views, it's common to hear publishers say—off the record, of course— that they have twice the traffic estimated by Nielsen. CNN and Fox execs recently made such claims to the New York Observer. In this case, you should believe the publishers. Rather than extrapolating from a sample, they log every single page request made to their servers and subtract the ones they don't think are valid (like robots or their own staff). They might adjust the count up for subscribers who read by e-mail, and adjust down for slide shows that rack up a dozen page-views for one article. They use the results to decide what topics are hot, which writers bring in a lot of readers, and whether slide shows are worth the effort. It's in the publishers' interest to get their page-view counts right, and they've got the server logs to do it.
If you're not an ad buyer, you probably only care how many readers a site has. That requires a unique-visitor count, where it's in the publisher's interest to exaggerate their reach to sway potential advertisers. Nielsen and comScore don't offer NASA-grade precision, but they're the most studiously calculated numbers available and the most consistent means to compare one site against another. How do you reconcile the factor-of-two difference between their Wired News counts? You accept the awkward truth: We aren't sure of the right answer, but we're closing in on it.
Penenberg called this "flying blind." I take the opposite view: It's transparency, one of the Net's basic strengths. Conflicting traffic reports confuse ad buyers, but they also give them more clues about where their money is really going. Mismatched ratings drive Web publishers crazy, but they can (and do) call Nielsen and comScore on the carpet to reconcile their methods against internal counts. I'm sure Harvey Weinstein can put the screws to the box-office people when he doesn't believe them, but he can't hit them with 25 terabytes of server logs.
The more I dig into how Web ratings work, the more I realize people in other media are in denial. Internet publishing is the most finely measurable medium ever invented; broadcast, movie, and print companies have no way of monitoring individual transactions from their end. Yet, while the Web guys admit they could be off by half, Nielsen claims its television ratings have a margin of error of 4 percent. If I were in the cast of Arrested Development, I'd demand a recount.