Firms' Web traffic data don't line up

In the chaotic world of modern marketing, the Internet is touted as the best medium for determining the effectiveness of ads.

So why is there so much controversy over a measurement as simple as how many people visit a website?

The independent firms that measure online traffic have been accused of undercounting minority Web surfers, overestimating visitors by more than double and ignoring Web users at work.

In an example of the sharp disparity that sometimes arises, in September three outside groups weighed in on how many visitors the San Francisco Chronicle's website, sfgate.com, received. ComScore Networks said 2.4 million, Nielsen/NetRatings counted 3.7 million and the Audit Bureau of Circulations came up with 7.6 million.

The dispute isn't just academic. Knowing the size of Internet audiences more precisely is important as advertisers try to decide where to allocate their money. They spent $16.9 billion in online ads in 2006, up 35% from the year before.

"Measurability is going to become more important," said Dave Martin, director of interactive media for Ignited Minds, an ad agency in Marina del Rey. "It would be great not to have to try and guess what traffic levels are."

That's why the two biggest firms measuring Web traffic, Nielsen/NetRatings and ComScore, have agreed to independent auditing of their numbers.

Last week, more than seven years after they first started measuring Web traffic, Nielsen and ComScore said they were on track to finish extensive audits by the Media Ratings Council, which also monitors print, radio and TV, within about a year.

Nearly 70% of advertisers said they would prefer to advertise on a site whose numbers had been audited by a third party, said Neal Lulofs, a spokesman with the Audit Bureau of Circulations. About 84% of advertisers said having this verification would become increasingly important.

Right now, there are hundreds of companies out there that count site visitors. Problem is, their statistics can vary by the millions. Nielsen/Net Ratings and ComScore have received most of the heat. To measure an audience, the two firms use panels that count eyeballs, similar to the way Nielsen measures TV viewers: They select a sample of people, use software to track what they do online and then extrapolate traffic patterns for the entire Internet.

"There are enormous questions about their reliability," said Benjamin Schachter, an analyst with UBS Investment Research.