That's because Web publishers use the ratings lists and site traffic reports to show advertisers how their sites rank in comparison with the competition, and the numbers are like gold.
To try to resolve this ongoing measurement issue, the Interactive Advertising Bureau and the Advertising Research Foundation are preparing a set of guidelines that ARF President Jim Spaeth hopes will solve the problem, "so there aren't wildly different numbers flying around out there as we have right now." The ARF's new Digital Media Measurement Council met for the first time last week to begin to hammer out the issues.
Media buyers say they want a way to compare data across media, not just on the Internet. That requires both online and offline measurement firms to agree on terms and definitions.
"Metrics have to be in some way comparable," Mr. Spaeth says. "If the measurement system for online media is out of step with other media, there's no way of making comparisons."
At the same time, there may be less impetus to reconcile data from competing Internet measurement firms than there was a few years ago. Media buyers have come to accept that panel-based data aren't perfect.
"I think there's a level of complacency that's crept into the marketplace that tends to treat the under- or over-reporting as similar to what we see in traditional media," says Rich LeFurgy, chairman of the IAB and partner at WaldenVC, a unit of Walden Group. "This is still an area that requires attention."
While audience measurement has taken the focus over the last few years, an emerging need is ad-delivery reconciliation, which would give advertisers exact information on whether their ad was delivered and how the audience interacted with it: Did they click on it, stop the download, hit the back button, or abort the site?
"Advertisers need to know, `What did I get for the money?' " says Robin Webster, CEO of the IAB. "In print, you get a tear sheet. In TV, you get a post-audit. For every media, you know your ad ran. For ours, it's confusing."
To help solve this ad-delivery-measurement problem, the IAB has commissioned a process audit by PricewaterhouseCoopers involving 10 companies from ad servers to Web publishers.
The audit will look at every possible scenario in which ads are delivered, and the parties will then look at the data to come up with voluntary industry guidelines to measure ad delivery.
"That's the basis of everything," Ms. Webster says. "When you get that done, you can bring the advertisers back in, and they will say, `I got what I paid for'."
Results of the audit are expected to be released at the IAB fall meeting Aug. 7.