An Internet advertising group has taken two Web ratings providers to task over their methodologies in an open letter, saying both companies have refused attempts to audit their data to explain inaccuracies in measurement.

Both the Interactive Advertising Bureau, which represents over 330 online advertisers (and is the author of the letter) and the Media Rating Council have been asking for these audits of Nielsen/NetRatings and comScore Media Metrix since 1999. However, both have so far refused to comply.

The IAB claims a significant number of its clients report their own server log data that is at odds with either company's estimates, and even each other. The group suspects this may have something to do with using outdated methodologies.

For example, either company could be using the "panel" method, a way of estimating ratings by using a small group of media consumers that was first developed in the 1930s and still is used today. However, that method has been found to be inaccurate.

"To persist in using panels that potentially undercount or ignore the diverse populations that are the future of consumer marketing is to deny marketers the insights they need to build their businesses," IAB President and CEO Randall Rothenberg wrote in the letter to both companies.

The Media Rating Council is a government organization that was created in the 1960s to oversee and ensure fairness in the ratings system. The IAB has worked with that group to develop a set of standards to govern the counting of ad impressions in new media advertising.

comScore said it will welcome a third-party to come in and investigate its methodologies, and argued that server logs often over-report due to consumers deleting cookies that are used to track site hits. Nielsen was not responding to requests for comment from the press.