Nielsen//NetRatings said it is in the process of securing third-party certification of its methodologies, and that it has already completed the Media Rating Council's pre-audit.
"The IAB has reviewed Nielsen//NetRatings' services in the past and we have over the last several years worked closely with the MRC, demonstrating our track record of transparency and cooperation," Nielsen//NetRatings President and CEO William Pulver said in the letter.
As part of its own efforts to achieve transparency, comScore said Monday that it recently opened its methodology to an evaluation by the Advertising Research Foundation, and said it plans to release the results publicly in the near future.
Although the IAB has spent years pushing the two major Web audience measurement services, an audit by the independent Media Rating Council is more likely now because there's a sense of urgency and greater industry support, said Randall Rothenberg, president and CEO of the IAB since January.
Media companies have often complained about major discrepancies between their own log files and panel companies' versions of their traffic.
In addition, the owners of sites serving niche audiences say their traffic can be ignored completely because panels represent a miniscule fraction of the total Web audience and do not accurately reflect all segments.
Added Pulver: "Since our inception in 1997, we have made significant investments to build a broad portfolio of patented panel- and site-centric measurement tools to create comprehensive products that report, among other things, audience estimates, demographic information and consumer behavior. NetRatings is taking a leadership position in integrating data from multiple sources to deliver accurate information on which both advertisers and publishers can rely."