• by Joe Mandese  @mp_joemandese, November 4, 2010

Nielsen Co. Thursday began informing clients that it has been erroneously reporting declines in Internet usage in its online media reports due to various problems with its system, including an inability to recognize Web sites that use "very long" URLs.

Nielsen said the glitch has definitely impacted declines reported by Nielsen's NetView service, but said it has not yet determined the "full effect" on its VideoCensus, MegaView Retail, MegaView Search, AdRelevance, and WebRF services, as well as various custom research reports.

"We currently believe these products are affected to a lesser degree," Nielsen said in a statement sent to a client this afternoon. "Our investigation has identified the major root cause of the decline in the U.S. and several contributing factors," Nielsen disclosed.

"The primary cause is an increasing incidence of websites using very long URLs that are not properly recognized by our systems. When our system attempts to process session data including the very long URLs, which are more than 2,000 bytes in size each, it intermittently does not recognize the session, causing an estimated average 22% decline in time spent year-over-year, which can vary at the domain level."

Nielsen said the erroneous estimates would be corrected in the data it reports for December, which will be delivered in January 2011. Nielsen said it is currently investigating whether similar problems have occurred in the Internet usage estimates for other countries. It also disclosed that it has uncovered "other contributing factors" that have resulted in smaller declines, and that it is working to resolve those problems, which were not disclosed.

"This investigation, covering every element of our Internet measurement methodology, including the panel, collection capabilities, and processes, and nature of the root cause indicates that we need to do a better job keeping pace with the rapid evolution of the Internet," Nielsen said, adding: "We are putting new processes in place to add greater rigor to the continuous testing of our methodology and expanding our monitoring for anomalous events. We have also engaged the Media Rating Council (MRC) to review our findings and the additional processes."

A Nielsen spokesman said the problems have no effect on its TV measurement reports, including its "Extended Screen" system, which will begin reporting Internet usage in its TV ratings sample next year.