Nielsen Admits to More Under-Counting Problems

By Andrew Gauthier 


Nielsen disclosed today that its online measurement system has been under-counting unique visitors to the top 1,000 sites on the web by an average of 5%, the latest in a series of problems for the company.

In a note to clients today, the company said that social gaming sites were most affected by the latest glitch. The note did not say how long the under-counting had gone on; last week the company said problems had impacted metrics over the past three months, but sources told Ad Age that Nielsen knew about them far earlier. More…