Nielsen admits to under-counting unique visitors by 5%

Nielsen has admitted to more discrepancies with is online measurement system – this time it has under-counted unique visitors to the top 1,000 sites on the web by an average of 5%.

In a note to clients today, the company said social gaming sites have been most affected by the problem. This news follows Nielsen’s first letter to clients less than two weeks ago, explaining that the system had failed to recognise long internet addresses and as such has been underestimating time spent online.

It is thought the issues with Nielsen’s measurement system have impacted data over the last three months, although Ad Age believes Nielsen knew about the problem far earlier.

However, on a more positive note, Nielsen claims to have successfully tested a fix for the initial problem, which will be implemented for the company’s November data.  The research company also hopes the problems are confined to the US market on the whole, although this has not been confirmed.

As a result of the issue with Nielsen’s data, UKOM was forced to postpone the planned release of its October data.