There are several methods for measuring net audiences which provide critical data for advertisers.

Cookies - small text files inserted on a user's computers by a web server and unique to that computer's browser - can be used for authenticating, tracking and maintaining information on users.

In comScore's study, an analysis of 400,000 home PCs in the US found that a hardcore minority of web users are clearing their cookies from their computers on a regular basis.

This causes servers to deposit new cookies which in turn could lead to an over-estimate of unique users to a particular website.

It found that 7% of computers accounted for 35% of all cookies, which extrapolated could mean the size of a site's audience is being overstated by as much as 150%, said comScore.

"It is clear that a certain segment of internet users clears its cookies very frequently. These 'serial resetters' have the potential to wildly inflate a site's internal unique visitor tally, because just one set of 'eyeballs' at the site may be counted as 10 or more unique visitors over the course of a month," explained comScore president Dr Magid Abraham.

Get the full story at BBC News