comScore found that deleting cookies may be (artificially) pumping up web traffic, PhysOrg reports:
A growing number of Internet users have taken to erasing their computer's cookies, which are unique identifiers inserted on a user's computer that reveal what Web sites someone has visited. While this might not seem like a catastrophic event, the deletion of these cookies could lead to inflated traffic reports for a particular Web site, according to the report.
Each time a user visits a Web site for the first time that Web site deposits a virtual checkmark, or cookie, onto the user's computer. That cookie prevents the Web site from cataloging repeat visits from the same user, thereby creating a more accurate count of new visitors to a particular online venue. If a user's cookies have been erased, however, that person's computer is registered as a new user when they visit a Web site, even if they have been there hundreds of times before.
ComScore evaluated a first-party Web site and a third party ad server that each receives more than 100 million hits each month.
Researchers found that 31 percent of U.S. Internet users erased their first-party cookies over the course of the month. As a result, Web sites could be inflating their web traffic by as much as 150 percent, according to comScore.