According to a new report from comScore, about 31 percent of U.S. Internet users cleared their cookies during the month. This figure, if correct, could lead to site visitor counts to be off by as much as 150%.

While comScore probably can’t be considered an unbiased source - they offer a browser-based approach to tracking user behavior - it’s clear that random cookies are becoming less and less useful as a tool for identifying unique users.

“There is a common perception that third-party cookie deletion rates should be significantly higher than first-party cookie deletion rates,” said comScore CEO Dr. Magid Abraham.  “These findings suggest that selective cookie management is not prevalent, a fact that comScore confirmed via a survey, with only 4 percent of Internet users indicating that they delete third-party but not first-party cookies.”

Some will use comScore’s numbers as a reason to slam Web analytics, or as justification for arguring that Web ad prices are inflated. comScore’s numbers aren’t entirely surprising, though, and serve as a reminder to make sure you’re measuring the right things. An example of this is Google’s AdSense, which tracks views, but charges you based on click-through actions.

Tags: statistics, trends