comScore released the results of a study analyzing the validity of using cookie-based data to measure the number of unique visitors to individual websites and ad servers.

comScore observed that 31 percent of U.S. Internet users cleared their first-party cookies during the month. This frequent cookie deletion leads to overstatements in audience sizes by a factor as high as 2.5, an overstatement of 150 percent.

Mohanbir Sawhney, a professor at the Kellogg School of Management, Northwestern University said the study clearly demonstrates the limitations of measuring Internet audiences based on cookies.

"To measure audiences more accurately, it is important to link visits to unique individuals, not unique cookies. As privacy programs become more entrenched, cookie-based audience counts will get even more unreliable," he said.