Publications

Frequent Cookie Deletion by 3 out of 10 U.S. Internet Users Leads to Overstatements in Audience Sizes by a Factor as High as 2.5

Implications for Advertising and Audience Measurement Deemed Significant by ARF and Industry Experts

RESTON, Va. , April 16, 2007 – comScore, a leader in measuring the digital world, today released the results of a study analyzing the validity of using cookie-based data to measure the number of unique visitors to individual Web sites or to gauge the number of unique users that were served an ad by an ad server. The study, based on an analysis of 400,000 home PC’s included in comScore’s U.S. sample during December 2006, examined both first-party and third-party cookies.

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Average Computer Receives 2.5 First-Party Cookies per Site Each Month

comScore observed that 31 percent of U.S. Internet users cleared their first-party cookies during the month. Within this user segment, the study found an average of 4.7 different cookies for the site. Among the 7-percent of computers with at least 4 cookie resets, comScore counted an average of 12.5 distinct first-party cookies per computer, accounting for 35 percent of all cookies observed in the analysis.

Using the total comScore sample as a basis, an average of 2.5 distinct first-party cookies were observed per computer for the site being examined. This indicates that Web site server logs that count unique cookies to measure unique visitors are likely to be exaggerating the size of the site’s audience by a factor as high as 2.5, or an overstatement of 150 percent.

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