Press Release

comScore Publishes White Paper on the Impact of Cookie Deletion on the Accuracy of Website Audience Measurement

Study Confirms that Cookie Deletion can Cause Large Overstatement in Site-Server Estimates of the Size of Online Audiences

RESTON, VA, June 6, 2007 – comScore, a leader in measuring the digital world, today announced that it is making available its white paper, “The Impact of Cookie Deletion on the Accuracy of Site-Server and Ad-Server Metrics: An Empirical comScore Study.” The study addresses the key sources of discrepancy between server-based and panel-based data and reveals that cookie deletion can lead to large overstatements in servers’ measurement of the size of online audiences. Without appropriate adjustments, site server audience reports can be inflated up to 2.5 times the actual number of unique visitors.

Topics covered in the white paper include:

  • Thorough discussion of the study’s methodology.
  • Quantification of the degree to which cookie deletion leads to an overstatement in site-servers’ estimates of the number of unique site visitors.
  • Comparison of first-party vs. third-party cookie deletion rates and the resulting overstatement in servers’ estimates of the size of site audiences and the reach of online advertising campaigns.
  • Examination of consumers’ awareness of, and attitudes towards, cookies.
  • Analysis of other factors leading to potentially inflated audience reports from site server data such as failure to remove international traffic from calculation of the domestic audience and the impact of cookie blocking.

If you would like to download a copy of the white paper, please visit:

About comScore

comScore, Inc. is a global leader in measuring the digital world. This capability is based on a massive, global cross-section of more than 2 million consumers who have given comScore permission to confidentially capture their browsing and transaction behavior, including online and offline purchasing. comScore panelists also participate in survey research that captures and integrates their attitudes and intentions. Through its proprietary technology, comScore measures what matters across a broad spectrum of behavior and attitudes. comScore consultants apply this deep knowledge of customers and competitors to help clients design powerful marketing strategies and tactics that deliver superior ROI. comScore services are used by more than 700 clients, including global leaders such as AOL, Microsoft, Yahoo!, Verizon, Best Buy, The Newspaper Association of America, Tribune Interactive, ESPN, Fox Sports, Nestlé, MBNA, Starcom USA, Universal McCann, the United States Postal Service, Merck and Expedia. For more information, please visit

Andrew Lipsman
comScore, Inc.
+1 312 775 6510