Press Release

comScore Submits Proposal to ABCe in U.K. for a new Person-Centric Measure for Web Site Server Measurement

New Approach Will Integrate comScore Panel and Site Server Data to Adjust for Potential Overstatement of Audience Size Caused by Cookie Deletion

LONDON, U.K., October 7th, 2009 – comScore, Inc. (NASDAQ: SCOR), a leader in measuring the digital world, today published an open white paper entitled “Proposal for a New Person-Centric Measure for Web Site Server Measurement in the U.K.” This proposal is based upon original comScore research which shows that by calculating cookie deletion rates from an online panel and applying the appropriate adjustments to Web-site server data, accurate user (i.e. people-based) metrics can be derived.

The results represent a major breakthrough in the reconciliation of panel-based and Web site server data to create a unified set of numbers for online audience measurement. This open white paper has been submitted to the Audit Bureau of Circulation’s electronic arm (ABCe) for consideration by its Internet Technical Group and by the Joint Industry Committee for Web Standards (JICWEBS) to help inform the current debate concerning the development of new industry-agreed audited metrics for the measurement of online audiences. comScore will also submit a closed white paper on the methodology used to create these measures using comScore’s Media Metrix 360 solution.

Overstatement and Resolution
comScore’s study on cookie deletion rates revealed that 23.5 percent of U.K. Internet users cleared their Web site (“first party”) cookies in a month (or had them cleared by automated software) with an average of 4.2 different cookies being observed for the same site within this user segment during the course of the month. For users who delete their cookies at least four times in a month there was an average of 12.8 unique cookies for the same site, indicating that these heavy cookie deleters can have an extremely high inflationary impact on server-side unique visitor counts.

Using the comScore U.K. panel, an average of 2.4 cookies per site appeared across all PCs during the course of a month, indicating that relying upon Web-site server data alone is likely to be exaggerating the size of a site’s audience by a factor as high as 2.4x, or an overstatement of 140 percent. ABCe delivers both monthly and daily data on Unique User/Browsers (UU/Bs) for all certified properties (based on the JICWEBS industry-agreed definitions).

These definitions expressly state that these are about devices measured, not people. Daily UU/B data go a long way to mitigating the cookie deletion issue in the context of site centric measurement for devices. However, by identifying the cookie deletion issue and integrating panel-centric data with site server files, comScore is able to resolve the cookie overstatement issue and arrive at a person-based view of the online audience, as required for media planning purposes.

comScore Cookie Deletion Analysis – 1st Party Cookies*
March 2009
Total U.K. – Home Locations
Source: comScore, Europe
Number of Cookie Deletions/Resets Percent of Computers Average Number of Cookies per Computer for Same Web Site Percent of Total Cookies
Total Sample 100.0% 2.4 100.0%
1 or more 23.5% 4.2 47.1%
4 or more 4.0% 12.8 25.8%

*Excludes Log-in cookies

“The limitations of using Web site server data in isolation have long been acknowledged by the industry,” said Mike Read, SVP and Managing Director, comScore Europe. “Counting cookies provides useful information, but without correction for cookie deletion this methodology can lead to potential overstatement in audience counts and as a consequence has been traditionally unable to arrive at an accurate person-level solution. In conjunction with the Audit Bureau of Circulation, we propose to resolve the issue of panel versus site server data once and for all, and arrive at an industry agreed-upon standard for the measurement of unique visitors.”

Available for Download
The comScore White paper, “Proposal for a New Person-Centric Measure for Web Site Server Measurement in the U.K.” can be downloaded for free via the following link:

Note to Editors:
comScore is submitting this proposal as an Associate Subscriber of ABCe for consideration through the standard industry channels of the ITG and JICWEBS. For the avoidance of doubt, comScore and ABCe are not “in partnership.”

About comScore
comScore, Inc. (NASDAQ: SCOR) is a global leader in measuring the digital world and preferred source of digital marketing intelligence. For more information, please visit

Ayaan Mohamud
comScore, Inc.
+ 44 (0) 207-099-1775

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