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Web Audience Measurement by Cookie Counting Considerably Overestimated
by Jack Loechner, Tuesday, April 24, 2007 7:15 AM
Web Audience Measurement by Cookie Counting Considerably Overestimated comScore recently released a study that analyzes the validity of using cookie-based data to measure
unique visitors and to guage the number of unique users that were served an ad by an ad server. The results indicate 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 A "cookie", a small text file inserted on a user's
computer by a Web server, is often used by web servers to identify and authenticate, track, and maintain specific personal information. First-party cookies are those left on a computer by a Web site
that has been visited, while third-party cookies are those left by a domain different than the site being visited. The Study 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, the study found an average of 12.5 distinct first-party cookies per computer, accounting for 35 percent of all cookies observed in the
analysis.
- An average of 2.5 distinct first-party cookies were observed per computer for the site being examined.
Cookie Deletion Analysis(1st Party Cookies December
2006)Number of Cookie Deletions/ResetsPercent of Computers
Avg. No. of Cookies per Computer Percent of
Cookies Total Sample 100% 2.5 100% 1 or more 31% 4.7 58% 4 or more
7% 12.5 35%
Source: comScore, Inc., April 2007 Dr. Magid Abraham, President and CEO of comScore, concludes that "...with just 7
percent of computers accounting for 35 percent of all cookies, it's clear that a certain segment of Internet users clears its cookies very frequently. These ‘serial resetters' have
the potential to wildly inflate a site's internal unique visitor tally, because just one set of ‘eyeballs' at the site may be counted as 10 or more unique visitors over the course of a
month. The result is a highly inflated estimate of unique visitors for sites that rely on cookies to count their audience." The analysis of third-party cookies revealed an average of 2.6
distinct cookies per computer in December, indicating a similar rate of overstatement as the first-party cookies.
Cookie Deletion Analysis(3rd Party Cookies December 2006)Number of Cookie Deletions/ResetsPercent of ComputersAvg. No. of Cookies per Computer Percent of
Cookies Total Sample 100%
2.6 100% 1 or more 27% 5.5 57% 4 or
more 7% 14.2 38%
Source: comScore, Inc., April, 2007 Dr. Abraham also noted that "There is a
common perception that third-party cookie deletion rates should be significantly higher than first-party cookie deletion rates. Because many PC users reset or delete their cookies using security
protection programs, conventional wisdom dictates that people are more likely to selectively expunge third-party cookies, which are generally deemed more invasive, while maintaining their first-party
cookies. But these findings suggest that selective cookie management is not prevalent... confirmed via a survey, with only 4 percent of Internet users indicating that they delete third-party but not
first-party cookies." Mohanbir Sawhney, McCormick Tribune Professor of Technology at the Kellogg School of Management, Northwestern University, commented "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."
And Bill Cook, PhD, Senior Vice President for Research and Standards at the Advertising Research Foundation, adds "Reach and frequency metrics are the cornerstone of any media
plan, and given the size of the discrepancies that can occur when counting cookies instead of people, the study underscores the importance of panel-based measurement. For the advertising
community, an accurate understanding of reach and frequency within a given target audience is vital." For more information, please
visit comScore here.