Cookie deletion overstates audience size: study

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Frequent cookie deletion by three out of 10 U.S. Internet users leads to overstatements in audience sizes by as much as 150 percent, according to a study by comScore Inc.

Reston, VA-based comScore said the study analyzed 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 who were served an ad by an ad server.

"While past studies from other research companies have shown a similar proportion of computers that clear their cookies, the comScore study is the first to highlight the disproportionately high percentage of cookies represented by those computers," said Magid Abraham, president/CEO of comScore, in a statement.

For example, he said that with just 7 percent of computers accounting for 35 percent of all cookies, it is clear that a certain segment of Internet users clears its cookies 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," he said. "The result is a highly inflated estimate of unique visitors for sites that rely on cookies to count their audience."

The study, based on an analysis of 400,000 home PCs included in comScore's U.S. sample during December 2006, examined both first- and third-party cookies.

For the purposes of this study, comScore analyzed the cookies from one prominent Web property and one third-party ad server, each representative of the total U.S. Internet audience and each reaching more than 100 million Internet users monthly. The study examined the degree to which users cleared cookies from their computers and thereby caused servers to deposit new cookies and potentially lead to overstated estimates of unique users when relying on cookie-based server data.

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 four 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 suggests that Web site server logs that count unique cookies to measure unique visitors are likely exaggerating the size of the site's audience by a factor as high as 2.5, or an overstatement of 150 percent.

ComScore's analysis of third-party cookies also revealed an average of 2.6 distinct cookies per computer in December, indicating a similar rate of overstatement as the first-party cookies. For those computers where at least one cookie reset occurred, the number of third-party cookies observed was slightly higher than first-party cookies at 5.5.

"There is a common perception that third-party cookie deletion rates should be significantly higher than first-party cookie deletion rates," Mr. Abraham said. "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, a fact that comScore confirmed via a survey, with only 4 percent of Internet users indicating that they delete third-party but not first-party cookies."

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