WebProNews

A controversial white paper on the impact of cookie deletion and the effectiveness of the analytics that depend on them has been made public. The comScore study contends that without appropriate adjustments, audience reports for websites could be inflated as much as two and a half times the actual number.

ComScore analyzed the first-party “B cookie” from Yahoo! and the third-party ad server persistent cookie from DoubleClick for its report. Each reaches over 100 million Internet users every month.

They looked for the frequency with which people clear these cookies from their browsers. These cookies would have to be placed on a system again when the cleared browser comes back to a property that uses them.

"This study by no means implies that Yahoo! or DoubleClick publish site-server data without adjusting for the overstatement caused by cookie deletion," comScore said in the study.

ComScore found an average of 2.5 distinct Yahoo cookies appearing per computer. A cookie-based approach to measuring an audience would be overstating those users by 150 percent.

For purposes of the study, comScore excluded computers on its panel if they were set to reject all cookies. Considering how many websites require cookies for people to use their pages, it's hard to believe many people set their PCs to do this routinely.

"However, for the 7 percent of computers on which four or more cookie resets occurred, the average number of cookies observed was 12.5 and these cookies accounted for 35 percent of all cookies observed in the study. In other words, a relatively small segment of PC users engaging in 'serial cookie deletion' has the potential to dramatically inflate cookie-based site server log data," comScore said of the regular cookie cleaners.

It sounds like the analytics industry may have some explaining to do to its clients.