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Cookie Deletion; Audience Measurement

Posted on 6.06.2007
comScore made available an important white paper today titled “The Impact of Cookie Deletion on the Accuracy of Site-Server and Ad-Server Metrics: An Empirical comScore Study.”  Sure, it may sound drier than dirt but it's a key piece of research that addresses the 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.  

From the official comscore statement: "Without appropriate adjustments, site server audience reports can be inflated up to 2.5 times the actual number of unique visitors."

And from the comscore summary: "This paper presents the results of a study of the rate at which cookies are deleted by computer users and examines the impact that cookie deletion has on the accuracy of site-server data for estimating the size of Web site audiences or for measuring the reach and frequency of online advertising campaigns."

Approximately 31 percent of U.S. computer users clear their first-party cookies in a month (or have them purged by automated software), with an average of 4.7 different cookies being observed for the same site within this user segment. Prior independent studies conducted by Belden Associates in 2004, by JupiterResearch in 2005 and by Nielsen/NetRatings in 2005 also concluded that cookies are deleted by at least 30 percent of Internet users in a month.

The relative accuracy of site-server data versus panel data for measuring Web site audiences has been hotly debated for some time, with server-based estimates of unique visitors often being quoted as higher than those provided by panel data. The results of the comScore study show that, without significant and careful adjustments for cookie deletion, site-server data can grossly overstate the number of unique visitors to a site and, as such, are simply not a reasonable surrogate for measuring the true number of people that visit a site.

Request the Cookie Deletion White Paper from Comscore.

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