THE IMPACT OF COOKIE DELETION ON THE ACCURACY OF SITE-SERVER AND AD-SERVER METRICS A STUDY FROM COMSCORE

Did anybody ese read the above report from comScore " The Impact of Cookie Deletion on The Accuracy of Site-Server And Ad-Server Metrics: An Empirical Comscore Study By Dr. Magid Abraham And Cameron Meierhoefer" who analyzed the first-party “B cookie” from Yahoo! and the thirdparty ad server persistent cookie from DoubleClick, The study examined the degree to which Internet users clear these cookies from their computers, thereby causing site-servers to deposit new cookies and potentially leading to overstated estimates of unique users in cookie-based site-server data.

The results of the study seemed to indicate that approximately 31 percent of U.S. computer users clear their first-party cookies in a month (or have them cleared by automated software), with an average of 4.7 different cookies being observed for the same site within this user segment. Using the comScore U.S. home sample as a base, an average of 2.5 distinct cookies were observed per computer for Yahoo! This finding indicates that, because of cookie deletion, a server-centric measurement system which uses cookies to measure the size of a site’s visitor base will typically overstate the true number of unique visitors by a factor of up to 2.5 times.

In a departure from conventional wisdom this study found that 3rd party cookies were not deleted more frequently than 1st party cookies. This is particularly interesting as in my work I observe that Doubleclick consistently reports Unique Visitors to be approx 20% higher than JDC web analytics solutions such as Coremetrics and Google Analytics. I believed this trend to be caused by greater deletion of 3rd party cookies - may be not?