In the comScore panel, we are able to see cookies passing between Web sites and panelists’ computers; and we are able to see and identify the different persons using those computers at those points in time. We recently conducted a study across some major online ad sales entities to determine how often the identifying cookie is associated with the behavior of a single user. Across 17 major entities — publishers, ad networks, and third-party services — we found an average of 44% of cookies were associated with a single user. Among the 56% of cookies that point to multiple users, if we assume that the wrong user is identified 50% of the time (which strikes as a conservative assumption), then on average, 28% of the time (half of 56%) the cookie involved in direct audience guarantees is pointing to the wrong person. Read more at www.mediapost.com