First let me say that I actually trust comscore data far more than most numbers i see. but I was reading their most recent update in their efforts to more accurately count online viewership and discovered this astounding statement:
One of the earliest – and perhaps most obvious – findings along the way is that we’ve seen ample evidence of the foibles of server-side analytics for measuring the number of unique visitors (i.e. people) who visit a site. Due to many inflationary factors, including cookie deletion and rejection, bot and spider traffic, and site visitation from multiple locations, we’ve found clear and direct evidence that web site servers routinely overstate actual people counts by a factor of two and higher. In particular, the inflationary impact of cookie deletion is consistent with independent research from a variety of other research companies, including Forrester, Belden, Jupiter and Nielsen. The inflation in server data has also now become apparent to the IAB (see their Audience Reach Measurement Guidelines) and academics such as Max Fomitchev, an assistant professor of Computer Science & Engineering at Pennsylvania State University who conducted an exhaustive study and concluded:
“Cookies are about just as inaccurate in estimating unique visitors as unique network addresses. This is the new and unrealized fact in the industry that has a direct impact on Internet advertising as currently reported unique visitor / core audience size numbers tend to overestimate the true audience size by a large factor (7-30, depending on the visitation frequency and the sampling period).”
As we have reached critical mass among websites participating in 360 programs, we have found some astounding statistics. In the month of December alone, we have seen over 1.5 Billion unique cookies just in the US. For instance, we have found 259 million cookies for AOL. That is a reach in excess of 125% compared to a true reach of 54%. These statistics mean that it is imperative to develop sophisticated methods to remove inflation caused by cookie deletion, rejection, multiple browsers, multiple devices and multiple locations of access such as home and work.