My friend Ian IM’d me last week and said he had confirmed comScore’s data on first-party cookie deletion. Since Ian is easily one of the sharpest people I know, I was immediately intrigued, given that he has been working on a methodology to restore deleted cookies using the browser’s cache and a dynamically generated script. Unfortunately Ian hasn’t been able to implement his strategy on a high-volume site, yet, but he did do a very robust comparison of measured site traffic data to comScore numbers.

What Ian saw by comparing the number of measured unique visitors based on accepted cookies to comScore data for the same site was, well, roughly a 2.5X inflation from panel to measured visitor counts. His monthly numbers ranged between 1.99X and 3.15X but he reports the average as 2.47X.

Ian also reported data for daily unique visitors where he saw an average inflation of 1.96X (range of 1.35X to 2.84X). Ian commented, and I agree, that the daily numbers are somewhat disconcerting given that they appear to support the notion that “serial deleters” are among the most engaged.

Keep in mind, these numbers are based on a direct comparison to the comScore panel-based numbers, numbers whose accuracy has long been questioned and continues to be questioned today.

As usual, Ian’s writing continues to be well thought out and well written, and I highly recommend reading him if you’re not already.  I also want to congratulate Ian on joining the team at WebSideStory/Visual Sciences.  The blogosphere loses a great practitioner but gains a great vendor/consultant (and to be fair, Ian has been a private consultant for as long as I have known him.)