The IAB and the MRC are working on a document with the working title, IAB Audience Reach Measurement Guidelines. The arduous work of crafting and wordsmithing is going on under the auspices of the IAB Audience Measurement Work Group, and the leadership of MRC Executive Director George Ivie (he’s the guy putting pen to paper).
This document, once ratified, will help to sort out some of the confusion in the online metrics space over panel-centric versus site-centric “Unique” estimates. The document essentially provides three sets of reach guidelines: one for user-centric measurement (such as is provided by Comscore); one for site-centric server data; and one for third-party ad-server networks. The Reach Definition Guidelines will clarify that all Uniques are not created equal. Specifically, the guidelines make clear that “Unique Cookies” (the Uniques counted by site-centric web analytics providers) are not the same thing as “Unique Visitors” (the Uniques counted by Comscore.)
I don’t want to go into too much detail, because the document is in draft form and subject to change; because it is not yet official; and because quite frankly it is not my place to “scoop” the IAB. But this passage from the most current draft I’ve seen proves instructive:
“Unique Cookies (unduplicated Cookied Browsers) do not generally represent users or people accessing Internet content or advertising due to several complexities surrounding use of browsers.”