Unique Visitors Come In Two Shapes And Sizes
Unique visitors come in two shapes and sizes -- the Web Analytics Association (WAA) version and the Interactive Advertising Bureau (IAB) version. For more than 10 years, the term unique visitors has been known as the count of unique cookies (de-duplicated over the period of analysis) to a Web site. As most sites do not require authentication or login, this unit of measurement (although not foolproof by any means) has been the de facto standard that most Web analytics vendors have used when configuring the unique visitors metric within their software.
In December 2006, the WAA published a standard for unique visitors that aimed to establish the definition of this metric around the use of counting unique cookies, with the option to use authenticated users if available. The goal of the standard was to educate the Web analyst to the most commonly used definition and to encourage vendors to openly document any variances from the standard, given that data collection and processing techniques may vary from vendor to vendor.
On February 23, 2009 the IAB published a set of Audience Reach Measurement Guidelines that specifically address the counting of unique users (also referred to as unique visitors in the document) as reported in panel-based measurement tools such as comScore and Nielsen, but they go on to say that census-based measurement tools should comply as well. The guidelines require that the measurement organization "must utilize in its identification and attribution processes underlying data that is, at least in a reasonable proportion, attributed directly to a person." Now keep in mind that the IAB's focus is to provide standards that are specifically applicable to the buying and selling of advertising, but its definition leaves one to wonder how census measurement tools (aka Web analytics vendors) will ever comply with this standard. Will they start to collect personally identifiable information? Will their upcoming product pipelines include the development of "algorithms and other data adjustment procedures" to meet the guideline? Could this guideline ever even be applicable to them -- or will it only ever apply to the audience measurement firms?
Bravo to the IAB for forcing the issue with audience measurement companies to standardize the way that they report uniques, but from a Web analyst's perspective -- and as a member of the WAA Standards committee -- I wish they would have not allowed the term "unique visitors" to be redefined in such a way as to allow for multiple definitions in the space. Web analysts and media planners today have a hard enough time trying to figure out which data source to use and which standard to apply when performing their job -- but that issue is now compounded even more by multiple definitions of unique visitors. In defense of the IAB, its membership is comprised of some heavy-hitter companies who are not about to change that "tab" in their reporting UI that says "Unique Visitors" on it. But in defense of WAA individual and company members, which include vendors such as Omniture and WebTrends (who were both listed as "Project Participants" on the IAB document, interestingly enough), neither are we. The term will live on in both places.
So where do we go from here? It is critical for those of you who use both Web analytics data AND use audience measurement data to know when to apply each standard.
Advertising: When buying or selling advertising and reporting the unique visitors number, you must use the IAB standard as your definition. This is most applicable when referring to your audience measurement data that is derived from panels that consist of identifiable people.
Web Site Analysis: When analyzing your Web site for any other purpose, the WAA standard is sufficient and is most commonly what you will find as the technical calculation in your Web analytics tool.
For those of you wondering, "Aren't you guys working together on standards?" Yes, we are talking, but we aren't approving each other's standards, only reviewing and raising the issues to each other and to our memberships when we agree or "agree to disagree." Our organizations differ greatly in composition and purpose and we aim to increase education and communication in the space as best we can, one metric at a time.