Controversy surrounding unique visitors, a core metric of Web analytics, has raised its head again. In a recent blog post, Web analytics consultant and author Eric Peterson called on the Web analytics industry to stop using the term "unique visitors" because it doesn't accurately reflect what's actually being measured.

Web analytics tools define unique visitors fundamentally different from an audience panel such as comScore and Nielsen/NetRatings. This is highly confusing for many people.

Web analytics systems define a "unique visitor" based on the presence of a cookie. If I visit a site using three different devices in a week (say a PC, a laptop, and a mobile phone), I'll be recorded as three different "unique visitors." If I also regularly delete the cookie, then I can appear to be a new visitor to the site and am therefore not counted as being "unique."

Audience measurement panels define a unique visitor based on the activity of an individual member of the panel. Web analytics tools measure all activity on a Web site (a so-called census-based approach), whereas a panel measures a proportion of the activity on a site (a sampling approach) and then uses that to estimate the total.

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