Unique users or unique visitor numbers are often seen in press reports, internal dashboards and case studies, and are used as the basis for making decisions.

But there is a problem with ‘uniques’; they are not what they seem, and while this may be known by those on the inside, business decision makers are being fed information that is really very misleading.

Exploring the issue

Have a look at the table below from Google Ad planner (January 2011). This table shows the number of unique visitors for Facebook.com in the USA calculated in two different ways, and the difference is startling.

Unique visitors (estimated cookies) is worked out using cookies and then equating one cookie to each ‘separate’ visitor. The unique visitors (users) number is Google’s estimate of the actual number of people who visited the site, and Google does not disclose its calculation method. 

To cross-check the estimate of people we can look at comScore, which uses a panel based methodology to estimate audience, and in January they published figures estimating 153m people visited Facebook.com in the month.

OK, not an exact match by any means, but the comScore estimate is close to the Google estimate.

Why is there such a gap?

What this data shows is that there is an enormous gulf between the two numbers, and what it means is that each person has more than four cookies per month.

The reason for the gap is that the assumed one cookie = one person model is broken, and I don’t think that really surprises anybody these days.

However, it is the size of the fracture which is impressive. Unique visitors (based on cookies) overestimates the real audience by 350%.

Those four cookies come from people using multiple devices, multiple browsers or deleting cookies.  As people start to access the web from mobile devices in ever greater numbers this trend is set to accelerate.  

The key thing to remember is cookies track browsers, and browsers are not people!

Does this matter?

Yes! If you use ‘uniques’ as a measure you can make the wrong decisions. I have had a debate with a senior business leader who was being told that his marketing team had saturated the existing market because the number of ‘uniques’ was almost equal to the known market size.

When the number of uniques was divided by four it told a very different story and the strategy was put under much closer scrutiny.

So what are you using unique visitor numbers for? How many of your tools rely on cookie based tracking? What do your business decision makers think a Unique user is?

I believe it is time to put the unique visitor metric behind us and to rethink the way we measure and analyse online data. What do you think?