I read an interesting post (Authenticated User IDs vs. Unique Visitors) on the Web Analytics Forum on Yahoo today, that I would like to comment on – the post goes like this:

“Hey Everyone, I have a challenge I hope you could help me out with. For one of our sites, we pass the Authenticated User-ID, e.g. aberlinger, via sProp1 in Omniture. EVERY (yes every) user must log-in access this site. Check out the stats below for Authenticated ID’s vs. Monthly Unique Visitors for March 2007:

30,710 unique visitors
19,708 authenticated user ID’s

How is this possible?

I realize people can remove cookies, but certainly not 30%! I also understand that “Unique Visitors” and sProp data are different reports. However, you there is nothing more unique than our User-ID’s. Has anyone else out there dealt with this issue regardless of the tool they use? I would appreciate the forums input.Cheers & Happy Holiday, Adam”

The answer to the first question: “have anyone else dealt with this issue?” – would be; I think everybody deal with this, if they try to compare their visitor metrics, but I do not see this as an error as such, merely a missing agreement between web analyst and web analytics vendor on what to expect from the following metrics:

• Visit
• Unique Visitors
• Authenticated Unique Visitors

Doing a similar metric check on an IndexTools client (the data I have available), where I filtered out all other activity than “signed-in”, we get the following for April 1st:

678,316 – Visits on April 1st (100% of all visits)
451,219 – Daily Unique Visitors on April 1st (66% of all visits)
418,764 – Authenticated Unique Visitors on April 1st (61% if all visits)
(Cookie’s Disabled: 1.32%)

This shows that the Authenticated Unique Visitors is 7.2% lower than the Unique Visitors (and have in mind that this percentage is of course higher should we look at a longer period) – let’s have a look at March, which is the period Adam looked at:

19,142,005 – Visits in March (100% of all visits)
12,644,738 – Daily Unique Visitors in March (66% of all visits)
7,860,645 – Monthly Unique Visitors in March (41% of all visits)
6,524,020 – Authenticated Unique Visitors in March (34% of all visits)
(Cookie’s Disabled: 1.19%)

Which shows that 17% of the unique visitors in March were NOT truly unique, if we agree for a second that a forced “sign-in” to the website is as close as we can get to a true unique visitor.

Dealing with the second question: “30,710 unique visitors vs. 19,708 authenticated user ID’s – How is this possible?”

Here Adam has 36% of his unique visitors in March that were NOT truly unique. Looking at the above example (and I am not concluding that this is a viable benchmark, but I think it is a good indication that Adam’s numbers could be “correct”). Then it is the definition of the metrics that are the issue. The direct answer is that, this is possible because:

1. User Reject Cookies
2. User Delete Cookies
3. User login from different computers (Home, Work, Hotels etc.)
4. Multiple Users use the same computer with different sign-in’s
5. …

CONCLUSION
When the Analytics Industry – vendors like me :-) use the term: Unique Visitors, we added a bit of sugar and marketing to it, because Unique Visitors is more accurately described as something like “Unique Computers”. With that said, it becomes unfair to compare unique visitor and authenticated visitor metrics as they simply do not represent the same aggregated total.

And amidst all this uncertainty (about the definition of the Unique Visitor metric) beware the vendor that says they can provide you with a even more accurate unique visitor sum than your current vendor.. because that is utterly nonsense. I wish I could tell you that, by shifting from Omniture to IndexTools I would be able to present you a number twice as accurate, but I cannot, and my metrics are very likely going to be no worse or no better than any of the other’s.

None of the presented metrics are flawed or inaccurate as such – they just represent different visit totals and perhaps it is time we rename the “unique visitor” metric to something more realistic :-)

Personally; I am a bigger fan of the Visit metric. But that’s me.

Have a great Easter!