Avinash Kaushik is going back and getting to the bottom of basic metrics like visitors and uniques in Standard Metrics Revisited: #1: Visitors.  I have suspected the main reason no two metrics packages give you the same numbers (even the same software, different version will usually come up with results that don’t match) is the way vistors, uniques and sessions are handled.  I suppose that visitors and sessions might even overlap.

"Yet it turns out that there is no standardization and often this metric (count of visitors) masquerades under different names. There is often a misunderstanding as to what it exactly is and how it is measured. "

The most basic metric of all, a visitor, is defined somewhat differently depending on who is programming the software and what they believe a visit/visitor show be counted.

"Most commonly prevalent names for visitor metrics are: Visitors, Visits, Total Visitors, Unique Visitors, Sessions, Cookies. (Please add the variations you have heard via comments below.)

It is a disservice to the world that so many names exist for the same metric. When we mention visits and unique visitors depending on the tool it is either the same thing or not, it confuses decision makers and sometimes it means that if we rip out Google Analytics and replace it with CoreMetrics we have to unlearn old definitions for the same thing. Quite sub optimal."

One problem is the vendors, according to Avinash; they’d have to relabel some of their metrics so all the packages are lined up.  The problem there is - there’s no reason for them to do that.  In fact, were StatCounter to do what Avinish suggests, they’d have to say they don’t track unique visitors and they probably don’t want to do that.  Why? most packages that are useful for advertising need to count uniques - if they don’t they will be lacking, that’s all.  Besides, what about all the downstream reports that will no longer be accurate.  

Example: we use StatCounter to total up unique visitors to do a monthly report and then Statcounter changes the label and says …ops! we do not count Uniques after all - we only count sessions.  

I think the idea is good, get all the vendors in on this and make them define what a visit is and how it will be measured so you can take different packages and get very close to the same numbers - but I doubt it will ever happen.