Hello,
In order to fully understand web site traffic analysis relating to unique visitors, it is important to know what it does and does not measure.
Unique Visitors - Everything You Know is Wrong
Jennifer Kyrnin's Web Design / HTML Bloghttp://webdesign.about.com/b/a/064965.htmQUOTE
I've known this for 8 years or more, the way that most log analysis tools collects "unique users" is wrong. But Marketing wants that information so badly, that even if it's wrong, it's still data.
A study was done to compare if signed in (thus verifiably unique) users were similar in number to IP and cookie-based numbers. "The results are staggering: the IP-based approach overestimated unique users by 7.6 times, whilst a cookie-based approach overestimated unique users by 2.3 times."
Note that Jennifer links in her post to an article that you have to create a free account to read.