How does Google Analytics calculate Absolute Unique Visitors?

Question

I understand the basics of this - there's a cookie, some javascript, and so on.

But Google Analytics appears* to calculate Absolute Unique Visitors over an arbitrary time period. To see this, run the Absolute Unique Visitors report over a period of about a month.

It looks to me like Google Analytics might be getting total Absolute Unique Visitors over an arbitrary time period by just adding up the absolute unique visitors from each day, which is clearly not going to give the right answer.

Other people have pointed out that calculating unique visitors over an arbitrary time period is actually quite a hard problem.

Am I misunderstanding whats going on? Can anyone shed light on this?

(* = In my Google Analytics report, over any given time period, the total unique visitors (from the Absolute Unique Visitors report) is always higher than the total visits for the same period. Which is impossible. Other people report the same observation.)

Answer 1

According to Webopedia:

In Web analytics, including Google Analytics, absolute unique visitor is a "visitor type" report that will count each visitor to your website only once during the date range you have selected. If the visitor was on your site prior to the date range selected this visitor would be sub-classified as a prior visitor. If the visitor did not land on your site prior to date range selected for the report they are classified as a first time visitor.