Unique Visitors

The big problem with counting unique visitors is that it’s impossible to figure
out from server logs who’s unique and who’s a visitor. Figure 2-7 deals with
this problem.
There are caveats aplenty here because you’re counting visits from unique
IP addresses, not actual people:
■■ Any sort of local area network connected by a single Internet gateway
may have several users with the same apparent IP address.
■■ A proxy server owned by an ISP that caches frequently accessed pages
will show up as one unique visitor even though it represents hundreds,
if not thousands, of users. You can put a no-cache directive on your pages,
but it works only if the proxy pays attention to it. And using such a directive
may slow your site for some users.
■■ In the home, it is very common to have more than one person using the
same computer. You may have three different people visiting from one
IP address.
■■ People visit from different places: from home, work, school, or from a
laptop at the coffee shop. What looks like four unique visitors may
actually be only one.
■■ People on dial-up change IP addresses almost every time they log in. If
a person visits every day from a different IP, that person looks like 20 or
30 people, depending on how the ISP assigns IP addresses

There isn’t much you can do about these issues. It’s the nature of the
beast — and log analyzers. Google Analytics is script-based, so it does not
have many of these problems, but it has a series of issues of its own. The bottom
line is that you can’t measure unique visitors with complete accuracy. You
measure unique visitors as well as you can and you make sure to compare
apples to apples. As far as the technology goes, AWStats Unique Visitors is the
number of unique IP addresses that made requests to your web server. It’s the
best measurement a log analyzer can provide of how many people visite

Yearly Summary
AWStats calculates its metrics on a monthly basis. To produce yearly metrics,
it adds the results from all months, with the warning While this strategy doesn’t affect the other metrics, it also doesn’t produce an accurate number of unique visitors. If a particular IP appeared in January, March, and July, it would add three unique visitors rather than just one. It’s not practical to save all the logs and run the analysis on one huge lump every time
the user wants a year-to-date. Suffice it to say that the AWStats unique-visitors
metric is not accurate in the aggregate.