I've been running both Google Analytics and AWStats on this site since the I started this blog in Drupal. The first full month that the site ran was November 2006. I have engaged in incremental improvements in SEO ranging from tagging, to meta tags, to a site map. I've also been trying to improve my markup to be more search engine friendly. In the two years that I've been tracking, site traffic has increased 1557% according to AWStats and 1272% in Google Analytics. On average, both services show, an increase in traffic of about 16% each month with some dips of up to 25% and increases as much as 92%.

The tracking is in lockstep--increases and decreases appear in both stats systems in the same place showing about the same level of increase or decrease--however, Google Analytics shows about 50% less traffic than AWStats.

Analytics requires pasting a javascript snippet on the site. It is probably fair to say that if a browser doesn't support javascript or if javascript is turned off, Analytics won't be able to log that visitor. AWStats uses the site's log files and filters the results by viewed traffic and not viewed traffic. Not viewed traffic includes bots, spiders, and so forth.

Page views in AWStats, on a Drupal site, I believe, are going to be wildly inaccurate because I understand that AWStats sees each php request as a page view. However, that doesn't explain why the unique visitors would be so very different.

Some users will come to a site multiple times--the cookie that Analytics relies upon is able to ignore the fact that a person may have a different IP address each time that they visit. AWStats is going to see each of those IP addresses as different unique users.

I'm not sure which service is more accurate, but given I tend to like to look at trends rather than specific numbers, using both services seems like a prudent course.