The best way to take web analytics data is with a healthy dose of salt. While analytics software packages have improved over the years, they are still best seen as providing an indicator of patterns over time, rather than representing accurate absolute numbers.
As I currently have access to traffic stats for the same website from three different analytics solutions, I thought I would share them.
|Visits by month|
As you can see, Web Trends and Google Urchin are roughly in line (not surprising, considering they are both work via logfile analysis), while Google Analytics is about five times lower than the others.
Another interesting comparison was on file downloads, as recorded by Urchin and Web Trends. With my limited sample, I found that Google Urchin undershot Web Trends in reported PDF download numbers by a factor of anywhere between 2 and 10. That is to say, Web Trends might report 28,000 downloads, and Urchin would report 2,500. Or Web Trends would say 5,000 and Urchin lists 2,300.