Ever since March 2004, I have been comparing analytics software built by the same company. The first entry I have on this was named Comparing Web Traffic Between Different Web Analytictical Tools which shows the huge gap in traffic when comparing Urchin 3.x to Urchin 5.x. Then the topic came up again in January 2005, with an entry named Web Analysis Tools & Consistancy. Which led me to an entry in late May of 2005 where I said, Web Analytics Needs Standards Bad.
For a visual reference check out this chart, you can click on it to enlarge. The red is Urchin and the blue is Google.
I started a thread on this topic at our forums and Matt, md_doc explains why this may be the case. He says;
Can you believe that it can affect on average, 20% of the visitors of this site? Who are you that are blocking those cookies? Do you even know you are blocking them? :)
Forum discussion at Search Engine Roundtable Forums.December Shows Urchin Has Higher Visitor Counts then Google Analytics
Thu 12/1 21.63% Fri 12/2 28.11% Sat 12/3 12.20% Sun 12/4 12.87% Mon 12/5 12.33% Tue 12/6 24.29% Wed 12/7 25.66% Thu 12/8 24.66% Fri 12/9 19.76% Sat 12/10 19.67% Sun 12/11 5.95% Mon 12/12 24.51% Tue 12/13 22.20% Wed 12/14 25.83% Thu 12/15 24.56% Fri 12/16 23.32% Sat 12/17 20.24% Sun 12/18 10.42% Mon 12/19 16.84% Tue 12/20 20.50% Wed 12/21 18.96% Thu 12/22 21.03% Fri 12/23 29.67% Sat 12/24 18.34% Sun 12/25 13.66% Mon 12/26 17.03% Tue 12/27 19.73% Wed 12/28 20.93% Thu 12/29 18.68% Fri 12/30 24.90% Sat 12/31 14.94%