Recent estimates put the number of sites that use Google Analytics within the 30-40% range. Google Analytics is a great system designed for website owners and marketers to help them make informed business decisions. I highly recommend that you use some sort of stats program, whether it be Google Analytics or a competing product, to help you better understand your website visitors and their activity on your site.
With that being said, Google Analytics is based on cookies and javascript so not every visitor to your site will be tracked. From time to time scripts break. It's definitely a good idea to have a backup stats system in place for these reasons.
A2 Hosting customers are in luck because AWStats, a statistics program, is included FREE with every cPanel account. AWStats isn't nearly as robust as Google Analytics, but it is based on the log files created by your web server and is much more accurate than Analytics. AWStats may not include many features of Analytics, but it is a perfect resource that you can call upon to determine if shifts in your Google Analytics data are a result of the script not working properly, or if there are in fact noteworthy traffic changes occurring on your site.
