Go to the Statistics > Traffic Stats section of your Control Panel to view detailed visitor statistics for each of your domains and subdomains.

The statistics for the www and non-www version of your site are gathered “under one roof” and are stored in the ‘non-www’ hostname.

This allows for the server to process data faster than splitting it in two.

In case you need an alternative, there are public services such as Google Analytics. More information could be found at:

http://www.google.com/analytics/

Of course, you need to have associated accounts with the respective vendor systems in order to gain access to these services.

Another alternative is Piwik. Installing it is easy and does not require much effort: http://piwik.org/docs/installation/

You can check the vendor FAQ as well: http://piwik.org/docs/installation/