How to accurately gauge the amount of visitors a website receives

How accurate is Statbrain? Statbrain estimates the number of visits that a website has based on offsite factors like backlinks, Alexa Rank etc. Statbrain does not have access to log files or any counter information. The number of visits that Statbrain estimates gives you an idea of the number of visits that a website has, but not the exact visitor number.
The number that Statbrain estimates are visits, is also known as visitor sessions and indicates the number of visits that a website has. The number is not the same as unique visitors which usually is a bit lower. Visits should not be mixed up with hits or page views which are entirely different numbers.

Link: http://www.statbrain.com/