How does Alexa get its traffic statistics?

I read on their site that people who use their alexa toolbar sends data to alexa for their traffic stats. Now, I could not find anywhere else on their site explaining any other means that they use to get their traffic data.

I don't know if this is true or not, but I was wondering if they also get traffic data from other toolbars as well, such as google's and yahoo's???

So that's my question here. Does anybody know where alexa get's their traffic statistics other than the alexa toolbar?

 

A gets stats but not very accurate stats, and hence the service they offer is not very creditable