[How do I measure web traffic of other people's website?]

I'm doing market research on several sites to determine the feasibility of a new business I'm working on. Since my project will be web-based, I need to gain good information on the amount and kind of traffic that goes to some other websites.

 

The question is, what's the best way to do this? I have tried Quantcast, Alexa and a slew of other sites but they each seem to give differing results, which in turn makes me doubt the validity of the information?

 

....................thanks

 

 

 

EDIT: again advertisement links and you will receive a ban