[grasshoper]

If you let them, they can directly measure the traffic of your website. If you sign up for their service, Quantcast's numbers will be very close to your own Google Analytics.

But, if you don't sign up, they'll still "measure" your traffic and come up with results that are no better than the other web traffic rankers. I don't know why anybody takes sites like these seriously anymore. Everybody I've met who had a website with decent traffic has said that these sites are always way off. I'd imagine that Alexa's main reason for reporting a ridiculous statistic like % of global/country traffic is that it makes it harder for website owners to realize how wrong they are.