Compete & Alexa heavily rely on browser toolbars (http://www.compete.com/help/s11 http://www.compete.com/help/s11) to estimate their data. They work hard to nor

Compete & Alexa heavily rely on browser toolbars (http://www.compete.com/help/s11 http://www.compete.com/help/s11) to estimate their data. They work hard to normalize their data, but you can probably imagine that the demographic of people with that kind of toolbar installed is heavily skewed (and some people argue loudly against it: http://venturebeat.com/2007/02/22/traffic-measuring-continued-why-compete-doesnt-work-and-why-quantcast-does/ http://venturebeat.com/2007/02/22/traffic-measuring-continue...) Other sites, like Comscore, use various methods like router-level analysis & client-side monitoring software (like Nielson w/ TVs). In the end, no method is 100% accurate and the best analysis comes from looking at various sources & taking it all with a grain of salt.