Mark has a great post questioning the statistical consistency of various ranking sites. He points to a report by VentureBeat that details findings from Quantcast, Alexa, Google Analytics and Compete on how they track traffic patterns.
It really is detrimental to the growth (and the maturation) of the blogosphere to not have a consistently accurate method for tracking sites. Without a common reference, each tracking service is all over the map and anyone can simply pick the rank they prefer, rather than getting any kind of accurate data.
Via: Mark Evans – Letâ€™s Have Some Statistical Consistency