As noted in a recent blog post, We see a large discrepancy between Google Analytics "visitors" and Quantcast "visitors".
Also, for reasons we have never figured out, Google Analytics just gets larger numbers than Quantcast. Right now GA is showing more visitors (15 million) on stackoverflow.com alone than Quantcast sees on the whole network (14 million):
Why? I don’t know. Either Google Analytics loses cookies sometimes, or Quantcast misses visitors. Counting is an inexact science.
We think this is because Quantcast uses a more conservative ratio of cookies-to-visitors. Whereas Google Analytics might consider every cookie a "visitor", Quantcast will only consider every 1.24 cookies a "visitor". This makes sense to me, as people may access our sites from multiple computers, multiple browsers, etcetera.
I have two closely related questions:
Is there an accepted standard ratio of cookies to visitors? This is obviously an inexact science, but is there any emerging rule of thumb?
Is there any more accurate way to count "visitors" to a website other than relying on browser cookies? Or is this just always going to be kind of a best-effort estimation crapshoot no matter how you measure it?