Hmm - I take your point, and potentially that could inflate the errors, like using IP numbers can.
But there is no way most sites can force a login/registration and still convert, which is what I assume you need to do the xref table... Hmm - or maybe there is and I just haven't had the courage to try it.
Are there any reliable studies about conversion rates on sites that force a login vs sites that don't? You would need a study that worked over time, since presumably people logging in can then be "kept more loyal" through email.
30-40% overlap between home and work use is, in itself, hugely significant. If that replicates across the Internet world, that means that potentially there are only 2/3rds of the number of real web users in the world than statistics are telling us. That is certainly significant.