Mark Glaser, writing in MediaShift, takes companies such as Interactive Advertising, Nielsen, and others, over the wide range of methods for measuring traffic and the flaws associated with the varied methods. Part of the issue is what do you want to measure: page views, unique visitors, time on site, all of which can be manipulated to make numbers better. How do you measure: IP address, cookies (making users like me 3 or 4 unique visitors per day). He makes a mention of a new company Quantcast, which has different ways to measure.
The deciding factor will be which measurement the advertisers trust. Glaser’s piece was in two parts (1, 2 )