Above is a link to an explanation of their methodology.
"..Hitwise has developed proprietary software that Internet Service Providers (ISPs) use to analyze website usage logs created on their network. The anonymous data sent to Hitwise from the ISPs include a range of industry standard metrics relating to the viewing of websites including page requests, visits and average visit length..."
They go on to say they are audited to maintain the integrity of their methods.
A competitor, Alexa.com relies on installed toolbars on individual browsers. Data is only collected from those with the toolbar. Basically you volunteer to install spyware on your computer so they can collect their data.
Hitwise gets their data from ISPs. Everybody has an ISP and no spyware on your machine is required.
15.14% of market share
How big is that pie? How big is that market?
Wonderful news in any case.
Great work, as usual, Kristinn.