I will agree that NPD retail sales number for PC are meaningless.
They estimate that 910.7 millions$ was spent in retail in 2007.
Well there's 2.5 millions Wow players in the US.
Those players alone spent 50$ (Burning crusade expansion) + 12*15$ ( monthly fee) = 230$.
That's 575 Million $ of PC games, only 125 Million of it that was accounted by NPD................








