And don't forget NPD only covers _roughly_ 60% of the North American market and nothing outside of US and Canada. I say roughly for two reasons:
1) They don't cover WalMart. My undrestanding is they extrapolate WalMart sales based on hand stock counts by NPD staff at 8 local Wal Marts to NPD.
2) When they claim to cover 60% of the market, do they mean 60% of gross unit volume, gross revenue, specific retailers, shelf space inches, or what? It's all very unclear, and I'm kinda in the know on these things. I've never had a straight answer from anybody I've every asked from marketing guru to Vice President of whosi-whatsis.
Point of data: On one of our games, NPD listed 179k units. Our royalty report from the publisher listed nearly 400k units. Using the traditional extrapolation of 60%, that's sales of 252k units to 400k units. Though my publisher may have artificially elevated their financial obligations to us in their royalty report, for some reason I doubt it.
Here's another few words along with steam that call NPD stats into question:
Amazon, Walmart.com, EBGAMES.com, etc.