I have some background in statistical information, and I am constantly impressed with the close accuracy of data on this website to highly specialized tracking agencies in the UK, Australia, USA, Canada, Japan, etc.  I don't know what kind of coverage ioi has of the retail chain, but I'm begginning to think that if he had about as much as some of the other agencies, his data would actually be more accurate than the sell-through data they have. 

Think of it this way - if NPD covers 60% of the US sales data, and and extrapolates for the rest of it, in addition to doing Canadian numbers. ioi's numbers were mostly within 15% of the US & Canada, with (taking a stab at it) 30x less (perhaps even less coverage) of sales information.  Granted, sales patterns are fairly uniform across the world at the moment, but the level of accuracy is not be underestimated.  My theory is that in recent weeks ioi has crossed the threshold needed to refine American weekly data accuracy, and that from here on out, we can expect continual refinements in data accuracy as coverage increases.  I think if even 5% of USA data was covered, the data would be 95%+ accurate. 

Eh, it depends. I think the issue is more with WHERE the data he is obtaining, is from. The real issue and challenge for ioi's VGchartz is where he's getting data from. IE, if 100% of the data was from Toys R Us, then the numbers we would get would be severely off. As refferenced in the games forum, the PS3s and 360s are barely selling. Realilistically, you coud get within 95, or even 98% of accuracy IF you had the perfect mix of various retail chains. This is impossible though, since if your getting data, your most likely farming the retail information chain-wide rather than a store here and there.

Yeah the spread is as important as the volume. We'll see how things develop, it's looking pretty good so far though. :)

Indeed, cross-sectioning is important. Whereas %age is moderately uniform despite geography, the outlet is important because people visit different retail outlets for different purposes, so the spending pattern at Target would be vastly different from gamestop