As was resported by Joystiq the other day, it appears that the NPD reports will finally integrate Walmart sales into their monthly sales data starting with their May Report which will be released on June 14th.
I don't see why the NPD's are used anymore
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As pointed out by another Joystiq article from as recent as August of last year, Richard Mitchell writes, "It's worth pointing out, however, that Steam, undoubtedly one of the largest digital distribution platforms, does not share its sales numbers".
NPD sales data does not incorporate Steam. As of two years ago that was thirty million accounts.
With the Steam Workshop integration of Portal 2 and Skyrim, and the increasing amount of independently made games sold on steam, the number can only be higher at this point.
Admittedly, it has been Valve's policy not to release the sales data from Steam, but regardless of the cause the effect is that it renders NPD reports for video games fairly impotent.
There are many games for whom a majority of their sales come from Steam, particularly independantly developed games.
When the use of the NPD reports is to try and determine the health of the industry, the fact that they ignore the incresingly large amount of indie devloped titles is evidence of their inability to do just that.
Every year it seems that more and more independant titles are released and with the rise of Kickstarter, that will only continue. The way in which the industry evaluates it's overall health, and the health of various intellectual properties and companies will need to change or evolve soon if people are expected to be able to take an accurate picture of its health.