A tougher question, and one I've been curious about. Do hardware sales take into account legitimate returns? If someone returns a PS2, say because their kid really wanted an Xbox and they didn't know, does that equal -1 in PS2 sales? Not that it really matters since returns would probably equal a statistic value of 0% of sales, but I wonder...
I know the system I used to work with quite possibly wouldn't have done so. It would +1 to inventory for a return, but if you queried the system for unit sales, it wouldn't reflect, because it would just be searching for a certain transaction type (sale), and total them. If you wanted true inventory movement, you'd have to query it differently. I wonder which data NPD gets...maybe it varies by retailer...
I don't know how NPD gets their data. Is it submitted by someone at each retail chain, or is it collected via some type of network hooked up to the POS systems? Anyone know?