Do we need an adjustment?

[crumas2]

Munkeh111 said:

crumas2 said:
I was under the impression that NPD was measuring sales to consumers in US-only, while VGChartz was measuring shipments from manufacturers for the Americas (not just the US).

Why are some posters claiming the numbers should match? Isn't this comparing apples to oranges?

 

VGC should be NPD + 10%, but the Xbox is 100% higher on VGC, and the PS3 is about 20% lower

 

Why 10%?  Is that an immutable number, or just a mean value of what it usually is when other countries and factors are taken into consideration?