Sorry about making a new thread, but the computer I'm on doesn't let me reply in existing threads (but I can create new ones O_o), otherwise I would have put this in the official NDP thread.

Firstly tolerance / margin for error: NPD is an estimate, VGC is an estimate, therefore there must be a built in tolerance for differences in numbers reported between NPD and VGC. From my perspective if the VGC and NPD numbers are within 10% that's a tight correlation, and no one really has justification for criticism. The general view is that NPD is more likely to be an accurate estimate than VGC, so a difference in numbers outside tolerance would suggest VGC numbers may need some adjustment.

With that said, lets look at June 2010 NPD and VGC data as reported here http://gamrconnect.vgchartz.com/thread.php?id=113069&page=3 thanks WilliamWatts for neatly putting NPD and VGC into a single post.

So here's my assesment of accuracy (my grades are purely arbitrary based on my opinion on the significance of the difference, any number within tolerance automatically gets an A):

                                                             % diff   Grade

PS3                                                      -29.0     D

X360                                                   0.3       A+

Wii                                                       -9.7      A-

DS                                                       -5.6      A

PSP                                                     -31.2    E

RED DEAD REDEMPTION 360        -19.5    C-

SUPER MARIO GALAXY 2              20.2     C-

RED DEAD REDEMPTION PS3        -21.5    C-

NEW SUPER MARIO BROS.            36.4     E-

JUST DANCE                                     -10.5    B+

HW overall grade         B+                                 

SW overall Grade         C-

As I see it Software is in for some significant adjustments, and not just the titles with numbers published by all accounts. On software it seems like week by week and even month by month sales should not be relied upon, but lifetime sales (with the regular ajustments that happen during a title's life) should be a reasonable representation of likely sales (accuracy would be greatly improved when publishers release game shipment data for example).

Hardware, the only adjustments I see as being needed are PS3 and PSP to be adjusted to come closer to NPD. That's not to say I think VGC should simply adopt the NPD numbers, given we are talking tolerances and estimates. Ideally VGC and NPD should be within 10%. But when they are not within tolerance what should be the adjustment? Should VGC simply adjust numbers so that the difference is 10%? My response would be no. After all NPD is also an estimate so its numbers also have a margin for error, only it will be a smaler MfE than VGC's. The balance of probilities is that the true number of sales, will fall somewhere between the VGC number and the NPD number.

So factoring the error of both sets of data, and depending on how heavily NPD data is weighted for the purposes of determining adjustments, I think VGC's PSP numbers for June could be adjusted up to betwen 107 - 113K and PS3 number adjusted up to between 268 - 283K.

Wow, NPD is getting good. Really getting close VGCs official numbers. Though your pluses and minuses are backwards.

also Canada/Mexico count for around 10 percent of NA sales no?

quiet or theyll lose the A mark for the xbox360 :)

I would call it margin of error instead of "tolerance." Furthermore, I would set NPD's numbers as the benchmark to compare everyone else's numbers to.

Finally, I am unsure of the reason for the differences between NPD and VGC's numbers. Does NPD track retailer sales to individual consumers while VGC tracks shipments to retailers?

Quite a bit of overtracking if your numbers are based off of shipments to retailers, but are useful. Shipments to retailers are more or less a measure of how desired the game is and is based off of retailer predictions at the organizational level, while retailer sales to consumers is the realization of how popular or unpopular a game is at the individual level.

Sources NPD sales predictions/data and NPD both US only AFAIK