Many of you have read the article stating that the 360 has reached an 8.9 software attach ratio according to NPD numbers. 

http://n4g.com/news/556411/npd-microsoft-attach-rate-hits-8-9-ps3-supply-improving if you missed the article.

Now consider that vgchartz has NA with an attach ratio of 10.91. If the NPD numbers are correct that would mean that vgchartz has over tracked 360 software sales by 47,599,137 units in NA alone. Now take into consideration that the 360 has been out for 240 weeks, which means each week the 360 sales figures for software have been over tracked by 198,330 units per week. 

Now for an example lets look at the May numbers for Super Mario Galaxy 2

With a launch date of 5/23 and Saturday tracking dates vgchartz has the sales number for 1 week at 779,015 while NPD has the same game over a 9 day period at 563,900. That is a difference of 215,115 units. 

http://www.vgchartz.com/game.php?id=35084 for Vgchartz Mario Galaxy 2 numbers
For May NPD numbers see link below
http://www.gamasutra.com/view/news/29131/NPD_Industry_Sales_Down_5_In_May_As_Red_Dead_Redemption_Tops_Chart.php

Since I do not know why these discrepancies occurred would someone with Vgchartz tracking knowledge please help explain to me the differences between the Americas tracking and NPD.

vgc NA is not the same as npd they count more area in NA numbers than just usa

This topic has come up a million times.

- NPD's US numbers don't include Canada; VGC does
- NPD is only a sample with estimation same as VGC, don't assume NPD is more accurate. NPD sample different retailers and have different models to produce the final number.
- NPD have been known to release incorrect public data and only correct it in the private, paid-for version. They also attribute sales data to the wrong month to correct internal discrepancies, leading to abnormally low then high numbers for two months or vice versa.
- They may treat bundled software differently to VGC

Thats what I thought, which would lead me to believe that the tie ratios for the systems would actually be lower than the NPD numbers. 

For example assume for a second that US sales make up 80% of NA sales. If the attach ration for 80% is 8.9 then when multiplied together you get 7.12. Which would mean the other 20% of NA must account for 3.79. Thus to find the attach ratio for the other 20% to come up with a total of 10.91 in the region then the areas outside of the US would need to have an attach ratio of 18.95. 

Now I am going to go out on a limb and assume that the US likely has the highest attach ratio in the NA region, which if true would mean that the Vgchartz attach ratio should be smaller than the one posted by NPD. 

AFAIK they don't count bundled software.

Assuming you add 2 bundled games to every unit sold then you still would need to assume that Canada, Mexico and any other region tracked in NA matched the US attach rate in order to reach vgchartz attach rate. 

Bundled software does make up a significant portion of the discrepancy for the numbers to match up you must assume that either NPD is under tracking, possible, or that vgchartz is over tracking also possible.

When considering PS3 numbers it seems that over tracking is still more likely considering a 9.1 attach rate in the Americas for PS3 and the PS3 has not had constant bundling to explain the difference.

Needless to say determining accurate tie ratios for these consoles will be difficult.

Please don't tell me GoW 3 is gonna get adjusted down...it was under-tracked by quite a bit in America in March and overtracked marginally in May and April.

What can we expect to be adjusted ?

Thanks for the breakdown IOI.

Now I have a question / suggestion why not include a country or regional breakdown in NA like you do in EMEAA. This would make an easier comparison for users especially for people interested in US vs. NPD tracking. 

so please track the arcade bundle... banjo viva pinata.

Yah I understood the impact of bundles, however disregarding bundles and assuming that other regions in NA do not match the US attach rate, I still have vgchartz about a a game and half higher overall.

Not saying who is correct but IOI could you tell me the difference in retailers used for each estimate. I assume that Gamestop, Walmart, Bestbuy, Target, Amazon and Toys R us are included in both.