because, like vgchartz, they're just extrapolating data from a sample population. extrapolating data is never 100% accurate, there will always be a degree of error. hence, why npd are sometimes off. december sales, comes to mind if i'm not mistaken.

While likely being sarcastic. It's possible they did it on purpose because they realized that they had been overtracking the PS3 and 360, and that if they kept things as they were the number totals might end up going of ther manufactured and shipped numbers... in which case they'd catch a hellstorm of complaint.

If you look at Famitsu and Mediacrate... when every they widley differ. Like say Media Crate says a game sold 60,000 and Famisu says 100,000 you'll see magically the next week Medicrate will often say it sold 60,000 when famitsu says 20,000.

All and all... who knows though.  You think Sony and MS would make a bigger fuss about the numbers if they were misrepresented.  Maybe the boost won't hapen till this next NPD... on the basis that the console boosts were caused by casuals who might not be able to get to the store the first 5 days or so. 

They don't get data for the nations largest retailer when it comes to videogame sales. (to my knowledge, they have a seperate deal with Wal-Mart, but not involved with this data to my knowledge.)

So NPD dont count the wall mart sales?

They extrapolate, just like anyone else who doesn't get their numbers.

we've already seen vgchartz adjust down after NPD although the numbers are still higher than NPD.

Well the only most of the people who bought the game already had a ps3 and xbox360. That games does not attract people to buy the system as do other games do!

They count them. They just base them off historical trends. It seemed to work well for the WiiFit numbers... which were even more huge amount.

It's still a hole in their data set however.  It could be totally possible that NPD totally overestimated the number of PS360's Wal-mart was selling and was forced to readjust for that during GTA4 and during the PS3 deal.

It's been said that big box stores like Wal-Mart greatly favor the wii while smaller specialty stores favor the PS360. 

Chad Warden? Seriously, that guy's just retarded. Why do you have random quotes from him as your signature? He should never have been popular. He should have been forgotten, the second his videos came on Youtube.

I really enjoy the content this site offers and I'm glad it exists even if numbers aren't 100% accurate but just face it: in case of doubt, NPD has 60% direct data and as for the 40% they estimate they've doing it for decades. If anyone is off, is this site.

Dolemit, there's a slight difference involved between extrapolating from 2% direct data and extrapolating from 60% direct data.