It could have been Sony asking them to do this.  I mean look at the last reporting period NPD released to the public, it had the PS3 in a distant third and it's likely not lost on Sony that many people are like lemmings and have a follow the leaders mentality.

Wow...Mod or not what a @$#@--

 I'm not saying you're wrong, but what makes you think Sony has that kind of pull with NPD?

I definitely agree Sony has the motive but I am not convinced they have the influence to get this  done. 

My theory: NPD releasing quarterly figures gives VGChartz a chance to deviate further from NPD, thus showing that there is a significant difference between NPD and VGChartz figures. With NPD the established "credible source" they can then discredit "other independently owned websites" as inaccurate and get the media to start paying for NPD figures again.

I didn't take statistics, although I helped my wife when she was taking the class in college (I'd read the lesson and try to understand the concept so I could help her, but otherwise, yeah it was a case of the blind leading the blind). But from what I picked up, I gained a real respect for the error ratios and how they were calculated, and how a small sample can statistically represent a large number. But as kn said, there are measures for how accurate your information probably is. And I think if ioi ran those numbers and published it to show how statistically he is tracking a large enough sample to make it valid, he could gain credibility and become an even more quoted news source. If ioi doesn't know statistics, then he could share that info with a trusted source that does know statistics and can do those calculations. Just my thoughts, anyway.

Well, at this point the problem isn't so much the sample size as its diversity.

If I poll 1% of the nation's population on something, I can get a pretty good read on the mood of the entire country, but if my sample consists only of 30-50 year old females that all goes out the window.

But it's something that's being worked on and will only get better with time.

I've added the NPD response to the news.

http://www.vgchartz.com/news/news.php?id=619

My Theory:

Especially because of the major platform having the right to release their numbers - Sony is not happy with how the sales discussion is effecting their PS3 market... PS3 is perceived poorly, performs poorly, people see that, it effects sales even further.

I put it down to something like that... they look BAD every time the NPD data comes out - therefore they don't want it there so they can't look bad.

Well, why would they make the change unless someone complained? And I really doubt that anyone with great sales would complain, so that does narrow down the candidates.

This will help VgChartz traffic tons! After the last NPDs go out, then people will have no where else to turn for numbers.

After reading the 1up article about this, I have to conclude that it's very likely Sony asked for the information not to be publicly released. Here are they key parts (emphasis mine):

1- "The NPD Group [...] consulted with the hardware manufacturers on temporarily changing their policy to accommodate them. "There was no push back [from Nintendo, Microsoft and Sony]," said Riley. "However, it's been a year, so we decided to pull this monthly feed

2- Regarding rogue sites that actively use existing data from market research firms, combine that data with estimates and then claim the data as their own -- it's true I'm no fan of sociopathic behavior and, as such, have real issues with sites like these but this has nothing to do with the change, either," he said.

So they consulted with hardware manufacturers one year ago and all was fine, and now suddenly they change it, and it has nothing to do with "rogue sites"? That likely means something changed with the manufacturers, and it's not hard to guess which one had problems with the media's and gamers' reactions to the data.