They don't understand what an estimate is and the attention that NPD gets has brainwashed people into thinking their numbers are "official."

I don't believe in any of these numbers without adquete proof. Neither VGChartz or NDP. However I am likely to use either as a base. I am also likely to use one that shows more stability unless otherwise noted proof is present.

I think it's because NPD is the only big source of numbers from the US, with a high percentage of the market covered.

Look at Japan for example. There are two widely respected tracking sources, Media Create and Famitsu/Enterbrain. Since they often publish quite different estimates, people understand that their figures aren't to be taken as gospel.

The real question is why people are unable to make a simple analogy of what this means for USA and NPD. The fact that there's only one big source should actually make it less accurate, but everyone assumes the exact opposite.

I don't trust either NPD or VGC completely. I prefer to wait for the game companies to provide me with sales information via press releases.

So they don't adjust their numbers? Instead they just add or subtract from the upcoming months?

We have no way of knowing really.

They HAVE to adjust their numbers because they don't track enough of the market to be as accurate as possible first time every time.... but as we only see the free sample figures, they may adjust old figures without our knowing (but the people who pay for the proper set of figures will know) OR they may as you say adjust by adding or subtracting to the following month.

Or a third option... they may adjust their proper figures like VGC does... but the free numbers given out are "falsified" to take into account past changes.

Ah right o cheers pal.

I've never truly trusted NPD. I remember when they had both April and May 2008 hardware sales for the ps3/360 below March, despite the release of GTAIV.

Of course, this would jibe with one of NJ5's theories:

3- They adjust for past errors by adjusting later months' data. In this case it's not possible to trust each month's data, since it may be wrong or adjusted up or down to account for past mistakes. At least in this case the sum of several months should be more close to the truth.

i will trust NPD before SONY or MS thats for sure

The last one has to be the case. Otherwide, their total sold would be different than the financials.