Sorry for the "newbie" question.

As you can tell by post count, I am still new at this. (But as you can tell by my name, I am not new at gaming).

Nevertheless, I remember once hearing that NPD measures about half of US retailers and then makes necessary calculations to come up with full sales figures. I also remember hearing that one of those that it did NOT measure was Wal-Mart.

Is this true? Also, if this is true, could non-inclusion of certain chains/stores be biasing the results.

What made me think of this was some of the great variations in software counts between VGChartz and NPD.

Thanks in advance

Mike from Morgantown

NDP does what ioi does here at vgchartz, but on a grander scale. NDP accounts for I believe 60% of retail, and then extrapolates the data to come up with their ESTIMATES. ioi does the same, but he has I believe 5% of retail, which is a large enough sample to base his numbers off.

NDP doesn't include Wal-Mart as they won't give their info out, typical slouches. It's possible there's variances because of this, but I trust vgchartz to be as accurate as possible for a fan site. :D

I'm simply seeing too much incorrect information going around, so allow me to explain.
Currently, The NPD Group tracks 65% of the retail market. This fluctuates based on retail panel size but it always hovers at around 65%, but keep in mind that it's not as simple as looking at one number (65%). For example, various retailers only sell so much of a certain type of category - eg, heavy sales in portables but few in consoles, heavy sales for one specific platform or publisher but not others, so there's much more to that number than you might think.
Retailers have a choice of whether or not to participate in the panel, but there are obvious benefits to being a participating retailer.
NPD covers just about everything sold to consumers but in terms of video games, it tracks far more than just software and hardware sales. Why don't you see or hear about this? All NPD can provide to the public is what you typically see in the public domain (hardware and software sales, and top-line info. from reports).
Regarding Wal-Mart, it pulled out of all third-party market research firms about six years ago. Will Wal-Mart ever come around? It depends on how it feels about the competition. If competition aggressively begins to nip away at its share ...
How does NPD project for the rest of the market to come up with 100%? It's not easy to explain, nor would I want to even begin to attempt it. It's a complicated process. Where NPD has the edge comes from its historical data. For example, Wal-Mart was once on NPD's panel and had been for a very long time. It's that historical data, along with its work with financial analysts, publishers and manufacturers that gives the industry an accurate measurement of market performance.
How accurate are NPD's numbers? NPD doesn't answer to anyone but its clients and The Street. NPD reports are issued to clients each month; typically the second Thursday of each month. Once clients receive the data, they'll review and certainly let NPD know if they feel something's "not right." Though restatements are rare, they do happen from time to time.
I hope this helps to clear things up.

Walmart and Sam's Club reached an agreement with NPD in... June?... I believe it was. I'm not sure if they're already being tracked, but they were supposed to start including their data soon after the deal, so NPD should be much more accurate.

 what are some of these benefits?  i can see how they adjust how their orders on--but they'll have to pay NPD anyway, so why participate?

 unless i guess participating is part of the contract, since when the data is pooled from multiple retailers it's more valuable... otherwise i have no idea why retailers would bother giving their secrets away.

Yes, Wal-Mart and Sam's Club did but that relationship involves Wal-Mart and Sam's using our consumer data. It's hard to explain but, long story short, no, Wal-Mart is not a part of the panel.
Regarding the benefits of market research, as I said in my earlier response, there's much more data and insight that we provide than simple sales lists and install base figures. Market research (not just NPD, btw) also gives marketing teams what they need to make more informed business decisions. From market-level (regional) information, to demographics, who buys what for whom and why they purchase it, etc., etc. When you combine consumer data with point-of-sale data, there's no end to what you can do with the info. That is, as long as you have the intelligence to do some digging and analysis.
Why do retailers "give their secrets away"? They aren't giving anything away. Retailers and e-tailers cannot gain access to competitor information, so they're protected. They can only view combined sales information (this is why accuracy is so important in market research). So, why bother? There are many reasons but I'll give you a simple example of why retailers participate. They have access to national-level sales data so they know how well (or not) various products are selling across the retail landscape and online, not just in their own stores.