People can't read:
"•A telephone survey of retailers across America. We called over 200 retailers (distributed across the country and across different chains in a proportion representative of market share) and asked how many Kinect units they had purchased and how many they had sold. Now 200 retailers from 30,000 isn't a lot, but one GameStop will sell similar amounts to another - each new store you call follows the law of diminishing returns in terms of accuracy improvement. Then, on a retailer by retailer basis, we took an average per store and multiply by the number of stores. We then scaled this with any missing retailers to match the overall marketshare. This gives one estimate of Kinect sell-through.
•A pre-order analysis using a typical pre-order to week-one ratio for a casual Xbox 360 title to arrive at a second estimate. An explanation of where our pre-order data comes from can form it's own editorial but a combination of retail pre-order info from best-seller lists at various stores and user pre-order data (purchase intent) via trawls of major sites like GameSpot, IGN, Eurogamer and so on are the two main contributors.
•Contact a small retail panel who provide regular data for VGChartz and weigh and scale that data to provide a third estimate. We have a lot of experience with this data so while it isn't particularly representitive we have a lookup table of scaling factors and adjustment for different types of game / hardware.
•A Gamercard analysis of more than 5 million Gamercards to calculate the proportion of players playing Kinect games (specifically Kinect Adventures), and from the raw figures we can scale up to produce a fourth estimate of sales via an analysis of gamercard to sales ratios for different types of game.
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NPD states they have data collected from 850 retailers but there's one big difference. VGCharts only focusses on games where NPD collects data on everydamn thing that's sold. Meaning out those 850 chains, how man actual sell games?
And for the most part they post similar numbers within a certain about of error. For example, I recall one of those Aliens games being quoted by NPD as selling 630k first month sales when the developer stated it just broke 1/2 million sales one week after NPD quote. That's an error of 26%. Higher than normal, yeah, just still means NPD's human just like the rest.