Tell him that for worldwide sales figures the only sources with any degree of accuracy are VGChartz, which uses the same methodology as trackers such as NPD, Chart Track and Enterbrain but on a smaller scale or in shipment figures, which only come every 3 months.
Because of the way European sales numbers are released to the public by the major regional tracker (Chart Track) they are impossible to use. This means the only actual tracker that releases usable European sales data to the public is VGC.
Combine this with the fact that compiling NPD, Media Create or Enterbrain data is useless as they never release any of the modifications they make to their data to the public, the data they release can only be used for that one month and compiling multiple months together to give an indication of sales is highly misleading.
Essentially for the individual periods of release the data for NPD, Enterbrain and Media Create is more accurate than VGC data but any compilation of the data is less accurate.
