Nielsen SoundScan
Since May 25, 1991, the Billboard 200's positions have been derived from Nielsen SoundScan sales data, as of 2008 contributed by approximately 14,000 music sellers. Because these numbers are supplied by a subset of sellers rather than record labels, it is common for these numbers to be substantially lower than those reported by the Recording Industry Association of America when Gold, Platinum and Diamond album awards are announced (RIAA awards reflect wholesale shipments, not retail sales).
Read more about this topic: Billboard 200