million units. Because of these changes in criteria, the sales level associated with a particular award depends on when the award was made.

Nielsen SoundScan figures are not used in RIAA certification; the RIAA system predates Nielsen SoundScan and includes sales outlets Nielsen misses. Prior to Nielsen SoundScan, RIAA certification was the only audited and verifiable system for tracking music sales in the U.S.; it is still the only system capable of tracking 100% of sales (albeit as shipments