Digital Song Sales
The Digital Song Sales ranks the best-selling digital songs in the United States, as compiled by Luminate and published by Billboard magazine. Although it originally started tracking song sales the week of October 30, 2004, it officially debuted in the issue dated January 22, 2005, and merged all versions of a song sold from digital music distributors. Its data was incorporated in the Hot 100 three weeks later. Since October 2004, digital sales have been incorporated into many of Billboard's music singles charts. The decision was based on the dramatic increase of the digital market while commercial single sales in a physical format were becoming negligible.
Source: Wikipedia
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