Billboard uses information Nielsen Music collects on sales, radio airplay and music streaming to determine the top 40 country songs. It develops the top 40 chart from its top 200 chart
Nielsen tracks airplay using its Broadcast Data System. The system recognizes the songs using a digital recognition system. The system monitors both radio stations and music video channels on television. Billboard uses Nielsen’s "gross impressions," which represent the number of times a song plays multiplied by the number of people who listen to the station.
Billboard also considers sales using data from Nielsen. Nielsen collects the data using SoundScan. SoundScan collects sales data using bar codes from cash registers of both retail and online stores. The system also considers concert sales.
As of 2015, Billboard places 20 percent weight on sales and 80 percent on airplay when ranking songs for its lists. The formula considers sales of singles and not albums.
Billboard began ranking songs in 1940. The company rates many different styles music. Before 1991, the company compiled its top 100 lists manually, with staff using the telephone to call record stores to see what music was selling and radio stations to determine the songs on their playlists. After 1991, the company began using Nielsen’s ratings.