Each week, Billboard puts together a chart of the top 100 most popular songs (as well as several other charts) based on a national sample of top 40 radio airplay, top 40 radio playlists, and music sales. Since the Top 40 comes from the Hot 100 chart, let's look at how the Hot 100 is compiled. As you can imagine, this is quite an undertaking.

First, there is airplay. What is actually being played on the radio and on music video channels on TV? Assuming program directors and disc jockeys have their finger on the pulse of popular music, this could be good measure of what people like. Airplay is tracked through Broadcast Data Systems (BDS), run by Nielsen. BDS uses digital pattern-recognition technology to identify songs that are played on radio stations and music video TV channels across the United States and Canada. This is done 24 hours a day, seven days a week, and captures over 100 million songs annually. BDS also provides "gross impressions," which is simply the number of people listening to a station multiplied by the number of times the record was played. When new songs are recorded, a copy of the recording is sent to BDS so it can be encoded and tracked by its system on the stations it monitors. This data is used not only by Billboard in compiling the weekly charts, but also by record company executives, radio stations, publishing firms, performance rights organizations (to calculate performance royalties), music retailers, independent promoters, film and TV producers, and artist managers.

Another measure of what music is hot is what people are buying. To find out what music is selling in record stores, Billboard goes to SoundScan. Nielsen SoundScan is an information system that tracks the sales of music and music videos throughout the United States and Canada. By scanning the bar codes, they can collect sales information from cash registers each week from over 14,000 retail, mass merchant, and non-traditional sources such as online stores, concert sales, etc. The data is compiled and available for subscribers every Wednesday. Like BDS data, the data from SoundScan is also very valuable for record companies, artists, concert promoters, and retailers.

Billboard's methodologies for compiling the charts have gone through several changes over the years. Since switching to Nielsen's BDS and SoundScan (see below for a little background), Billboard changed the weighting of airplay versus sales. Because tracking a single song through album sales isn't exactly accurate, singles sales have always been used to track the sales side of song popularity. But, since only about 20% of people actually buy singles and over 90% listen to the radio, it made sense to alter the ratio of points. Now, the overall points are weighted to 20% sales and 80% airplay.

But, if Billboard bases its charts on what is already being played on the radio and purchased in music stores, how do radio stations find out about new music?

Some Chart History
On July 27, 1940, Billboard Magazine published the first national charts to rank the top 10 songs by individual artists according to popularity. There were actually three different charts, the Best Sellers in Stores, the Most Played in Juke Boxes, and the Most Played by Jockeys. On November 12, 1955, however, Tom Noonan, who was in charge of Billboard's chart department, came up with the idea of listing the overall top 100 songs. This list was called the Hot 100 and was compiled based on: the top selling singles from all genres of music, the most played on the radio, the most played in juke boxes, and the most sales in sheet music. The sheet music sales were later dropped, though.

Because the Hot 100 only tracked sales of "singles," a separate chart was set up to track album sales. This chart had several names (Best-Selling Popular Albums, Top LPs, Top LPs and Tapes, and Top Pop Albums) until 1991, when new technology was introduced to track sales and playtime and compile the charts. Then the name was changed to the Billboard 200. In addition to a separate album chart, Billboard also started separate charts for different types of music such as country, R&B, rap, and others.

Prior to 1991, the Hot 100 was compiled manually, by actual people. Billboard staff spent countless hours on the telephone with record stores to find out what music was selling, and more hours on the telephone with radio stations to find out what songs were on their playlists and what songs had been added that week. On November 30, 1991, however, Billboard switched to two data collection services -- Broadcast Data Systems (BDS) and SoundScan -- both developed by Nielsen.