Tracking CD sales is done for two main purposes. The first is to measure how many CDs are sold overall, as a measure of how well the industry is holding up during changes in the economy or the emergence of alternative ways of obtaining music. The second is to measure which CDs sold best during a particular period, most notably in the form of weekly charts.
There are several ways to measure CD sales across the industry. One is to take the figures reported by stores themselves. Another is to use the figures produced by the record labels. Generally the latter method will be slightly more accurate since in most markets there is a smaller range of record labels than of stores selling music, so it’s easier to get an accurate picture from fewer sources. For example, in the United States alone the four largest record companies are responsible for more than 80 percent of the CD sales each year.
Weekly charts have historically been measured by taking records from a sample set of stores. This is designed to find a balance of producing accurate results without the costs of gathering the data being too high. Using a sample is generally seen as acceptable as it doesn’t necessarily matter exactly how many copies of each CD are sold as long as they can ranked in order of sales with enough accuracy.
Usually the full sales figures from the sample stores will be extrapolated to produce the overall results. The stores will be selected to give a reasonable overview of the entire market, so will need to cover different geographic areas as well as different audiences such as “serious” music lovers and a more casual, mainstream audience. The selection of stores can be tweaked when producing specialist charts such as those for a particular genre of music.
Some countries now include sales of digital downloads as well as CD sales in their official figures. These sales can be tracked more accurately as each site has precise figures about how many “copies” of an album have been downloaded, in a more accurate form than some retail chains have readily available. There are different ways of incorporating download sales data. One way is to take the actual number of paid downloads and then adjust this figure so that it makes up the “correct” proportion of overall sales based on what is known about the balance between CD and download sales in a market.
The use of digital download figures can drastically affect the make-up of the music charts. On one hand, it means that an extremely popular release doesn’t run the risk of selling out in stores as can happen with CD sales if demand is underestimated. On the other hand, older songs can and do sell well. This is particularly common when a musician dies and their back catalog suddenly sells very well online. This isn’t always reflected so quickly with CDs as it may take time for publishers to reissue albums to meet the spike in demand.