Record charts are compiled using a variety of criteria. These commonly include sales of records, cassettes and compact discs; amount of radio airplay; requests to radio disk jockeys; and, recently, number of downloads.
Some charts are specific to a particular musical genre and most to a particular geographical location (although download charts are not easily pinned down in this way). The most common period of time covered by a chart is one week, with the chart being printed or broadcast at the end of this time. Summary charts for years and decades are then calculated from their component weekly charts. Component charts have become an increasingly important way to measure the commercial success of individual songs.