Data for Billboard's sales charts -- which include all of our album charts -- are compiled by Nielsen SoundScan from a universe of merchants that represents more than 90% of the U.S. music retail market. The sample includes not only music stores and the music departments at electronics and department stores, but also direct-to-consumer transactions and Internet sales (both physical albums via Internet, and ones bought via digital downloads). A limited array of verifiable sales from concert venues is also tabulated.

All sales charts use the entire Nielsen SoundScan panel, with the exception of the R&B;/Hip-Hop charts which uses a panel of core stores that specialize in the genre. The Nielsen SoundScan system utilizes that same point-of-sale that music merchants use to track their inventory, so you can think of itemized receipt from your last trip to the music store as a ballot cast for our charts.

Billboard's radio charts are compiled using information tracked by from Nielsen Broadcast Data Systems (BDS), which electronically monitors radio stations in more than 120 markets across the United States. The BDS system looks for an audio fingerprint -- a characteristic that differentiates a song from all of the other ones that it tracks -- using the same technology that was once used to track submarines.

Some airplay charts -- such as Modern Rock or Adult Top 40 -- are based only on the number of times that stations in a format played each song. Others -- including Hot Country Songs or Hot Latin Songs -- are based on audience.

The audience charts cross-reference BDS data with listener information compiled by the Arbitron ratings system to determine the approximate number of audience impressions made for plays in each daypart. Thus, a song that plays at 4:00 a.m. does not count as much as one played at 4:00 p.m., and a station with a large audience will influence the chart more than either a station in a smaller market or one with a specialized format that attracts less audience.

With very few exceptions, stations tracked for Billboard by BDS are commercial stations.

Most of the charts in Billboard are either sales charts or radio charts. The only exceptions in which we try to mingle sales numbers with radio data are three of our signature charts: The Billboard Hot 100, Hot R&B;/Hip-Hop Songs and the recently launched Billboard Pop 100.

We use both pools of data because while the consumer's decision to purchase is a significant vote of popularity, singles have a job that extends beyond being a sales vehicle: to capture radio play and, hopefully, stimulate album sales. Beyond that, in today's competitive market, radio programmers do not make music decisions lightly, but rather use extensive research to play songs their audiences want to hear.

These three hybrid charts each use formulas to mix Nielsen SoundScan sales with BDS audience. The Hot 100 and the Pop 100 each utilize the a la carte sale of downloaded tracks with sales of the few retail-available singles that are still shipped to stores. The former chart also factors in audience from all popular formats monitored by BDS -- from top 40 and hip-hop to country, Latin and rock -- while the Pop 100 confines its radio panel to mainstream top 40 stations.

The Hot R&B;/Hip-Hop Songs chart meshes audience data from R&B; and hip-hop stations with sales from the core R&B;/hip-hop panel.

There are a few Billboard charts that are made up of data from different sources. Here's how our other charts are compiled:

Billboard's Hot Ringtones and Hot RingMasters charts use sales data from Nielsen Mobile's RingScan service, which compiles sales information from the majority of ringtone retailers in the U.S. market. The Hot Ringtones chart ranks sales of polyphonic and monophonic ringtones with the Hot RingMasters chart featuring master recording-based sales.

The Hot Dance/Club Play chart is compiled reports from a nationwide panel of club DJs, detailing the tracks that elicit the most audience response.

Most of Billboard's DVD/Video charts use information provided by Nielsen VideoScan, which tracks sales from video outlets as Nielsen SoundScan does for music outlets. One DVD/Video chart, Top Music Video utilizes data collected by Nielsen SoundScan.

Generally, charts reflect sales and airplay between Monday and Sunday of any given week. However, the mixed data charts (The Billboard Hot 100, Pop 100 and Hot R&B;/Hip-Hop Songs) use an airplay cycle of Wednesday through Tuesday.

Charts are available Thursday on Billboard.com and Billboard.biz and reflect the date of the Billboard issue in which they appear. In the case of online-only charts, it is the same issue date as all other charts reflecting data from the same seven-day period.

The printed magazine first reaches newsstands on Friday. Each issue is dated based on the end of its publication week. Thus the Billboard that reaches newsstands on Friday, Sept. 9, for example, is dated Sept. 17.