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Mechanisms for Calculating Ratings (United States) 
There are two companies that gather audience measurements for ratings broadcast programming in the United States:
- Arbitron - The Arbitron gathers its statistics via a series of continuing surveys of radio listeners in major broadcast markets. Participants list the stations they're listening to every quarter hour; the results are tabulated into quarterly "ratings books" for each covered market.
- The A. C. Nielsen Company measures ratings for television.
The ratings industry breaks their statistics down in several ways, to express different measurements:
- The Share is the percentage of radio listeners tuned in to a given station at a given time. For example - a "1.4 Share" means that that 1.4% of all people listening to the radio at a specified time are tuned into the specified station or program.
- Rating is the percentage of potential audience members - whether tuned into any program or not - who are tuned into a particular program or station at a given time.
- Cume or Cumulative Audience is the number of unique people tuned into a program or station at a given time. Cume is usually expressed as the estimated number of listeners in any given quarter-hour.
- Average Quarter Hour (AQH)
- The AQH figure is the average number of audience members during a typical quarter-hour (measured as quarters of the hour from :00 to :15, :15 to :30, :30 to :45, and :45 after the hour to the top of the hour, respectively.
- Time Spent Listening (TSL)
- The amount of time an average listener spends listening to the station or program before channel surfing. This measurement drives both the frequency of radio commercial breaks, as well as the station's programming strategy. Stations whose radio formats tend to have short TSLs (music stations) as well as dayparts (see below) with short TSLs (morning drive time, for example) will have more frequent commercial breaks; formats and times with longer TSLs will schedule breaks less frequently.
All of the other ratings are broken down by demographics. Stations that might have weak overall ("12+") demographics may have strong enough ratings within a given, desired "demographic" to be attactive to advertisers, and thus profitable.
For example, talk radio stations frequently have lower-than-average "12+" ratings, but much higher numbers among males age 35-54. Since the 35-54 male demographic is highly coveted by advertisers, such a station can be quite profitable.
In addition to demographics, a key breakdown in ratings is the "Daypart", or segment of the broadcast day.
In radio (and to a lesser extent television), the key dayparts are:
- Morning Drive: characterized by short TSLs (the time a person spends in their car) and, for the well-programmed station, high cumes, Morning Drive (usually from 5AM to around 9AM) is a key revenue generator.
- Mid-Mornings, Mid-Days: With lower listenership (measured by lower average quarter hours) but generally longer TSLs, the middle of the day is a place where stations frequently experiment with longer music, television programming, and longer television commercial sets.
- Afternoon Drive: Again, high listenership but short TSL, combined with a more-tired listenership that is driving home from work, combines to make afternoon drive time (usually from 3PM to 7PM) a time of short sweeps and, in talk radio, brief conversational snips.
- Evenings and Overnights: Lower listenership means lower revenues - but a standout program frequently can draw a niche audience that, if it shows up in the ratings book, can mean solid revenue.
Influencing ratings 
While it is illegal (under US law, enforced by the Federal Communications Commission) and unethical for a broadcaster to directly ask listeners to fill in ratings diaries for their stations and programs, they do use a number of tactics to "game the system".
Staggered Breaks 
Ratings diaries for radio stations break the hour into quarter-hour periods, at :00, :15, :30 and :45 after the hour. Listeners are expected to enter the station's identification information (see below) for every station they listen to in a given quarter hour.
As a result, radio stations will frequently run their commercial breaks slightly after each quarter hour; a listener that listens from :10 after to :16 after the hour, and then tunes away when a break starts, counts as a listener for two quarter hours rather than just one. This raises the "AQH" for both quarter hours.
Sweeps Weeks 
Radio and television stations will frequently save their best programming ideas - the biggest cliffhangers, the most outrageous stunts - as well as their promotional budgets, for "Sweeps Weeks", the four annual periods when the ratings services perform their most extensive surveying. A stunt or program that draws a bigger audience during a Sweeps Week will translate into a higher "cume" measurement for the station.
When processing results from user-submitted ratings diaries, Arbitron tabulates any of four different pieces of user-entered data:
- The station's call letters (e.g. WXYZ-FM)
- The station's frequency (1280 AM)
- The station's slogan ("Hot Rockin' 105")
- The name of the air talent or program ("Bill Michaels", "The K-Rock Morning Zoo").
To ensure that diarykeepers write the correct information into their diaries, stations do a number of things:
- Play a variety of recorded station ID jingles, voice-overs or "liners" before, during and after commercial breaks to ensure the station's four key identifiers are always perceptible.
- Whenever air talent (disk jockeys, talk show hosts or other announcers) begin or end a segment, they will usually begin (or end) with one or several of the four key identifiers.
If diarykeepers happen to write down the name of the talent or program, but do not identify what station it is on, Arbitron will automatically default listenership to the local market station that carries the program. So if the person writing the entry is not listening to their local market affiliate, but instead something like a high powered station from another nearby market transmitting the same program, or that program simulcasting on satellite radio, the carrier that the diarykeeper was actually listening to will not get credit.