Typically, in the USA, it refers to Nielsen Ratings, a system developed by Nielsen Media Research for determining how many viewers are watching a program.
Viewership in the USA is measured with two numbers: ratings points and share. Ratings points measure the percentage of all households (or demographic audience) viewing a program, while the share measures the percentage of households (or demographic audience) with sets in use (so if there are 100 households, 50 of them watching television, and 25 watching a specific program, that program has a 25 rating and a 50 share). At present, each ratings point equals approximately 1,128,000 households.
Ratings are measured by statistical sampling, with data gathered from electronic devices as well as written surveys about viewing habits. The written surveys are being phased out and are currently only used for local station ratings in smaller markets. Though they strive to represent all regions, age groups, genders, ethnicities, etc, there are often criticisms about ratings underreporting various groups (which in turn results in less programming aimed at those groups).
With the increasing popularity of DVR's Nielsen now provides multiple types of ratings data called streams to account for different types of viewing. Each day they release the previous night's traditional live viewing ratings and the live+same day ratings which include all DVR viewings on the same day (defined as 3AM to 3AM) of the initial airing The live+same day ratings are the ratings you see on most websites as that is Nielsen's defualt stream. Then two weeks after the end of the ratings week (Sunday) they release live+three day ratings which include all DVR viewings within the three days after the initial airing and live+seven day ratings which include all DVR viewings within seven days after the first airing. In addition to their traditional program ratings Nielsen now also offers commercial minute ratings which are an average rating for all national commercial minutes during a program. They offer this commercial rating for all four ratings streams. Starting with the 2007-2008 season most ad buys are based on the commercial minute ratings from the live+three day DVR rating stream. This combination has become known as the C3 rating short for commercial and three day.
On January 1, 1997 a system of content ratings for television were introduced in the United States by the FCC due to complaints by various consumer groups of the increasing level of sex, violence and other adult content on televsion. It is based on the MPAA film rating system but with a few notable differences—the main one being that there is no ratings board, and each network sets its own ratings. The television rating system is also much more versatile in describing the content and has more rating levels than the system used for movies. It is designed specifically to work with the V-chip, which in 2000 was mandated to be installed in all new televisions.
The basic rating levels are:
TV-Y (content suitable for young children)
TV-Y7 (content may not be suitable for children under 7)
TV-G (content suitable for all audiences)
TV-PG (parental guidance suggested)
TV-14 (parental guidance suggested for children under 14)
TV-MA (content suitable for mature audiences or adults only)
In addition to the basic ratings levels a series of letters can be added to the bottom of the ratings box to signify the exact nature of the content. These include:
- fantasy violence (only used with the TV-Y7 rating)
- sexual situations
- coarse language
- highly suggestive dialogue
The V, L, and S ratings are used with TV-PG, TV-14, and TV-MA but the D rating is used with TV-PG and TV-14 only.
At first, TV ratings were displayed only at the beginning of every program. However, since early Summer 2006, the rating is displayed on screen after every commercial break. In addition, the rating at the beginning of the program has been increased in size by 70%. This was a move instituted by the National Cable Television Association to dissuade the FCC from possibly censoring cable television as they do with terrestrial television.
The V-Chip can be programmed to block all content above a certain ratings level to assist parents in monitoring their childrens viewing habits.