Sweeps

Nielsen ratings are the audience measurement systems developed by Robert F. Elder and Louis F. Woodruff and sold to Nielsen Company, in an effort to determine the audience size and composition of television programming in the United States. Nielsen Media Research was founded by Arthur C. Nielsen, a market analyst whose career had begun in the 1920s with brand advertising analysis and had expanded into radio market analysis during the 1930s, culminating in Nielsen ratings of radio programming, which was meant to provide statistics as to the markets of radio shows. The first Nielsen ratings for radio programs were released the first week of December 1947. They measured the top 20 programs in four areas: total audience, average audience, cumulative audience and homes per dollar spent for time and talent.

In 1950, Nielsen moved to television, developing a ratings system using the methods he and his company had developed for radio. That method has since become the primary source of audience measurement information in the television industry around the world.

Nielsen television ratings are gathered in one of two ways:

Changing systems of viewing have impacted Nielsen's methods of market research. In 2005, Nielsen began measuring the usage of digital video recording devices such as TiVo. Initial results indicated that time-shifted viewing will have a significant impact on television ratings. A year later, the networks were not yet figuring these new results into their ad rates because of the resistance of advertisers.

The most commonly cited Nielsen results are reported in two measurements: ratings points and share, usually reported as: "ratings points/share". As of 2013^[update], there were an estimated 115.6 million television households in the United States, up 1.2% from the previous year because of the inclusion of televisions that receive content over the Internet. A single national ratings point represents 1% of the total number, or 1,156,000 households for the 2013–14 season. Nielsen re-estimates the number of television-equipped households each August for the upcoming television season.

...
Wikipedia