Short answers, pulled from the story.
Nielsen ratings are audience measurement figures produced by Nielsen Media Research that estimate how many American television households watched a given program. The rating is calculated as a percentage of all TV-equipped households tuned to a show, while "share" measures the percentage of households that were actively watching television at that time. For the 2017-18 season, the total universe of TV homes was 119.6 million.
Nielsen Media Research began producing TV ratings in the United States with the 1950-51 television season. The company had previously measured radio audiences, releasing its first Nielsen Radio Index results in the first week of December 1947.
Nielsen sweeps were four measurement periods each year, held in February, May, July, and November, during which Nielsen gathered diary data from households across the country. The term dates from 1954, when Nielsen collected diaries in the Eastern United States first and swept westward. Local TV stations used sweeps data to set advertising rates and schedule programming, making November, February, and May the most commercially significant periods.
Nielsen retired its paper TV diary service on the 28th of June 2018. The replacement relies on a hierarchy of electronic tools: Local People Meters in the 25 largest markets, Set Meters combined with Return Path Data in mid-sized markets, and data modeling in the smallest markets. In January 2025, Nielsen also gained Media Rating Council accreditation for its Big Data + Panel system, which covers person-level estimates across 45 million households and 75 million devices.
Specific demographic groups, particularly viewers aged 18-49, command higher advertising rates regardless of total audience size. During the 2007-08 season, ABC charged $419,000 per commercial during Grey's Anatomy versus $248,000 for a spot on CSI: Crime Scene Investigation, even though CSI had nearly five million more total viewers, because Grey's Anatomy performed stronger in younger demographics.
Nielsen lost accreditation from the Media Rating Council in 2022 due to inaccurate data reporting during the COVID-19 pandemic. It regained accreditation in April 2023.