The U.S. television "sweeps" are a feature of local television market viewer preference measurement which are conducted by A.C. Nielsen Media Research. Several times each year, Nielsen Station Index (Nielsen Media Research's local market measurement service) collects demographic viewing data from sample homes in every one of the 210 television markets in the United States. Each home in the sample maintains a paper viewing diary for one week. Each household member writes down what programs they and their guests watch in their home during the course of that week.

"Sweeps" have existed since the beginning of TV viewer preference measurement. The measurement periods are called "sweeps" because Nielsen Media Research mails out diaries to certain households around the country, then collects and processes the diaries in a specific order. The diaries from the Northeast regions are processed first and then "swept up" in a geographical progression throughout the U.S., from the South to the Midwest and then finally to the West. The standard sweep months are November, February, May and July of each year.

In some of the larger markets, there are as many as three additional months (October, January and March) during which diaries are used to provide viewer information. Standard reports based on the diary data are produced and issued regularly to clients. This viewing information is used by local television stations, cable systems, advertisers and their agencies to buy and sell commercial advertising as well as make programming decisions.

Following, as an example of the different periods and the respective numbers of local markets they cover, are the Nielsen "sweep" periods for the 1999-2000 TV season. Three of the periods include all local markets, one additional period includes all but 3 of the total markets, and the remaining three periods cover only specific local markets.

• October, 25 Markets
• November, All 210 Markets
• January, 17 Markets
• February, All 210 Markets
• March, 4 Markets
• May, All 210 Markets
• July, 207 Markets (excluding Fairbanks, Honolulu and Juneau)

Special programming is often broadcast during the sweep periods, in an attempt to attract as many people as possible to watch television. To television viewers, sweeps usually means new episodes of network shows, big specials from the networks, exciting news stories on local and nation news broadcasts, and the biggest interviews on the talk shows. The traditional "sweeps" months are November and May, which is why one will very rarely find a rerun on in those months.

Nielsen ratings are tracked all year long, but networks set their advertising rates by how high their ratings are during the sweep months, so the ratings from these periods make a difference.

However, the ratings measure much more than the simple numbers of viewers. Both networks and advertisers also consider such things as what kind of audience (or demographics) the show brings in; what night the show will be broadcast; which time slot it is in; how well competing shows are doing — or their predecessors had done — in that slot; how a specific program retains, increases, or loses audience from its lead-in program, and whether a program generally builds audience from week to week.

Also factored in are things like what kind of publicity or other media attention the show is generating, and whether its stars seem to be "breaking out" — from their tv program roles onto magazine covers and the big screen (which would be very useful cross-promotion for the series).