Four times a year, American television viewers are subjected to sweeps periods. The sweeps are when the A.C. Nielsen Company measures the audiences in all 210 US television markets.  Nielsen continuously measures national programming, but local audiences are only measured in November, February, May, and July-August. The ratings gathered during these periods are used to set ad-rates and to make decisions about local programming. During sweeps months, the networks schedule new episodes of programs, specials, original productions, and other shows that are likely to draw a larger-than-ordinary audience. In non-sweeps months, viewers get a lot of reruns.

The measurement of these local markets was not begun by Neilsen, but rather by a competitor, the American Research Bureau, now known as Arbitron. By 1961 Arbitron was measuring every television market in America at least twice a year. It comes from the metaphor of sweeping up or gathering the data. From Newsweek, 30 November 1970:

There is a temptation to look one’s best during sweeps, but the practice of “loading,” or temporarily beefing up programming, is specifically forbidden by Federal unfair-competition regulations.

(Source: Oxford English Dictionary, 2nd Edition)

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