Like HowStuffWorks on Facebook!

How Sweeps Week Works

How TV Ratings Work

TV ratings are a form of audience measurement adopted from the early days of radio in the 1930s when surveyors would randomly call households and ask about their listening habits [source: Gross]. The object of audience measurement has always been the same, to accurately gauge the audience size for a network, station or individual show and use that information to sell on-air advertising, what we call commercials.

Commercials are the backbone of broadcast TV; they pay the bills. A half-hour TV show contains roughly 22 minutes of entertainment and eight minutes of commercials. Six minutes of those commercials are national ads sold by the network itself (such as ABC, CBS, NBC or Fox). The local affiliate station sells other two minutes of commercials. Networks rely on this network of independently owned affiliates to broadcast the network's shows in local markets nationwide.

The TV networks set prices for their national commercials based on national ratings data provided by Nielsen. National ratings are expressed as a percentage of all of the households in America with a TV set that are watching the network at a given time. Ratings are further broken down by demographic segments. Adults 18-49 years old, for example, are a favorite demographic for advertisers, since consumers in that age group tend to buy more stuff.

Nielsen collects its national TV ratings data via two methods: set meters, which capture which channel the TV is tuned to, and its patented "people meter" system. With the latter device, each member of the household has his or her own button on a special remote control to indicate when they are tuning in to a particular station. These people meters track the viewing habits of approximately 45,000 people in 20,000 homes in the 56 largest TV markets [sources: James and Nielsen].

For Sweeps Week, Nielsen still sends out millions of paper diaries to collect viewing data on local TV audiences, except for the very largest markets [source: Manly]. This geographically specific data is what local affiliates use to set their own advertising rates. Since paper diaries take much longer to process, Nielsen confines Sweeps Weeks to four, four-week periods throughout the traditional television season with Sweeps in November, February, May and July.

Next we'll explore why Sweeps Week has been blamed for some of the most sensational moments in TV.