Actors from the CW show 'The Flash' speak at an 'upfront.' Networks show their programs to advertisers in hopes that they'll buy commercials 'up front' or in advance of the fall season. It's always held at the end of the May sweeps period. See pictures of TV Shows Kevin Mazur/WireImage/Getty Images

Sweeps Week is a throwback to simpler times, when Americans watched TV shows on actual TVs — rather than laptops, tablets and smartphones — and 50 million people sat down to the same episode of "I Love Lucy" at the same time on the same channel all across the country. In 2013, a show on ABC was lucky to get 5 million viewers [source: Stelter].

Sweeps Week was the brainchild of the A.C. Nielsen Company, which popularized TV ratings in the 1950s and whose TV audience numbers are still the industry standard. In 1954, Nielsen sent "TV diaries" to households across America, asking them to record their precise TV viewing habits for a week. Then, in a geographic "sweep" starting in the Northeast and moving to the West, Nielsen collected the booklets, compiled the data and published the first accurate reports on the viewing habits of American families [source: Fletcher].



Why does it matter how many people watch "Modern Family" or "Real Housewives of Moscow" on a Wednesday night in Topeka, Kansas? Because both the national TV networks and local TV stations both depend on advertising to make money. With detailed information about the size of the local TV audience, the local TV station can charge advertisers more for commercial spots during the most popular shows and the most active times of day.

Today, Sweeps Week is four separate four-week periods spaced throughout the year. ("Sweeps Month" is more like it.) Sweeps Week remains an important gauge for TV advertisers, even though the entertainment landscape looks very different than it did in the days of "The Honeymooners" and the "Ed Sullivan Show." In the 1950s, TV viewers chose between three or four broadcast channels. Today, the typical home includes cable or satellite TV with hundreds of channels, plus online access to streaming TV via computers and mobile devices.

As Nielsen and its competitors struggle to get an accurate headcount of a highly fragmented TV audience, some TV networks and local affiliates are losing their faith in the institution of Sweeps Week. Keep reading for a closer look at how TV ratings work and to learn how Sweeps Week is fighting for survival in the Internet era.