Share is a comparative tool. It allows station and network executives to determine how well their programs are doing when compared to competing programs on other broadcast or cable channels.

     Share is closely associated with "rating," another measurement term. Both terms are derived from the same estimates of audience size, but the percentage quotient is calculated differently. Share measures the percentage of active TV viewers who are watching a particular program, while the rating for a program calculates the percentage of all television households­ both those using TV and those not using TV.

     For example, station WXXX  airs  Jeopardy!  at 7:00 P.M. Sample data estimate that 10,000 of the city's 100,000 TV households (10 percent) are viewing that program. Some 40,000 households are viewing other programs, but another 50,000 are not using their TV sets. Since 10,000 of the 50,000 active viewers (20 percent) are watching Jeopardy!, that program has a share of 20, even though its rating (the percentage of TV households) is only 10 .

     Electronic media trade journals generally report both rating and share. Rating is expressed first and is given to the nearest tenth of a percent. Share follows and is rounded to the nearest whole percent. For example, an audience estimate for 60 Minutes may report a 13.0/28, that is, 13 percent of the total TV households (the rating) and 28 percent of the viewing audience (the share).

     If every television household were using TV during a given time period, the share and the rating would be equal. But since this never happens, the share for any program is always greater than its rating because different divisors are used to calculate the two equations. The gap between share and rating is greatest during periods of very light viewing. An early morning news­ cast with a share of 30 and a rating of only 3 is competing very well against other programs in the same time block, even though the total number of viewers for all programs is small.

     Share is useful as a comparative tool during virtually any portion of the day, however. When a program gains share, it usually does so at the expense of competing programs since the total audience for television during any given daypart is relatively stable.

     Share can also be used to illustrate programming trends. One network may average its share of successive programs to illustrate its dominance on a particular weekday night. A new broadcast or cable network may average its share across an entire season to illustrate its increasingly competitive position over a previous season.

     Share can be used to demonstrate industry trends. For example, the combined share of the American Broadcasting Company (ABC), the Columbia Broadcasting System (CBS), and the National Broadcasting Company (NBC) for the 1980-81 programming year was 90. This meant that 90 percent of the viewing audience was watching one of these three networks . The remaining 10 percent of the audience was distributed among independent stations, public television, and the few cable networks then in operation. By 1993-94, combined network share had dropped to 60, primarily because the cable networks collectively had captured one-third of the network viewers.

     This erosion has continued, and during the 2001--02 season,  combined  network  share  for  the  four major U.S. networks (ABC, CBS, NBC, and FOX) fell below 50 for the first time. While the combined share for these four was 49, additional share for The WB and the United Paramount Network (UPN) raised the total share for "over-the-air" networks to 57.

     Some industry observers predict that network share will continue to decline; others assert that network share has "bottomed out" but may begin to show a slight gain in years to come. A study of network share measures the competition between traditional broadcasters and their new technology competitors.

     Unless otherwise specified, share refers to the total universe of television households. Share can be used in demographics breakouts, however, a morning talk show may have a 2.2/20 for women 18 to 34 years of age. That would be the rating and share for this particular demographic grouping.