Rating Services - Nothing is free in this world, not even television (though it seems to be). With the exception of cable services or PBS stations which rely on cash contributions or charges to finance programming, other network television is provided courtesy of money supplied by advertisers who pay a premium to local/national network stations to have their products hawked via the format known as a "commercial" (an advertisement broadcast on radio or television). To evaluate who is watching what programs and thus the sponsor's commercials, marketing measurement agencies have come into existence (first on radio) to determine the number of audience members viewing any program at any one time. With this knowledge, the television networks/local stations can calculate what to charge advertisers. The more viewers a particular program/channel commands, the higher prices can be charged to those companies or persons seeking air time to sell their wares. In the 1950s an audience rating system called "Trendex" made random phone calls to 1,000 people in some fifteen cities to solicit opinions on what the audience was watching. See also "A.C. Nielsen Rating Service", "Arbitron Rating Service"