<P> Commercial stations program the format of the station to gain as large a slice of the demographic audience as possible . </P> <P> A station's value is usually measured as a percentage of market share in a market of a certain size . The measurement in U.S. markets has historically been by Arbitron, a commercial statistical service that uses listener diaries . Arbitron diaries were historically collected on Thursdays, and for this reason, most radio stations have run special promotions on Thursdays, hoping to persuade last - minute Arbitron diarists to give them a larger market - share . Stations are contractually prohibited from mentioning Arbitron on the air . </P> <P> Market share is not always a consideration, because not all radio stations are commercial . Public radio is funded by government and private donors . Since most public broadcasting operations don't have to make a profit, no commercials are necessary . (In fact, because most public broadcasting stations operate under noncommercial licenses from their country's broadcasting regulator, they may not be allowed to sell advertising at all .) Underwriting spots, which mention the name of a sponsor and some information but cannot include "calls to action" attempting to convince the listener to patronize the sponsor, may be allowed . </P> <P> Also, satellite radio either charges subscribers or is operated by a public broadcasting service . Therefore, satellite radio rarely carries commercials or tries to raise money from donors . The lack of commercial interruptions in satellite radio is an important advantage . Often the only breaks in a satellite music station's programming are for station identification and DJ introductions . </P>

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