By now, nearly every major market in the U.S. has a radio station sporting the newfangled “Jack” format, in which blocks of hit music selected for its familiarity to an “average” national audience is interspersed with prerecorded voice tracks. “When a new Jack or Bob or Mike station enters a market, there tends to be a spike in ratings. But according to a new study… Jack and Bob face two problematic trends. At many such stations the audience size has diminished as the novelty of the format wears off, and the time each person spends listening to the station – an important statistic for advertisers – is fairly low, suggesting that people tune in for the fun of the songs but tune out in a short time for what other stations offer: on-air personalities and local news, perhaps.”