Classic rock or oldies formats have been described as having the weakness of not playing new artists. This is true in a creative sense, but not a commercial one. Conventional wisdom in the radio industry is that stations will not get good ratings or revenue if they frequently play songs unfamiliar to their audience. This is why "Top 40" stations played only the biggest hits and why oldies and classic rock formats do the same for the eras they cover. An inherent danger to this philosophy is that closed-campus work environments, such as retail, foster an environment where listeners hear the station continuously for several hours, multiple days in a row, which can become unpleasant after a short amount of time. Oldies and related formats do have an inherent advantage in that they have a much broader time frame (up to 30 years, compared to the current hits of a top-40 station) from which to draw their playlist and can thus play a greater variety of songs than a station bound to devote the majority of its spins to a limited list. The ideal "classic" station (of whatever format) finds the balance between playing listener favorites frequently enough to develop a base while at the same time cultivating a playlist broad enough not to breed contempt. Nevertheless, there seems to be a cottage industry of Internet stations specializing in specific forms of classic rock and oldies, particularly psychedelic rock and progressive rock.