K.M., does anyone know which radio station was the first to use "research" to determine which songs to play? In the 1950s-60s, there were hundreds of top-40 stations but the first top-40 stations were broadcasting in the 1920s-30s, although the term "top 40" had yet to be created. I'm guessing that the announcers in the early years of radio could play any song they wanted...
By the 1950s-60s, we had thousands more stations and some played top 40, some played country, some played r&b, some played MOR, some played jazz, some played classical, some played beautiful music.....and most of them started doing "research." A few stations, such as KFWB, would allow the DJs to vote each week on which new songs they wanted to add to the playlist, but most stations started relying on research and music tests.
The earliest research by all-recorded music "Hit Parade" recorded music stations was, in fact, the genesis of the format: jukebox plays. As Storz added stations and McLendon created more still, stations began to call the record shops as well as watching Billboard and Cash Box and Record World. This would be the standard well into the 70's.
In the mid-70's as the sale of 45's dwindled, folks like McCoy and Wallace came up with callout as a way to find out the hot currents. Soon, as stations added more and more gold to their lists, auditorium tests were developed.
Prior to the 1952 birth of Top 40, stations had musical "interludes" between drama, comedy, news and other such shows. Think of it as being more like the radio version of The Lawrence Welk Show than Top 40. Different hosts, different bands, many versions of the same hit songs but by different bands and singers. And quite a bit of live music, courtesy of the pressure of the AFM.