The study, written by Stan Liebowitz, an economics professor at the University of Texas at Dallas, compared record sales and music radio listening in some 100 American cities from 1998 to 2003. It found that, very roughly, an hour’s worth of radio listening per person per day, over the course of a year, corresponded with a 0.75 drop in the number of albums purchased per capita in a given city. Professor Liebowitz has proposed that people use radio listening as a substitute for buying music.
Heading over to the U of T, it looks like his homepage is down, and here’s the only link I can find for the study at UTD. Luckily, the Google cache gets me through to at least this — The Elusive Symbiosis: The Impact of Radio on the Record Industry; Review of Economic Research on Copyright Issues, 2004, v.1(1), pp.92-118 (local copy)