r5r5 writes "European Commission's Institute for Prospective Technological Studies has published a study which concludes that the impact of piracy on the legal sale of music is virtually nonexistent or even slightly positive. The study's results suggest that Internet users do not view illegal downloading as a substitute for legal digital music and that a 10% increase in clicks on illegal downloading websites leads to a 0.2% increase in clicks on legal purchase websites. Online music streaming services are found to have a somewhat larger (but still small) effect on the purchases of digital sound recordings, suggesting a complementary relationship between these two modes of music consumption. According to the results, a 10% increase in clicks on legal streaming websites leads to up to a 0.7% increase in clicks on legal digital purchase websites."
It's worth noting that this study only measured the effect of piracy on online purchases, not on revenue from physical formats.