From the paper, which apparently nobody here actually bothered to skim: > To study this phenomenon, I must overcome the fact that file sharing is endogenous in

From the paper, which apparently nobody here actually bothered to skim: > To study this phenomenon, I must overcome the fact that file sharing is endogenous in its determination of an album’s sales because an album that is popular in file-sharing networks will also be popular in retail markets. This occurs because file-sharing and retail demand are both driven by unobserved album quality. To address this endogeneity, I exploit exogenous variation inhow widely available an album was prior to its official release date. An album’s ease of availability in file-sharing networks is positively correlated with the number of times that it is downloaded because availability determines the supply of the album in file-sharing networks. Availability is not independently correlated with an album’s popularity in retail markets because pre-release filesharing is driven by leaks, which are “crimes of opportunity” that occur at some stage of the production or album marketing process (Williams, 2009).