"In an ideal world, having multiple human experts annotate each song with a large, extensible vocabulary of tags would likely lead to the most accurate music index. How- ever, this is an extremely expensive and unrealistic idea. To illustrate this point, consider that Pandora has been annotating music for over a decade and yet has only annotated only about 5% of the music that is available for sale on the Apple iTunes store. Social tagging offers an improvement in terms of scalability by crowd-sourcing the task of music annotation; however, it suf- fers from strong popularity bias and inconsistent annotation behavior. Music annotation games also rely on crowd-sourcing but have the potential to pro- vide more consistent annotations for a controlled set of songs. However, to date, none of the games have been successful at attracting and sustaining a large group of users"