a study by economists felix oberholzer-gee and koleman strumpf in 2004 concluded that music file sharing's effect on sales was "statistically indistinguishable from zero". this research was disputed by other economists, most notably stan liebowitz, who said oberholzer-gee and strumpf had made multiple assumptions about the music industry "that are just not correct." in june 2010, "billboard" reported that oberholzer-gee and strumpf had "changed their minds", now finding "no more than 20% of the recent decline in sales is due to sharing". however, citing nielsen soundscan as their source, the co-authors maintained that illegal downloading had not deterred people from being original. "in many creative industries, monetary incentives play a reduced role in motivating authors to remain creative. data on the supply of new works are consistent with the argument that file sharing did not discourage authors and publishers. since the advent of file sharing, the production of music, books, and movies has increased sharply." glenn peoples of "billboard" disputed the underlying data, saying "soundscan's number for new releases in any given year represents new commercial titles, not necessarily new creative works." the riaa likewise responded that "new releases" and "new creative works" are two separate things. "[t]his figure includes re-releases, new compilations of existing songs, and new digital-only versions of catalog albums. soundscan has also steadily increased the number of retailers (especially non-traditional retailers) in their sample over the years, better capturing the number of new releases brought to market. what oberholzer and strumpf found was better ability to track new album releases, not greater incentive to create them."