Then I would collect data on piracy rate vs initial buy rate vs long tail and find product types that share those. Then, with all that data, one can start looking into DRM and individual products from groups with shared economic traits but which differ primarily on the DRM question. With a decent sample size, some kind of answer should pop out.
That would be my guess of a simplified study. Statisticians and economics has surely more to say on this and where the corner cases are. There might also be smaller studies to do if one accept some early assumptions.