how many books used v how heavily are they used
see Knowlton who thinks the difference is negligible, what matters is only whether it gets used or doesn’t.
Levine-Clark wants to measure both things separately for e-books, though it seems like only the former could be used in a comparison to print.
Slater 2009 looks at concentration of use. E-book use will probably look more concentrated than print use because for print we don’t know how many times each user referred to a book while they had it checked out. I’m not clear what numbers he is comparing. I might say to look at what percentage of the books account for 10% of use, or 50% of use. Slater also asks what percentage of used books were only used once (for netLibrary and equivalent print titles).