[Merrill]

Libraries have two problems - guessing what books their patrons need and should be acquired, and guessing what books they have that their patrons no longer need.

The second process, "weeding the collection", at least can be informed by circulation data as to whether a book has been loaned in the last several years, as well as other criteria such as rarity, historical importance, being part of a special collection, availability from network libraries, and conditions under which it may have been gifted to the library.