Abstract
ARL interlibrary loan statistics and most other interlibrary loan cost studies provide only the mean and median unit cost. The author suggests that future statistical survey include marginal cost in addition to unit cost. Another important factor that is not included in the current interlibrary loan cost studies is the copyright fee, or royalty fee. When the sixth article from a journal is requested for copying by a library in the same year, a copyright fee is incurred, and it can be more than $50 per article. However, the copyright fee is not listed as a separate category in any of the current interlibrary loan cost studies. Since the copyright fee is an additional cost to the borrowing library, it is a marginal cost, not a fixed cost (such as staff salaries). Zhou's formula to include the copyright fee into the marginal cost study is presented, and the marginal analysis and the copyright fee are incorporated into the interlibrary loan cost study for the first time.