...any copying of copyrighted material done for a limited and “transformative” purpose, such as to comment upon, criticize, or parody a copyrighted work.Most fair use analysis are usually either commentary and criticism or parody.
Composition and Copyright by The American Library Association Office for Information Technology Policy created a Fair Use Evaluator to help you determine whether your proposed use of copyrighted materials falls within the guidelines of Fair Use.
Circular 21: Reproduction of Copyrighted Works by Educators and Librarians provides some guidelines but the courts are not bound by these guidelines and the Copyright Act contains no such guidelines!
There are no similar guidelines for factual and scientific information. Think about how you would want your work used. The article may be 20 pages long, but if you use the most important page or section of the article, it may not be fair use.
The advice and information on copyrighted materials provided by the library is not intended to substitute for and is in no way legal advice.