The usual purpose of preliminary exams is to determine whether a student has a sufficient grasp of the canon to be able to teach. An honors performance on a preliminary exam rewards not only a solid basic grasp of a field but also demonstrated capacity for original thought.
Most departments use written exams or a combinations of written exams and an oral. The substitution of research papers for exams has been shown to produce narrowness and to consume extra time. Typically the time to degree increases when students are required to submit a research paper in lieu of an exam. Please see our Prelim guide for more information.