A teaching assistant (TA) is a junior scholar employe on a temporary contract by a college or university in teaching-related responsibilities. TA responsibilities vary greatly and may include tutoring; holding office hours; grading homework or exams; invigilating tests or exams; assisting a professor with a large lecture class by teaching students in recitation, laboratory, or discussion sessions; and even teaching their own classes. In some universities (such as the University of Michigan), they are known as graduate student instructors (GSIs). In New Zealand, Australian, and some Canadian universities, they are known as tutors. At Harvard College they are known as teaching fellows (TFs). TAs include graduate teaching assistants (GTAs), who are graduate students, and undergraduate teaching assistants (UTAs), who are undergraduate students. While the term assistant implies that they assist with a class, most (about two-thirds) GTAs serve as the sole instructor for one or more classes each semester; although these GTAs may work under a supervisor or course coordinator, they have the responsibility to prepare and teach class, make and grade homework and quizzes, and even create and administer their own exams. Like professors, GTAs generally have a fixed salary determined by each contract period, usually an academic school year.