<p>bclintonk is correct. Only 3% of Michigan classes are taught by TAs. Those are almost all lower level language and mathematics classes. The remaining TAs are primarily used as discussion leaders. The same is true at any major research university. The only way a university like Columbia or Harvard or Penn, with its 10,000+ graduate students, can put them to work is by making them research assistants or teacher assistants. For some reason, private universities are ashamed of admitting that graduate students work as TAs.</p>