Do PhD students have written exams? I thought they did research and wrote papers? How can you write an exam to test a PhD student when they are the ones coming up with the knowledge in the area?
PhD students take classes, and some of them do have written exam (in CS theory more than other parts of CS). In the US, where the PhD is 5 years, you spent a good portion of the first 2 years taking classes.
Before coming up with new knowledge you need to know what's already out there.
