[tjl]

I've known students who defended their thesis and were told to do major revisions. Typically, it's because their thesis supervisor didn't do their job properly as they should know not to send that student to defend.

You're right in that they weed Ph.D. students out earlier, during their comprehensive exam. How it's done varies from department to department and university to university. My comprehensive was a lengthy oral exam by my committee with two rounds of questions. The first on background and the second on the written thesis proposal I submitted. I went for 3.5 hours straight, basically until the committee wanted lunch.

Equating a research group to a startup isn't a bad analogy. One of the professors in my department basically uses his students to do research for his company. He even makes them sign over the IP rights to him. Other professors have a continuing line of research across a number of students. Even my Ph.D. thesis was the latest in a number of theses on the same topic, each getting progressively more advanced. My thesis basically finished that line, with other related ones opening up as a result.