Matt Might, a professor of Computer Science at the University of Utah, created a set of circular diagrams to explain the significance and impact of a doctoral degree. He uses them to explain to incoming doctoral students what they're pursuing. The first, a circle, represents the sum of all human knowledge. Click on the link to view the sequence.
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Personally, I think I could have done better given $50,000 and free reign, (thinking paying people in the field to apprentice, etc.), but I'm sure others need the structure of university.
You just need to find out which sort of person you are.
Depends where you get it from and what
you study.
There's still the opportunity cost of going to school for an additional six years instead of working, and on average people do not make that money back. If you're in it for the money, the PhD isn't worth it.