PhD students can't outwork low stipends, but they can outthink them. Identify tasks faculty avoid, propose taking them on, and negotiate better pay. Some universities restrict outside work, but formal petitions often succeed.
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While you can't outwork a low stipend, you might be able to outthink it by doing things like hanging around with the faculty members, seeing what they don't want to do that you might be able to do, looking for opportunities. But I talked to someone recently and their university would not let them do any other work than what they're being paid for. They had to actually ask and petition their university to go into an opportunity and get permission to do so. They were able to do it, but be aware that they may limit your ability to do other work. Join our free Dissertation Help community and we'll provide as much support as possible.