The dropout rate varies significantly by discipline. My experience is only with the natural sciences where in North America at least, most students are funded b

The dropout rate varies significantly by discipline. My experience is only with the natural sciences where in North America at least, most students are funded by their program+advisor, compared to the humanities where funding opportunities are few and far in between. I would guess that students who are funded by their advisor are much more likely to be exploited compared to the self-funded student because of the monetary exchange going on. But funded students are also less likely to quit because they have a relatively comfy job for five years that protects them from the "real world".