How common are self-funded PhDs in the STEM fields in U.S. universities? I met a PhD student that is completely self-funded at our engineering college .  Our university is notorious for being one of the most expensive schools to attend; I can't imagine someone funding his own graduate studies for five/six years, right after paying for an undergrad education.

He did land an important internship at a large hedge fund, though, so I'm guessing in the long run, if he lands in finance, it could be all worth it.

My question is: are self-funded PhDs in the STEM fields at U.S. universities pretty common?
From the Survey of Earned Doctorates, a large-scale survey of students who earn doctoral degrees from U.S. institutions, 


In the life sciences, 9% report "own resources" as primary source of support.
In the physical sciences, 3.8% report "own resources" as primary source of support.
In the social sciences, 25.3% report "own resources" as primary source of support. 
In engineering, 3.8% report "own resources" as primary source of support.


See TABLE 35. Doctorate recipients' primary source of financial support, by broad field of study, sex, citizenship status, ethnicity, and race: 2014.