What are the 25th-75th percentile ranges for the annual income for graduate students in the United States?

Note that those statistics are often averaged over all possible disciplines, and therefore, since there is such a wide disparity of stipend levels between different schools, and between different disciplines at the same school.

At the institution I attended for graduate school, engineers had stipends approximately 30% higher than the science majors (chemistry, physics, math, etc.). Similarly, at the undergraduate institution I attended, a similar disparity existed between science and humanities graduate students.

It is also important to ask if master's students, who often don't receive a salary at all, are included in that average. (And, since humanities students tend to stay longer, they may skew the statistics even more than one might expect.)

(One final note to directly address the question: competitive national fellowships in the US currently pay between $30,000 and $35,000 per year as a stipend. I would estimate, then, that most stipends are significantly below that amount. I'd say something in the range of $20,000-$30,000 would be appropriate in STEM fields, and probably $15,000-$20,000 for full-time humanities PhD's.)

I would suggest the major driver of differential graduate student incomes is the varying sources of funding for different research areas. Some funding agencies list the amounts they fund explicitly on their website (e.g., NIH NRSA grants gives $22,032 for graduate students, other numbers available on their site), while others require the amounts to be listed by the PI in the grant proposal (e.g., Air Force Office of Scientific Research grants do not list any specific amounts in the call for proposals).

Given the large number of funding sources across all disciplines, finding actual numbers (beyond anecdotal reporting) would be a major undertaking. If you do research and publish the numbers, though, I'd love to see them.

Here is a nice list of PhD stipends in different fields and schools in the USA:

• Brown University – 20500 (Political Sciences in 2013), 27500 (BioMed:Biology in 2013)
• California Institute of Technology – 21321 (Physics in 2014)
• Carnegie Mellon University – 27600 (Biological Sciences in 2013)
• Columbia University – 32447 (GSAS in 2014), 31666 (GSAS in 2012)
• Cornell University – 36080 (Life Sciences/Physical Sciences/Engineering in 2014), 31293 (Humanities/Art in 2014), 30533 (Policy in 2013), 30533 (BMCB in 2013), 29067 (MBG in 2010)
• Duke University - 28773 (Arts & Sciences in 2014), 29420 (Medical Sciences & Nursing in 2014), 27850 (Engineering in 2014), 27934 (Arts & Sciences in 2013), 28700 (Medical Sciences & Nursing in 2013), 26910 (Engineering in 2013)
• Emory University – 28000 (BCDB in 2014), 28000 (Biomed in 2013)
• Harvard University – 32232 (SEAS in 2014), 31284 (SEAS in 2012), 32616 (OEB, GSAS in 2012), 26795 (GSAS in 2009),
• John Hopkins University – 29218 (Biochemistry, biophysics, cellular, developmental, molecular biology in 2014), 22000 (Political Science in 2014), 25000 (Biostats in 2014), 21000 (Political Science in 2013), 20000 (Biostats in 2013)
• MIT – 31968 (Engineering PhD RA in 2014), 30888 (Science/English PhD RA in 2012), 30630 (Chemistry PhD RA in 2012), 28236 (English MS RA in 2012), 31644 (Science/English TA in 2012), 28524 (English TA in 2012)
• Northwestern University – 22992 (average in 2014), 22428 (avg. in 2013), 28000 (IBIS in 2013), 21936 (Physics and Astronomy in 2013)
• Princeton University – 32000 (Molecular Biology in 2014), 26784 (Humanities in 2011), 27504 (Natural sciences in 2011)
• Rice University – 26000 (Chemistry in 2014), 24720 (Chemistry in 2013), 28000 (Biochemistry and Cell Biology in 2013)
• Stanford University – 36500 (PhD with SGF in 2014), 29500 (Neuroscience in 2010), 29250 (GSBS in 2010)
• University of California Berkeley – 31000 (Molecular and Cell biology in 2014), 30500 (PMB in 2013), 30000 (MCB in 2013)
• University of California Los Angeles (UCLA) – 26000 (Ecology and Evolutionary Bio in 2014)
• University of California San Francisco (UCSF) – 31000 (Neuroscience in 2014)
• University of Chicago – 29000 + $500 relocation fee (BSD in 2014), 28500 (BSD in 2013), 28000 (Humanities in 2013), 28000 (BSD in 2012)
• University of Notre Dame – 25333 (Biological Sciences in 2013)
• University of Pennsylvania – 30500 (Biochemistry and Biophysics in 2014), 27615 (Art & Sciences in 2013)
• University of South Carolina – 17500 – 22000 without tuition
• Vanderbilt University – 25500 (CPB in 2011), 25500 (Biomedical Sciences in 2009), 20000 (Chemistry in 2005)
• Washington University at St. Louis – 28500 (DBBS in 2014), 28500 (Biology/Biomed Sciences in 2013)
• Yale University – 32500 (BBS in 2014), 31700 (BBS in 2012), 35700 (BBS with NIH or NSF funding in 2012)