Why do PhD admission committees focus on your bachelor's degree grades when you have better master's degree grades?

Question

I've heard various occasions where students with mediocre bachelor's degree grades, but superb master's degree grades, get rejected by PhD programs due to their poor bachelor's degree grades.

What could be the rationale behind focusing on the now likely outdated bachelor's degree grades when more recent and up-to-date master's degree grades are available?

(Note I am not guessing they were rejected due to their bachelor's degree grades, the professor who interviewed and wanted to take the student in, explicitly stated that he/she could not due to the student's poor bachelor's degree grades)

Answer 1

In the U.S. and in math, at least in my experience, graduate grades tend to be less reliable measures than undergraduate grades for how one did in a class. Graduate grades tend to be A -- "good" to "excellent", B -- "total-incompetence-but-attended-class" to "acceptable", C -- essentially you failed (for some programs, you're gone if you accumulate 3 C's).

Universities and departments range from "B's are rarely given and C's are almost never given" to "A's tend to be given for work consistent with passing the Ph.D. qualifying exam in the subject", B's tend to be "not horrible" to "good-but-not-exhibiting-future-Ph.D.-potential-performance", and perhaps a few C's are given for especially bad work.

Nonetheless, I would think nearly all A's and well above average recommendations in a Masters program would undo all but the very worst of undergraduate grades, and maybe even those. But a 3.3 Masters average and typical-good-sounding recommendations are not going to undo a 2.8 undergraduate average in one's major.

Answer 2

In the US, graduate class grades tend to be inflated. If you're applying to a prestigious place, it might be the case that they can't accept everyone who got straight-A's in the master's program and are forced to use the undergrad grades as differentiation criteria in the category of course work.

Otherwise, an upward trend in grades can generally help you, but the trend has to be robust under the correction for the higher average grades in grad programs.