I have been searching for PhD completion rate information off and on for a few weeks now, and I finally found a broad brushstroke of what I was looking for, here.  The long and short of it- after ten years, the PhD completion rate for math and physical sciences stands at 54.7%, at least from students who entered their programs in 1992-1994.  This is crazy!  How do universities get away with this?  People coming into graduate school have no idea that the completion rate is so low, and then feel guilty or like a complete failure for dropping out, when in truth nearly half of entering students do just that.  No one holds individual programs accountable, but they should.  Apparently Britain publishes the retention rates for each of their universities, which has probably forced the universities there to at least work on their numbers.  This information would at least allow people to make more informed choices.  Even if perspective students made the exact same choices, they would at least psychologically prepare themselves for the possibility of incompletion.