I've looked. I can't find it.
Not here: http://www.northeastern.edu/clms/publications/
Not here, either: http://www.employmentpolicy.org/people/andrew-m-sum
I can find lots of blogs that quote these wonderful statistics, but not one that references the source of these statistics in the form of a white paper or journal paper. The only references provided are links to Sum's home page.
I am very curious: How did he count graduate students?
A good percentage of physical science (physics, chemistry, geology, meteorology, ...) undergrads proceed on to graduate school rather than getting a job armed only with their undergraduate degree.