I saw the PhD transcript of a colleague graduated from a US university. There was no score for the dissertation, just stated Pass with no effect on the cumulative GPA.
Is it common?
How the dissetation quality controls the final GPA?
Isn't the major academic credit (40 - 60%) of a PhD program its dissertation?
Yes, this is near-universal in the United States. Dissertations are not graded, and the GPA includes only course grades.
For the most part, people do not care about course grades in Ph.D. programs once you've completed your dissertation. The GPA is understood to refer only to courses, so the GPA is also not considered important. In particular, it is not viewed as a summary score for the whole program, but rather only for the initial coursework.
Dissertation quality is measured by reading it, gauging where it was published, or reading letters of recommendation. For any job where the hiring committee cares how good the dissertation was, they will be reading recommendation letters, so there would be little benefit in having a numerical score in addition to the letters.
Is it common?
Yes
How the dissertation quality controls the final GPA? Isn't the major academic credit (40 - 60%) of a PhD program its dissertation?
The quality of the dissertation has no impact on the final GPA because nobody cares about the PhD GPA.
The only real indicator of the quality of a PhD student's work is the research he/she produced over the course of the PhD, which can't be reduced in a meaningful way to a single number.
It is my (not-well-founded) belief that PhD dissertations are not graded because grading is (or should be considered) essentially foreign to academia, and it is industry, or the commercial world, which has the basic interest in placing people on a unidimensional numeric scale of who's better than whom. It is (somewhat) humiliated to be stamped with a number or grade. When you've made a significant research contribution, you are respected by not being graded any more.
