The correlation between GPA in grad school and research ability In the US graduate universities, Ph.D. students have to take courses. Can a student with low GPA do good research? I think doing research is solving problems. If someone cannot solve a homework or an exam, how the person can even solve more complicated problems to publish?
In the US graduate universities, Ph.D. students have to take courses. Can a student with low GPA do good research?


In short, yes. GPA is a measure of your ability to follow instructions, do homework on time, and perform on exams. If the grading scheme has enough of a curve it barely does that. In any case, it's not a meaningful measure of research ability.

In the US graduate program I'm familiar with (in physics), several of the classes you take are actually not that closely related to your own research. Spending too much time on those classes can actively hurt your research output, while being somewhat beneficial to your GPA. Often there are also teaching duties to handle, so time management can be crucial. Sometimes you hear the advice to get good enough grades*, and spend the rest of your time on research. It also seems that potential employers are not too interested in the graduate GPA - the degree itself and your research tend to be more important factors.

*The grades should be enough to remain in good academic standing, and count towards your degree.


  I think doing research is solving problems. If someone cannot solve a homework or an exam, how the person can even solve more complicated problems to publish?


To some extent this is true, particularly for the courses relevant to one's research area. Understanding that subject matter and being able to solve problems is clearly worthwhile. Usually that translates to good grades in those classes, but I don't think it's guaranteed. One can have personal issues that semester, suffer from time pressure on a final exam, etc. 

On the other hand, research problems are not always just more complicated versions of the problems studied in class. They may require new, creative methods that you wouldn't see in a course. They may require learning specific procedures that just aren't covered in standard courses. Not to mention that being able to pick a good open problems to work on is an art in itself. Certainly, getting good scores in a course does not guarantee one becoming a successful, independent researcher.

In conclusion, there might be some correlation between graduate GPA and research ability for large enough population sizes, but I don't think it holds any predictive power on the individual level.