<p>Doing poorly/failing a class related to your field of interest is an obvious red flag, but then that has to do only with specific classes, not your overall GPA.</p>
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Your argument seems to stem from a hypothetical situation where no one has research experience, only grades. In that case, well, of course you might think the student who did well in computer architecture (CoAr) is better prepared to do research on that topic than the student who failed/did not take the course. However, without research experience, the student with the A+ basically stands no chance of being accepted to CoAr programs anyway.</p>
<p>If the “good” student and the “bad” student both have successful CoAr research experience, then I think their performance in the CoAr class becomes somewhat irrelevant. Their research experiences are proof enough that they can put CoAr research into practice, and the poor grade/lack of a grade from the “bad” student could be for a myriad of (legitimate) reasons. As I said before, too many factors are involved in class grades to make them useful indicators of research ability, which is why a good grade in a specific class will never be an “indispensable trait” in prospective PhDs. With all else being equal (including their success in CoAr research), I think the “good” student would have only slight advantage over the “bad” student.</p>