Should PTAs be allowed to mark exam scripts?
In large courses, it is impossible for one person to be able to grade all the exams—it takes far too long to grade hundreds (or thousands) of exams. Even a class of 70 students typically would take me 20 hours to mark up a single exam.

However, if a single person is assigned responsibility for devising a rubric for and grading a problem across an entire exam—a “latitudinal” approach—it should be fair, because it treats all students equally. A “longitudinal” approach in which graders are responsible for completely grading a portion of the exams is tougher to make fair, since then grader bias becomes a possibility. To get around that, you’d then need some system to “equate” grades between the graders, such as ensuring their means and standard deviations are normalized to the same values to reduce the impact of bias.