"[Myth] that exams are objectively graded. Daniel Stark and Edward Elliot sent two English essays to 200 high school teachers for grading. They got back 142 grades. For one paper, the grades ranged from 50 to 99; for the other, the grades went from 64 to 99. But English is not an "objective" subject, you say. Well, they did the same thing for an essay answer in mathematics and got back grades ranging from 28 to 95. Though most of the grades they received in both cases fell into the middle ground, it was evident that a good part of any grade was the result of who marked the exam and not of who took it"
The study the author is referring to is: "Starch D, Elliott EC. Reliability of grading work in mathematics." http://www.jstor.org/stable/1076246?seq=1#page_scan_tab_contents
This is pretty consistent with my experience, both as a grader and a student. The cited experiment was done in 1913, however. I am curious about more recent, broader studies about testing the variance of grading - especially for math exams such as those we give in college algebra or calculus.
The same studies are referenced here: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4041495/
The only recent reproduction I have found was for the Starch/Elliot experiment about english papers, not math exams: http://pareonline.net/getvn.asp?v=16&n=17