So recently I myself partook in assigning grades to students in a class. The process for physics 2210 went something like this. First we looked at the rough distribution of scores and compared them to the distribution of total scores for previous years of the class. We (though basically just professor Ailion) decided on good ranges to set breaks between grade levels based both on the actual scores and guided to some extent by looking for gaps in the continuum of percentages. With 297 people in the class a "large" gap between scores was 7/10ths of a point.
After deciding on the rough placement of the dividing lines between grades we looked at the people on either side of the dividing lines and talked about whether or not to bump them over the line or under it. For instance in this particular 2210 class the first test was by far the easiest and it had an average score of 74 and quite a number of people got 100's on the first test. The way the class is set up you are allowed to drop your lowest tests. So if you hadn't taken the first test (because of adding the class late etc) but had done well on the other exams we took that as a reason to bump up a person's grade.
Of course we spent much more time arguing about where to put the A to A- transition line than we did about where to put the D to D- line even though somewhat ironically the lower end of the scores was more densely populated than the top so that a difference of a point towards the A region might affect 3 or 4 people but a change of a point down towards the B- to C+ range might encompass 20 people.
But I find the contrast between this realm of grading and the graduate realm of grading rather amusing. The grades for my graduate E+M course for the entire class are posted on the class web page (thinly disguised behind student ID numbers). In the class there was quite a wide range of performances going from over 300 points down to only half that at around 160 points. I have however here omitted an anomaly. There is a student enrolled in the course who does not come to class and rarely turns in homework though the student does come to take the tests. At any rate this student got only 60 some odd points in the class... almost a third of the points of the second lowest score in the class and less than 20% of the total possible points in the class.
I admit that I was expecting this student to receive a failing grade in the course. But graduate classes are a very different world than the world of the 300 student undergraduate physics course. For someone in the P.H.D program a B- is a failing grade because only B's and above can be counted as progress towards a degree. I had internalized this knowledge already but had somehow assumed that surely one could achieve a lower score with a truly dismal performance in a class... But now that I have evidence of a truly dismal performance and I see it rightly paired with its failing grade... it is just that now that failing grade is a B-.