If we deal with all this by the use of statistics, which the sociologist, etc. are fond to do, then we expect a gaussian distribution (Bell Curve) of scores having a certain width (related to the standard deviation) and a position (the mean score).
Even in a larger class, say 100 people, there are strange looking distributions of scores. In fact if we divide the range from 0 to 100 into bins of a given width the distribution of scores can be made to look more or less gaussian. A gaussian distribution is the basis of the idea of "grading on a curve." That is, the distribution is divided into segments the top 1/4 being A and the bottom 1/4 assigned F (NC in Brown's lingo), etc. The use of this scheme, whose grade divisions are also arbitrary is based on the notion that the fundamentals of statistics prevail. I do not think they really do because of so many variables. Each class may be more or less prepared than the class the year before, the questions one year may be less difficult than in other years, each exam is taken by an individual and that individual may have a bad or good day, and on and on. This is what I call a lack of statistical control over the experiment and in my opinion it is the flaw in the work of sociologists, medical statisticians, epidemiologists, Deans of the Faculty, Provosts, etc. and has led to lots of problems. In physical science we deal with inanimate objects and have a greater measure of statistical control over experiments. Even then troubles can arise. This is the reason I reject "curving", or using the Bell Curve for grade determination. As far as I am concerned everyone can have an A if everyone knows the material. Of course everyone can have an NC too.