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Curving helps control for different degrees of harshness in grading between different exams and between different instructors. Curving also allows professors to choose any level of difficulty on their exams, instead of being forced to choose basic questions that everyone is expected to find straightforward, and many students find boring. For example, I had a chem class where the median exam grades were often around 30%. The exam questions took the concepts covered in the textbooks and expanded those concepts to more complex applications, making most exam questions far more challenging than the textbook examples. There were also too many questions for most students to finish within class period, so speed/intuition was tested as well as knowledge. The exams were curved such that the ~30% median grade was a B/B+. </p>
<p>There is nothing about the definition of curving that prevents everyone from getting high grades. A professor could think everyone scored high enough to show mastery of the material and curve everyone to a high grade, although I’ve never seen curves applied without at least a few students getting Cs or lower. In general, more advanced classes with a higher percentage of stellar students tend to be curved to higher grades than more basic classes (more advanced classes are less likely to have a formerly applied curved at all, which partially relates to smaller class sizes). A similar pattern emerges with more selective colleges tending to curve exams such that a larger portion of the class receives high grades than at less selective colleges (in minority of classes for which a formal curve is applied) . For example, Princeton is probably the school that is most often mentioned on this forum during discussions of grade deflation and harsh grading, yet the median grades in Princeton classes are far higher than the equivalent classes at most less selective state schools.</p>