  We study letter grading schemes, which are routinely employed for evaluating
student performance. Typically, a numerical score obtained via one or more
evaluations is converted into a letter grade (e.g., A+, B-, etc.) by
associating a disjoint interval of numerical scores to each letter grade.
  We propose the first model for studying the (de)motivational effects of such
grading on the students and, consequently, on their performance in future
evaluations. We use the model to compare uniform letter grading schemes, in
which the range of scores is divided into equal-length parts that are mapped to
the letter grades, to numerical scoring, in which the score is not converted to
any letter grade (equivalently, every score is its own letter grade).
  Theoretically, we identify realistic conditions under which numerical scoring
is better than any uniform letter grading scheme. Our experiments confirm that
this holds under even weaker conditions, but also find cases where the converse
occurs.
