QUOTE : If the option of taking the course pass-fail (without a letter grade) is available to all students, it is usually observed that there are some who will elect pass-fail no matter how many others do, some who will elect letter grades no matter how many elect pass-fail, and an intermediate group who will elect pass-fail if enough do but will choose letter grades if pass-fail is uncommon. Notice that the first and second groups’ behavior is independent of how the third group chooses, but not vice versa; the people whose behavior is uninfluenced nevertheless influence others.