Distribution Gap Method
This widely-used method of assigning test or course grades is based on the relative ranking of students in the form of a frequency distribution or tally of student exam scores. The frequency distribution is carefully scrutinized for gaps, several consecutive scores which have zero frequency. A horizontal line is drawn at the top of the first gap ("Here are the A's") and a second gap is sought. The process continues until all possible grade ranges (A-E) are identified.
The major fallacy with this technique is the dependence on "chance" to form the gaps. The gaps are random because measurement errors (due to guessing, poorly written items, etc.) dictate where gaps will or will not appear. If scores from an equivalent test could be obtained from the same group, the gaps would likely appear in different places. Some students would get higher grades, some would get lower grades, and many grades would remain unchanged. Unless the instructor has additional achievement data to reevaluate borderline cases, many students could see their fate determined more by chance than performance.