When is 'grading on a curve' appropriate? Wikipedia defines 'grading on a curve' as a 

a statistical method of assigning grades designed to yield a
  pre-determined distribution of grades among the students in a class.

The article does not say much about when it's appropriate/inappropriate to grade on a curve, but it does state 

The ultimate objective of grading curves is to minimize or eliminate
  the influence of variation between different instructors of the same
  course

I take it then that if a course has only one instructor then that would remove an argument for grading on a curve. It also seems to me that it might be inappropriate to grade on a curve when a course cohort is extremely small, since it would then be hard to say what grades 'should' be given.
What other factors make grading on a curve more/less appropriate? Are there factors that make it absolutely inappropriate?
 A: I grade on a curve because I want a grade that is relative to the performance of all of the students in that class.  If the test is too difficult for any students to get 90% or higher, absolute grades are not appropriate and a curve should be used.  I design the test such that it is nearly impossible for anyone to get 100%.  I design it that way to produce a score that tells the difference between the stronger and weaker students.  Therefore, I grade against the average of scores, and use standard deviations to delineate letter grades.
Another issue would be the life threatening nature of the subject matter.  I would think a driver's license test should have an absolute grade, because if you do not know what a stop sign is, people die.  The purpose here is to test to a "hurdle rate", a minimum set of required information the student must know, not to differentiate between good and bad drivers.
Similar situation for engineering classes when designing bridges, cars, planes, etc.  You must design the bridge right or people could die.  These classes have an absolute grading scale.  There is also no partial credit.  You either built the bridge right or you didn't, getting the general idea right doesn't work.
