Tuesday, September 18, 2007

Life under a normal curve

If there is one thing that you are going to hate in ISB it is the Normal curve. ISB follows a relative grading system and uses a "bell curve" grading system, wherein the grades are assigned according to the frequency distribution known as the Normal distribution. While it does benefits a few, its victims easily outnumber the beneficiaries.

Why should grades be normative? When scores are referenced to the performance of group member, there will always be students who have lower scores than this, even if the individual student scores are themselves quite high when evaluated against specific performance criteria or standards. Conversely, if all students perform poorly relative to a larger population, even the highest graded students may be failing to meet standards.

More fundamentally, this whole process is based on a deep misunderstanding of statistics. Many measurements can be approximated by the normal distribution, not the other way around. A class’s marks are not an imperfect observation of what should be the normal distribution. The marks are what they are. The normal distribution might or might not approximate that. If it does not, mathematically transforming the grades to match it is meaningless. Instructors should grade papers according to their intrinsic merit and give out whatever grades result even if the distribution results in a lot of A's or F's. On tests, an instructor should know, before looking at the results, what score will be required for each grade and what grade this paper deserves. How can the performance of one student go down just because the someone else has also performed well? Why should good grades be a commodity in short supply?

I have no grudges against using the normal distribution where it is appropriate (e.g., in estimating confidence intervals from random samples). To make this correct usage clear, it might be wise to revert to the original phrase used to describe the normal curve - "Normal curve of error." This would make it clear that the normal bell curve is "normal" only if we are dealing with random errors. Social life, grading, IQ, attitudes etc. however, are not a lottery, and there is no reason to expect these variables to be normally distributed.

I hope the law of averages will catch up and today's victims (Me) will benefit someday from the same stupid concept.