B is for Bell Curve

Ken BeattyDr. Ken Beatty

First, let me leave nothing to the imagination: I hate the Bell Curve. Because I teach assessment statistics to graduate students, I know I shouldn’t callously bully an innocent graph of achievement, but it isn’t the tool itself I object to, but the wicked uses to which it is put.

Bad beginnings

The Bell Curve was first called “the normal curve of error” by Abraham de Moivre in 1733. He used it to explain games of chance, but by the 19th century, the Bell Curve was being misapplied to justify differences in society such as to support Francis Galton’s theories of eugenics, a pseudo-scientific movement to breed humans to produce a master race. It was only the Nazi party’s horrific love of the idea that led to its belated rejection (Goertzel & Fashing, 1981).

In time, the Bell Curve swept into classrooms as a popular means for quantifying levels of student performance. The assumption was that among any group of students, about 10 percent of the weakest ones inhabit the low end of the curve, most linger comfortably in the middle, and 10 percent can be assumed to be highly competent.

The Bell Curve in practice

At one university, my fellow teachers and I were forced to apply the Bell Curve to the grades of each of our classes. Practically speaking, this meant 3 of 30 students would get the lowest D and E failing grades, another 3 or so would get the top A grades, and the bulk in the middle, 24 students, could expect to earn C and B grades. Through considerable protests and prayers, management might permit us to squeeze the middle of the curve and award more A grades as well as save our weakest students from expiring in an assessment train wreck, but we had to beg.

Regardless of final figure manipulations, a bad taste was left in the mouths of both students and teachers. Students felt the Bell Curve inherently made the classroom unnecessarily competitive and that success depended not so much on an individual doing well as on others doing badly. It wouldn’t matter if each and every member of my class were the secret spawn of an Einstein cloning experiment; degrees of difference would be conjured up, and otherwise brilliant students would be spread across the Bell Curve like falling blossoms on wet pavement.

At the same time, teachers felt their work was devalued because even the most highly educated, experienced, and dedicated teachers offering the most innovative lessons could not hope to have their students score higher than those of their less-inspired colleagues. Any gains in student achievement attributable to a superb teacher’s work would be washed away in the Bell Curve. Teachers who rebelled against the obvious unfairness of it all faced the subtle punishment of having to provide detailed justifications of deviations from the so-called norm. From the administrative perspective, the Bell Curve helped even out teachers’ grades and avoid grade inflation, the creeping tendency for teachers to award higher marks, in some cases to garner positive evaluations. Continue reading