Page:Popular Science Monthly Volume 78.djvu/409

This page has been proofread, but needs to be validated.

The curve for these facts was printed and sent to each instructor with an explanation of its meaning, and a superimposed red curve showing in each case precisely how the instructor's distribution differed from the norm. A table was prepared showing the distribution of grades for all courses having 80 or more students.[1] The range for each grade in

PSM V78 D409 Mean and extreme distributions of grades a and e 10.png

percentages was as follows:

A 0.7-20
B 6-39
C 27-62
D 0-31
E 0-20

Accordingly, we have scientific grounds for assuming that a theoretically correct distribution of the grades of college students will approach the normal surface of frequency (Fig. 9, A and B) unless the group is subject to selection. In that case the curve would be skewed negatively or positively as in Fig. 10.

  1. In Bulletin 368 of the University of Wisconsin, Professor Dearborn attempts to justify the normal distribution of grades "from the fact that it is used in actual practise." Two objections may be made to this contention: first, very few instructors do closely approximate the normal distribution; second, as their practises have no scientific basis, any one of them could only by accident indicate the theoretically correct distribution. If, however, all of Professor Dearborn's curves were represented by one, made from thousands of grades by scores of instructors, it would conform more closely to the general biological law of variation than any of the curves he presents.