Page:Popular Science Monthly Volume 78.djvu/409

Rh 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. The range for each grade in

percentages was as follows:

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.