van Ormer, E. B., & Williams, C. O. (1940). The normal probability curve applied to social statistics. In E. B. van Ormer & C. O. Williams, Elementary statistics for students of education and psychology (pp. 26-33). Ann Arbor, MI, US: Edwards Brothers.
http://dx.doi.org/10.1037/13595-007
This chapter discusses the normal probability curve as it applies to social statistics. The normal probability curve represents the relative frequency of occurrence of the possible effects (in a very large number of occasions of a certain event) of various combinations of a very large number of independent and random factors, when the chances of the occurrence or non-occurrence of a given factor are equal. When a great many independent and random factors, equally likely to occur or not occur, enter into the way in which a particular event will take place, there will be a certain combination of factors which will occur most frequently and the other combinations will occur less and less frequently, some occurring very, very rarely. (PsycINFO Database Record (c) 2016 APA, all rights reserved)