
Source: The Concept and the Role of the Model in Mathematics and Natural and Social Sciences (1961), p. 79; Partly cited in: Norman L. Johnson and Samuel Kotz (1977) Urn Models and Their Application: an. Approach to Modern Discrete Probability Theory http://dis.unal.edu.co/~gjhernandezp/sim/hide/Urn%20Models%20and%20Their%20Application%20-%20An%20approach%20to%20modern%20discrete%20probability%20theory_Norman%20L.Johnson(Wiley%201977%20413s).pdf, John Wiley & Sons.