Ill] ITS VARIABILITY 79 



value by the mean, we get a figure which is independent of 

 any particular units, and which is called the Coefl&cient of Varia- 

 bility. (It is usually multiphed by 100, to make it of a more 

 convenient amount ; and we may then define this coefficient, C, 

 as = a/M X 100.) 



In regard to the growth of man, Pearson has determined this 

 coefficient of variabihty as follows : in male new-born infants, 

 the coefficient in regard to weight is 15-66, and in regard to 

 stature, 6-50 ; in male adults, for weight 10-83, and for stature, 3'66. 

 The amount of variabihty tends, therefore, to decrease with 

 growth or age. 



Similar determinations have been elaborated by Bowditqh, by 

 Boas and Wissler, and by other writers for intermediate ages, 

 especially from about five years old to eighteen, so covering a 

 great part of the whole period of growth in man*. 



Coefficient of Variability (ujM x 100) in Man, at various ages. 



Age 5 6 ■? 8 9 



Stature (Bowditch) ... 4-76 4-60 4-42 4-49 4-40 



„ (Boas and Wissler) 4-15 4-14 4-22 4-37 4-33 



Weight (Bowditch) ... 11-56 10-28 11-08 9-92 11-04 



Age 10 11 12 13 14 



Stature (Bowditch) ... 4-55 4-70 4-90 5-47 5-79 



„ (Boas and Wissler) 4-36 4-54 4-73 5-16 5-57 



Weight (Bowditch) ... 11-60 11-76 13-72 13-60 16-80 



Age 15 16 17 18 



Stature (Bowditch) .... 5-57 4-50 4-55 3-69 



„ (Boas and Wissler) 5-50 4-69 4-27 3-94 



Weight (Bowditch) ... 15-32 13-28 12-96 10-40 



The result is very curious indeed. We see, from Fig. 11, 

 that the curve of variabihty is very similar to what we have called 

 the acceleration-curve (Fig. 4) : that is to say, it descends when the 

 rate of growth diminishes, and rises very markedly again when, in 

 late boyhood, the rate of growth is temporarily accelerated. We 



* Cf. Fr. Boas, Growth of Toronto Children, Bep. of U.S. Comm. of Education, 

 1896-7, pp. 1541-1599, 1898; Boas and Clark Wissler, Statistics of Growth, 

 Education Re}]. 1904, pp. 25-132, 1906 ; H. P. Bowditch, Rep. Mass. State Board 

 of Health, 1877 ; K. Pearson, On the Magnitude of certain coefficients of Correlation 

 in Man, Pr. R. S. lxvi, 1900. 



